https://www.coastalwiki.org/w/api.php?action=feedcontributions&user=Juliettejackson&feedformat=atomCoastal Wiki - User contributions [en]2024-03-28T20:48:29ZUser contributionsMediaWiki 1.31.7https://www.coastalwiki.org/w/index.php?title=Tsunami&diff=22538Tsunami2008-09-01T15:15:13Z<p>Juliettejackson: </p>
<hr />
<div>{{<br />
Definition|title=Tsunami<br />
|definition= A Tsunami is a seismically generated gravity waves, characterised by [[wave period]]s that are in the order of minutes rather than seconds.}}</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Tsunami&diff=22537Tsunami2008-09-01T15:14:57Z<p>Juliettejackson: </p>
<hr />
<div>{{<br />
Definition|title=Tsunami<br />
|definition= A Tsunami is a seismically generated gravity waves, characterised by [[wave period]] that are in the order of minutes rather than seconds.}}</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Tsunami&diff=22536Tsunami2008-09-01T15:14:39Z<p>Juliettejackson: </p>
<hr />
<div>{{<br />
Definition|title=Tsunami<br />
|definition= A Tsunami is a seismically generated gravity waves, characterised by [[wave periods]] that are in the order of minutes rather than seconds.}}</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Tsunami&diff=22535Tsunami2008-09-01T15:14:02Z<p>Juliettejackson: New page: {{ Definition|title=Tsunami |definition= A Tsunami is a seismically generated gravity waves, characterised by wave periods that are in the order of minutes rather than seconds.}}</p>
<hr />
<div>{{<br />
Definition|title=Tsunami<br />
|definition= A Tsunami is a seismically generated gravity waves, characterised by wave periods that are in the order of minutes rather than seconds.}}</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Help:Basic_setup,_rules_and_guidelines&diff=19103Help:Basic setup, rules and guidelines2008-01-17T12:46:56Z<p>Juliettejackson: </p>
<hr />
<div>Read this article carefully, before you start to edit or write an article. This article explains the rules of the Coastal Wiki, how to write an article and what an excellent article looks like. For more information on editing an article, see [[how to edit an article]]. <br />
<br />
==Basic setup==<br />
===Creating an account===<br />
For obtaining an editing authorisation you may contact your [http://www.encora.eu/networks.php national coordination office] or you may send an email to info@encora.eu. You will receive a request to provide contact information for the Wiki Contact Database. Your account is linked with your personal record in the ENCORA Contact Database. To find your name in the contact database enter your name under 'Your full name'. If you cannot find your name there, contact info@encora.eu, and ask to be added to the contact database. After you have been added to the contact database you will be able to create an account by clinking on the 'create an account' button on the top right of the page. Fill in the details and ensure that when you type in your full name you click on your name which will appear in a dropdown box - this action will create the link between the Wiki and the contact database.<br />
<br />
===Write a new article===<br />
Anonymous contributions to the Coastal Wiki are precluded. Authors, co-authors and editors of articles are explicitly acknowledged. Your contribution will be reviewed by the coordinator of the theme where your article best fits. The theme coordinators also check that your contribution is adequately linked to existing articles on similar or related topics. First, have a search around you topic of interest to check that your proposed article is not duplicating the information contained within an existing article. If this is the case, instead of adding your article, please complement or revise the existing article (see the subsection "Edit other articles" below). If your proposed article is sufficiently different from the current content, identify which theme (from the 10 themes at the [[Main Page]]) your article fits the best and contact the theme-coordinator. You will find the contact information of theme-coordinators at the main page of each theme. If the theme-coordinator confirms that your article would fill a gap, he/she may suggest where to position your article and create a link to a new article for you or you may create a new article yourself. To create a new article, enter the title in the '''search window''' and click on '''Go'''. If an article with this title does not yet exist you may create the article by clicking on '''create this page'''. It is also possible to create a new article by typing the article name between brackets in an existing article: <nowiki>[[Name of new article]]</nowiki>. Click the '''Save page''' button and follow the link which appears in red. If you need to change your title do it before editing; otherwise use '''Move Page''' function in the '''Edit''' box. This creates a new article, to which users are redirected. Always check "what links here" (in the toolbox at the left-hand) before moving a page. Eventually, adjust internal links to the new title. <br />
<br />
Make sure that your article fits the fabric of articles dealing with related topics, especially articles with a more general scope. Refer to these articles by introducing links; you should also consider re-editing related articles to introduce appropriate links to your article (but not more often than strictly necessary). Submit your article for peer review to expert colleagues before entering it in the Coastal Wiki. See also the rules and guidelines below.<br />
<br />
===Edit other articles===<br />
To start editing this or any other page either click the edit link at the top of the page or use the '''edit''' button at the beginning of each section. This takes you to the edit page: a page with a text box containing the wikitext, i.e. the editable source code from which the server produces the webpage. We recommend to contact the author; you will find contact details by clicking on history. In the case of conflicting evidence use the discussion page.<br />
<br />
===Editing and saving===<br />
Have an article prepared in a word document or work as you go. See the [[Basic setup, rules and guidelines#Coastal Wiki rules|Rules]] and [[Basic setup, rules and guidelines#Guidelines|Guidelines]] how a Wiki-article should look like. Make use of [[How to edit an article#Basic Wiki-formatiing|basic Wiki-formatting]] to make links and do simple formatting. You can preview your work as you go, click '''Show preview''' to see how your changes will look before you make them permanent. Repeat the edit/preview process until you are satisfied, then click '''Save page''' and your changes will be immediately applied to the article. It is also important to '''Save page''' often while you are working. The buttons '''Show preview''' and '''Save page''' are found at the bottom of the edit page.<br />
<br />
==Coastal Wiki rules==<br />
Check this rules first, before starting editing and writing!<br />
# Check if an article on the same of a very similar topic exists already (by using the search function). In that case, instead of adding your article, it might be better to complement or revise this existing article. We recommend to contact the author; you will find contact details by clicking on history. In the case of conflicting evidence use the discussion page. <br />
# Make sure that your article fits the fabric of articles dealing with related topics, especially articles with a more general scope. Refer to these articles by introducing links; you should also consider re-editing related articles to introduce appropriate links to your article (but not more often than strictly necessary). <br />
# Important claims or statements need to be substantiated; adding an authoritative reference (or a link to authoritative Internet source) is often better than providing proof in the paper. <br />
# It is recommended to upload important background documents serving as reference material in the CoastWeb Archive (CoastWeb link on the portal). <br />
# Submit your article for peer review to expert colleagues before entering it in the Coastal Wiki; we encourage Coastal Wiki authors to work in teams and to review each other’s contributions. <br />
# Do not copy-and-paste any material that is subject to copyright. <br />
# Coastal Wiki are well-focused and short (typically 500-1000 words); structure your article according to the instructions given in the Guidelines.<br />
<br />
==Guidelines==<br />
# '''Title''': The title should be as short as possible. Note: If you need to change your title do it before editing; otherwise use '''Move Page''' function in the '''Edit''' box. This creates a new article, to which users are redirected. Always check "what links here" (in the toolbox at the left-hand) before moving a page. Eventually, adjust internal links to the new title. <br />
# '''Lead''': An article should start with a concise lead that summarizes the entire topic and prepares the reader for the higher level of detail in the subsequent sections. The lead section should appear before the table of contents, without a heading. So, do not type ‘introduction’ as a heading – just write the introductory paragraph at the start of article. You may include an introduction as well, but this has a different purpose than the summary. <br />
# '''Structure of main text''': an article preferably starts with an introduction, which provides a relevant context (policy, practice, science). Consider also important interactions with other components of the coastal system. After the main text enter a section “See also” with “Internal links”, “External links” and “Further reading”. Close the article with a list with cited references, credit the author and add categories to the article. <br />
# '''Content of an article''': An article should contain illustrations, boxes and tables, if they are appropriate to the subject, with succinct captions and acceptable copyright status. It is of appropriate length (between a few hundred to a few thousand words), staying focused on the main topic without going into unnecessary detail. Appropriate links are created with related articles for information on broader context or on specific details. Don’t enter into details if good references and links to detailed information are available. General tips:<br />
## Separate major sections of article with section headlines and use a hierarchical heading structure <br />
## Use wiki markup, see also format<br />
## Define terms, in a separate, individual definition page, see [[definitions]]<br />
## Add relevant images or graphics, tables with explanatory captions. Preferably an article contains 1 image for every 500 words. <br />
## Use analogies and comparisons to illustrate<br />
<br />
==Excellent article==<br />
An excellent Coastal Wiki article is well written, comprehensive, factually accurate, neutral and stable.<br />
# '''Well written''' means that the prose is compelling, understandable for non-experts, avoids unnecessary technical terms and unexplained abbreviations. <br />
# '''Comprehensive''' means that the article provides relevant context and relationships. <br />
# '''Factually accurate''' means that claims are verifiable against reliable sources and accurately present the related body of published knowledge. Claims are supported with specific evidence and external citations; this involves the provision of a "References" section in which sources are set out and, where appropriate, complemented by online citations. See also the Coastal Wiki Rules. <br />
# '''Neutral''' means that the article presents views fairly and without bias; however, articles need not give minority views equal coverage. <br />
# '''Stable''' means that the article is not the subject of ongoing edit wars and that its content does not change significantly from day to day.<br />
<br />
==See also==<br />
See also the page [[Coastal Wiki:Why, What and for Whom?]] for more information about the background and audience of the Coastal Wiki. <br />
<br />
See [[How to edit an article]] for more information on the editing of an article.</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Help:Basic_setup,_rules_and_guidelines&diff=19102Help:Basic setup, rules and guidelines2008-01-17T12:23:42Z<p>Juliettejackson: </p>
<hr />
<div>Read this article carefully, before you start to edit or write an article. This article explains the rules of the Coastal Wiki, how to write an article and what an excellent article looks like. For more information on editing an article, see [[how to edit an article]]. <br />
<br />
==Basic setup==<br />
===Creating an account===<br />
For obtaining an editing authorisation you may contact your [http://www.encora.eu/networks.php national coordination office] or you may send an email to info@encora.eu. You will receive a request to provide contact information for the Wiki Contact Database. Your account is linked with your personal record in the ENCORA Contact Database. To find your name in the contact database enter your name under 'Your full name'. If you cannot find your name there, contact info@encora.eu, and ask to be added to the contact database. After you have been added to the contact database you will be able to create an account by clinking on the 'create an account' button on the top right of the page. Fill in the details and ensure that when you type in your full name you click on your name which will appear in a dropdown box - this action will create the link between the Wiki and the contact database.<br />
<br />
===Write a new article===<br />
Anonymous contributions to the Coastal Wiki are precluded. Authors, co-authors and editors of articles are explicitly acknowledged. Your contribution will be reviewed by the coordinator of the theme where your article best fits. The theme coordinators also check that your contribution is adequately linked to existing articles on similar or related topics. First, have a search around you topic of interest to check that your proposed article is not a duplicate an existing article. If this is the case, instead of adding your article, please complement or revise this existing article (see the subsection "Edit other articles" below). If your proposed article is sufficiently different from the current content, identify first in which theme (from the 10 themes at the [[Main Page]]) your article fits the best and contact the theme-coordinator. You will find the contact information of theme-coordinators at the main page of each theme. If the theme-coordinator confirms that your article would fill a gap, he/she may create a link to a new article for you or you may create a new article yourself. To create a new article, enter the title in the '''search window''' and click on '''Go'''. If an article with this title does not yet exist you may create the article by clicking on '''create this page'''. It is also possible to create a new article by typing the article name between brackets in an existing article: <nowiki>[[Name of new article]]</nowiki>. Click the '''Save page''' button and follow the link which appears in red. If you need to change your title do it before editing; otherwise use '''Move Page''' function in the '''Edit''' box. This creates a new article, to which users are redirected. Always check "what links here" (in the toolbox at the left-hand) before moving a page. Eventually, adjust internal links to the new title. <br />
<br />
Make sure that your article fits the fabric of articles dealing with related topics, especially articles with a more general scope. Refer to these articles by introducing links; you should also consider re-editing related articles to introduce appropriate links to your article (but not more often than strictly necessary). Submit your article for peer review to expert colleagues before entering it in the Coastal Wiki. See also the rules and guidelines below.<br />
<br />
===Edit other articles===<br />
To start editing this or any other page either click the edit link at the top of the page or use the '''edit''' button at the beginning of each section. This takes you to the edit page: a page with a text box containing the wikitext, i.e. the editable source code from which the server produces the webpage. We recommend to contact the author; you will find contact details by clicking on history. In the case of conflicting evidence use the discussion page.<br />
<br />
===Editing and saving===<br />
Have an article prepared in a word document or work as you go. See the [[Basic setup, rules and guidelines#Coastal Wiki rules|Rules]] and [[Basic setup, rules and guidelines#Guidelines|Guidelines]] how a Wiki-article should look like. Make use of [[How to edit an article#Basic Wiki-formatiing|basic Wiki-formatting]] to make links and do simple formatting. You can preview your work as you go, click '''Show preview''' to see how your changes will look before you make them permanent. Repeat the edit/preview process until you are satisfied, then click '''Save page''' and your changes will be immediately applied to the article. It is also important to '''Save page''' often while you are working. The buttons '''Show preview''' and '''Save page''' are found at the bottom of the edit page.<br />
<br />
==Coastal Wiki rules==<br />
Check this rules first, before starting editing and writing!<br />
# Check if an article on the same of a very similar topic exists already (by using the search function). In that case, instead of adding your article, it might be better to complement or revise this existing article. We recommend to contact the author; you will find contact details by clicking on history. In the case of conflicting evidence use the discussion page. <br />
# Make sure that your article fits the fabric of articles dealing with related topics, especially articles with a more general scope. Refer to these articles by introducing links; you should also consider re-editing related articles to introduce appropriate links to your article (but not more often than strictly necessary). <br />
# Important claims or statements need to be substantiated; adding an authoritative reference (or a link to authoritative Internet source) is often better than providing proof in the paper. <br />
# It is recommended to upload important background documents serving as reference material in the CoastWeb Archive (CoastWeb link on the portal). <br />
# Submit your article for peer review to expert colleagues before entering it in the Coastal Wiki; we encourage Coastal Wiki authors to work in teams and to review each other’s contributions. <br />
# Do not copy-and-paste any material that is subject to copyright. <br />
# Coastal Wiki are well-focused and short (typically 500-1000 words); structure your article according to the instructions given in the Guidelines.<br />
<br />
==Guidelines==<br />
# '''Title''': The title should be as short as possible. Note: If you need to change your title do it before editing; otherwise use '''Move Page''' function in the '''Edit''' box. This creates a new article, to which users are redirected. Always check "what links here" (in the toolbox at the left-hand) before moving a page. Eventually, adjust internal links to the new title. <br />
# '''Lead''': An article should start with a concise lead that summarizes the entire topic and prepares the reader for the higher level of detail in the subsequent sections. The lead section should appear before the table of contents, without a heading. So, do not type ‘introduction’ as a heading – just write the introductory paragraph at the start of article. You may include an introduction as well, but this has a different purpose than the summary. <br />
# '''Structure of main text''': an article preferably starts with an introduction, which provides a relevant context (policy, practice, science). Consider also important interactions with other components of the coastal system. After the main text enter a section “See also” with “Internal links”, “External links” and “Further reading”. Close the article with a list with cited references, credit the author and add categories to the article. <br />
# '''Content of an article''': An article should contain illustrations, boxes and tables, if they are appropriate to the subject, with succinct captions and acceptable copyright status. It is of appropriate length (between a few hundred to a few thousand words), staying focused on the main topic without going into unnecessary detail. Appropriate links are created with related articles for information on broader context or on specific details. Don’t enter into details if good references and links to detailed information are available. General tips:<br />
## Separate major sections of article with section headlines and use a hierarchical heading structure <br />
## Use wiki markup, see also format<br />
## Define terms, in a separate, individual definition page, see [[definitions]]<br />
## Add relevant images or graphics, tables with explanatory captions. Preferably an article contains 1 image for every 500 words. <br />
## Use analogies and comparisons to illustrate<br />
<br />
==Excellent article==<br />
An excellent Coastal Wiki article is well written, comprehensive, factually accurate, neutral and stable.<br />
# '''Well written''' means that the prose is compelling, understandable for non-experts, avoids unnecessary technical terms and unexplained abbreviations. <br />
# '''Comprehensive''' means that the article provides relevant context and relationships. <br />
# '''Factually accurate''' means that claims are verifiable against reliable sources and accurately present the related body of published knowledge. Claims are supported with specific evidence and external citations; this involves the provision of a "References" section in which sources are set out and, where appropriate, complemented by online citations. See also the Coastal Wiki Rules. <br />
# '''Neutral''' means that the article presents views fairly and without bias; however, articles need not give minority views equal coverage. <br />
# '''Stable''' means that the article is not the subject of ongoing edit wars and that its content does not change significantly from day to day.<br />
<br />
==See also==<br />
See also the page [[Coastal Wiki:Why, What and for Whom?]] for more information about the background and audience of the Coastal Wiki. <br />
<br />
See [[How to edit an article]] for more information on the editing of an article.</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Help:Basic_setup,_rules_and_guidelines&diff=19101Help:Basic setup, rules and guidelines2008-01-17T11:54:33Z<p>Juliettejackson: </p>
<hr />
<div>Read this article carefully, before you start to edit or write an article. This article explains the rules of the Coastal Wiki, how to write an article and what an excellent article looks like. For more information on editing an article, see [[how to edit an article]]. <br />
<br />
==Basic setup==<br />
===Creating an account===<br />
For obtaining an editing authorisation you may contact your [http://www.encora.eu/networks.php national coordination office] or you may send an email to info@encora.eu. You will receive a request to provide contact information for the Wiki Contact Database. Anonymous contributions to the Coastal Wiki are precluded. Authors, co-authors and editors of articles are explicitly acknowledged. Your contribution will be reviewed by the coordinator of the theme where your article best fits. The theme coordinators also check that your contribution is adequately linked to existing articles on similar or related topics. Your account is linked with your personal record in the ENCORA Contact Database. To find your name in the contact database enter your name under 'Your full name'. If you cannot find your name there, contact info@encora.eu, and ask for an account in the contact database.<br />
<br />
===Write a new article===<br />
First, check if an article about a similar topic already exists. If this is the case, instead of adding your article, it might be better to complement or revise this existing article (see the subsection "Edit other articles" below). If you want to write an article, identify first in which theme (from the 10 themes at the [[Main Page]]) your article fits the best and contact the theme-coordinator. You will find the contact information of theme-coordinators at the main page of each theme. If the theme-coordinator confirms that your article would fill a gap, he/she may create a link to a new article for you or you may create a new article yourself. To create a new article, enter the title in the '''search window''' and click on '''Go'''. If an article with this title does not yet exist you may create the article by clicking on '''create this page'''. It is also possible to create a new article by typing the article name between brackets in an existing article: <nowiki>[[Name of new article]]</nowiki>. Click the '''Save page''' button and follow the link which appears in red. If you need to change your title do it before editing; otherwise use '''Move Page''' function in the '''Edit''' box. This creates a new article, to which users are redirected. Always check "what links here" (in the toolbox at the left-hand) before moving a page. Eventually, adjust internal links to the new title. <br />
<br />
Make sure that your article fits the fabric of articles dealing with related topics, especially articles with a more general scope. Refer to these articles by introducing links; you should also consider re-editing related articles to introduce appropriate links to your article (but not more often than strictly necessary). Submit your article for peer review to expert colleagues before entering it in the Coastal Wiki. See also the rules and guidelines below.<br />
<br />
===Edit other articles===<br />
To start editing this or any other page either click the edit link at the top of the page or use the '''edit''' button at the beginning of each section. This takes you to the edit page: a page with a text box containing the wikitext, i.e. the editable source code from which the server produces the webpage. We recommend to contact the author; you will find contact details by clicking on history. In the case of conflicting evidence use the discussion page.<br />
<br />
===Editing and saving===<br />
Have an article prepared in a word document or work as you go. See the [[Basic setup, rules and guidelines#Coastal Wiki rules|Rules]] and [[Basic setup, rules and guidelines#Guidelines|Guidelines]] how a Wiki-article should look like. Make use of [[How to edit an article#Basic Wiki-formatiing|basic Wiki-formatting]] to make links and do simple formatting. You can preview your work as you go, click '''Show preview''' to see how your changes will look before you make them permanent. Repeat the edit/preview process until you are satisfied, then click '''Save page''' and your changes will be immediately applied to the article. It is also important to '''Save page''' often while you are working. The buttons '''Show preview''' and '''Save page''' are found at the bottom of the edit page.<br />
<br />
==Coastal Wiki rules==<br />
Check this rules first, before starting editing and writing!<br />
# Check if an article on the same of a very similar topic exists already (by using the search function). In that case, instead of adding your article, it might be better to complement or revise this existing article. We recommend to contact the author; you will find contact details by clicking on history. In the case of conflicting evidence use the discussion page. <br />
# Make sure that your article fits the fabric of articles dealing with related topics, especially articles with a more general scope. Refer to these articles by introducing links; you should also consider re-editing related articles to introduce appropriate links to your article (but not more often than strictly necessary). <br />
# Important claims or statements need to be substantiated; adding an authoritative reference (or a link to authoritative Internet source) is often better than providing proof in the paper. <br />
# It is recommended to upload important background documents serving as reference material in the CoastWeb Archive (CoastWeb link on the portal). <br />
# Submit your article for peer review to expert colleagues before entering it in the Coastal Wiki; we encourage Coastal Wiki authors to work in teams and to review each other’s contributions. <br />
# Do not copy-and-paste any material that is subject to copyright. <br />
# Coastal Wiki are well-focused and short (typically 500-1000 words); structure your article according to the instructions given in the Guidelines.<br />
<br />
==Guidelines==<br />
# '''Title''': The title should be as short as possible. Note: If you need to change your title do it before editing; otherwise use '''Move Page''' function in the '''Edit''' box. This creates a new article, to which users are redirected. Always check "what links here" (in the toolbox at the left-hand) before moving a page. Eventually, adjust internal links to the new title. <br />
# '''Lead''': An article should start with a concise lead that summarizes the entire topic and prepares the reader for the higher level of detail in the subsequent sections. The lead section should appear before the table of contents, without a heading. So, do not type ‘introduction’ as a heading – just write the introductory paragraph at the start of article. You may include an introduction as well, but this has a different purpose than the summary. <br />
# '''Structure of main text''': an article preferably starts with an introduction, which provides a relevant context (policy, practice, science). Consider also important interactions with other components of the coastal system. After the main text enter a section “See also” with “Internal links”, “External links” and “Further reading”. Close the article with a list with cited references, credit the author and add categories to the article. <br />
# '''Content of an article''': An article should contain illustrations, boxes and tables, if they are appropriate to the subject, with succinct captions and acceptable copyright status. It is of appropriate length (between a few hundred to a few thousand words), staying focused on the main topic without going into unnecessary detail. Appropriate links are created with related articles for information on broader context or on specific details. Don’t enter into details if good references and links to detailed information are available. General tips:<br />
## Separate major sections of article with section headlines and use a hierarchical heading structure <br />
## Use wiki markup, see also format<br />
## Define terms, in a separate, individual definition page, see [[definitions]]<br />
## Add relevant images or graphics, tables with explanatory captions. Preferably an article contains 1 image for every 500 words. <br />
## Use analogies and comparisons to illustrate<br />
<br />
==Excellent article==<br />
An excellent Coastal Wiki article is well written, comprehensive, factually accurate, neutral and stable.<br />
# '''Well written''' means that the prose is compelling, understandable for non-experts, avoids unnecessary technical terms and unexplained abbreviations. <br />
# '''Comprehensive''' means that the article provides relevant context and relationships. <br />
# '''Factually accurate''' means that claims are verifiable against reliable sources and accurately present the related body of published knowledge. Claims are supported with specific evidence and external citations; this involves the provision of a "References" section in which sources are set out and, where appropriate, complemented by online citations. See also the Coastal Wiki Rules. <br />
# '''Neutral''' means that the article presents views fairly and without bias; however, articles need not give minority views equal coverage. <br />
# '''Stable''' means that the article is not the subject of ongoing edit wars and that its content does not change significantly from day to day.<br />
<br />
==See also==<br />
See also the page [[Coastal Wiki:Why, What and for Whom?]] for more information about the background and audience of the Coastal Wiki. <br />
<br />
See [[How to edit an article]] for more information on the editing of an article.</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Incident_wave&diff=18918Incident wave2007-12-19T12:37:08Z<p>Juliettejackson: New page: {{Definition|title=Incident wave |definition= Wave moving landward <ref name="Simm">JD, Simm, Brampton, AH, Beech NW, Brooke, JS. 1996. “Beach Management Manual”. CIRIA, Report 153, 27...</p>
<hr />
<div>{{Definition|title=Incident wave<br />
|definition= Wave moving landward <ref name="Simm">JD, Simm, Brampton, AH, Beech NW, Brooke, JS. 1996. “Beach Management Manual”. CIRIA, Report 153, 27pp.</ref>.<br />
}}<br />
<br />
<br />
==References==<br />
<references/></div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Waves&diff=18917Waves2007-12-19T12:36:23Z<p>Juliettejackson: </p>
<hr />
<div>There is typically a distinction between short waves, which are waves with periods less than approximately 20 s, and long waves or long period oscillations, which are oscillations with periods between 20-30 s and 40 min. Water-level oscillations with periods or recurrence intervals larger than around 1 hour, such as [[tide|astronomical tide]] and [[storm surge]], are referred to as water-level variations. The short waves are wind waves and swell, whereas long waves are divided into [[Surf beat|surf-beats]], harbour resonance, [[seiche]] and tsunamis. Natural waves can be viewed as a wave field consisting of a large number of single wave components each characterised by a [[wave height]], a [[wave period]] and a propagation direction. Wave fields with many different wave periods and heights are called irregular, and wave fields with many wave directions are called directional. A wave field can be more or less irregular and more or less directional.<br />
<br />
==Short Waves==<br />
===Types of short waves===<br />
<br />
[[Image:irregular storm a.jpg|thumb|200px|Fig. 1a. Irregular directional storm waves (including white capping)]]<br />
[[Image:irregular storm b.jpg|thumb|200px|Fig. 1b. Regular unidirectional swell.]]<br />
<br />
<ref name="Aagard">Written by Aagard, Troels. 2007.</ref>Short waves are waves, generated by the wind that propagate towards the beach. They can be either actively forced by the wind (wind waves - see below) or they can have left their generation area (swell waves - see below). [[Incident wave|Incident waves]] are the primary source of energy input to the beach. On their way from deep water towards the shoreline they undergo refraction and shoaling processes. In deep water, incident waves are nearly sinusoidal; as they propagate into shallower water (shoaling), their celerity and wave length decrease and as the total energy flux should remain constant (according to linear theory and neglecting bottom friction), the wave height must increase while the wavelength decreases. <br />
<br />
<ref name="Aagard"/>As the waves propagate towards the shoreline, the wave shape becomes increasingly skewed with peaked wave crests and longer, rounded wave troughs, and wave orbital velocities become larger under crests than under troughs. This is a characteristic of fundamental importance to sediment transport, especially seaward of the wave breakpoint as there will be a tendency for the incident waves to push sediment towards the beach. <br />
<br />
The short waves are the single most important parameter in coastal morphology. Wave conditions vary considerably from site to site, depending mainly on the wind climate and on the type of water area. The short waves are divided into:<br />
<br />
*'''Wind waves''', also called storm waves, or sea. These are waves generated and influenced by the local wind field. Wind waves are normally relatively steep (high and short) and are often both irregular and directional, for which reason it is difficult to distinguish defined wave fronts. The waves are also referred to as short-crested. Wind waves tend to be destructive for the coastal profile because they generate an offshore (as opposed to onshore) movement of sediments, which results in a generally flat shoreface and a steep foreshore.<br />
*'''Swell''' are waves, which have been generated by wind fields far away and have travelled long distances over deep water away from the wind field, which generated the waves. Their direction of propagation is thus not necessarily the same as the local wind direction. Swell waves are often relatively long, of moderate height, regular and unidirectional. Swell waves tend to build up the coastal profile to a steep shoreface.<br />
<br />
<br />
<br />
<br />
====Wave breaking====<br />
<ref name="Aagard"/>Depth-limited wave breaking is the prerequisite for the generation of nearshore currents and secondary wave phenomena. Seaward of the surf zone, any wave energy losses primarily occur through whitecapping and friction against the sea bed. As the waves approach the beach, however, depth-limited breaking will occur when orbital velocities, increasing towards the beach exceed the wave phase speed which decreases in the landward direction. The breaking wave height, H<sub>b</sub> is related to the water depth at breaking, h<sub>b</sub>, through<br />
<br />
:H<sub>b</sub>=γh<sub>b</sub><br />
<br />
<br />
where γ is the breaker index. In nature waves are irregular and random and using H<sub>rms</sub> as the measure of wave height, the maximum time-averaged value of the breaker index (<γ<sub>rms</sub>>) is in the order of 0.35-0.5. The fact that wave heights within the surf zone are depth-limited means that wave heights approach a linear function of water depth. <br />
<br />
The predominant type of wave breaking depends on wave steepmness and beach slope expressed through the surf scaling parameter:<br />
<br />
:&epsilon;=&pi;H / Tgtan<sup>2</sup>&beta;<br />
<br />
<br />
where T is wave period, g is the acceleration of gravity and β is the beach slope. With spilling breakers, ε > 20, plunging occurs for 2.5 < ε < 20 and surging breakers predominate when ε < 2.5. <br />
<br />
As waves propagate towards the beach, short wave energy is gradually lost through breaking and long infragravity waves become more important.<br />
<br />
===Wave generation===<br />
Wind waves are generated as a result of the action of the wind on the surface of the water. The wave height, wave period, propagation direction and duration of the wave field at a certain location depend on:<br />
<br />
#The wind field (speed, direction and duration)<br />
#The fetch of the wind field (meteorological fetch) or the water area (geographical fetch)<br />
#The water depth over the wave generation area.<br />
<br />
Swell is, as previously stated, wind waves generated elsewhere but transformed as they propagate away from the generation area. The dissipation processes, such as wave-breaking, attenuate the short period much more than the long period components. This process acts as a filter, whereby the resulting long-crested swell will consist of relatively long waves with moderate wave height.<br />
<br />
===Wave transformation===<br />
[[Wave transformation]]: The types of transformation discussed here are mainly related to wave phenomena occurring in the natural environment. When the waves approach the shoreline, they are affected by the seabed through processes such as refraction, shoaling, bottom friction and wave-breaking. However, wave-breaking also occurs in deep water when the waves are too steep. If the waves meet major structures or abrupt changes in the coastline, they will be transformed by diffraction. If waves meet a submerged reef or structure, they will overtop the reef - please follow the link to article.<br />
<br />
===Statistical description of wave parameters===<br />
[[Statistical description of wave parameters]]: Because of the random nature of natural waves, a statistical description of the waves is normally always used. The individual wave heights often follow the Rayleigh-distribution. Statistical wave parameters are calculated based on this distribution. The most commonly used variables in coastal engineering are described in this section - please follow the link to the article.<br />
<br />
===Wave climate classification according to wind climate===<br />
The different wind climates, which dominate different oceans and regions, cause correspondingly characteristic wave climates. These characteristic wave climates can be classified as follows:<br />
*Storm wave climate. <br />
*Swell climate. <br />
*Monsoon wave climate. <br />
*Tropical cyclone climate. <br />
<br />
For details on these classifications follow the link [[Wave climate classification according to wind climate]].<br />
<br />
==Long Waves==<br />
The long waves are primarily second order phenomena of shallow water wave processes. The four main types of long waves are described in the following.<br />
===Surf beat===<br />
[[image:wave set up_a.jpg|thumb|300px|Fig. 2. Wave set-up]]<br />
Natural waves often show a tendency to wave grouping, where a series of high waves follows a series of low waves. This is especially pronounced on open sea-coasts, where the incoming waves may be of different origins and will thus have a large spreading in wave heights, wave directions, and wave periods (or frequencies). Wave grouping will cause oscillations in the wave set-up with a period corresponding to approx. 6 – 8 times the mean wave period; this phenomenon is called surf-beats. Surf-beats near port entrances are very important in relation to mooring conditions in the port basins and sedimentation in the port entrance.<br />
<br style="clear:both;"/><br />
<br />
===Harbour resonance===<br />
[[image:wave set up_b.jpg|thumb|Fig. 3. Surf beat generated harbour resonance, recorded by a tide gauge]]Harbour resonance is forced oscillation of a confined water body (e.g. a harbour basin or a lagoon) connected to a larger water body (the sea). If long-period oscillations are present in the sea, e.g. due to wave grouping or surf-beats or seiche, large oscillations at the natural frequency of the confined water body may occur. Oscillations at the first harmonic, which are the simplest mode of resonance, are often called the pumping or Helmholz mode. <br />
<br />
Harbour resonance normally has periods in the range of 2 to 10 minutes. It is especially important in connection with the mooring conditions for large vessels, as their resonance period for the so-called surge motion is often close to that of the harbour resonance. In addition the associated water exchange may cause siltation.<br />
<br />
[[image:wave set up_c.jpg|thumb|right|Fig. 4. Circulation caused by the gradient in the wave set-up]]<br />
===Seiche=== <br />
A seiche is the free oscillation of a water body, probably caused by rapid variations in the wind conditions. Seiche can occur in closed water areas, such as lakes or lagoons, and in semi-closed water bodies, such as bays. The period of the seiche oscillation is typically in the range of 2 to 40 minutes. Seiche can influence a port in the same manner as surf-beats. It is important to establish whether seiche is present in an area through field investigations, and if so, to take it into account in the layout of the port. Surf-beat influence within a port is often caused by an inexpedient layout. The influence of surf-beat is not applicable for seiche, as seiche is not limited to the nearshore zone. This means that if seiche motion is present in an area, it will inevitably penetrate the entrance. However, its impact on the port may be minimised through a proper layout.<br />
<br />
===Tsunami===<br />
<!--Threats to the coastal zone, Section 6 links here --><br />
A tsunami is a single wave, which is generated by sub-sea earthquakes; it typically has a period of 5 to 60 minutes. Tsunami waves can travel long distances across the oceans; they are similar to shallow water waves, which means that the speed v is calculated as the square root of the product of the water depth and the acceleration of gravity, v = (gh)<sup>1/2</sup>. Consequently, tsunamis travel very fast in the deep oceans. If the water depth is 5000 m, the speed will be more than 200 m/s or about 800 km/hour. A tsunami is normally not very high in deep water, but when it approaches the coastline, the wave will be shoaling and can reach a height of more than 10 m. Tsunamis are rare and coastal projects seldom take them into account. However, in very sensitive projects, such as nuclear power plants located in the coastal hinterland, the risk must be considered.<br />
<br />
==<ref name="Aagard"/>Infragravity waves==<br />
[[Image:infragravity.jpg|thumb|Fig. 5. Iinfragravity wave orbital velocities at two Danish beaches, plotted as a function of relative water depth. h/h<sub>b</sub> = 0 is at the [[shoreline]], and h/h<sub>b</sub> = 1 indicates the mean position of the wave breakpoint.]]Infragravity waves are waves that are forced by difference interactions in the incident wave frequency band and consequently they have frequencies which are lower than the frequencies of incident waves ~0.005-0.05 Hz. From a morphodynamic viewpoint, much of the interest in infragravity waves is due to the fact that they are often standing in the cross-shore direction and sometimes also alongshore, therefore resulting in a stationary drift velocity field in the bottom boundary layer. They can therefore potentially provide a mechanism for nearshore bar formation and the generation of three-dimensional features such as rip currents and rhythmic bars. Apart from quasi-steady drift velocities in the boundary layer, orbital velocities associated with these motions can generate oscillatory sediment fluxes which have been demonstrated to be important to the net sediment transport in the surf zone. <br />
<br />
Nearshore-standing infragravity waves may occur as either leaky mode waves which are two-dimensional standing waves having a succession of antinodes and nodes away from the point of reflection (e.g. the shoreline), or as edge waves which are three-dimensional waves trapped against the nearshore by reflection and refraction and which can propagate alongshore (progressive edge waves) or be longshore-standing (standing edge waves). Edge waves have a finite number of nodes/antinodes in the cross-shore direction (the number of cross-shore surface elevation nodes is called the mode number, ''n''), and a theoretically infinite number of nodes/antinodes in the longshore dimension. <br />
<br />
Infragravity wave heights and orbital velocities increase towards the shoreline (see Fig. 5. below), while incident wave heights decrease landward due to wave breaking. Infragravity waves should therefore be of increasing relative importance with proximity to the shoreline and sediment resuspension and transport become increasingly affected by infragravity motions. A more comprehensive treatment of nearshore infragravity waves can be found in e.g. Aagaard and Masselink (1999)<ref>Aagaard, T. and Masselink, G., 1999. The Surf Zone. In: A.D.Short (ed) Handbook of Beach and Shoreface Morphodynamics, Wiley Interscience, pp.72-118.</ref>.<br />
<br />
==References==<br />
<references/><br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
<br />
[[category:Theme 5]]<br />
[[Category:Coastal processes, interactions and resources]]<br />
[[Category:Hydrodynamics]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Wave_period&diff=18916Wave period2007-12-19T12:33:12Z<p>Juliettejackson: New page: {{Definition|title=Wave period |definition= The time taken for two successive wave crests to pass at the same point <ref name="Simm">JD, Simm, Brampton, AH, Beech NW, Brooke, JS. 1996. “...</p>
<hr />
<div>{{Definition|title=Wave period<br />
|definition= The time taken for two successive wave crests to pass at the same point <ref name="Simm">JD, Simm, Brampton, AH, Beech NW, Brooke, JS. 1996. “Beach Management Manual”. CIRIA, Report 153, 27pp.</ref>.<br />
}}<br />
<br />
<br />
==References==<br />
<references/></div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Wave_height&diff=18915Wave height2007-12-19T12:32:08Z<p>Juliettejackson: New page: {{Definition|title=Wave height |definition= The vertical distance between the trough and the following crest<ref name="Simm">JD, Simm, Brampton, AH, Beech NW, Brooke, JS. 1996. “Beach Ma...</p>
<hr />
<div>{{Definition|title=Wave height<br />
|definition= The vertical distance between the trough and the following crest<ref name="Simm">JD, Simm, Brampton, AH, Beech NW, Brooke, JS. 1996. “Beach Management Manual”. CIRIA, Report 153, 27pp.</ref>.<br />
}}<br />
<br />
<br />
==References==<br />
<references/></div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Waves&diff=18914Waves2007-12-19T12:31:08Z<p>Juliettejackson: </p>
<hr />
<div>There is typically a distinction between short waves, which are waves with periods less than approximately 20 s, and long waves or long period oscillations, which are oscillations with periods between 20-30 s and 40 min. Water-level oscillations with periods or recurrence intervals larger than around 1 hour, such as [[tide|astronomical tide]] and [[storm surge]], are referred to as water-level variations. The short waves are wind waves and swell, whereas long waves are divided into [[Surf beat|surf-beats]], harbour resonance, [[seiche]] and tsunamis. Natural waves can be viewed as a wave field consisting of a large number of single wave components each characterised by a [[wave height]], a [[wave period]] and a propagation direction. Wave fields with many different wave periods and heights are called irregular, and wave fields with many wave directions are called directional. A wave field can be more or less irregular and more or less directional.<br />
<br />
==Short Waves==<br />
===Types of short waves===<br />
<br />
[[Image:irregular storm a.jpg|thumb|200px|Fig. 1a. Irregular directional storm waves (including white capping)]]<br />
[[Image:irregular storm b.jpg|thumb|200px|Fig. 1b. Regular unidirectional swell.]]<br />
<br />
<ref name="Aagard">Written by Aagard, Troels. 2007.</ref>Short waves are waves, generated by the wind that propagate towards the beach. They can be either actively forced by the wind (wind waves - see below) or they can have left their generation area (swell waves - see below). Incident waves are the primary source of energy input to the beach. On their way from deep water towards the shoreline they undergo refraction and shoaling processes. In deep water, incident waves are nearly sinusoidal; as they propagate into shallower water (shoaling), their celerity and wave length decrease and as the total energy flux should remain constant (according to linear theory and neglecting bottom friction), the wave height must increase while the wavelength decreases. <br />
<br />
<ref name="Aagard"/>As the waves propagate towards the shoreline, the wave shape becomes increasingly skewed with peaked wave crests and longer, rounded wave troughs, and wave orbital velocities become larger under crests than under troughs. This is a characteristic of fundamental importance to sediment transport, especially seaward of the wave breakpoint as there will be a tendency for the incident waves to push sediment towards the beach. <br />
<br />
The short waves are the single most important parameter in coastal morphology. Wave conditions vary considerably from site to site, depending mainly on the wind climate and on the type of water area. The short waves are divided into:<br />
<br />
*'''Wind waves''', also called storm waves, or sea. These are waves generated and influenced by the local wind field. Wind waves are normally relatively steep (high and short) and are often both irregular and directional, for which reason it is difficult to distinguish defined wave fronts. The waves are also referred to as short-crested. Wind waves tend to be destructive for the coastal profile because they generate an offshore (as opposed to onshore) movement of sediments, which results in a generally flat shoreface and a steep foreshore.<br />
*'''Swell''' are waves, which have been generated by wind fields far away and have travelled long distances over deep water away from the wind field, which generated the waves. Their direction of propagation is thus not necessarily the same as the local wind direction. Swell waves are often relatively long, of moderate height, regular and unidirectional. Swell waves tend to build up the coastal profile to a steep shoreface.<br />
<br />
<br />
<br />
<br />
====Wave breaking====<br />
<ref name="Aagard"/>Depth-limited wave breaking is the prerequisite for the generation of nearshore currents and secondary wave phenomena. Seaward of the surf zone, any wave energy losses primarily occur through whitecapping and friction against the sea bed. As the waves approach the beach, however, depth-limited breaking will occur when orbital velocities, increasing towards the beach exceed the wave phase speed which decreases in the landward direction. The breaking wave height, H<sub>b</sub> is related to the water depth at breaking, h<sub>b</sub>, through<br />
<br />
:H<sub>b</sub>=γh<sub>b</sub><br />
<br />
<br />
where γ is the breaker index. In nature waves are irregular and random and using H<sub>rms</sub> as the measure of wave height, the maximum time-averaged value of the breaker index (<γ<sub>rms</sub>>) is in the order of 0.35-0.5. The fact that wave heights within the surf zone are depth-limited means that wave heights approach a linear function of water depth. <br />
<br />
The predominant type of wave breaking depends on wave steepmness and beach slope expressed through the surf scaling parameter:<br />
<br />
:&epsilon;=&pi;H / Tgtan<sup>2</sup>&beta;<br />
<br />
<br />
where T is wave period, g is the acceleration of gravity and β is the beach slope. With spilling breakers, ε > 20, plunging occurs for 2.5 < ε < 20 and surging breakers predominate when ε < 2.5. <br />
<br />
As waves propagate towards the beach, short wave energy is gradually lost through breaking and long infragravity waves become more important.<br />
<br />
===Wave generation===<br />
Wind waves are generated as a result of the action of the wind on the surface of the water. The wave height, wave period, propagation direction and duration of the wave field at a certain location depend on:<br />
<br />
#The wind field (speed, direction and duration)<br />
#The fetch of the wind field (meteorological fetch) or the water area (geographical fetch)<br />
#The water depth over the wave generation area.<br />
<br />
Swell is, as previously stated, wind waves generated elsewhere but transformed as they propagate away from the generation area. The dissipation processes, such as wave-breaking, attenuate the short period much more than the long period components. This process acts as a filter, whereby the resulting long-crested swell will consist of relatively long waves with moderate wave height.<br />
<br />
===Wave transformation===<br />
[[Wave transformation]]: The types of transformation discussed here are mainly related to wave phenomena occurring in the natural environment. When the waves approach the shoreline, they are affected by the seabed through processes such as refraction, shoaling, bottom friction and wave-breaking. However, wave-breaking also occurs in deep water when the waves are too steep. If the waves meet major structures or abrupt changes in the coastline, they will be transformed by diffraction. If waves meet a submerged reef or structure, they will overtop the reef - please follow the link to article.<br />
<br />
===Statistical description of wave parameters===<br />
[[Statistical description of wave parameters]]: Because of the random nature of natural waves, a statistical description of the waves is normally always used. The individual wave heights often follow the Rayleigh-distribution. Statistical wave parameters are calculated based on this distribution. The most commonly used variables in coastal engineering are described in this section - please follow the link to the article.<br />
<br />
===Wave climate classification according to wind climate===<br />
The different wind climates, which dominate different oceans and regions, cause correspondingly characteristic wave climates. These characteristic wave climates can be classified as follows:<br />
*Storm wave climate. <br />
*Swell climate. <br />
*Monsoon wave climate. <br />
*Tropical cyclone climate. <br />
<br />
For details on these classifications follow the link [[Wave climate classification according to wind climate]].<br />
<br />
==Long Waves==<br />
The long waves are primarily second order phenomena of shallow water wave processes. The four main types of long waves are described in the following.<br />
===Surf beat===<br />
[[image:wave set up_a.jpg|thumb|300px|Fig. 2. Wave set-up]]<br />
Natural waves often show a tendency to wave grouping, where a series of high waves follows a series of low waves. This is especially pronounced on open sea-coasts, where the incoming waves may be of different origins and will thus have a large spreading in wave heights, wave directions, and wave periods (or frequencies). Wave grouping will cause oscillations in the wave set-up with a period corresponding to approx. 6 – 8 times the mean wave period; this phenomenon is called surf-beats. Surf-beats near port entrances are very important in relation to mooring conditions in the port basins and sedimentation in the port entrance.<br />
<br style="clear:both;"/><br />
<br />
===Harbour resonance===<br />
[[image:wave set up_b.jpg|thumb|Fig. 3. Surf beat generated harbour resonance, recorded by a tide gauge]]Harbour resonance is forced oscillation of a confined water body (e.g. a harbour basin or a lagoon) connected to a larger water body (the sea). If long-period oscillations are present in the sea, e.g. due to wave grouping or surf-beats or seiche, large oscillations at the natural frequency of the confined water body may occur. Oscillations at the first harmonic, which are the simplest mode of resonance, are often called the pumping or Helmholz mode. <br />
<br />
Harbour resonance normally has periods in the range of 2 to 10 minutes. It is especially important in connection with the mooring conditions for large vessels, as their resonance period for the so-called surge motion is often close to that of the harbour resonance. In addition the associated water exchange may cause siltation.<br />
<br />
[[image:wave set up_c.jpg|thumb|right|Fig. 4. Circulation caused by the gradient in the wave set-up]]<br />
===Seiche=== <br />
A seiche is the free oscillation of a water body, probably caused by rapid variations in the wind conditions. Seiche can occur in closed water areas, such as lakes or lagoons, and in semi-closed water bodies, such as bays. The period of the seiche oscillation is typically in the range of 2 to 40 minutes. Seiche can influence a port in the same manner as surf-beats. It is important to establish whether seiche is present in an area through field investigations, and if so, to take it into account in the layout of the port. Surf-beat influence within a port is often caused by an inexpedient layout. The influence of surf-beat is not applicable for seiche, as seiche is not limited to the nearshore zone. This means that if seiche motion is present in an area, it will inevitably penetrate the entrance. However, its impact on the port may be minimised through a proper layout.<br />
<br />
===Tsunami===<br />
<!--Threats to the coastal zone, Section 6 links here --><br />
A tsunami is a single wave, which is generated by sub-sea earthquakes; it typically has a period of 5 to 60 minutes. Tsunami waves can travel long distances across the oceans; they are similar to shallow water waves, which means that the speed v is calculated as the square root of the product of the water depth and the acceleration of gravity, v = (gh)<sup>1/2</sup>. Consequently, tsunamis travel very fast in the deep oceans. If the water depth is 5000 m, the speed will be more than 200 m/s or about 800 km/hour. A tsunami is normally not very high in deep water, but when it approaches the coastline, the wave will be shoaling and can reach a height of more than 10 m. Tsunamis are rare and coastal projects seldom take them into account. However, in very sensitive projects, such as nuclear power plants located in the coastal hinterland, the risk must be considered.<br />
<br />
==<ref name="Aagard"/>Infragravity waves==<br />
[[Image:infragravity.jpg|thumb|Fig. 5. Iinfragravity wave orbital velocities at two Danish beaches, plotted as a function of relative water depth. h/h<sub>b</sub> = 0 is at the [[shoreline]], and h/h<sub>b</sub> = 1 indicates the mean position of the wave breakpoint.]]Infragravity waves are waves that are forced by difference interactions in the incident wave frequency band and consequently they have frequencies which are lower than the frequencies of incident waves ~0.005-0.05 Hz. From a morphodynamic viewpoint, much of the interest in infragravity waves is due to the fact that they are often standing in the cross-shore direction and sometimes also alongshore, therefore resulting in a stationary drift velocity field in the bottom boundary layer. They can therefore potentially provide a mechanism for nearshore bar formation and the generation of three-dimensional features such as rip currents and rhythmic bars. Apart from quasi-steady drift velocities in the boundary layer, orbital velocities associated with these motions can generate oscillatory sediment fluxes which have been demonstrated to be important to the net sediment transport in the surf zone. <br />
<br />
Nearshore-standing infragravity waves may occur as either leaky mode waves which are two-dimensional standing waves having a succession of antinodes and nodes away from the point of reflection (e.g. the shoreline), or as edge waves which are three-dimensional waves trapped against the nearshore by reflection and refraction and which can propagate alongshore (progressive edge waves) or be longshore-standing (standing edge waves). Edge waves have a finite number of nodes/antinodes in the cross-shore direction (the number of cross-shore surface elevation nodes is called the mode number, ''n''), and a theoretically infinite number of nodes/antinodes in the longshore dimension. <br />
<br />
Infragravity wave heights and orbital velocities increase towards the shoreline (see Fig. 5. below), while incident wave heights decrease landward due to wave breaking. Infragravity waves should therefore be of increasing relative importance with proximity to the shoreline and sediment resuspension and transport become increasingly affected by infragravity motions. A more comprehensive treatment of nearshore infragravity waves can be found in e.g. Aagaard and Masselink (1999)<ref>Aagaard, T. and Masselink, G., 1999. The Surf Zone. In: A.D.Short (ed) Handbook of Beach and Shoreface Morphodynamics, Wiley Interscience, pp.72-118.</ref>.<br />
<br />
==References==<br />
<references/><br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
<br />
[[category:Theme 5]]<br />
[[Category:Coastal processes, interactions and resources]]<br />
[[Category:Hydrodynamics]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Seiche&diff=18913Seiche2007-12-19T12:29:45Z<p>Juliettejackson: New page: {{Definition|title=Seiche |definition= Standing wave oscillation in an effectively closed body of water <ref name="Simm">JD, Simm, Brampton, AH, Beech NW, Brooke, JS. 1996. “Beach Manage...</p>
<hr />
<div>{{Definition|title=Seiche<br />
|definition= Standing wave oscillation in an effectively closed body of water <ref name="Simm">JD, Simm, Brampton, AH, Beech NW, Brooke, JS. 1996. “Beach Management Manual”. CIRIA, Report 153, 26pp.</ref>.<br />
}}<br />
<br />
<br />
==References==<br />
<references/></div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Waves&diff=18912Waves2007-12-19T12:28:34Z<p>Juliettejackson: </p>
<hr />
<div>There is typically a distinction between short waves, which are waves with periods less than approximately 20 s, and long waves or long period oscillations, which are oscillations with periods between 20-30 s and 40 min. Water-level oscillations with periods or recurrence intervals larger than around 1 hour, such as [[tide|astronomical tide]] and [[storm surge]], are referred to as water-level variations. The short waves are wind waves and swell, whereas long waves are divided into [[Surf beat|surf-beats]], harbour resonance, [[seiche]] and tsunamis. Natural waves can be viewed as a wave field consisting of a large number of single wave components each characterised by a wave height, a wave period and a propagation direction. Wave fields with many different wave periods and heights are called irregular, and wave fields with many wave directions are called directional. A wave field can be more or less irregular and more or less directional.<br />
<br />
==Short Waves==<br />
===Types of short waves===<br />
<br />
[[Image:irregular storm a.jpg|thumb|200px|Fig. 1a. Irregular directional storm waves (including white capping)]]<br />
[[Image:irregular storm b.jpg|thumb|200px|Fig. 1b. Regular unidirectional swell.]]<br />
<br />
<ref name="Aagard">Written by Aagard, Troels. 2007.</ref>Short waves are waves, generated by the wind that propagate towards the beach. They can be either actively forced by the wind (wind waves - see below) or they can have left their generation area (swell waves - see below). Incident waves are the primary source of energy input to the beach. On their way from deep water towards the shoreline they undergo refraction and shoaling processes. In deep water, incident waves are nearly sinusoidal; as they propagate into shallower water (shoaling), their celerity and wave length decrease and as the total energy flux should remain constant (according to linear theory and neglecting bottom friction), the wave height must increase while the wavelength decreases. <br />
<br />
<ref name="Aagard"/>As the waves propagate towards the shoreline, the wave shape becomes increasingly skewed with peaked wave crests and longer, rounded wave troughs, and wave orbital velocities become larger under crests than under troughs. This is a characteristic of fundamental importance to sediment transport, especially seaward of the wave breakpoint as there will be a tendency for the incident waves to push sediment towards the beach. <br />
<br />
The short waves are the single most important parameter in coastal morphology. Wave conditions vary considerably from site to site, depending mainly on the wind climate and on the type of water area. The short waves are divided into:<br />
<br />
*'''Wind waves''', also called storm waves, or sea. These are waves generated and influenced by the local wind field. Wind waves are normally relatively steep (high and short) and are often both irregular and directional, for which reason it is difficult to distinguish defined wave fronts. The waves are also referred to as short-crested. Wind waves tend to be destructive for the coastal profile because they generate an offshore (as opposed to onshore) movement of sediments, which results in a generally flat shoreface and a steep foreshore.<br />
*'''Swell''' are waves, which have been generated by wind fields far away and have travelled long distances over deep water away from the wind field, which generated the waves. Their direction of propagation is thus not necessarily the same as the local wind direction. Swell waves are often relatively long, of moderate height, regular and unidirectional. Swell waves tend to build up the coastal profile to a steep shoreface.<br />
<br />
<br />
<br />
<br />
====Wave breaking====<br />
<ref name="Aagard"/>Depth-limited wave breaking is the prerequisite for the generation of nearshore currents and secondary wave phenomena. Seaward of the surf zone, any wave energy losses primarily occur through whitecapping and friction against the sea bed. As the waves approach the beach, however, depth-limited breaking will occur when orbital velocities, increasing towards the beach exceed the wave phase speed which decreases in the landward direction. The breaking wave height, H<sub>b</sub> is related to the water depth at breaking, h<sub>b</sub>, through<br />
<br />
:H<sub>b</sub>=γh<sub>b</sub><br />
<br />
<br />
where γ is the breaker index. In nature waves are irregular and random and using H<sub>rms</sub> as the measure of wave height, the maximum time-averaged value of the breaker index (<γ<sub>rms</sub>>) is in the order of 0.35-0.5. The fact that wave heights within the surf zone are depth-limited means that wave heights approach a linear function of water depth. <br />
<br />
The predominant type of wave breaking depends on wave steepmness and beach slope expressed through the surf scaling parameter:<br />
<br />
:&epsilon;=&pi;H / Tgtan<sup>2</sup>&beta;<br />
<br />
<br />
where T is wave period, g is the acceleration of gravity and β is the beach slope. With spilling breakers, ε > 20, plunging occurs for 2.5 < ε < 20 and surging breakers predominate when ε < 2.5. <br />
<br />
As waves propagate towards the beach, short wave energy is gradually lost through breaking and long infragravity waves become more important.<br />
<br />
===Wave generation===<br />
Wind waves are generated as a result of the action of the wind on the surface of the water. The wave height, wave period, propagation direction and duration of the wave field at a certain location depend on:<br />
<br />
#The wind field (speed, direction and duration)<br />
#The fetch of the wind field (meteorological fetch) or the water area (geographical fetch)<br />
#The water depth over the wave generation area.<br />
<br />
Swell is, as previously stated, wind waves generated elsewhere but transformed as they propagate away from the generation area. The dissipation processes, such as wave-breaking, attenuate the short period much more than the long period components. This process acts as a filter, whereby the resulting long-crested swell will consist of relatively long waves with moderate wave height.<br />
<br />
===Wave transformation===<br />
[[Wave transformation]]: The types of transformation discussed here are mainly related to wave phenomena occurring in the natural environment. When the waves approach the shoreline, they are affected by the seabed through processes such as refraction, shoaling, bottom friction and wave-breaking. However, wave-breaking also occurs in deep water when the waves are too steep. If the waves meet major structures or abrupt changes in the coastline, they will be transformed by diffraction. If waves meet a submerged reef or structure, they will overtop the reef - please follow the link to article.<br />
<br />
===Statistical description of wave parameters===<br />
[[Statistical description of wave parameters]]: Because of the random nature of natural waves, a statistical description of the waves is normally always used. The individual wave heights often follow the Rayleigh-distribution. Statistical wave parameters are calculated based on this distribution. The most commonly used variables in coastal engineering are described in this section - please follow the link to the article.<br />
<br />
===Wave climate classification according to wind climate===<br />
The different wind climates, which dominate different oceans and regions, cause correspondingly characteristic wave climates. These characteristic wave climates can be classified as follows:<br />
*Storm wave climate. <br />
*Swell climate. <br />
*Monsoon wave climate. <br />
*Tropical cyclone climate. <br />
<br />
For details on these classifications follow the link [[Wave climate classification according to wind climate]].<br />
<br />
==Long Waves==<br />
The long waves are primarily second order phenomena of shallow water wave processes. The four main types of long waves are described in the following.<br />
===Surf beat===<br />
[[image:wave set up_a.jpg|thumb|300px|Fig. 2. Wave set-up]]<br />
Natural waves often show a tendency to wave grouping, where a series of high waves follows a series of low waves. This is especially pronounced on open sea-coasts, where the incoming waves may be of different origins and will thus have a large spreading in wave heights, wave directions, and wave periods (or frequencies). Wave grouping will cause oscillations in the wave set-up with a period corresponding to approx. 6 – 8 times the mean wave period; this phenomenon is called surf-beats. Surf-beats near port entrances are very important in relation to mooring conditions in the port basins and sedimentation in the port entrance.<br />
<br style="clear:both;"/><br />
<br />
===Harbour resonance===<br />
[[image:wave set up_b.jpg|thumb|Fig. 3. Surf beat generated harbour resonance, recorded by a tide gauge]]Harbour resonance is forced oscillation of a confined water body (e.g. a harbour basin or a lagoon) connected to a larger water body (the sea). If long-period oscillations are present in the sea, e.g. due to wave grouping or surf-beats or seiche, large oscillations at the natural frequency of the confined water body may occur. Oscillations at the first harmonic, which are the simplest mode of resonance, are often called the pumping or Helmholz mode. <br />
<br />
Harbour resonance normally has periods in the range of 2 to 10 minutes. It is especially important in connection with the mooring conditions for large vessels, as their resonance period for the so-called surge motion is often close to that of the harbour resonance. In addition the associated water exchange may cause siltation.<br />
<br />
[[image:wave set up_c.jpg|thumb|right|Fig. 4. Circulation caused by the gradient in the wave set-up]]<br />
===Seiche=== <br />
A seiche is the free oscillation of a water body, probably caused by rapid variations in the wind conditions. Seiche can occur in closed water areas, such as lakes or lagoons, and in semi-closed water bodies, such as bays. The period of the seiche oscillation is typically in the range of 2 to 40 minutes. Seiche can influence a port in the same manner as surf-beats. It is important to establish whether seiche is present in an area through field investigations, and if so, to take it into account in the layout of the port. Surf-beat influence within a port is often caused by an inexpedient layout. The influence of surf-beat is not applicable for seiche, as seiche is not limited to the nearshore zone. This means that if seiche motion is present in an area, it will inevitably penetrate the entrance. However, its impact on the port may be minimised through a proper layout.<br />
<br />
===Tsunami===<br />
<!--Threats to the coastal zone, Section 6 links here --><br />
A tsunami is a single wave, which is generated by sub-sea earthquakes; it typically has a period of 5 to 60 minutes. Tsunami waves can travel long distances across the oceans; they are similar to shallow water waves, which means that the speed v is calculated as the square root of the product of the water depth and the acceleration of gravity, v = (gh)<sup>1/2</sup>. Consequently, tsunamis travel very fast in the deep oceans. If the water depth is 5000 m, the speed will be more than 200 m/s or about 800 km/hour. A tsunami is normally not very high in deep water, but when it approaches the coastline, the wave will be shoaling and can reach a height of more than 10 m. Tsunamis are rare and coastal projects seldom take them into account. However, in very sensitive projects, such as nuclear power plants located in the coastal hinterland, the risk must be considered.<br />
<br />
==<ref name="Aagard"/>Infragravity waves==<br />
[[Image:infragravity.jpg|thumb|Fig. 5. Iinfragravity wave orbital velocities at two Danish beaches, plotted as a function of relative water depth. h/h<sub>b</sub> = 0 is at the [[shoreline]], and h/h<sub>b</sub> = 1 indicates the mean position of the wave breakpoint.]]Infragravity waves are waves that are forced by difference interactions in the incident wave frequency band and consequently they have frequencies which are lower than the frequencies of incident waves ~0.005-0.05 Hz. From a morphodynamic viewpoint, much of the interest in infragravity waves is due to the fact that they are often standing in the cross-shore direction and sometimes also alongshore, therefore resulting in a stationary drift velocity field in the bottom boundary layer. They can therefore potentially provide a mechanism for nearshore bar formation and the generation of three-dimensional features such as rip currents and rhythmic bars. Apart from quasi-steady drift velocities in the boundary layer, orbital velocities associated with these motions can generate oscillatory sediment fluxes which have been demonstrated to be important to the net sediment transport in the surf zone. <br />
<br />
Nearshore-standing infragravity waves may occur as either leaky mode waves which are two-dimensional standing waves having a succession of antinodes and nodes away from the point of reflection (e.g. the shoreline), or as edge waves which are three-dimensional waves trapped against the nearshore by reflection and refraction and which can propagate alongshore (progressive edge waves) or be longshore-standing (standing edge waves). Edge waves have a finite number of nodes/antinodes in the cross-shore direction (the number of cross-shore surface elevation nodes is called the mode number, ''n''), and a theoretically infinite number of nodes/antinodes in the longshore dimension. <br />
<br />
Infragravity wave heights and orbital velocities increase towards the shoreline (see Fig. 5. below), while incident wave heights decrease landward due to wave breaking. Infragravity waves should therefore be of increasing relative importance with proximity to the shoreline and sediment resuspension and transport become increasingly affected by infragravity motions. A more comprehensive treatment of nearshore infragravity waves can be found in e.g. Aagaard and Masselink (1999)<ref>Aagaard, T. and Masselink, G., 1999. The Surf Zone. In: A.D.Short (ed) Handbook of Beach and Shoreface Morphodynamics, Wiley Interscience, pp.72-118.</ref>.<br />
<br />
==References==<br />
<references/><br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
<br />
[[category:Theme 5]]<br />
[[Category:Coastal processes, interactions and resources]]<br />
[[Category:Hydrodynamics]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Surf_beat&diff=18911Surf beat2007-12-19T12:16:26Z<p>Juliettejackson: </p>
<hr />
<div>{{Definition|title=Surf beat<br />
|definition= Independent long wave caused by reflection of bound long wave <ref name="Simm">JD, Simm, Brampton, AH, Beech NW, Brooke, JS. 1996. “Beach Management Manual”. CIRIA, Report 153, 27pp.</ref>.<br />
}}<br />
<br />
<br />
==References==<br />
<references/></div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Surf_beat&diff=18910Surf beat2007-12-19T12:16:01Z<p>Juliettejackson: </p>
<hr />
<div>{{Definition|title=Surf Beat<br />
|definition= Independent long wave caused by reflection of bound long wave <ref name="Simm">JD, Simm, Brampton, AH, Beech NW, Brooke, JS. 1996. “Beach Management Manual”. CIRIA, Report 153, 27pp.</ref>.<br />
}}<br />
<br />
<br />
==References==<br />
<references/></div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Surf_beat&diff=18909Surf beat2007-12-19T12:14:19Z<p>Juliettejackson: New page: {{Definition|title=Surf Beat |definition= Independent long wave caused by reflection of bound long wave <ref name="Karsten">Mangor, Karsten. 2004. “Beach Management Manual”. CIRIA, 27p...</p>
<hr />
<div>{{Definition|title=Surf Beat<br />
|definition= Independent long wave caused by reflection of bound long wave <ref name="Karsten">Mangor, Karsten. 2004. “Beach Management Manual”. CIRIA, 27pp.</ref>.<br />
}}<br />
<br />
<br />
==References==<br />
<references/></div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Waves&diff=18908Waves2007-12-19T12:11:36Z<p>Juliettejackson: </p>
<hr />
<div>There is typically a distinction between short waves, which are waves with periods less than approximately 20 s, and long waves or long period oscillations, which are oscillations with periods between 20-30 s and 40 min. Water-level oscillations with periods or recurrence intervals larger than around 1 hour, such as [[tide|astronomical tide]] and [[storm surge]], are referred to as water-level variations. The short waves are wind waves and swell, whereas long waves are divided into [[Surf beat|surf-beats]], harbour resonance, seiche and tsunamis. Natural waves can be viewed as a wave field consisting of a large number of single wave components each characterised by a wave height, a wave period and a propagation direction. Wave fields with many different wave periods and heights are called irregular, and wave fields with many wave directions are called directional. A wave field can be more or less irregular and more or less directional.<br />
<br />
==Short Waves==<br />
===Types of short waves===<br />
<br />
[[Image:irregular storm a.jpg|thumb|200px|Fig. 1a. Irregular directional storm waves (including white capping)]]<br />
[[Image:irregular storm b.jpg|thumb|200px|Fig. 1b. Regular unidirectional swell.]]<br />
<br />
<ref name="Aagard">Written by Aagard, Troels. 2007.</ref>Short waves are waves, generated by the wind that propagate towards the beach. They can be either actively forced by the wind (wind waves - see below) or they can have left their generation area (swell waves - see below). Incident waves are the primary source of energy input to the beach. On their way from deep water towards the shoreline they undergo refraction and shoaling processes. In deep water, incident waves are nearly sinusoidal; as they propagate into shallower water (shoaling), their celerity and wave length decrease and as the total energy flux should remain constant (according to linear theory and neglecting bottom friction), the wave height must increase while the wavelength decreases. <br />
<br />
<ref name="Aagard"/>As the waves propagate towards the shoreline, the wave shape becomes increasingly skewed with peaked wave crests and longer, rounded wave troughs, and wave orbital velocities become larger under crests than under troughs. This is a characteristic of fundamental importance to sediment transport, especially seaward of the wave breakpoint as there will be a tendency for the incident waves to push sediment towards the beach. <br />
<br />
The short waves are the single most important parameter in coastal morphology. Wave conditions vary considerably from site to site, depending mainly on the wind climate and on the type of water area. The short waves are divided into:<br />
<br />
*'''Wind waves''', also called storm waves, or sea. These are waves generated and influenced by the local wind field. Wind waves are normally relatively steep (high and short) and are often both irregular and directional, for which reason it is difficult to distinguish defined wave fronts. The waves are also referred to as short-crested. Wind waves tend to be destructive for the coastal profile because they generate an offshore (as opposed to onshore) movement of sediments, which results in a generally flat shoreface and a steep foreshore.<br />
*'''Swell''' are waves, which have been generated by wind fields far away and have travelled long distances over deep water away from the wind field, which generated the waves. Their direction of propagation is thus not necessarily the same as the local wind direction. Swell waves are often relatively long, of moderate height, regular and unidirectional. Swell waves tend to build up the coastal profile to a steep shoreface.<br />
<br />
<br />
<br />
<br />
====Wave breaking====<br />
<ref name="Aagard"/>Depth-limited wave breaking is the prerequisite for the generation of nearshore currents and secondary wave phenomena. Seaward of the surf zone, any wave energy losses primarily occur through whitecapping and friction against the sea bed. As the waves approach the beach, however, depth-limited breaking will occur when orbital velocities, increasing towards the beach exceed the wave phase speed which decreases in the landward direction. The breaking wave height, H<sub>b</sub> is related to the water depth at breaking, h<sub>b</sub>, through<br />
<br />
:H<sub>b</sub>=γh<sub>b</sub><br />
<br />
<br />
where γ is the breaker index. In nature waves are irregular and random and using H<sub>rms</sub> as the measure of wave height, the maximum time-averaged value of the breaker index (<γ<sub>rms</sub>>) is in the order of 0.35-0.5. The fact that wave heights within the surf zone are depth-limited means that wave heights approach a linear function of water depth. <br />
<br />
The predominant type of wave breaking depends on wave steepmness and beach slope expressed through the surf scaling parameter:<br />
<br />
:&epsilon;=&pi;H / Tgtan<sup>2</sup>&beta;<br />
<br />
<br />
where T is wave period, g is the acceleration of gravity and β is the beach slope. With spilling breakers, ε > 20, plunging occurs for 2.5 < ε < 20 and surging breakers predominate when ε < 2.5. <br />
<br />
As waves propagate towards the beach, short wave energy is gradually lost through breaking and long infragravity waves become more important.<br />
<br />
===Wave generation===<br />
Wind waves are generated as a result of the action of the wind on the surface of the water. The wave height, wave period, propagation direction and duration of the wave field at a certain location depend on:<br />
<br />
#The wind field (speed, direction and duration)<br />
#The fetch of the wind field (meteorological fetch) or the water area (geographical fetch)<br />
#The water depth over the wave generation area.<br />
<br />
Swell is, as previously stated, wind waves generated elsewhere but transformed as they propagate away from the generation area. The dissipation processes, such as wave-breaking, attenuate the short period much more than the long period components. This process acts as a filter, whereby the resulting long-crested swell will consist of relatively long waves with moderate wave height.<br />
<br />
===Wave transformation===<br />
[[Wave transformation]]: The types of transformation discussed here are mainly related to wave phenomena occurring in the natural environment. When the waves approach the shoreline, they are affected by the seabed through processes such as refraction, shoaling, bottom friction and wave-breaking. However, wave-breaking also occurs in deep water when the waves are too steep. If the waves meet major structures or abrupt changes in the coastline, they will be transformed by diffraction. If waves meet a submerged reef or structure, they will overtop the reef - please follow the link to article.<br />
<br />
===Statistical description of wave parameters===<br />
[[Statistical description of wave parameters]]: Because of the random nature of natural waves, a statistical description of the waves is normally always used. The individual wave heights often follow the Rayleigh-distribution. Statistical wave parameters are calculated based on this distribution. The most commonly used variables in coastal engineering are described in this section - please follow the link to the article.<br />
<br />
===Wave climate classification according to wind climate===<br />
The different wind climates, which dominate different oceans and regions, cause correspondingly characteristic wave climates. These characteristic wave climates can be classified as follows:<br />
*Storm wave climate. <br />
*Swell climate. <br />
*Monsoon wave climate. <br />
*Tropical cyclone climate. <br />
<br />
For details on these classifications follow the link [[Wave climate classification according to wind climate]].<br />
<br />
==Long Waves==<br />
The long waves are primarily second order phenomena of shallow water wave processes. The four main types of long waves are described in the following.<br />
===Surf beat===<br />
[[image:wave set up_a.jpg|thumb|300px|Fig. 2. Wave set-up]]<br />
Natural waves often show a tendency to wave grouping, where a series of high waves follows a series of low waves. This is especially pronounced on open sea-coasts, where the incoming waves may be of different origins and will thus have a large spreading in wave heights, wave directions, and wave periods (or frequencies). Wave grouping will cause oscillations in the wave set-up with a period corresponding to approx. 6 – 8 times the mean wave period; this phenomenon is called surf-beats. Surf-beats near port entrances are very important in relation to mooring conditions in the port basins and sedimentation in the port entrance.<br />
<br style="clear:both;"/><br />
<br />
===Harbour resonance===<br />
[[image:wave set up_b.jpg|thumb|Fig. 3. Surf beat generated harbour resonance, recorded by a tide gauge]]Harbour resonance is forced oscillation of a confined water body (e.g. a harbour basin or a lagoon) connected to a larger water body (the sea). If long-period oscillations are present in the sea, e.g. due to wave grouping or surf-beats or seiche, large oscillations at the natural frequency of the confined water body may occur. Oscillations at the first harmonic, which are the simplest mode of resonance, are often called the pumping or Helmholz mode. <br />
<br />
Harbour resonance normally has periods in the range of 2 to 10 minutes. It is especially important in connection with the mooring conditions for large vessels, as their resonance period for the so-called surge motion is often close to that of the harbour resonance. In addition the associated water exchange may cause siltation.<br />
<br />
[[image:wave set up_c.jpg|thumb|right|Fig. 4. Circulation caused by the gradient in the wave set-up]]<br />
===Seiche=== <br />
A seiche is the free oscillation of a water body, probably caused by rapid variations in the wind conditions. Seiche can occur in closed water areas, such as lakes or lagoons, and in semi-closed water bodies, such as bays. The period of the seiche oscillation is typically in the range of 2 to 40 minutes. Seiche can influence a port in the same manner as surf-beats. It is important to establish whether seiche is present in an area through field investigations, and if so, to take it into account in the layout of the port. Surf-beat influence within a port is often caused by an inexpedient layout. The influence of surf-beat is not applicable for seiche, as seiche is not limited to the nearshore zone. This means that if seiche motion is present in an area, it will inevitably penetrate the entrance. However, its impact on the port may be minimised through a proper layout.<br />
<br />
===Tsunami===<br />
<!--Threats to the coastal zone, Section 6 links here --><br />
A tsunami is a single wave, which is generated by sub-sea earthquakes; it typically has a period of 5 to 60 minutes. Tsunami waves can travel long distances across the oceans; they are similar to shallow water waves, which means that the speed v is calculated as the square root of the product of the water depth and the acceleration of gravity, v = (gh)<sup>1/2</sup>. Consequently, tsunamis travel very fast in the deep oceans. If the water depth is 5000 m, the speed will be more than 200 m/s or about 800 km/hour. A tsunami is normally not very high in deep water, but when it approaches the coastline, the wave will be shoaling and can reach a height of more than 10 m. Tsunamis are rare and coastal projects seldom take them into account. However, in very sensitive projects, such as nuclear power plants located in the coastal hinterland, the risk must be considered.<br />
<br />
==<ref name="Aagard"/>Infragravity waves==<br />
[[Image:infragravity.jpg|thumb|Fig. 5. Iinfragravity wave orbital velocities at two Danish beaches, plotted as a function of relative water depth. h/h<sub>b</sub> = 0 is at the [[shoreline]], and h/h<sub>b</sub> = 1 indicates the mean position of the wave breakpoint.]]Infragravity waves are waves that are forced by difference interactions in the incident wave frequency band and consequently they have frequencies which are lower than the frequencies of incident waves ~0.005-0.05 Hz. From a morphodynamic viewpoint, much of the interest in infragravity waves is due to the fact that they are often standing in the cross-shore direction and sometimes also alongshore, therefore resulting in a stationary drift velocity field in the bottom boundary layer. They can therefore potentially provide a mechanism for nearshore bar formation and the generation of three-dimensional features such as rip currents and rhythmic bars. Apart from quasi-steady drift velocities in the boundary layer, orbital velocities associated with these motions can generate oscillatory sediment fluxes which have been demonstrated to be important to the net sediment transport in the surf zone. <br />
<br />
Nearshore-standing infragravity waves may occur as either leaky mode waves which are two-dimensional standing waves having a succession of antinodes and nodes away from the point of reflection (e.g. the shoreline), or as edge waves which are three-dimensional waves trapped against the nearshore by reflection and refraction and which can propagate alongshore (progressive edge waves) or be longshore-standing (standing edge waves). Edge waves have a finite number of nodes/antinodes in the cross-shore direction (the number of cross-shore surface elevation nodes is called the mode number, ''n''), and a theoretically infinite number of nodes/antinodes in the longshore dimension. <br />
<br />
Infragravity wave heights and orbital velocities increase towards the shoreline (see Fig. 5. below), while incident wave heights decrease landward due to wave breaking. Infragravity waves should therefore be of increasing relative importance with proximity to the shoreline and sediment resuspension and transport become increasingly affected by infragravity motions. A more comprehensive treatment of nearshore infragravity waves can be found in e.g. Aagaard and Masselink (1999)<ref>Aagaard, T. and Masselink, G., 1999. The Surf Zone. In: A.D.Short (ed) Handbook of Beach and Shoreface Morphodynamics, Wiley Interscience, pp.72-118.</ref>.<br />
<br />
==References==<br />
<references/><br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
<br />
[[category:Theme 5]]<br />
[[Category:Coastal processes, interactions and resources]]<br />
[[Category:Hydrodynamics]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Coastal_Hydrodynamics_And_Transport_Processes&diff=18904Coastal Hydrodynamics And Transport Processes2007-12-18T15:28:57Z<p>Juliettejackson: </p>
<hr />
<div>The hydrodynamic conditions or processes, that come about from [[Tidal wave|waves]] transforming over a coastal profile and generating wave set up and [[Longshore current|longshore currents]], will result in movement and transport of the sediments (e.g. sand) present in the profile. This is referred to as ''littoral transport processes'' and is the main subject of this article. However, transport of fine sediments will also be discussed, but only very briefly.<br />
<br />
==Sediment transport in general==<br />
The sediment on the seabed is transported when it is exposed to large enough forces, or ''shear stresses'', by the water movements. These movements can be caused by the current or by the wave orbital velocities or a combination of both, the latter being the most important situation. The relevant parameters for the description of the sediment transport along a shoreline or in a coastal area are therefore the following:<br />
<br />
*The wave conditions at the site and the possible variations over the site plus the adjoining areas<br />
*The current conditions as well as the variations of these over the area <br />
*The water-level conditions, i.e. tide, storm surge and wave set-up<br />
*The bathymetry (the depth variations) in the area <br />
*The sediment characteristics over the area <br />
*The sources and sinks of sediment, such as rivers, eroding coasts or tidal inlets<br />
<br />
<br />
<ref name="Aagard">Written by Aagard, Troels. 2007.</ref>The link between hydrodynamic forcing and the morphological response of the beach is realized through the transport of sediment. This transport may occur as either bedload or as suspended load. Even though modelling efforts suggest that suspended load is the more important within the surf zone, particularly under high energy conditions, there is still no consensus on the subject, partly because bedload is very difficult to measure under field conditions. <br />
<br />
<ref name="Aagard"/>While suspended sediment transport may be easier to measure then bedload it is not a simple matter to predict, one of the main reasons being that the velocity field in the nearshore zone is oscillatory and that sediment resuspension (and transport) responds non-linearly to the forcing; further the existence of bedforms may induce phase changes between velocity and sediment concentration. <br />
<br />
<ref name="Aagard"/>Sediment concentrations within the surf zone have been found to depend upon elevation above the bed, bed configuration and local bed shear stress as well as wave breaker type. Sediment is mainly resuspended by the large shear stresses generated by wave motions while mean currents play a subordinate role. The reason is that in the nearshore region, oscillatory velocities are generally significantly larger than mean current velocities and the wave boundary layer thickness is small (resulting in steep velocity gradients) compared to the thickness of the current boundary layer. <br />
<br />
<ref name="Aagard"/>Physically, sediment can be resuspended from the seabed in a number of ways. Close to the bed, small turbulent eddies created by bed friction mobilize the sediment which can be mixed to higher elevations by turbulence generated by breaking waves. Hence, sediment concentration profile gradients are different under breaking and non-breaking waves. In the latter case, suspended sediment is confined to a relatively thin layer near the bed. <br />
<br />
<ref name="Aagard"/>The distribution of sediment in the water column may occur either as a diffusive process which generally occurs when the bed is flat and devoid of bedforms, or as a convective process whereby sediment is suspended and lifted upward in coherent packages e.g. due to vortex shedding from ripples at the bed. In both cases, the vertical distribution of sediment in the water column can be approximated by:<br />
<br />
<br />
<br />
:<math>w_{s}c+\varepsilon_{s}(\delta_{c}/\delta_{z})= 0</math> <br />
<br />
<br />
where <math>w_{s}</math> is the fall velocity of the sediment, c is the sediment concentration, z is elevation above the bed and <math>\varepsilon_{s}</math> is an eddy diffusion coefficient representing the scale of turbulent mixing. This equation predicts that in a steady state situation, the downward flux of sediment (<math>w_{s}c</math>) must be balanced by an upward flux given by the product of the concentration gradient and the sediment diffusion.<br />
<br />
==Littoral Transport==<br />
[[Image:Littoral_b.jpg|thumb|400px|Fig. 1. Variation in littoral transport with wave exposure and wave incidence angle]]''Littoral transport'' is the term used for the transport of non-cohesive sediments, i.e. mainly sand, in the littoral zone along a shoreline mainly due to the action of breaking waves. The littoral transport is also called the longshore transport or the ''littoral drift''. <br />
===Description===<br />
Littoral transport is often described under the assumption that the shoreline is nearly straight with nearly parallel depth contours. This assumption is very often valid, especially if the sections of the shore are not too long and if a gradual transition between such sections is assumed. Under these circumstances, the littoral transport can briefly be described as follows.<br />
<br />
When waves approach the shoreline obliquely, refraction tends to turn the wave fronts so that they are almost parallel to the shoreline. At the same time, when approaching the breaker-zone, they undergo shoaling, which means that they become steeper and higher. Finally, the waves break. During the breaking process, the associated turbulence causes some of the seabed sediments to be brought into suspension. These suspended sediments, plus some of the sediments on the seabed, are then carried along the shoreline by the longshore current, which has its maximum near the breaker-line. The two transport modes are referred to as suspended transport and bed load, respectively. The sum of these is the littoral drift. <br />
<br />
<br />
===Distribution of the Littoral Transport===<br />
[[Image:littoral distribution2.jpg|thumb|right|Fig. 2a ]]<br />
[[Image:littoral distribution5.jpg||thumb|right||Fig. 2b. Distribution of the littoral transport over a coastal profile for grain sizes <math>D_{50}</math> = 0.2 mm and 0.5 mm and for the wave heights HS = 1.0 m, 3.0 m and 5.0 m. Equilibrium profiles corresponding to the grain sizes have been used, refer to Fig. 6. Emperical width of littoral zone. ''Angle of incidence: \alpha; = 30^{0}. Calculated by LITPACK.'']]<br />
The magnitude of the littoral transport or drift, Q, depends on parameters, the most important of which are:<br />
<br />
*''Wave height.'' The littoral drift is proportional to the wave height to the power of approximately 3.<br />
*''Grain size.'' The littoral drift is inversely proportional to the grain size to the power of approximately 3.<br />
*''Wave incidence angle.'' The littoral drift is approximately proportional to <math>sin^{2.5}(2\alpha)</math>, where &alpha; is the wave incidence angle.<br />
<br />
It can be seen that the littoral drift varies strongly with several parameters. It is therefore crucial to have exact data when making littoral drift calculations. It is an important point that the littoral drift over the coastal profile depends not only on the hydrodynamics but also very much on the variation of the sediment characteristics over the profile. Hence, the sediment distribution along the coastal profile should be taken into account whenever possible.<br />
<br />
===Littoral Drift Budgets===<br />
A ''littoral drift budget'' for a coastal profile is the sum of littoral transport contributions caused by all the possible combinations of wave heights and directions, as well as tide and storm surge.<br />
<br />
Consider, for example, a coastline oriented north-south with the sea to the west. All wave components from south to west will yield northward littoral drift contributions, and all wave components from west to north will yield southward littoral drift contributions. The sum of the northward drift components is called the ''northward littoral drift'', and similarly is the sum of the southward drift components referred to as the ''southward littoral drift''. The difference between the northward and the southward littoral drifts is called the ''net littoral drift'', which is associated with a ''net littoral drift direction''. The sum of the northward and the southward drift rates is called the ''gross littoral drift'', which has no direction.<br />
<br />
====Littoral drift parameters====<br />
Littoral drift budgets can be made for any period relevant for the site under study as long as there are sufficient data. An overview of the magnitude of littoral drift is provided in the following table as a function of the following parameters:<br />
*The significant wave heights <math>H_{s}</math><br />
*The angle of incidence at 'deep water' <math>\alpha_{0}</math> (20m has been used as 'deep water')<br />
*A duration of 24 hours <br />
*Beach sand with <math>d_{50}= 0.25mm </math> <br />
*Calculations performed by LITPACK on the equilibrium profile corresponding to <math>d_{50}= 0.25mm </math> <br />
<br />
<br />
{|border="1" cellpadding="2" align="center"<br />
|+Table: Littoral transport rates Q as a function of <math>5_{s}= 0.25mm </math> and angle of incidence <math>\alpha_{0}</math> at deep water (20 m). Calculated by LITPACK. ''Note: <math>D_{50}</math>, H and MWD at 20m water depth''<br />
<br />
!rowspan="2" colspan="2"| Q[m<sup>3</sup>/24hrs]<br />
!colspan="6" align="center"|&alpha;<sub>0</sub><br />
|-<br />
! width="50"|5!! width="50"|15!! width="50"|30!! width="50"|45!! width="50"|60!! width="50"|75<br />
|-<br />
!rowspan="5"|H<sub>s</sub><br />
!1.0<br />
|50|| 100|| 300|| 350|| 300|| 150<br />
|-<br />
!2.0<br />
|400|| 1000|| 2,000|| 3,000|| 2,500|| 1,000<br />
|-<br />
!3.0<br />
|1,500||4,000|| 10,000||15,000||10000|| 5,000<br />
|-<br />
!4.0<br />
|4,000|| 10,000||30,000||40,000||35,000||15,000<br />
|-<br />
!5.0<br />
|8,000|| 25,000||65,000||100,000||85,000||35,000<br />
|}<br />
<br />
<br />
[[Image:drift budget_c.jpg|350px|thumb|right|Fig. 3.]]<br />
An important parameter in relation to the littoral drift conditions is the variation of the net transport with varying orientation of the coastline. If e.g. a groyne is constructed, this will initially block the transport resulting in net zero transport at this location. This means that the sand will accrete upstream of the groyne forming a coastline with the orientation, which gives zero transport. The efficiency of the groyne depends very much on the angle between the present orientation of the coastline and the orientation of net zero transport. If this angle is small, the groyne will be efficient, as it will be able to hold a long sand filet. If the angle is large, which is the case with a very oblique wave exposure, the groyne will only be able to hold a very short sand filet, which means that a groyne will not be an applicable type of coast protection in this case.<br />
<br />
====Net littoral drift====<br />
When discussing the littoral transport along a coastline in general, it is always the net littoral drift that is referred to unless otherwise specified. Gradients in the net littoral drift along a section of coast lead to '''coastline erosion or accretion'''. The gross littoral drift is important for backfilling of channels/trenches across the littoral drift zone, as all littoral drift situations lead to backfilling of the channel/trench.<br />
<br />
====What does littoral drift depend on?====<br />
The littoral drift also depends on the sea current, although to a much smaller extent than it depends on the longshore current. This means that the most important hydraulic parameter for the littoral transport is the wave conditions.<br />
<br />
The water-level mainly determines where in the coastal profile the transport will take place, but the water-level only influences the magnitude of the littoral drift to a lesser extent. However, the tide may have significant influence on the transport conditions for macro-tidal environments. Positive or negative correlation between the waves and the water-level variations may be of importance for sedimentation patterns near large structures. <br />
<br />
====Sediments and littoral drift====<br />
At many locations there is a considerable variation in the grain size depending on the distance from the coastline. Typically the sediments become finer with increasing distance from the coastline. This will, to some extent, blur the picture of the littoral drift given above.<br />
<br />
The fine cohesive sediments, which may be present in the outer part of the profile, will be in suspension over the entire water column and will also tend to spread over the entire coastal profile during strong wave exposure. The transport which this gives rise to is normally not considered a part of the littoral drift, as this only takes the non-cohesive sediments into consideration. The transport of the cohesive sediments thus only plays an indirect role in the stability of the coastal profile. The existence of this transport of fine suspended sediments will, however, be of importance in relation to sedimentation in ports and in trenches.<br />
<br />
[[Image:tombolo2.jpg|thumb|right|Fig. 4. Tombolo formation behind coastal breakwater]]<br />
The longshore transport is, as already mentioned, characterised by a combination of sediment moved along the seabed, the so-called ''bed load transport'', and of sediment in suspension, the so-called ‘’suspended load’’. Even when the sand is in suspension it is still relatively close to the seabed because of the relatively high ''fall velocity'' of sand grains. This means that any change in the hydrodynamics or bathymetric conditions will "immediately" result in a corresponding change in the transport capacity and therefore also in the morphology. This results for instance in the typical accumulation of sand behind even a relatively short, detached, coastal breakwater, as the accumulation of sand reflects the “immediate” response on the attenuated transport capacity behind the breakwater. It is not possible to guide the sand between the coastal breakwater and the shoreline if the breakwater has a length of more than around 0.5 times the distance from the shoreline. If the length of the breakwater is more than approximately 0.8 times the distance from the shoreline so much sand will be trapped that the breakwater will be connected to land by a tombolo formation. This immediate morphological response to even small changes in the littoral transport is also the reason why many attempts to construct island-ports with zero impact on the shoreline have failed. Most of them have been connected to land by [[tombolo]] formations.<br />
<br />
==Onshore and Offshore Transport and Equilibrium Coastal Profile==<br />
Varying wave conditions result in varying onshore and offshore transports over the coastal profile. These transports are, to some extent, reversible and therefore non critical in terms of long term coastal stability. However, extreme storm surge and wave exposure result in coastal erosion.<br />
<br />
===Erosion and rebuilding sequence===<br />
When the coastal profile is exposed to non extreme waves and storm surge, the sediments near the shoreline will be transported offshore and typically be deposited in a bar resulting in an overall flattening of the slope of the shoreface. However, the inner part of the shoreface as well as the foreshore will become steeper in this process, and the shoreline will recede. During the following periods of smaller waves, swell and normal water-level conditions, the bar will travel very slowly towards the coastline again, practically rebuilding the original coastal profile.<br />
<br />
During such a sequence of profile erosion and rebuilding, certain parts of the coastal profile may experience temporary erosion. This may not be recorded in profile surveys, because some rebuilding will already have taken place before it is possible to carry out surveys after the storm. It is important to take such temporary profile fluctuations into account when designing structures in the coastal zone. It is particularly important to have a sufficiently wide beach so that the temporary beach erosion will not cause erosion of the coast.<br />
<br />
===Coastal profile===<br />
This onshore and offshore transport is closely related to the form of the coastal profile. Several investigations have revealed that a coastal profile possesses an average, characteristic form, which is referred to as the theoretical equilibrium profile. The equilibrium profile has been defined as "a statistical average profile, which maintains its form apart from small fluctuations, including seasonal fluctuations". The depth d [meters] in the equilibrium profile increases exponentially with the distance x from the shoreline according to the equation<ref name=Dean/><br />
<br />
:<math>d=Ax^{m}</math> [x and d in meters]<br />
<br />
where A is the dimensionless steepness parameter and m is a dimensionless exponent. Based on fitting to natural upper shoreface profiles, Dean, 1987<ref name="Dean">Dean, R.G., 1987. "Coastal Sediment Processes: Toward engineering solutions." Proceedings Coastal Sediments '87, Am. So. Civ. Eng., 1-24.</ref> has suggested an average value of m = 0.67. However the value of m is subject to large variability dependent of the beach type expressed by the dimensionless fall velocity <math>\Omega=H_{0}/\omega_{s}T</math> where <math>H_{0}</math> is the deep water wave height, T is the wave period and <math>\omega s</math> is the sediment fall velocity. The value of m varies typically between m ~ 0.4 for reflective beaches (<math>\Omega<1.5</math>) and m ~ 0.8 for dissipative beaches (<math>\Omega>5.5</math>).<ref>Cowell, P.J., Hanslow, D.J. and Meleo, J.F., 1999. The Shoreface: In: A.D. Short (editor), Handbook of Beach and Shoreface Morphodynamics, Wiley and Sons, Chichester, 29-71.</ref><ref>Masselink, G. and Huges, M. G., 2003. Introduction to Coastal Processes adn Geomorphology. Published by Hodder Arnold. ISBN 0340764104.</ref>.<br />
<br />
<br />
The steepness parameter A has empirically been related<ref name="Dean"/> to the sediment fall velocity <br />
<math>\omega_{s}</math> as follows:<br />
<br />
:<math>A=0.067\omega_{s}^{0.44}</math> [<math>\omega_{s}</math> in cm <math>s^{-1}</math>]<br />
<br />
Values for A as a function of the mean grain size <math>d_{50}</math> is shown in the table below.<br />
<br />
{|border="1" align="center"<br />
|+Table: Correlation between mean grain size d<sub>50</sub> in mm and the constant A in Dean’s equilibrium profile equation<br />
!width="100"|d50<br />
!width="75"|0.10<br />
!width="75"|0.15<br />
!width="75"|0.20<br />
!width="75"|0.25<br />
!width="75"|0.30<br />
!width="75"|0.50<br />
!width="75"|1.00<br />
!width="75"|2.00<br />
!width="75"|5.00<br />
!width="75"|10.00<br />
|-align="center"<br />
!width="100"|A<br />
|0.043||0.062||0.080||0.092||0.103||0.132||0.178||0.234||0.318||0.390<br />
|}<br />
<br />
<br />
It is seen that the equilibrium profile does not depend on the wave height. The reason for this is that the water depth limits the wave height inside the breaker zone. However, the wave height decides the width of the littoral zone, within which the equilibrium shoreface concept is valid. Thus, the equilibrium profile is only valid for the littoral zone, i.e. out to the [[Closure depth]] d<sub>l</sub>:<br />
<br />
:<math>d_{1} = 2.28H_{S,12h/y} - 68.5^{H^{2}_{S,12h/y}}gT^{2}_{s}</math><br />
<br />
<br />
[[Image:equilibrium profiles.jpg|thumb|Fig. 5. Equilibrium profiles for grain sizes 0.15, 0.2, 0.3, 0.5, 1.0, 10 and 30 mm.]]<br />
[[Image:emperical width.jpg|thumb|Fig. 6. Empirical width of littoral zone as a function of the mean grain size for various wave climates represented by <math>H_{S,12h/y}</math>]]<br />
<br />
The width of the littoral zone and the slope of the shoreface thus depend on the mean grain size as well as on the wave conditions.<br />
<br />
The equilibrium profile becomes increasingly steeper with increasing grain size. Typical equilibrium profiles for different grain size characteristics are presented in Fig. 5. <br />
<br />
The width of the littoral zone as a function of the mean grain size and for different wave climates, represented by <math>H_{S,12h/y}</math>, is presented in Fig. 6.<br />
<br />
These figures can be used in preliminary design considerations for artificial beaches and reclamation areas fronted by natural slopes.<br />
<br />
It is evident from these correlations between grain size, equilibrium profile and wave conditions that it is very important in beach nourishment to use materials as coarse as or coarser than the native material. Otherwise the nourished sand will immediately be transported offshore in nature’s attempt to form the new and flatter equilibrium profile, which fits the finer sand.<br />
<br />
In the real world there is often a sorting of the sediments in the active coastal profile; the mean grain size decreases with increasing distance from the shoreline. If this variation is introduced into the considerations concerning the equilibrium profile, a more accurate representation of the equilibrium profile at a specific site is obtained.<br />
<br />
The concept of equilibrium profiles is a rather crude representation of the coastal profile conditions since it neither includes nor explains the occurrence of bar formations, etc. However, the concept of the equilibrium profile is a rather practical “tool” for the analysis of coastal conditions and, as already mentioned, for preliminary design considerations. <br />
<br />
If the geological coastal profile at a location is flatter than the calculated equilibrium profile, the wave action in the profile will tend to form the equilibrium profile, which means that material will be moved towards the shore. However, at a certain location towards the shore, there is not sufficient wave energy to move the sand any further and a barrier with a corresponding lagoon is formed.<br />
<br />
The equilibrium concept can also explain why shore and coast erosion take place at locations where the equilibrium profile is already established, when such profiles are exposed to the combined action of storm waves and storm surge (tidal wave). The increased water level will correspond to a profile, which is too steep compared to the equilibrium profile. At a certain distance from the shoreline the water will consequently be too deep relative to the equilibrium depth. Nature will compensate by transporting sand from the beach towards the sea in an attempt to re-establish the equilibrium profile, which fits the temporary high water level. This will result in setback of the shoreline; however, if the beach is not sufficiently wide for this adjustment, the sediment will be taken from the cliff or dunes. The amount of erosion during a storm thus depends primarily upon the magnitude of the storm surge and its duration. It is evident from this description that a wide beach is a precondition for a stable coastline. Coast protection can thus be established by providing a wide beach through beach or foreshore nourishment. After the storm, the material, which was brought offshore during the storm surge conditions, will to a great extent be transported slowly back to the beach, however extreme storm surge/wave events will result in a permanent offshore loss of material.<br />
<br />
===Cross-shore sediment transport===<br />
<ref name="Aagard"/>Once the sediment is brought up into the water column it becomes available for transport by the various hydrodynamic processes. In the longshore dimension, the transport is accomplished almost exclusively by mean currents because the longshore component of wave orbital motions is small. Longshore sediment transport gradients are small on long straight coasts and hence the morphological impact is small except in the vicinity of shoreline discontinuities. <br />
<br />
<ref name="Aagard"/>In the cross-shore dimension, however, the transport is considerably more complex. The net sediment transport at a given point in the profile is often a balance between an onshore transport caused by skewed incident short wave motions, an offshore transport caused by mean currents and a transport caused by long waves which can be either onshore or offshore directed<ref name="Aagard, Masselink">Aagaard, T. and Masselink, G., 1999. The Surf Zone. In: A.D.Short (ed) Handbook of Beach and Shoreface Morphodynamics, Wiley Interscience, pp.72-118. </ref>. Consequently shore-normal sediment transport gradients can become large and morphological changes created by such transport gradients are often considerable, spatially as well as temporally. <br />
<br />
<ref name="Aagard"/>Accretion occurs in zones of sediment transport convergence whereas erosion occurs in zones of divergence. As an example, longshore bars are often formed near, or in the zone of wave breaking because the sediment transport outside the breakpoint is often dominated by the short waves and onshore directed because of the wave skewness while inside the breakpoint, sediment transport is dominated by the offshore directed undertow. The position of the bars in the profile can then fluctuate in phase with the wave energy conditions: When waves are small, the breakpoint is located close to the shoreline and the bars tend to move onshore while large waves break far from the shoreline and the bars move offshore.<br />
<br />
==Transport of non-cohensive sediments==<br />
Most of the transport of non-cohesive sediments (sand) takes place in wave-dominated environments. There are, however, locations where the transport of sand is mainly dominated by the current. In relation to coastal morphology the ''tidal inlet'' is the most important. <br />
<br />
===Description of tidal inlets===<br />
A tidal inlet is the connection between the sea and a lagoon, which is exposed to shifting tidal currents.<br />
<br />
Flood tide causes the tidal current to run from the sea into the lagoon and ebb tide goes in the opposite direction. The exchanged water mass during a tidal cycle is called the ''tidal volume.'' This can roughly be calculated as the surface area of the lagoon times the tidal range.<br />
<br />
The tidal current in a tidal inlet on a coastline is responsible for the exchange of sand between the littoral zone and the lagoon. This sand transport typically results in varying depths and shifting locations of the inlets and in the formation of lagoon shoals ''(flood shoals)'' and offshore shoals ''(ebb shoals)''. At the same time the longshore transport interferes in these processes resulting in curved bar formations crossing the inlet as well as the formation of sand spits and possible shifting of the tidal inlet along the shore. All in all, it is a very mobile environment, which is not recommended for development of any kind.<br />
<br />
Tidal inlets are very often regulated and fixed by inlet jetties and they are frequently dredged to allow navigation. An important aspect in relation to regulated tidal inlets on littoral transport coastlines is that the jetties constitute a blockage of the littoral transport. This results in sand accumulation on the updrift side and lee side erosion along the downdrift coastline, unless special precautions are taken. When the sand accumulation on the updrift side reaches the tip of the jetty, the sand will start to bypass and this will cause sedimentation in the inlet. Normally the sand does not pass the dredged channel and therefore it does not nourish the lee side beach.<br />
<br />
===Added complexities===<br />
The above description of tidal inlets is greatly simplified and presents only the mechanisms in a very broad outline. The hydrodynamic and sediment transport conditions in tidal inlets are very complicated, as a tidal inlet always constitutes a delicate balance between the “forces” which keep it open, namely the tidal exchange, and the “forces” which tend to close it, namely the littoral transport processes. It adds to the complexity of tidal inlets that many different time scales are involved, the most important of which are outlined below:<br />
<br />
*Semi-diurnal and diurnal tidal components<br />
*Neap and spring with fortnightly periods<br />
*Seasonal variations in water-level, storm surge and wave conditions<br />
*Very wide time scales for the wave conditions: from seconds for single waves to days for storm duration to seasons for variations in general wave climate to years for the recurrence of extreme wave events<br />
<br />
===Studies on tidal inlets===<br />
Tidal inlet studies can be performed at many levels, from<br />
*a parametric empirical stability analysis involving only the main parameters such as the tidal parameters, the cross section area of the inlet and the wave energy; to <br />
*a complete study involving numerical modelling of hydrodynamics, waves, sediment transport and morphological evolution<br />
<br />
==References==<br />
<references/><br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
[[category:Theme 5]]<br />
[[Category:Coastal processes, interactions and resources]]<br />
[[Category:Hydrodynamics]]<br />
[[category:Coastal erosion]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Coastal_Hydrodynamics_And_Transport_Processes&diff=18903Coastal Hydrodynamics And Transport Processes2007-12-18T15:14:35Z<p>Juliettejackson: </p>
<hr />
<div>The hydrodynamic conditions or processes, that come about from [[Tidal wave|waves]] transforming over a coastal profile and generating wave set up and longshore currents, will result in movement and transport of the sediments (e.g. sand) present in the profile. This is referred to as ''littoral transport processes'' and is the main subject of this article. However, transport of fine sediments will also be discussed, but only very briefly.<br />
<br />
==Sediment transport in general==<br />
The sediment on the seabed is transported when it is exposed to large enough forces, or ''shear stresses'', by the water movements. These movements can be caused by the current or by the wave orbital velocities or a combination of both, the latter being the most important situation. The relevant parameters for the description of the sediment transport along a shoreline or in a coastal area are therefore the following:<br />
<br />
*The wave conditions at the site and the possible variations over the site plus the adjoining areas<br />
*The current conditions as well as the variations of these over the area <br />
*The water-level conditions, i.e. tide, storm surge and wave set-up<br />
*The bathymetry (the depth variations) in the area <br />
*The sediment characteristics over the area <br />
*The sources and sinks of sediment, such as rivers, eroding coasts or tidal inlets<br />
<br />
<br />
<ref name="Aagard">Written by Aagard, Troels. 2007.</ref>The link between hydrodynamic forcing and the morphological response of the beach is realized through the transport of sediment. This transport may occur as either bedload or as suspended load. Even though modelling efforts suggest that suspended load is the more important within the surf zone, particularly under high energy conditions, there is still no consensus on the subject, partly because bedload is very difficult to measure under field conditions. <br />
<br />
<ref name="Aagard"/>While suspended sediment transport may be easier to measure then bedload it is not a simple matter to predict, one of the main reasons being that the velocity field in the nearshore zone is oscillatory and that sediment resuspension (and transport) responds non-linearly to the forcing; further the existence of bedforms may induce phase changes between velocity and sediment concentration. <br />
<br />
<ref name="Aagard"/>Sediment concentrations within the surf zone have been found to depend upon elevation above the bed, bed configuration and local bed shear stress as well as wave breaker type. Sediment is mainly resuspended by the large shear stresses generated by wave motions while mean currents play a subordinate role. The reason is that in the nearshore region, oscillatory velocities are generally significantly larger than mean current velocities and the wave boundary layer thickness is small (resulting in steep velocity gradients) compared to the thickness of the current boundary layer. <br />
<br />
<ref name="Aagard"/>Physically, sediment can be resuspended from the seabed in a number of ways. Close to the bed, small turbulent eddies created by bed friction mobilize the sediment which can be mixed to higher elevations by turbulence generated by breaking waves. Hence, sediment concentration profile gradients are different under breaking and non-breaking waves. In the latter case, suspended sediment is confined to a relatively thin layer near the bed. <br />
<br />
<ref name="Aagard"/>The distribution of sediment in the water column may occur either as a diffusive process which generally occurs when the bed is flat and devoid of bedforms, or as a convective process whereby sediment is suspended and lifted upward in coherent packages e.g. due to vortex shedding from ripples at the bed. In both cases, the vertical distribution of sediment in the water column can be approximated by:<br />
<br />
<br />
<br />
:<math>w_{s}c+\varepsilon_{s}(\delta_{c}/\delta_{z})= 0</math> <br />
<br />
<br />
where <math>w_{s}</math> is the fall velocity of the sediment, c is the sediment concentration, z is elevation above the bed and <math>\varepsilon_{s}</math> is an eddy diffusion coefficient representing the scale of turbulent mixing. This equation predicts that in a steady state situation, the downward flux of sediment (<math>w_{s}c</math>) must be balanced by an upward flux given by the product of the concentration gradient and the sediment diffusion.<br />
<br />
==Littoral Transport==<br />
[[Image:Littoral_b.jpg|thumb|400px|Fig. 1. Variation in littoral transport with wave exposure and wave incidence angle]]''Littoral transport'' is the term used for the transport of non-cohesive sediments, i.e. mainly sand, in the littoral zone along a shoreline mainly due to the action of breaking waves. The littoral transport is also called the longshore transport or the ''littoral drift''. <br />
===Description===<br />
Littoral transport is often described under the assumption that the shoreline is nearly straight with nearly parallel depth contours. This assumption is very often valid, especially if the sections of the shore are not too long and if a gradual transition between such sections is assumed. Under these circumstances, the littoral transport can briefly be described as follows.<br />
<br />
When waves approach the shoreline obliquely, refraction tends to turn the wave fronts so that they are almost parallel to the shoreline. At the same time, when approaching the breaker-zone, they undergo shoaling, which means that they become steeper and higher. Finally, the waves break. During the breaking process, the associated turbulence causes some of the seabed sediments to be brought into suspension. These suspended sediments, plus some of the sediments on the seabed, are then carried along the shoreline by the longshore current, which has its maximum near the breaker-line. The two transport modes are referred to as suspended transport and bed load, respectively. The sum of these is the littoral drift. <br />
<br />
<br />
===Distribution of the Littoral Transport===<br />
[[Image:littoral distribution2.jpg|thumb|right|Fig. 2a ]]<br />
[[Image:littoral distribution5.jpg||thumb|right||Fig. 2b. Distribution of the littoral transport over a coastal profile for grain sizes <math>D_{50}</math> = 0.2 mm and 0.5 mm and for the wave heights HS = 1.0 m, 3.0 m and 5.0 m. Equilibrium profiles corresponding to the grain sizes have been used, refer to Fig. 6. Emperical width of littoral zone. ''Angle of incidence: \alpha; = 30^{0}. Calculated by LITPACK.'']]<br />
The magnitude of the littoral transport or drift, Q, depends on parameters, the most important of which are:<br />
<br />
*''Wave height.'' The littoral drift is proportional to the wave height to the power of approximately 3.<br />
*''Grain size.'' The littoral drift is inversely proportional to the grain size to the power of approximately 3.<br />
*''Wave incidence angle.'' The littoral drift is approximately proportional to <math>sin^{2.5}(2\alpha)</math>, where &alpha; is the wave incidence angle.<br />
<br />
It can be seen that the littoral drift varies strongly with several parameters. It is therefore crucial to have exact data when making littoral drift calculations. It is an important point that the littoral drift over the coastal profile depends not only on the hydrodynamics but also very much on the variation of the sediment characteristics over the profile. Hence, the sediment distribution along the coastal profile should be taken into account whenever possible.<br />
<br />
===Littoral Drift Budgets===<br />
A ''littoral drift budget'' for a coastal profile is the sum of littoral transport contributions caused by all the possible combinations of wave heights and directions, as well as tide and storm surge.<br />
<br />
Consider, for example, a coastline oriented north-south with the sea to the west. All wave components from south to west will yield northward littoral drift contributions, and all wave components from west to north will yield southward littoral drift contributions. The sum of the northward drift components is called the ''northward littoral drift'', and similarly is the sum of the southward drift components referred to as the ''southward littoral drift''. The difference between the northward and the southward littoral drifts is called the ''net littoral drift'', which is associated with a ''net littoral drift direction''. The sum of the northward and the southward drift rates is called the ''gross littoral drift'', which has no direction.<br />
<br />
====Littoral drift parameters====<br />
Littoral drift budgets can be made for any period relevant for the site under study as long as there are sufficient data. An overview of the magnitude of littoral drift is provided in the following table as a function of the following parameters:<br />
*The significant wave heights <math>H_{s}</math><br />
*The angle of incidence at 'deep water' <math>\alpha_{0}</math> (20m has been used as 'deep water')<br />
*A duration of 24 hours <br />
*Beach sand with <math>d_{50}= 0.25mm </math> <br />
*Calculations performed by LITPACK on the equilibrium profile corresponding to <math>d_{50}= 0.25mm </math> <br />
<br />
<br />
{|border="1" cellpadding="2" align="center"<br />
|+Table: Littoral transport rates Q as a function of <math>5_{s}= 0.25mm </math> and angle of incidence <math>\alpha_{0}</math> at deep water (20 m). Calculated by LITPACK. ''Note: <math>D_{50}</math>, H and MWD at 20m water depth''<br />
<br />
!rowspan="2" colspan="2"| Q[m<sup>3</sup>/24hrs]<br />
!colspan="6" align="center"|&alpha;<sub>0</sub><br />
|-<br />
! width="50"|5!! width="50"|15!! width="50"|30!! width="50"|45!! width="50"|60!! width="50"|75<br />
|-<br />
!rowspan="5"|H<sub>s</sub><br />
!1.0<br />
|50|| 100|| 300|| 350|| 300|| 150<br />
|-<br />
!2.0<br />
|400|| 1000|| 2,000|| 3,000|| 2,500|| 1,000<br />
|-<br />
!3.0<br />
|1,500||4,000|| 10,000||15,000||10000|| 5,000<br />
|-<br />
!4.0<br />
|4,000|| 10,000||30,000||40,000||35,000||15,000<br />
|-<br />
!5.0<br />
|8,000|| 25,000||65,000||100,000||85,000||35,000<br />
|}<br />
<br />
<br />
[[Image:drift budget_c.jpg|350px|thumb|right|Fig. 3.]]<br />
An important parameter in relation to the littoral drift conditions is the variation of the net transport with varying orientation of the coastline. If e.g. a groyne is constructed, this will initially block the transport resulting in net zero transport at this location. This means that the sand will accrete upstream of the groyne forming a coastline with the orientation, which gives zero transport. The efficiency of the groyne depends very much on the angle between the present orientation of the coastline and the orientation of net zero transport. If this angle is small, the groyne will be efficient, as it will be able to hold a long sand filet. If the angle is large, which is the case with a very oblique wave exposure, the groyne will only be able to hold a very short sand filet, which means that a groyne will not be an applicable type of coast protection in this case.<br />
<br />
====Net littoral drift====<br />
When discussing the littoral transport along a coastline in general, it is always the net littoral drift that is referred to unless otherwise specified. Gradients in the net littoral drift along a section of coast lead to '''coastline erosion or accretion'''. The gross littoral drift is important for backfilling of channels/trenches across the littoral drift zone, as all littoral drift situations lead to backfilling of the channel/trench.<br />
<br />
====What does littoral drift depend on?====<br />
The littoral drift also depends on the sea current, although to a much smaller extent than it depends on the longshore current. This means that the most important hydraulic parameter for the littoral transport is the wave conditions.<br />
<br />
The water-level mainly determines where in the coastal profile the transport will take place, but the water-level only influences the magnitude of the littoral drift to a lesser extent. However, the tide may have significant influence on the transport conditions for macro-tidal environments. Positive or negative correlation between the waves and the water-level variations may be of importance for sedimentation patterns near large structures. <br />
<br />
====Sediments and littoral drift====<br />
At many locations there is a considerable variation in the grain size depending on the distance from the coastline. Typically the sediments become finer with increasing distance from the coastline. This will, to some extent, blur the picture of the littoral drift given above.<br />
<br />
The fine cohesive sediments, which may be present in the outer part of the profile, will be in suspension over the entire water column and will also tend to spread over the entire coastal profile during strong wave exposure. The transport which this gives rise to is normally not considered a part of the littoral drift, as this only takes the non-cohesive sediments into consideration. The transport of the cohesive sediments thus only plays an indirect role in the stability of the coastal profile. The existence of this transport of fine suspended sediments will, however, be of importance in relation to sedimentation in ports and in trenches.<br />
<br />
[[Image:tombolo2.jpg|thumb|right|Fig. 4. Tombolo formation behind coastal breakwater]]<br />
The longshore transport is, as already mentioned, characterised by a combination of sediment moved along the seabed, the so-called ''bed load transport'', and of sediment in suspension, the so-called ‘’suspended load’’. Even when the sand is in suspension it is still relatively close to the seabed because of the relatively high ''fall velocity'' of sand grains. This means that any change in the hydrodynamics or bathymetric conditions will "immediately" result in a corresponding change in the transport capacity and therefore also in the morphology. This results for instance in the typical accumulation of sand behind even a relatively short, detached, coastal breakwater, as the accumulation of sand reflects the “immediate” response on the attenuated transport capacity behind the breakwater. It is not possible to guide the sand between the coastal breakwater and the shoreline if the breakwater has a length of more than around 0.5 times the distance from the shoreline. If the length of the breakwater is more than approximately 0.8 times the distance from the shoreline so much sand will be trapped that the breakwater will be connected to land by a tombolo formation. This immediate morphological response to even small changes in the littoral transport is also the reason why many attempts to construct island-ports with zero impact on the shoreline have failed. Most of them have been connected to land by [[tombolo]] formations.<br />
<br />
==Onshore and Offshore Transport and Equilibrium Coastal Profile==<br />
Varying wave conditions result in varying onshore and offshore transports over the coastal profile. These transports are, to some extent, reversible and therefore non critical in terms of long term coastal stability. However, extreme storm surge and wave exposure result in coastal erosion.<br />
<br />
===Erosion and rebuilding sequence===<br />
When the coastal profile is exposed to non extreme waves and storm surge, the sediments near the shoreline will be transported offshore and typically be deposited in a bar resulting in an overall flattening of the slope of the shoreface. However, the inner part of the shoreface as well as the foreshore will become steeper in this process, and the shoreline will recede. During the following periods of smaller waves, swell and normal water-level conditions, the bar will travel very slowly towards the coastline again, practically rebuilding the original coastal profile.<br />
<br />
During such a sequence of profile erosion and rebuilding, certain parts of the coastal profile may experience temporary erosion. This may not be recorded in profile surveys, because some rebuilding will already have taken place before it is possible to carry out surveys after the storm. It is important to take such temporary profile fluctuations into account when designing structures in the coastal zone. It is particularly important to have a sufficiently wide beach so that the temporary beach erosion will not cause erosion of the coast.<br />
<br />
===Coastal profile===<br />
This onshore and offshore transport is closely related to the form of the coastal profile. Several investigations have revealed that a coastal profile possesses an average, characteristic form, which is referred to as the theoretical equilibrium profile. The equilibrium profile has been defined as "a statistical average profile, which maintains its form apart from small fluctuations, including seasonal fluctuations". The depth d [meters] in the equilibrium profile increases exponentially with the distance x from the shoreline according to the equation<ref name=Dean/><br />
<br />
:<math>d=Ax^{m}</math> [x and d in meters]<br />
<br />
where A is the dimensionless steepness parameter and m is a dimensionless exponent. Based on fitting to natural upper shoreface profiles, Dean, 1987<ref name="Dean">Dean, R.G., 1987. "Coastal Sediment Processes: Toward engineering solutions." Proceedings Coastal Sediments '87, Am. So. Civ. Eng., 1-24.</ref> has suggested an average value of m = 0.67. However the value of m is subject to large variability dependent of the beach type expressed by the dimensionless fall velocity <math>\Omega=H_{0}/\omega_{s}T</math> where <math>H_{0}</math> is the deep water wave height, T is the wave period and <math>\omega s</math> is the sediment fall velocity. The value of m varies typically between m ~ 0.4 for reflective beaches (<math>\Omega<1.5</math>) and m ~ 0.8 for dissipative beaches (<math>\Omega>5.5</math>).<ref>Cowell, P.J., Hanslow, D.J. and Meleo, J.F., 1999. The Shoreface: In: A.D. Short (editor), Handbook of Beach and Shoreface Morphodynamics, Wiley and Sons, Chichester, 29-71.</ref><ref>Masselink, G. and Huges, M. G., 2003. Introduction to Coastal Processes adn Geomorphology. Published by Hodder Arnold. ISBN 0340764104.</ref>.<br />
<br />
<br />
The steepness parameter A has empirically been related<ref name="Dean"/> to the sediment fall velocity <br />
<math>\omega_{s}</math> as follows:<br />
<br />
:<math>A=0.067\omega_{s}^{0.44}</math> [<math>\omega_{s}</math> in cm <math>s^{-1}</math>]<br />
<br />
Values for A as a function of the mean grain size <math>d_{50}</math> is shown in the table below.<br />
<br />
{|border="1" align="center"<br />
|+Table: Correlation between mean grain size d<sub>50</sub> in mm and the constant A in Dean’s equilibrium profile equation<br />
!width="100"|d50<br />
!width="75"|0.10<br />
!width="75"|0.15<br />
!width="75"|0.20<br />
!width="75"|0.25<br />
!width="75"|0.30<br />
!width="75"|0.50<br />
!width="75"|1.00<br />
!width="75"|2.00<br />
!width="75"|5.00<br />
!width="75"|10.00<br />
|-align="center"<br />
!width="100"|A<br />
|0.043||0.062||0.080||0.092||0.103||0.132||0.178||0.234||0.318||0.390<br />
|}<br />
<br />
<br />
It is seen that the equilibrium profile does not depend on the wave height. The reason for this is that the water depth limits the wave height inside the breaker zone. However, the wave height decides the width of the littoral zone, within which the equilibrium shoreface concept is valid. Thus, the equilibrium profile is only valid for the littoral zone, i.e. out to the [[Closure depth]] d<sub>l</sub>:<br />
<br />
:<math>d_{1} = 2.28H_{S,12h/y} - 68.5^{H^{2}_{S,12h/y}}gT^{2}_{s}</math><br />
<br />
<br />
[[Image:equilibrium profiles.jpg|thumb|Fig. 5. Equilibrium profiles for grain sizes 0.15, 0.2, 0.3, 0.5, 1.0, 10 and 30 mm.]]<br />
[[Image:emperical width.jpg|thumb|Fig. 6. Empirical width of littoral zone as a function of the mean grain size for various wave climates represented by <math>H_{S,12h/y}</math>]]<br />
<br />
The width of the littoral zone and the slope of the shoreface thus depend on the mean grain size as well as on the wave conditions.<br />
<br />
The equilibrium profile becomes increasingly steeper with increasing grain size. Typical equilibrium profiles for different grain size characteristics are presented in Fig. 5. <br />
<br />
The width of the littoral zone as a function of the mean grain size and for different wave climates, represented by <math>H_{S,12h/y}</math>, is presented in Fig. 6.<br />
<br />
These figures can be used in preliminary design considerations for artificial beaches and reclamation areas fronted by natural slopes.<br />
<br />
It is evident from these correlations between grain size, equilibrium profile and wave conditions that it is very important in beach nourishment to use materials as coarse as or coarser than the native material. Otherwise the nourished sand will immediately be transported offshore in nature’s attempt to form the new and flatter equilibrium profile, which fits the finer sand.<br />
<br />
In the real world there is often a sorting of the sediments in the active coastal profile; the mean grain size decreases with increasing distance from the shoreline. If this variation is introduced into the considerations concerning the equilibrium profile, a more accurate representation of the equilibrium profile at a specific site is obtained.<br />
<br />
The concept of equilibrium profiles is a rather crude representation of the coastal profile conditions since it neither includes nor explains the occurrence of bar formations, etc. However, the concept of the equilibrium profile is a rather practical “tool” for the analysis of coastal conditions and, as already mentioned, for preliminary design considerations. <br />
<br />
If the geological coastal profile at a location is flatter than the calculated equilibrium profile, the wave action in the profile will tend to form the equilibrium profile, which means that material will be moved towards the shore. However, at a certain location towards the shore, there is not sufficient wave energy to move the sand any further and a barrier with a corresponding lagoon is formed.<br />
<br />
The equilibrium concept can also explain why shore and coast erosion take place at locations where the equilibrium profile is already established, when such profiles are exposed to the combined action of storm waves and storm surge (tidal wave). The increased water level will correspond to a profile, which is too steep compared to the equilibrium profile. At a certain distance from the shoreline the water will consequently be too deep relative to the equilibrium depth. Nature will compensate by transporting sand from the beach towards the sea in an attempt to re-establish the equilibrium profile, which fits the temporary high water level. This will result in setback of the shoreline; however, if the beach is not sufficiently wide for this adjustment, the sediment will be taken from the cliff or dunes. The amount of erosion during a storm thus depends primarily upon the magnitude of the storm surge and its duration. It is evident from this description that a wide beach is a precondition for a stable coastline. Coast protection can thus be established by providing a wide beach through beach or foreshore nourishment. After the storm, the material, which was brought offshore during the storm surge conditions, will to a great extent be transported slowly back to the beach, however extreme storm surge/wave events will result in a permanent offshore loss of material.<br />
<br />
===Cross-shore sediment transport===<br />
<ref name="Aagard"/>Once the sediment is brought up into the water column it becomes available for transport by the various hydrodynamic processes. In the longshore dimension, the transport is accomplished almost exclusively by mean currents because the longshore component of wave orbital motions is small. Longshore sediment transport gradients are small on long straight coasts and hence the morphological impact is small except in the vicinity of shoreline discontinuities. <br />
<br />
<ref name="Aagard"/>In the cross-shore dimension, however, the transport is considerably more complex. The net sediment transport at a given point in the profile is often a balance between an onshore transport caused by skewed incident short wave motions, an offshore transport caused by mean currents and a transport caused by long waves which can be either onshore or offshore directed<ref name="Aagard, Masselink">Aagaard, T. and Masselink, G., 1999. The Surf Zone. In: A.D.Short (ed) Handbook of Beach and Shoreface Morphodynamics, Wiley Interscience, pp.72-118. </ref>. Consequently shore-normal sediment transport gradients can become large and morphological changes created by such transport gradients are often considerable, spatially as well as temporally. <br />
<br />
<ref name="Aagard"/>Accretion occurs in zones of sediment transport convergence whereas erosion occurs in zones of divergence. As an example, longshore bars are often formed near, or in the zone of wave breaking because the sediment transport outside the breakpoint is often dominated by the short waves and onshore directed because of the wave skewness while inside the breakpoint, sediment transport is dominated by the offshore directed undertow. The position of the bars in the profile can then fluctuate in phase with the wave energy conditions: When waves are small, the breakpoint is located close to the shoreline and the bars tend to move onshore while large waves break far from the shoreline and the bars move offshore.<br />
<br />
==Transport of non-cohensive sediments==<br />
Most of the transport of non-cohesive sediments (sand) takes place in wave-dominated environments. There are, however, locations where the transport of sand is mainly dominated by the current. In relation to coastal morphology the ''tidal inlet'' is the most important. <br />
<br />
===Description of tidal inlets===<br />
A tidal inlet is the connection between the sea and a lagoon, which is exposed to shifting tidal currents.<br />
<br />
Flood tide causes the tidal current to run from the sea into the lagoon and ebb tide goes in the opposite direction. The exchanged water mass during a tidal cycle is called the ''tidal volume.'' This can roughly be calculated as the surface area of the lagoon times the tidal range.<br />
<br />
The tidal current in a tidal inlet on a coastline is responsible for the exchange of sand between the littoral zone and the lagoon. This sand transport typically results in varying depths and shifting locations of the inlets and in the formation of lagoon shoals ''(flood shoals)'' and offshore shoals ''(ebb shoals)''. At the same time the longshore transport interferes in these processes resulting in curved bar formations crossing the inlet as well as the formation of sand spits and possible shifting of the tidal inlet along the shore. All in all, it is a very mobile environment, which is not recommended for development of any kind.<br />
<br />
Tidal inlets are very often regulated and fixed by inlet jetties and they are frequently dredged to allow navigation. An important aspect in relation to regulated tidal inlets on littoral transport coastlines is that the jetties constitute a blockage of the littoral transport. This results in sand accumulation on the updrift side and lee side erosion along the downdrift coastline, unless special precautions are taken. When the sand accumulation on the updrift side reaches the tip of the jetty, the sand will start to bypass and this will cause sedimentation in the inlet. Normally the sand does not pass the dredged channel and therefore it does not nourish the lee side beach.<br />
<br />
===Added complexities===<br />
The above description of tidal inlets is greatly simplified and presents only the mechanisms in a very broad outline. The hydrodynamic and sediment transport conditions in tidal inlets are very complicated, as a tidal inlet always constitutes a delicate balance between the “forces” which keep it open, namely the tidal exchange, and the “forces” which tend to close it, namely the littoral transport processes. It adds to the complexity of tidal inlets that many different time scales are involved, the most important of which are outlined below:<br />
<br />
*Semi-diurnal and diurnal tidal components<br />
*Neap and spring with fortnightly periods<br />
*Seasonal variations in water-level, storm surge and wave conditions<br />
*Very wide time scales for the wave conditions: from seconds for single waves to days for storm duration to seasons for variations in general wave climate to years for the recurrence of extreme wave events<br />
<br />
===Studies on tidal inlets===<br />
Tidal inlet studies can be performed at many levels, from<br />
*a parametric empirical stability analysis involving only the main parameters such as the tidal parameters, the cross section area of the inlet and the wave energy; to <br />
*a complete study involving numerical modelling of hydrodynamics, waves, sediment transport and morphological evolution<br />
<br />
==References==<br />
<references/><br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
[[category:Theme 5]]<br />
[[Category:Coastal processes, interactions and resources]]<br />
[[Category:Hydrodynamics]]<br />
[[category:Coastal erosion]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Coastal_Hydrodynamics_And_Transport_Processes&diff=18901Coastal Hydrodynamics And Transport Processes2007-12-18T15:10:27Z<p>Juliettejackson: </p>
<hr />
<div>The hydrodynamic conditions or processes, that come about from [[Tidal wave:waves]] transforming over a coastal profile and generating wave set up and longshore currents, will result in movement and transport of the sediments (e.g. sand) present in the profile. This is referred to as ''littoral transport processes'' and is the main subject of this article. However, transport of fine sediments will also be discussed, but only very briefly.<br />
<br />
==Sediment transport in general==<br />
The sediment on the seabed is transported when it is exposed to large enough forces, or ''shear stresses'', by the water movements. These movements can be caused by the current or by the wave orbital velocities or a combination of both, the latter being the most important situation. The relevant parameters for the description of the sediment transport along a shoreline or in a coastal area are therefore the following:<br />
<br />
*The wave conditions at the site and the possible variations over the site plus the adjoining areas<br />
*The current conditions as well as the variations of these over the area <br />
*The water-level conditions, i.e. tide, storm surge and wave set-up<br />
*The bathymetry (the depth variations) in the area <br />
*The sediment characteristics over the area <br />
*The sources and sinks of sediment, such as rivers, eroding coasts or tidal inlets<br />
<br />
<br />
<ref name="Aagard">Written by Aagard, Troels. 2007.</ref>The link between hydrodynamic forcing and the morphological response of the beach is realized through the transport of sediment. This transport may occur as either bedload or as suspended load. Even though modelling efforts suggest that suspended load is the more important within the surf zone, particularly under high energy conditions, there is still no consensus on the subject, partly because bedload is very difficult to measure under field conditions. <br />
<br />
<ref name="Aagard"/>While suspended sediment transport may be easier to measure then bedload it is not a simple matter to predict, one of the main reasons being that the velocity field in the nearshore zone is oscillatory and that sediment resuspension (and transport) responds non-linearly to the forcing; further the existence of bedforms may induce phase changes between velocity and sediment concentration. <br />
<br />
<ref name="Aagard"/>Sediment concentrations within the surf zone have been found to depend upon elevation above the bed, bed configuration and local bed shear stress as well as wave breaker type. Sediment is mainly resuspended by the large shear stresses generated by wave motions while mean currents play a subordinate role. The reason is that in the nearshore region, oscillatory velocities are generally significantly larger than mean current velocities and the wave boundary layer thickness is small (resulting in steep velocity gradients) compared to the thickness of the current boundary layer. <br />
<br />
<ref name="Aagard"/>Physically, sediment can be resuspended from the seabed in a number of ways. Close to the bed, small turbulent eddies created by bed friction mobilize the sediment which can be mixed to higher elevations by turbulence generated by breaking waves. Hence, sediment concentration profile gradients are different under breaking and non-breaking waves. In the latter case, suspended sediment is confined to a relatively thin layer near the bed. <br />
<br />
<ref name="Aagard"/>The distribution of sediment in the water column may occur either as a diffusive process which generally occurs when the bed is flat and devoid of bedforms, or as a convective process whereby sediment is suspended and lifted upward in coherent packages e.g. due to vortex shedding from ripples at the bed. In both cases, the vertical distribution of sediment in the water column can be approximated by:<br />
<br />
<br />
<br />
:<math>w_{s}c+\varepsilon_{s}(\delta_{c}/\delta_{z})= 0</math> <br />
<br />
<br />
where <math>w_{s}</math> is the fall velocity of the sediment, c is the sediment concentration, z is elevation above the bed and <math>\varepsilon_{s}</math> is an eddy diffusion coefficient representing the scale of turbulent mixing. This equation predicts that in a steady state situation, the downward flux of sediment (<math>w_{s}c</math>) must be balanced by an upward flux given by the product of the concentration gradient and the sediment diffusion.<br />
<br />
==Littoral Transport==<br />
[[Image:Littoral_b.jpg|thumb|400px|Fig. 1. Variation in littoral transport with wave exposure and wave incidence angle]]''Littoral transport'' is the term used for the transport of non-cohesive sediments, i.e. mainly sand, in the littoral zone along a shoreline mainly due to the action of breaking waves. The littoral transport is also called the longshore transport or the ''littoral drift''. <br />
===Description===<br />
Littoral transport is often described under the assumption that the shoreline is nearly straight with nearly parallel depth contours. This assumption is very often valid, especially if the sections of the shore are not too long and if a gradual transition between such sections is assumed. Under these circumstances, the littoral transport can briefly be described as follows.<br />
<br />
When waves approach the shoreline obliquely, refraction tends to turn the wave fronts so that they are almost parallel to the shoreline. At the same time, when approaching the breaker-zone, they undergo shoaling, which means that they become steeper and higher. Finally, the waves break. During the breaking process, the associated turbulence causes some of the seabed sediments to be brought into suspension. These suspended sediments, plus some of the sediments on the seabed, are then carried along the shoreline by the longshore current, which has its maximum near the breaker-line. The two transport modes are referred to as suspended transport and bed load, respectively. The sum of these is the littoral drift. <br />
<br />
<br />
===Distribution of the Littoral Transport===<br />
[[Image:littoral distribution2.jpg|thumb|right|Fig. 2a ]]<br />
[[Image:littoral distribution5.jpg||thumb|right||Fig. 2b. Distribution of the littoral transport over a coastal profile for grain sizes <math>D_{50}</math> = 0.2 mm and 0.5 mm and for the wave heights HS = 1.0 m, 3.0 m and 5.0 m. Equilibrium profiles corresponding to the grain sizes have been used, refer to Fig. 6. Emperical width of littoral zone. ''Angle of incidence: \alpha; = 30^{0}. Calculated by LITPACK.'']]<br />
The magnitude of the littoral transport or drift, Q, depends on parameters, the most important of which are:<br />
<br />
*''Wave height.'' The littoral drift is proportional to the wave height to the power of approximately 3.<br />
*''Grain size.'' The littoral drift is inversely proportional to the grain size to the power of approximately 3.<br />
*''Wave incidence angle.'' The littoral drift is approximately proportional to <math>sin^{2.5}(2\alpha)</math>, where &alpha; is the wave incidence angle.<br />
<br />
It can be seen that the littoral drift varies strongly with several parameters. It is therefore crucial to have exact data when making littoral drift calculations. It is an important point that the littoral drift over the coastal profile depends not only on the hydrodynamics but also very much on the variation of the sediment characteristics over the profile. Hence, the sediment distribution along the coastal profile should be taken into account whenever possible.<br />
<br />
===Littoral Drift Budgets===<br />
A ''littoral drift budget'' for a coastal profile is the sum of littoral transport contributions caused by all the possible combinations of wave heights and directions, as well as tide and storm surge.<br />
<br />
Consider, for example, a coastline oriented north-south with the sea to the west. All wave components from south to west will yield northward littoral drift contributions, and all wave components from west to north will yield southward littoral drift contributions. The sum of the northward drift components is called the ''northward littoral drift'', and similarly is the sum of the southward drift components referred to as the ''southward littoral drift''. The difference between the northward and the southward littoral drifts is called the ''net littoral drift'', which is associated with a ''net littoral drift direction''. The sum of the northward and the southward drift rates is called the ''gross littoral drift'', which has no direction.<br />
<br />
====Littoral drift parameters====<br />
Littoral drift budgets can be made for any period relevant for the site under study as long as there are sufficient data. An overview of the magnitude of littoral drift is provided in the following table as a function of the following parameters:<br />
*The significant wave heights <math>H_{s}</math><br />
*The angle of incidence at 'deep water' <math>\alpha_{0}</math> (20m has been used as 'deep water')<br />
*A duration of 24 hours <br />
*Beach sand with <math>d_{50}= 0.25mm </math> <br />
*Calculations performed by LITPACK on the equilibrium profile corresponding to <math>d_{50}= 0.25mm </math> <br />
<br />
<br />
{|border="1" cellpadding="2" align="center"<br />
|+Table: Littoral transport rates Q as a function of <math>5_{s}= 0.25mm </math> and angle of incidence <math>\alpha_{0}</math> at deep water (20 m). Calculated by LITPACK. ''Note: <math>D_{50}</math>, H and MWD at 20m water depth''<br />
<br />
!rowspan="2" colspan="2"| Q[m<sup>3</sup>/24hrs]<br />
!colspan="6" align="center"|&alpha;<sub>0</sub><br />
|-<br />
! width="50"|5!! width="50"|15!! width="50"|30!! width="50"|45!! width="50"|60!! width="50"|75<br />
|-<br />
!rowspan="5"|H<sub>s</sub><br />
!1.0<br />
|50|| 100|| 300|| 350|| 300|| 150<br />
|-<br />
!2.0<br />
|400|| 1000|| 2,000|| 3,000|| 2,500|| 1,000<br />
|-<br />
!3.0<br />
|1,500||4,000|| 10,000||15,000||10000|| 5,000<br />
|-<br />
!4.0<br />
|4,000|| 10,000||30,000||40,000||35,000||15,000<br />
|-<br />
!5.0<br />
|8,000|| 25,000||65,000||100,000||85,000||35,000<br />
|}<br />
<br />
<br />
[[Image:drift budget_c.jpg|350px|thumb|right|Fig. 3.]]<br />
An important parameter in relation to the littoral drift conditions is the variation of the net transport with varying orientation of the coastline. If e.g. a groyne is constructed, this will initially block the transport resulting in net zero transport at this location. This means that the sand will accrete upstream of the groyne forming a coastline with the orientation, which gives zero transport. The efficiency of the groyne depends very much on the angle between the present orientation of the coastline and the orientation of net zero transport. If this angle is small, the groyne will be efficient, as it will be able to hold a long sand filet. If the angle is large, which is the case with a very oblique wave exposure, the groyne will only be able to hold a very short sand filet, which means that a groyne will not be an applicable type of coast protection in this case.<br />
<br />
====Net littoral drift====<br />
When discussing the littoral transport along a coastline in general, it is always the net littoral drift that is referred to unless otherwise specified. Gradients in the net littoral drift along a section of coast lead to '''coastline erosion or accretion'''. The gross littoral drift is important for backfilling of channels/trenches across the littoral drift zone, as all littoral drift situations lead to backfilling of the channel/trench.<br />
<br />
====What does littoral drift depend on?====<br />
The littoral drift also depends on the sea current, although to a much smaller extent than it depends on the longshore current. This means that the most important hydraulic parameter for the littoral transport is the wave conditions.<br />
<br />
The water-level mainly determines where in the coastal profile the transport will take place, but the water-level only influences the magnitude of the littoral drift to a lesser extent. However, the tide may have significant influence on the transport conditions for macro-tidal environments. Positive or negative correlation between the waves and the water-level variations may be of importance for sedimentation patterns near large structures. <br />
<br />
====Sediments and littoral drift====<br />
At many locations there is a considerable variation in the grain size depending on the distance from the coastline. Typically the sediments become finer with increasing distance from the coastline. This will, to some extent, blur the picture of the littoral drift given above.<br />
<br />
The fine cohesive sediments, which may be present in the outer part of the profile, will be in suspension over the entire water column and will also tend to spread over the entire coastal profile during strong wave exposure. The transport which this gives rise to is normally not considered a part of the littoral drift, as this only takes the non-cohesive sediments into consideration. The transport of the cohesive sediments thus only plays an indirect role in the stability of the coastal profile. The existence of this transport of fine suspended sediments will, however, be of importance in relation to sedimentation in ports and in trenches.<br />
<br />
[[Image:tombolo2.jpg|thumb|right|Fig. 4. Tombolo formation behind coastal breakwater]]<br />
The longshore transport is, as already mentioned, characterised by a combination of sediment moved along the seabed, the so-called ''bed load transport'', and of sediment in suspension, the so-called ‘’suspended load’’. Even when the sand is in suspension it is still relatively close to the seabed because of the relatively high ''fall velocity'' of sand grains. This means that any change in the hydrodynamics or bathymetric conditions will "immediately" result in a corresponding change in the transport capacity and therefore also in the morphology. This results for instance in the typical accumulation of sand behind even a relatively short, detached, coastal breakwater, as the accumulation of sand reflects the “immediate” response on the attenuated transport capacity behind the breakwater. It is not possible to guide the sand between the coastal breakwater and the shoreline if the breakwater has a length of more than around 0.5 times the distance from the shoreline. If the length of the breakwater is more than approximately 0.8 times the distance from the shoreline so much sand will be trapped that the breakwater will be connected to land by a tombolo formation. This immediate morphological response to even small changes in the littoral transport is also the reason why many attempts to construct island-ports with zero impact on the shoreline have failed. Most of them have been connected to land by [[tombolo]] formations.<br />
<br />
==Onshore and Offshore Transport and Equilibrium Coastal Profile==<br />
Varying wave conditions result in varying onshore and offshore transports over the coastal profile. These transports are, to some extent, reversible and therefore non critical in terms of long term coastal stability. However, extreme storm surge and wave exposure result in coastal erosion.<br />
<br />
===Erosion and rebuilding sequence===<br />
When the coastal profile is exposed to non extreme waves and storm surge, the sediments near the shoreline will be transported offshore and typically be deposited in a bar resulting in an overall flattening of the slope of the shoreface. However, the inner part of the shoreface as well as the foreshore will become steeper in this process, and the shoreline will recede. During the following periods of smaller waves, swell and normal water-level conditions, the bar will travel very slowly towards the coastline again, practically rebuilding the original coastal profile.<br />
<br />
During such a sequence of profile erosion and rebuilding, certain parts of the coastal profile may experience temporary erosion. This may not be recorded in profile surveys, because some rebuilding will already have taken place before it is possible to carry out surveys after the storm. It is important to take such temporary profile fluctuations into account when designing structures in the coastal zone. It is particularly important to have a sufficiently wide beach so that the temporary beach erosion will not cause erosion of the coast.<br />
<br />
===Coastal profile===<br />
This onshore and offshore transport is closely related to the form of the coastal profile. Several investigations have revealed that a coastal profile possesses an average, characteristic form, which is referred to as the theoretical equilibrium profile. The equilibrium profile has been defined as "a statistical average profile, which maintains its form apart from small fluctuations, including seasonal fluctuations". The depth d [meters] in the equilibrium profile increases exponentially with the distance x from the shoreline according to the equation<ref name=Dean/><br />
<br />
:<math>d=Ax^{m}</math> [x and d in meters]<br />
<br />
where A is the dimensionless steepness parameter and m is a dimensionless exponent. Based on fitting to natural upper shoreface profiles, Dean, 1987<ref name="Dean">Dean, R.G., 1987. "Coastal Sediment Processes: Toward engineering solutions." Proceedings Coastal Sediments '87, Am. So. Civ. Eng., 1-24.</ref> has suggested an average value of m = 0.67. However the value of m is subject to large variability dependent of the beach type expressed by the dimensionless fall velocity <math>\Omega=H_{0}/\omega_{s}T</math> where <math>H_{0}</math> is the deep water wave height, T is the wave period and <math>\omega s</math> is the sediment fall velocity. The value of m varies typically between m ~ 0.4 for reflective beaches (<math>\Omega<1.5</math>) and m ~ 0.8 for dissipative beaches (<math>\Omega>5.5</math>).<ref>Cowell, P.J., Hanslow, D.J. and Meleo, J.F., 1999. The Shoreface: In: A.D. Short (editor), Handbook of Beach and Shoreface Morphodynamics, Wiley and Sons, Chichester, 29-71.</ref><ref>Masselink, G. and Huges, M. G., 2003. Introduction to Coastal Processes adn Geomorphology. Published by Hodder Arnold. ISBN 0340764104.</ref>.<br />
<br />
<br />
The steepness parameter A has empirically been related<ref name="Dean"/> to the sediment fall velocity <br />
<math>\omega_{s}</math> as follows:<br />
<br />
:<math>A=0.067\omega_{s}^{0.44}</math> [<math>\omega_{s}</math> in cm <math>s^{-1}</math>]<br />
<br />
Values for A as a function of the mean grain size <math>d_{50}</math> is shown in the table below.<br />
<br />
{|border="1" align="center"<br />
|+Table: Correlation between mean grain size d<sub>50</sub> in mm and the constant A in Dean’s equilibrium profile equation<br />
!width="100"|d50<br />
!width="75"|0.10<br />
!width="75"|0.15<br />
!width="75"|0.20<br />
!width="75"|0.25<br />
!width="75"|0.30<br />
!width="75"|0.50<br />
!width="75"|1.00<br />
!width="75"|2.00<br />
!width="75"|5.00<br />
!width="75"|10.00<br />
|-align="center"<br />
!width="100"|A<br />
|0.043||0.062||0.080||0.092||0.103||0.132||0.178||0.234||0.318||0.390<br />
|}<br />
<br />
<br />
It is seen that the equilibrium profile does not depend on the wave height. The reason for this is that the water depth limits the wave height inside the breaker zone. However, the wave height decides the width of the littoral zone, within which the equilibrium shoreface concept is valid. Thus, the equilibrium profile is only valid for the littoral zone, i.e. out to the [[Closure depth]] d<sub>l</sub>:<br />
<br />
:<math>d_{1} = 2.28H_{S,12h/y} - 68.5^{H^{2}_{S,12h/y}}gT^{2}_{s}</math><br />
<br />
<br />
[[Image:equilibrium profiles.jpg|thumb|Fig. 5. Equilibrium profiles for grain sizes 0.15, 0.2, 0.3, 0.5, 1.0, 10 and 30 mm.]]<br />
[[Image:emperical width.jpg|thumb|Fig. 6. Empirical width of littoral zone as a function of the mean grain size for various wave climates represented by <math>H_{S,12h/y}</math>]]<br />
<br />
The width of the littoral zone and the slope of the shoreface thus depend on the mean grain size as well as on the wave conditions.<br />
<br />
The equilibrium profile becomes increasingly steeper with increasing grain size. Typical equilibrium profiles for different grain size characteristics are presented in Fig. 5. <br />
<br />
The width of the littoral zone as a function of the mean grain size and for different wave climates, represented by <math>H_{S,12h/y}</math>, is presented in Fig. 6.<br />
<br />
These figures can be used in preliminary design considerations for artificial beaches and reclamation areas fronted by natural slopes.<br />
<br />
It is evident from these correlations between grain size, equilibrium profile and wave conditions that it is very important in beach nourishment to use materials as coarse as or coarser than the native material. Otherwise the nourished sand will immediately be transported offshore in nature’s attempt to form the new and flatter equilibrium profile, which fits the finer sand.<br />
<br />
In the real world there is often a sorting of the sediments in the active coastal profile; the mean grain size decreases with increasing distance from the shoreline. If this variation is introduced into the considerations concerning the equilibrium profile, a more accurate representation of the equilibrium profile at a specific site is obtained.<br />
<br />
The concept of equilibrium profiles is a rather crude representation of the coastal profile conditions since it neither includes nor explains the occurrence of bar formations, etc. However, the concept of the equilibrium profile is a rather practical “tool” for the analysis of coastal conditions and, as already mentioned, for preliminary design considerations. <br />
<br />
If the geological coastal profile at a location is flatter than the calculated equilibrium profile, the wave action in the profile will tend to form the equilibrium profile, which means that material will be moved towards the shore. However, at a certain location towards the shore, there is not sufficient wave energy to move the sand any further and a barrier with a corresponding lagoon is formed.<br />
<br />
The equilibrium concept can also explain why shore and coast erosion take place at locations where the equilibrium profile is already established, when such profiles are exposed to the combined action of storm waves and storm surge (tidal wave). The increased water level will correspond to a profile, which is too steep compared to the equilibrium profile. At a certain distance from the shoreline the water will consequently be too deep relative to the equilibrium depth. Nature will compensate by transporting sand from the beach towards the sea in an attempt to re-establish the equilibrium profile, which fits the temporary high water level. This will result in setback of the shoreline; however, if the beach is not sufficiently wide for this adjustment, the sediment will be taken from the cliff or dunes. The amount of erosion during a storm thus depends primarily upon the magnitude of the storm surge and its duration. It is evident from this description that a wide beach is a precondition for a stable coastline. Coast protection can thus be established by providing a wide beach through beach or foreshore nourishment. After the storm, the material, which was brought offshore during the storm surge conditions, will to a great extent be transported slowly back to the beach, however extreme storm surge/wave events will result in a permanent offshore loss of material.<br />
<br />
===Cross-shore sediment transport===<br />
<ref name="Aagard"/>Once the sediment is brought up into the water column it becomes available for transport by the various hydrodynamic processes. In the longshore dimension, the transport is accomplished almost exclusively by mean currents because the longshore component of wave orbital motions is small. Longshore sediment transport gradients are small on long straight coasts and hence the morphological impact is small except in the vicinity of shoreline discontinuities. <br />
<br />
<ref name="Aagard"/>In the cross-shore dimension, however, the transport is considerably more complex. The net sediment transport at a given point in the profile is often a balance between an onshore transport caused by skewed incident short wave motions, an offshore transport caused by mean currents and a transport caused by long waves which can be either onshore or offshore directed<ref name="Aagard, Masselink">Aagaard, T. and Masselink, G., 1999. The Surf Zone. In: A.D.Short (ed) Handbook of Beach and Shoreface Morphodynamics, Wiley Interscience, pp.72-118. </ref>. Consequently shore-normal sediment transport gradients can become large and morphological changes created by such transport gradients are often considerable, spatially as well as temporally. <br />
<br />
<ref name="Aagard"/>Accretion occurs in zones of sediment transport convergence whereas erosion occurs in zones of divergence. As an example, longshore bars are often formed near, or in the zone of wave breaking because the sediment transport outside the breakpoint is often dominated by the short waves and onshore directed because of the wave skewness while inside the breakpoint, sediment transport is dominated by the offshore directed undertow. The position of the bars in the profile can then fluctuate in phase with the wave energy conditions: When waves are small, the breakpoint is located close to the shoreline and the bars tend to move onshore while large waves break far from the shoreline and the bars move offshore.<br />
<br />
==Transport of non-cohensive sediments==<br />
Most of the transport of non-cohesive sediments (sand) takes place in wave-dominated environments. There are, however, locations where the transport of sand is mainly dominated by the current. In relation to coastal morphology the ''tidal inlet'' is the most important. <br />
<br />
===Description of tidal inlets===<br />
A tidal inlet is the connection between the sea and a lagoon, which is exposed to shifting tidal currents.<br />
<br />
Flood tide causes the tidal current to run from the sea into the lagoon and ebb tide goes in the opposite direction. The exchanged water mass during a tidal cycle is called the ''tidal volume.'' This can roughly be calculated as the surface area of the lagoon times the tidal range.<br />
<br />
The tidal current in a tidal inlet on a coastline is responsible for the exchange of sand between the littoral zone and the lagoon. This sand transport typically results in varying depths and shifting locations of the inlets and in the formation of lagoon shoals ''(flood shoals)'' and offshore shoals ''(ebb shoals)''. At the same time the longshore transport interferes in these processes resulting in curved bar formations crossing the inlet as well as the formation of sand spits and possible shifting of the tidal inlet along the shore. All in all, it is a very mobile environment, which is not recommended for development of any kind.<br />
<br />
Tidal inlets are very often regulated and fixed by inlet jetties and they are frequently dredged to allow navigation. An important aspect in relation to regulated tidal inlets on littoral transport coastlines is that the jetties constitute a blockage of the littoral transport. This results in sand accumulation on the updrift side and lee side erosion along the downdrift coastline, unless special precautions are taken. When the sand accumulation on the updrift side reaches the tip of the jetty, the sand will start to bypass and this will cause sedimentation in the inlet. Normally the sand does not pass the dredged channel and therefore it does not nourish the lee side beach.<br />
<br />
===Added complexities===<br />
The above description of tidal inlets is greatly simplified and presents only the mechanisms in a very broad outline. The hydrodynamic and sediment transport conditions in tidal inlets are very complicated, as a tidal inlet always constitutes a delicate balance between the “forces” which keep it open, namely the tidal exchange, and the “forces” which tend to close it, namely the littoral transport processes. It adds to the complexity of tidal inlets that many different time scales are involved, the most important of which are outlined below:<br />
<br />
*Semi-diurnal and diurnal tidal components<br />
*Neap and spring with fortnightly periods<br />
*Seasonal variations in water-level, storm surge and wave conditions<br />
*Very wide time scales for the wave conditions: from seconds for single waves to days for storm duration to seasons for variations in general wave climate to years for the recurrence of extreme wave events<br />
<br />
===Studies on tidal inlets===<br />
Tidal inlet studies can be performed at many levels, from<br />
*a parametric empirical stability analysis involving only the main parameters such as the tidal parameters, the cross section area of the inlet and the wave energy; to <br />
*a complete study involving numerical modelling of hydrodynamics, waves, sediment transport and morphological evolution<br />
<br />
==References==<br />
<references/><br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
[[category:Theme 5]]<br />
[[Category:Coastal processes, interactions and resources]]<br />
[[Category:Hydrodynamics]]<br />
[[category:Coastal erosion]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Currents&diff=18167Currents2007-12-08T12:54:30Z<p>Juliettejackson: </p>
<hr />
<div>The various types of currents in the sea, which may be important to coastal processes in one way or another, are described in the following.<br />
==Currents in the Open Sea==<br />
===Tidal===<br />
[[image:tidal currents.jpg|thumb|Fig. 1. Tidal currents in tidal inlet (Caravelas in Brazil).]]<br />
[[Tidal current|Tidal currents]] are strongest in large water depths away from the coastline and in straits where the current is forced into a narrow area. The most important tidal currents in relation to coastal morphology are the currents generated in tidal inlets. Typical maximum current speeds in tidal inlets are approx. 1 m/s, whereas tidal current speeds in straits and estuaries can reach speeds as high as approx. 3 m/s.<br />
<br />
===Wind-generated===<br />
Wind-generated currents are caused by the direct action of the wind shear stress on the surface of the water. The wind-generated currents are normally located in the upper layer of the water body and are therefore not very important from a morphological point of view. In very shallow coastal waters and lagoons, the wind-generated current can, however, be of some importance. Wind-generated current speeds are typically less than 5 per cent of the wind speed.<br />
<br />
===Storm surge===<br />
Storm surge current is the current generated by the total effect of the wind shear stress and the barometric pressure gradients over the entire area of water affected by a specific storm. This type of current is similar to the tidal currents. The horizontal current velocity follows a logarithmic distribution in the water profile and has the same characteristics as the tidal current. It is strongest at large water depths away from the coastline and in confined areas, such as straits and tidal inlets.<br />
<br />
==Current in the Nearshore Zone==<br />
Nearshore mean currents which occur within the surf zone are principally driven by the breaking waves. For purposes of simplification, nearshore mean currents are usually separated into their cross-shore and longshore components: Undertows and rip currents have their principal axes oriented perpendicular to the beach (offshore) while longshore currents act parallel to the beach. These currents are all driven by cross- and/or longshore components of [[radiation stress]] gradients (in practive wave energy gradients) that arise through wave breaking. <br />
<br />
===Shore-parallel currents===<br />
''The longshore current'' is the dominant current in the nearshore zone. The longshore current is generated by the shore-parallel component of the stresses associated with the breaking process for obliquely incoming waves, the so-called ''radiation stresses'', and by the surplus water which is carried across the breaker-zone towards the coastline. This current has its maximum close to the breaker-line. During storms the longshore current can reach speeds exceeding 2.5 m/s. The longshore current carries sediment along the shoreline, the so-called littoral drift; this mechanism will be discussed further in [[Coastal Hydrodynamics And Transport Processes]].<br />
<br />
The longshore current is generally parallel to the coastline and it varies in strength approximately proportional to the square root of the wave height and with sin2&alpha;<sub>b</sub>, where &alpha;<sub>b</sub> is the wave incidence angle at breaking. As the position of the breaking line constantly shifts due to the irregularity of natural wave fields and since the distance to the breaker-line varies with the wave height, the distribution of the longshore current in the coastal profile will vary accordingly.<br />
<br />
===Shore-normal currents===<br />
====Undertow====<br />
[[Image:undertow velocities.jpg|thumb|300px|Fig. 2. Undertow velocities measured on a Danish beach during high and moderate wave conditions. Velocities are seaward direcetd and hence negative in the figure.</]]<br />
<ref name="Aagard"/>The undertow is defined as a longshore homogeneous current flowing offshore near the seabed and it is driven by the cross-shore setup gradient, i.e. the radiation stress decay. The offshore discharge of water is compensated by the onshore directed mass transport and roller transport in the upper layers of the water column. Fig. 2. below shows typically occurring undertow velocities on a beach during moderate and high wave conditions. During moderate conditions, only few waves break on the outer bar, and undertow velocities are small. In conditions with large waves, undertow velocities may be up to about 50 cm/s.<br />
<br style="clear:both;"/><br />
<br />
====Rip currents====<br />
[[image:longshore distribution.jpg|thumb|350px|Fig. 3. Distribution in longshore current in a coastal profile and rip current pattern.]]<br />
At certain intervals along the coastline, the longshore current will form a rip current. It is a local current directed away from the shore, bringing the surplus water carried over the bars in the breaking process back into deep water. The rip opening in the bars will often form the lowest section of the coastal profile; a local setback in the shoreline is often seen opposite the rip opening. The rip opening travels slowly downstream. <br />
<br />
<ref name="Aagard">Aagard, Troels. 2007.</ref>Rip currents are narrow, jet-like currents which are directed seaward across the surf zone. They are often located in topographic depressions in nearshore bars and thus topographically constricted. A cell circulation system consists of a slow onshore directed mass transport across bars and two longshore directed feeder currents in the trough that converge on the rip current ''per se''. The rip current is again subdivided into the rip neck, located in the rip channel across the bar, and the rip head seaward of the bar where the rip current expands and slows down. Rip currents are often rhythmically spaced along the beach, having wavelengths of approximately 100-1000 m and they are forced by longshore setup gradients. Such setup gradients occur in the case of longshore wave height gradients or in the case when the topography is non-uniform alongshore. Such non-uniform alongshore topography can consist of alternating shoals/bar horns where wave dissipation is strong, and depressions in the bar where dissipation is weaker. Longshore gradients in wave dissipation create longshore gradients in setup that force the rip currents. As rip currents tend to scour out the depressions in the bar, a positive morphodynamic feedback can exist between bathymetry and hydrodynamics.<br />
<br />
====Cross-currents along the shore-normal coastal profile====<br />
Cross-currents occur especially in the surf-zone. Three contributions balance each other:<br />
*'''Mass transport''', or wave drift, is a phenomenon occurring during wave motion over both sloping and horizontal beds. Water particles near the surface will be transported in the direction of wave propagation when waves travel over an area. This phenomenon is called the mass transport. In the surf-zone the mass transport is directed towards the coast.<br />
*'''Surface roller drift'''. When the waves break, water is transported in the surface rollers towards the coast. This is the so-called surface roller drift.<br />
*'''Undertow'''. In the surf-zone, the above two contributions are concentrated near the surface. As the net flow is zero, they are compensated for by a return flow in the offshore direction, which is concentrated near the bed. This is the so-called undertow. The undertow is important in the formation of bars.<br />
<br />
===Two-dimensional currents in the nearshore zone===<br />
Along a straight shoreline, the above-mentioned shore-parallel and shore-normal current patterns dominate. The currents discussed here are two-dimensional in the horizontal plane due to complex bathymetries and structures in the nearshore zone. <br />
<br />
Two-dimensional current patterns occur, especially in the following situations:<br />
<br />
#When the bathymetry is irregular and very different from the smooth shore-parallel pattern of depth contours characteristic of sandy shorelines, and also when the coastline is very irregular. This can, for example, be at partially rocky coastlines or along coastlines where coral reefs or other hard reefs are present. Irregular depth contours give rise to irregular wave patterns, which again can cause special current phenomena important to the understanding of the coastal morphology. Irregular bathymetry combined with an irregular coastline adds further to the complexity of the wave and current pattern. Reefs provide partial protection against wave action. However, they also generate overtopping of water and compensation currents behind the reef. At low sections of the reef or in gaps in the reef, the surplus water returns to the sea in rip-like jets. This is the pattern for both submerged reefs and emerged reefs with overtopping during storms. Such current systems are of great importance to the morphology behind the reef. Changes in reef structure, natural or man-made, can cause great changes in the morphology. <br />
#In the vicinity of coastal structures, such as groynes, coastal breakwaters and port structures. Such structures influence the current pattern in two principally different ways: by obstructing the shore-parallel current and by setting up secondary circulation currents. <br />
<br />
<br />
====Obstructions====<br />
[[image:lee circulation_both.jpg|200px|thumb|Fig. 4. Lee circulation patterns for a coastal breakwater and a small port. The optimal shape of a small port, avoiding the lee area.]]<br />
The nature of the obstruction of the shore-parallel currents of course depends on the extension and shape of the coastal structure. If the structure is located within the breaker-zone, the obstruction leads to offshore-directed jet-like currents, which cause loss of beach material. If the structure is a port, the current will follow the upstream breakwater and finally reach the entrance area. The currents in the entrance area will both influence the navigation conditions and cause sedimentation, consequently the design of the entrance is important. It must provide a smooth and predictable current pattern so its impact on navigation is acceptable, sedimentation must be minimised and the bypass of sand must be optimised. The answer is a smooth layout of the main and secondary breakwaters combined with a narrow entrance pointing towards the prevailing waves.<br />
<br />
=====Leeward side=====<br />
At the leeward side of coastal structures, special current patterns caused by the sheltering effect of the structure in the diffraction area can develop. Sheltered or partly sheltered areas may result in circulation currents along the inner shoreface as well as return currents leading to deep water. The reason for this is that the wave set-up in the sheltered areas is smaller than in the adjacent exposed areas and this generates a gradient in the water-level towards the sheltered areas. These circulation currents in the sheltered areas can be dangerous for swimmers who are using the sheltered area for swimming during rough weather. Another problem is that the sheltered areas will be exposed to sedimentation and such areas must, therefore, be avoided when planning small ports.<br />
<br />
=====Beyond breaker zone=====<br />
If the structure extends beyond the breaker-zone, the shore-parallel current will be directed along the structure, where the increasing depth will decrease the speed. The current will deposit the sand in a shoal off the breaker-zone upstream of the structure. In the case of a major port, the longshore current will not reach the entrance area. In the lee area of a major coastal structure, the effect of return currents towards the sheltered area will also be pronounced, but the current circulation pattern will be smoother and less dangerous for swimmers. The sheltered areas will act as a sedimentation area adding severely to effects of the lee side erosion outside the sheltered area of such structures. Once again, sheltered areas should be avoided.<br />
<br />
<br />
=====Special morphological features=====<br />
[[image:ebb and flood.jpg|thumb|Fig. 5. Ebb and flood shoals at tidal channel, Cay Calker, Belize. This area is mainly exposed to the tidal currents, whereas the wave climate is very mild.]]<br />
Adjacent to special morphological features such as sand spits, river mouths and tidal inlets. The current patterns and the associated sediment transport at such locations can be very complicated. Only a few general comments will be given in this overview of currents and their impacts.<br />
<br />
In tidal inlets and river mouths there are often concentrated currents in the gorge section of the mouth, but seawards of this area the current pattern expands and the current speed decreases. This is also the case landwards of the gorge section in tidal inlets. The gorge section is often deep and narrow, whereas the expanding currents on either side tend to form the ebb and flood shoals respectively. The ebb shoal tends to form a dome-shaped bar on littoral transport shorelines, on which the littoral transport bypasses the mouth/inlet.<br />
<br style="clear:both;"/><br />
<br />
==References==<br />
<references/><br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
[[category:Theme 5]]<br />
[[Category:Coastal processes, interactions and resources]]<br />
[[Category:Hydrodynamics]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Tidal_current&diff=18166Tidal current2007-12-08T12:51:32Z<p>Juliettejackson: </p>
<hr />
<div>{{Definition|title=Tidal currents<br />
|definition= Tidal currents are formed by the gravitational forces of the sun, the moon and the planets. These currents are of oscillatory nature with typical periods of around 12 or 24 hours, the so-called ''semi-diurnal'' and ''diurnal'' tidal currents. }}<br />
<br />
<br />
==References==<br />
Karsten Mangor</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Tidal_current&diff=18165Tidal current2007-12-08T12:50:37Z<p>Juliettejackson: </p>
<hr />
<div><br />
{{Definition|title=Tidal currents<br />
|definition= Tidal currents are formed by the gravitational forces of the sun, the moon and the planets. These currents are of oscillatory nature with typical periods of around 12 or 24 hours, the so-called ''semi-diurnal'' and ''diurnal'' tidal currents. <ref name="Karsten">Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment</ref>.<br />
}}<br />
<br />
<br />
==References==<br />
<references/></div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Currents&diff=18164Currents2007-12-08T12:44:39Z<p>Juliettejackson: </p>
<hr />
<div>The various types of currents in the sea, which may be important to coastal processes in one way or another, are described in the following.<br />
==Currents in the Open Sea==<br />
===Tidal===<br />
[[image:tidal currents.jpg|thumb|Fig. 1. Tidal currents in tidal inlet (Caravelas in Brazil).]]<br />
[[Tidal current|tidal currents]] are strongest in large water depths away from the coastline and in straits where the current is forced into a narrow area. The most important tidal currents in relation to coastal morphology are the currents generated in tidal inlets. Typical maximum current speeds in tidal inlets are approx. 1 m/s, whereas tidal current speeds in straits and estuaries can reach speeds as high as approx. 3 m/s.<br />
<br />
===Wind-generated===<br />
Wind-generated currents are caused by the direct action of the wind shear stress on the surface of the water. The wind-generated currents are normally located in the upper layer of the water body and are therefore not very important from a morphological point of view. In very shallow coastal waters and lagoons, the wind-generated current can, however, be of some importance. Wind-generated current speeds are typically less than 5 per cent of the wind speed.<br />
<br />
===Storm surge===<br />
Storm surge current is the current generated by the total effect of the wind shear stress and the barometric pressure gradients over the entire area of water affected by a specific storm. This type of current is similar to the tidal currents. The horizontal current velocity follows a logarithmic distribution in the water profile and has the same characteristics as the tidal current. It is strongest at large water depths away from the coastline and in confined areas, such as straits and tidal inlets.<br />
<br />
==Current in the Nearshore Zone==<br />
Nearshore mean currents which occur within the surf zone are principally driven by the breaking waves. For purposes of simplification, nearshore mean currents are usually separated into their cross-shore and longshore components: Undertows and rip currents have their principal axes oriented perpendicular to the beach (offshore) while longshore currents act parallel to the beach. These currents are all driven by cross- and/or longshore components of [[radiation stress]] gradients (in practive wave energy gradients) that arise through wave breaking. <br />
<br />
===Shore-parallel currents===<br />
''The longshore current'' is the dominant current in the nearshore zone. The longshore current is generated by the shore-parallel component of the stresses associated with the breaking process for obliquely incoming waves, the so-called ''radiation stresses'', and by the surplus water which is carried across the breaker-zone towards the coastline. This current has its maximum close to the breaker-line. During storms the longshore current can reach speeds exceeding 2.5 m/s. The longshore current carries sediment along the shoreline, the so-called littoral drift; this mechanism will be discussed further in [[Coastal Hydrodynamics And Transport Processes]].<br />
<br />
The longshore current is generally parallel to the coastline and it varies in strength approximately proportional to the square root of the wave height and with sin2&alpha;<sub>b</sub>, where &alpha;<sub>b</sub> is the wave incidence angle at breaking. As the position of the breaking line constantly shifts due to the irregularity of natural wave fields and since the distance to the breaker-line varies with the wave height, the distribution of the longshore current in the coastal profile will vary accordingly.<br />
<br />
===Shore-normal currents===<br />
====Undertow====<br />
[[Image:undertow velocities.jpg|thumb|300px|Fig. 2. Undertow velocities measured on a Danish beach during high and moderate wave conditions. Velocities are seaward direcetd and hence negative in the figure.</]]<br />
<ref name="Aagard"/>The undertow is defined as a longshore homogeneous current flowing offshore near the seabed and it is driven by the cross-shore setup gradient, i.e. the radiation stress decay. The offshore discharge of water is compensated by the onshore directed mass transport and roller transport in the upper layers of the water column. Fig. 2. below shows typically occurring undertow velocities on a beach during moderate and high wave conditions. During moderate conditions, only few waves break on the outer bar, and undertow velocities are small. In conditions with large waves, undertow velocities may be up to about 50 cm/s.<br />
<br style="clear:both;"/><br />
<br />
====Rip currents====<br />
[[image:longshore distribution.jpg|thumb|350px|Fig. 3. Distribution in longshore current in a coastal profile and rip current pattern.]]<br />
At certain intervals along the coastline, the longshore current will form a rip current. It is a local current directed away from the shore, bringing the surplus water carried over the bars in the breaking process back into deep water. The rip opening in the bars will often form the lowest section of the coastal profile; a local setback in the shoreline is often seen opposite the rip opening. The rip opening travels slowly downstream. <br />
<br />
<ref name="Aagard">Aagard, Troels. 2007.</ref>Rip currents are narrow, jet-like currents which are directed seaward across the surf zone. They are often located in topographic depressions in nearshore bars and thus topographically constricted. A cell circulation system consists of a slow onshore directed mass transport across bars and two longshore directed feeder currents in the trough that converge on the rip current ''per se''. The rip current is again subdivided into the rip neck, located in the rip channel across the bar, and the rip head seaward of the bar where the rip current expands and slows down. Rip currents are often rhythmically spaced along the beach, having wavelengths of approximately 100-1000 m and they are forced by longshore setup gradients. Such setup gradients occur in the case of longshore wave height gradients or in the case when the topography is non-uniform alongshore. Such non-uniform alongshore topography can consist of alternating shoals/bar horns where wave dissipation is strong, and depressions in the bar where dissipation is weaker. Longshore gradients in wave dissipation create longshore gradients in setup that force the rip currents. As rip currents tend to scour out the depressions in the bar, a positive morphodynamic feedback can exist between bathymetry and hydrodynamics.<br />
<br />
====Cross-currents along the shore-normal coastal profile====<br />
Cross-currents occur especially in the surf-zone. Three contributions balance each other:<br />
*'''Mass transport''', or wave drift, is a phenomenon occurring during wave motion over both sloping and horizontal beds. Water particles near the surface will be transported in the direction of wave propagation when waves travel over an area. This phenomenon is called the mass transport. In the surf-zone the mass transport is directed towards the coast.<br />
*'''Surface roller drift'''. When the waves break, water is transported in the surface rollers towards the coast. This is the so-called surface roller drift.<br />
*'''Undertow'''. In the surf-zone, the above two contributions are concentrated near the surface. As the net flow is zero, they are compensated for by a return flow in the offshore direction, which is concentrated near the bed. This is the so-called undertow. The undertow is important in the formation of bars.<br />
<br />
===Two-dimensional currents in the nearshore zone===<br />
Along a straight shoreline, the above-mentioned shore-parallel and shore-normal current patterns dominate. The currents discussed here are two-dimensional in the horizontal plane due to complex bathymetries and structures in the nearshore zone. <br />
<br />
Two-dimensional current patterns occur, especially in the following situations:<br />
<br />
#When the bathymetry is irregular and very different from the smooth shore-parallel pattern of depth contours characteristic of sandy shorelines, and also when the coastline is very irregular. This can, for example, be at partially rocky coastlines or along coastlines where coral reefs or other hard reefs are present. Irregular depth contours give rise to irregular wave patterns, which again can cause special current phenomena important to the understanding of the coastal morphology. Irregular bathymetry combined with an irregular coastline adds further to the complexity of the wave and current pattern. Reefs provide partial protection against wave action. However, they also generate overtopping of water and compensation currents behind the reef. At low sections of the reef or in gaps in the reef, the surplus water returns to the sea in rip-like jets. This is the pattern for both submerged reefs and emerged reefs with overtopping during storms. Such current systems are of great importance to the morphology behind the reef. Changes in reef structure, natural or man-made, can cause great changes in the morphology. <br />
#In the vicinity of coastal structures, such as groynes, coastal breakwaters and port structures. Such structures influence the current pattern in two principally different ways: by obstructing the shore-parallel current and by setting up secondary circulation currents. <br />
<br />
<br />
====Obstructions====<br />
[[image:lee circulation_both.jpg|200px|thumb|Fig. 4. Lee circulation patterns for a coastal breakwater and a small port. The optimal shape of a small port, avoiding the lee area.]]<br />
The nature of the obstruction of the shore-parallel currents of course depends on the extension and shape of the coastal structure. If the structure is located within the breaker-zone, the obstruction leads to offshore-directed jet-like currents, which cause loss of beach material. If the structure is a port, the current will follow the upstream breakwater and finally reach the entrance area. The currents in the entrance area will both influence the navigation conditions and cause sedimentation, consequently the design of the entrance is important. It must provide a smooth and predictable current pattern so its impact on navigation is acceptable, sedimentation must be minimised and the bypass of sand must be optimised. The answer is a smooth layout of the main and secondary breakwaters combined with a narrow entrance pointing towards the prevailing waves.<br />
<br />
=====Leeward side=====<br />
At the leeward side of coastal structures, special current patterns caused by the sheltering effect of the structure in the diffraction area can develop. Sheltered or partly sheltered areas may result in circulation currents along the inner shoreface as well as return currents leading to deep water. The reason for this is that the wave set-up in the sheltered areas is smaller than in the adjacent exposed areas and this generates a gradient in the water-level towards the sheltered areas. These circulation currents in the sheltered areas can be dangerous for swimmers who are using the sheltered area for swimming during rough weather. Another problem is that the sheltered areas will be exposed to sedimentation and such areas must, therefore, be avoided when planning small ports.<br />
<br />
=====Beyond breaker zone=====<br />
If the structure extends beyond the breaker-zone, the shore-parallel current will be directed along the structure, where the increasing depth will decrease the speed. The current will deposit the sand in a shoal off the breaker-zone upstream of the structure. In the case of a major port, the longshore current will not reach the entrance area. In the lee area of a major coastal structure, the effect of return currents towards the sheltered area will also be pronounced, but the current circulation pattern will be smoother and less dangerous for swimmers. The sheltered areas will act as a sedimentation area adding severely to effects of the lee side erosion outside the sheltered area of such structures. Once again, sheltered areas should be avoided.<br />
<br />
<br />
=====Special morphological features=====<br />
[[image:ebb and flood.jpg|thumb|Fig. 5. Ebb and flood shoals at tidal channel, Cay Calker, Belize. This area is mainly exposed to the tidal currents, whereas the wave climate is very mild.]]<br />
Adjacent to special morphological features such as sand spits, river mouths and tidal inlets. The current patterns and the associated sediment transport at such locations can be very complicated. Only a few general comments will be given in this overview of currents and their impacts.<br />
<br />
In tidal inlets and river mouths there are often concentrated currents in the gorge section of the mouth, but seawards of this area the current pattern expands and the current speed decreases. This is also the case landwards of the gorge section in tidal inlets. The gorge section is often deep and narrow, whereas the expanding currents on either side tend to form the ebb and flood shoals respectively. The ebb shoal tends to form a dome-shaped bar on littoral transport shorelines, on which the littoral transport bypasses the mouth/inlet.<br />
<br style="clear:both;"/><br />
<br />
==References==<br />
<references/><br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
[[category:Theme 5]]<br />
[[Category:Coastal processes, interactions and resources]]<br />
[[Category:Hydrodynamics]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Waves&diff=17198Waves2007-11-30T19:50:47Z<p>Juliettejackson: </p>
<hr />
<div>There is typically a distinction between short waves, which are waves with periods less than approximately 20 s, and long waves or long period oscillations, which are oscillations with periods between 20-30 s and 40 min. Water-level oscillations with periods or recurrence intervals larger than around 1 hour, such as [[tide|astronomical tide]] and [[storm surge]], are referred to as water-level variations. The short waves are wind waves and swell, whereas long waves are divided into surf-beats, harbour resonance, seiche and tsunamis. Natural waves can be viewed as a wave field consisting of a large number of single wave components each characterised by a wave height, a wave period and a propagation direction. Wave fields with many different wave periods and heights are called irregular, and wave fields with many wave directions are called directional. A wave field can be more or less irregular and more or less directional.<br />
<br />
==Short Waves==<br />
===Types of short waves===<br />
<br />
[[Image:irregular storm a.jpg|thumb|200px|Fig. 1a. Irregular directional storm waves (including white capping)]]<br />
[[Image:irregular storm b.jpg|thumb|200px|Fig. 1b. Regular unidirectional swell.]]<br />
<br />
<ref name="Aagard">Written by Aagard, Troels. 2007.</ref>Short waves are waves, generated by the wind that propagate towards the beach. They can be either actively forced by the wind (wind waves - see below) or they can have left their generation area (swell waves - see below). Incident waves are the primary source of energy input to the beach. On their way from deep water towards the shoreline they undergo refraction and shoaling processes. In deep water, incident waves are nearly sinusoidal; as they propagate into shallower water (shoaling), their celerity and wave length decrease and as the total energy flux should remain constant (according to linear theory and neglecting bottom friction), the wave height must increase while the wavelength decreases. <br />
<br />
<ref name="Aagard"/>As the waves propagate towards the shoreline, the wave shape becomes increasingly skewed with peaked wave crests and longer, rounded wave troughs, and wave orbital velocities become larger under crests than under troughs. This is a characteristic of fundamental importance to sediment transport, especially seaward of the wave breakpoint as there will be a tendency for the incident waves to push sediment towards the beach. <br />
<br />
The short waves are the single most important parameter in coastal morphology. Wave conditions vary considerably from site to site, depending mainly on the wind climate and on the type of water area. The short waves are divided into:<br />
<br />
*'''Wind waves''', also called storm waves, or sea. These are waves generated and influenced by the local wind field. Wind waves are normally relatively steep (high and short) and are often both irregular and directional, for which reason it is difficult to distinguish defined wave fronts. The waves are also referred to as short-crested. Wind waves tend to be destructive for the coastal profile because they generate an offshore (as opposed to onshore) movement of sediments, which results in a generally flat shoreface and a steep foreshore.<br />
*'''Swell''' are waves, which have been generated by wind fields far away and have travelled long distances over deep water away from the wind field, which generated the waves. Their direction of propagation is thus not necessarily the same as the local wind direction. Swell waves are often relatively long, of moderate height, regular and unidirectional. Swell waves tend to build up the coastal profile to a steep shoreface.<br />
<br />
<br />
<br />
<br />
====Wave breaking====<br />
<ref name="Aagard"/>Depth-limited wave breaking is the prerequisite for the generation of nearshore currents and secondary wave phenomena. Seaward of the surf zone, any wave energy losses primarily occur through whitecapping and friction against the sea bed. As the waves approach the beach, however, depth-limited breaking will occur when orbital velocities, increasing towards the beach exceed the wave phase speed which decreases in the landward direction. The breaking wave height, H<sub>b</sub> is related to the water depth at breaking, h<sub>b</sub>, through<br />
<br />
:H<sub>b</sub>=γh<sub>b</sub><br />
<br />
<br />
where γ is the breaker index. In nature waves are irregular and random and using H<sub>rms</sub> as the measure of wave height, the maximum time-averaged value of the breaker index (<γ<sub>rms</sub>>) is in the order of 0.35-0.5. The fact that wave heights within the surf zone are depth-limited means that wave heights approach a linear function of water depth. <br />
<br />
The predominant type of wave breaking depends on wave steepmness and beach slope expressed through the surf scaling parameter:<br />
<br />
:&epsilon;=&pi;H / Tgtan<sup>2</sup>&beta;<br />
<br />
<br />
where T is wave period, g is the acceleration of gravity and β is the beach slope. With spilling breakers, ε > 20, plunging occurs for 2.5 < ε < 20 and surging breakers predominate when ε < 2.5. <br />
<br />
As waves propagate towards the beach, short wave energy is gradually lost through breaking and long infragravity waves become more important.<br />
<br />
===Wave generation===<br />
Wind waves are generated as a result of the action of the wind on the surface of the water. The wave height, wave period, propagation direction and duration of the wave field at a certain location depend on:<br />
<br />
#The wind field (speed, direction and duration)<br />
#The fetch of the wind field (meteorological fetch) or the water area (geographical fetch)<br />
#The water depth over the wave generation area.<br />
<br />
Swell is, as previously stated, wind waves generated elsewhere but transformed as they propagate away from the generation area. The dissipation processes, such as wave-breaking, attenuate the short period much more than the long period components. This process acts as a filter, whereby the resulting long-crested swell will consist of relatively long waves with moderate wave height.<br />
<br />
===Wave transformation===<br />
[[Wave transformation]]: The types of transformation discussed here are mainly related to wave phenomena occurring in the natural environment. When the waves approach the shoreline, they are affected by the seabed through processes such as refraction, shoaling, bottom friction and wave-breaking. However, wave-breaking also occurs in deep water when the waves are too steep. If the waves meet major structures or abrupt changes in the coastline, they will be transformed by diffraction. If waves meet a submerged reef or structure, they will overtop the reef - please follow the link to article.<br />
<br />
===Statistical description of wave parameters===<br />
[[Statistical description of wave parameters]]: Because of the random nature of natural waves, a statistical description of the waves is normally always used. The individual wave heights often follow the Rayleigh-distribution. Statistical wave parameters are calculated based on this distribution. The most commonly used variables in coastal engineering are described in this section - please follow the link to the article.<br />
<br />
===Wave climate classification according to wind climate===<br />
The different wind climates, which dominate different oceans and regions, cause correspondingly characteristic wave climates. These characteristic wave climates can be classified as follows:<br />
*Storm wave climate. <br />
*Swell climate. <br />
*Monsoon wave climate. <br />
*Tropical cyclone climate. <br />
<br />
For details on these classifications follow the link [[Wave climate classification according to wind climate]].<br />
<br />
==Long Waves==<br />
The long waves are primarily second order phenomena of shallow water wave processes. The four main types of long waves are described in the following.<br />
===Surf beat===<br />
[[image:wave set up_a.jpg|thumb|300px|Fig. 2. Wave set-up]]<br />
Natural waves often show a tendency to wave grouping, where a series of high waves follows a series of low waves. This is especially pronounced on open sea-coasts, where the incoming waves may be of different origins and will thus have a large spreading in wave heights, wave directions, and wave periods (or frequencies). Wave grouping will cause oscillations in the wave set-up with a period corresponding to approx. 6 – 8 times the mean wave period; this phenomenon is called surf-beats. Surf-beats near port entrances are very important in relation to mooring conditions in the port basins and sedimentation in the port entrance.<br />
<br style="clear:both;"/><br />
<br />
===Harbour resonance===<br />
[[image:wave set up_b.jpg|thumb|Fig. 3. Surf beat generated harbour resonance, recorded by a tide gauge]]Harbour resonance is forced oscillation of a confined water body (e.g. a harbour basin or a lagoon) connected to a larger water body (the sea). If long-period oscillations are present in the sea, e.g. due to wave grouping or surf-beats or seiche, large oscillations at the natural frequency of the confined water body may occur. Oscillations at the first harmonic, which are the simplest mode of resonance, are often called the pumping or Helmholz mode. <br />
<br />
Harbour resonance normally has periods in the range of 2 to 10 minutes. It is especially important in connection with the mooring conditions for large vessels, as their resonance period for the so-called surge motion is often close to that of the harbour resonance. In addition the associated water exchange may cause siltation.<br />
<br />
[[image:wave set up_c.jpg|thumb|right|Fig. 4. Circulation caused by the gradient in the wave set-up]]<br />
===Seiche=== <br />
A seiche is the free oscillation of a water body, probably caused by rapid variations in the wind conditions. Seiche can occur in closed water areas, such as lakes or lagoons, and in semi-closed water bodies, such as bays. The period of the seiche oscillation is typically in the range of 2 to 40 minutes. Seiche can influence a port in the same manner as surf-beats. It is important to establish whether seiche is present in an area through field investigations, and if so, to take it into account in the layout of the port. Surf-beat influence within a port is often caused by an inexpedient layout. The influence of surf-beat is not applicable for seiche, as seiche is not limited to the nearshore zone. This means that if seiche motion is present in an area, it will inevitably penetrate the entrance. However, its impact on the port may be minimised through a proper layout.<br />
<br />
===Tsunami===<br />
<!--Threats to the coastal zone, Section 6 links here --><br />
A tsunami is a single wave, which is generated by sub-sea earthquakes; it typically has a period of 5 to 60 minutes. Tsunami waves can travel long distances across the oceans; they are similar to shallow water waves, which means that the speed v is calculated as the square root of the product of the water depth and the acceleration of gravity, v = (gh)<sup>1/2</sup>. Consequently, tsunamis travel very fast in the deep oceans. If the water depth is 5000 m, the speed will be more than 200 m/s or about 800 km/hour. A tsunami is normally not very high in deep water, but when it approaches the coastline, the wave will be shoaling and can reach a height of more than 10 m. Tsunamis are rare and coastal projects seldom take them into account. However, in very sensitive projects, such as nuclear power plants located in the coastal hinterland, the risk must be considered.<br />
<br />
==<ref name="Aagard"/>Infragravity waves==<br />
[[Image:infragravity.jpg|thumb|Fig. 5. Iinfragravity wave orbital velocities at two Danish beaches, plotted as a function of relative water depth. h/h<sub>b</sub> = 0 is at the [[shoreline]], and h/h<sub>b</sub> = 1 indicates the mean position of the wave breakpoint.]]Infragravity waves are waves that are forced by difference interactions in the incident wave frequency band and consequently they have frequencies which are lower than the frequencies of incident waves ~0.005-0.05 Hz. From a morphodynamic viewpoint, much of the interest in infragravity waves is due to the fact that they are often standing in the cross-shore direction and sometimes also alongshore, therefore resulting in a stationary drift velocity field in the bottom boundary layer. They can therefore potentially provide a mechanism for nearshore bar formation and the generation of three-dimensional features such as rip currents and rhythmic bars. Apart from quasi-steady drift velocities in the boundary layer, orbital velocities associated with these motions can generate oscillatory sediment fluxes which have been demonstrated to be important to the net sediment transport in the surf zone. <br />
<br />
Nearshore-standing infragravity waves may occur as either leaky mode waves which are two-dimensional standing waves having a succession of antinodes and nodes away from the point of reflection (e.g. the shoreline), or as edge waves which are three-dimensional waves trapped against the nearshore by reflection and refraction and which can propagate alongshore (progressive edge waves) or be longshore-standing (standing edge waves). Edge waves have a finite number of nodes/antinodes in the cross-shore direction (the number of cross-shore surface elevation nodes is called the mode number, ''n''), and a theoretically infinite number of nodes/antinodes in the longshore dimension. <br />
<br />
Infragravity wave heights and orbital velocities increase towards the shoreline (see Fig. 5. below), while incident wave heights decrease landward due to wave breaking. Infragravity waves should therefore be of increasing relative importance with proximity to the shoreline and sediment resuspension and transport become increasingly affected by infragravity motions. A more comprehensive treatment of nearshore infragravity waves can be found in e.g. Aagaard and Masselink (1999)<ref>Aagaard, T. and Masselink, G., 1999. The Surf Zone. In: A.D.Short (ed) Handbook of Beach and Shoreface Morphodynamics, Wiley Interscience, pp.72-118.</ref>.<br />
<br />
==References==<br />
<references/><br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
<br />
[[category:Theme 5]]<br />
[[Category:Coastal processes, interactions and resources]]<br />
[[Category:Hydrodynamics]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Revetments&diff=17175Revetments2007-11-30T16:17:55Z<p>Juliettejackson: /* See also */</p>
<hr />
<div>The following article discusses the fixing of the coastline by revetments.<br />
<br />
==Method==<br />
[[Revetment|Revetments]] can be an exposed structure as well as a buried structure. <br />
<br />
===Exposed revetments===<br />
Revetments are always made as sloping structures and are very often constructed as permeable structures using natural stones or concrete blocks, thereby enhancing wave energy absorption and minimising reflection and wave run-up. <br />
<br />
However, revetments can also consist of different kinds of concrete slabs, some of them permeable and interlocking. In this way their functionality is increased in terms of absorption and strength. An example of a permeable and interlocking concrete slap is the so-called Flex Slap.<br />
<br />
Net mesh stone-filled mattresses, such as Gabions, are also used; however, they are only recommended for use at fairly protected locations. <br />
<br />
Revetments can also consist of sand-filled geotextile fabric bags, mattresses and tubes. Such structures must be protected against UV-light to avoid weathering of the fabric. Sand-bagging is often used as emergency protection. Geotextile fabric revetments are fragile against mechanical impact and vandalism, and their appearance is not natural. <br />
[[Image:Revetments.jpg|400px|thumb|center|Fig. 1. Examples of revetments.]]<br />
<br />
<br />
===Buried revetments===<br />
[[Image: Emergency revetments.jpg|thumb| Fig. 2. An example of an emergency revetment constructed in concrete blocks, the revetment will later be buried into an artificial dune. (Danish Coastal Authority<ref>Danish Coastal Authority, 1998. "Menneske, Hav, Kyst og Sand". (in Danish), (Man, Sea Coast and Sand in English). Kystinspektoratet 1973-1998.</ref>)]]<br />
<br />
A buried revetment can be constructed as part of a soft protection, e.g. as a hard emergency protection built into a strengthened dune which acts as shore protection and/or sea defence.<br />
<br />
==Functional characteristics==<br />
All types of revetments have the inherent function of beach degradation as they are used at locations where the coast is exposed to erosion. A revetment will fix the location of the coastline, but it will not arrest the ongoing erosion in the coastal profile, and the beach in front of the revetment will gradually disappear. However, as a revetment is often made as a permeable, sloping structure, it will normally not accelerate the erosion, as did seawalls; on the contrary, rubble revetments are often used as reinforcement for seawalls which have been exposed due to the disappearance of the beach. Such reinforcement protects the foot of the seawall and minimises the reflection. <br />
<br />
A revetment, like a seawall, will decrease the release of sediments from the section it protects, for which reason it will have a negative impact on the sediment budget along adjacent shorelines.<br />
<br />
==Applicability==<br />
A revetment is a passive structure, which protects against erosion caused by wave action, storm surge and currents. The main difference in the function of a seawall and a revetment is that a seawall protects against erosion and flooding, whereas a revetment only protects against erosion. A revetment is thus a passive coastal protection measure and is used at locations exposed to erosion or as a supplement to seawalls or dikes at locations exposed to both erosion and flooding. Revetments are used on all types of coasts, however mainly types 1 - 4.<br />
<br />
Rubble revetments and similar structures have a permeable and fairly steep slope; normally a 1:2 slope is used. This slope is suitable neither for recreational use nor for the landing or hauling of small fishing boats. Consequently, this kind of structure should not be used at locations, where the beach is used for recreation or fishing activities. For such locations, other types of protection measures must be considered, but if a revetment is required, a more gently sloping structure with a smooth surface is recommended.<br />
<br />
==References==<br />
<references/><br />
<br />
==See also==<br />
[[Theme 5#Coastal protection techniques|Coastal protection techniques]]<br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
[[Category:Theme 5]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Beach_scraping&diff=17171Beach scraping2007-11-30T16:16:10Z<p>Juliettejackson: </p>
<hr />
<div>{{Definition|title=Beach scraping <br />
|definition=Beach scraping is recovering material from the berm at the foreshore and placing it on the backshore at the foot of the dunes or the cliff.<br />
}}</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Shore_nourishment&diff=17169Shore nourishment2007-11-30T16:14:24Z<p>Juliettejackson: /* See also */</p>
<hr />
<div>Nourishment can be divided into backshore nourishment, beach nourishment and shoreface nourishment. <br />
<br />
==What is nourishment?==<br />
Nourishment can be regarded as a very natural way of combating coastal erosion and shore erosion as it artificially replaces a deficit in the sediment budget over a certain stretch with a corresponding volume of sand. However, as the cause of the erosion is not eliminated, erosion will continue in the nourished sand. It is thus inherent in the nourishment concept that the nourished sand is gradually sacrificed. This means that nourishment as a stand-alone method normally requires a long-term maintenance effort. In general, nourishment is only suited for major sections of shoreline; otherwise the loss of sand to neighbouring sections will be too large. Regular nourishment requires a permanent well-functioning organisation, which makes nourishment as a stand-alone solution unsuitable for privately owned coastlines.<br />
<br />
The success of a nourishment scheme depends very much on the grain size of the nourished sand, the so-called borrow material, relative to the grain size of the native sand. As described in [[Coastal Hydrodynamics And Transport Processes#Onshore and Offshore Transport and Equilibrium Coastal Profile|Onshore and Offshore Transport and Equilibrium Coastal Profile]], the characteristics of the sand determine the overall shape of the coastal profile expressed in the equilibrium profile concept. Furthermore, in nature the hydrodynamic processes tend to sort the sediments in the profile so that the grain size decreases with increasing water depth.<br />
<br />
==Equilibrium conditions==<br />
When borrow sand is placed in a coastal profile, neither the profile nor the grain size distribution will match the equilibrium conditions. Nature will attempt to re-establish a new equilibrium profile so changes will always occur in the nourished profile. There will also be changes caused by the continued long-term erosion trend and the profile response to individual events. This means that in practice it is neither possible to perform a short-term nor a long-term stable nourishment at an eroding coast. It is inherently unstable on eroding shorelines. These are the basic realities, which the public, the politicians and those who fund the projects, find it hard to accept. On the other hand, as environmental concerns and requirements for sustainability are gaining in importance, nourishment has gradually increased its share of shoreline management schemes over the last decades.<br />
<br />
===Grain size===<br />
[[Image:nourished beaches equilibrium.jpg|350px|right|thumb|Fig. 1. Equilibrium conditions for nourished beaches required to obtain an additional beach width of &delta;w with borrow sand, which is finer and coarser than the native sand (upper and lower, respectively).]]<br />
As mentioned above, the performance of a nourishment scheme very much depends on the grain size of the borrow material relative to the grain size of the native material; see the discussion on [[Coastal Hydrodynamics And Transport Processes#Onshore and Offshore Transport and Equilibrium Coastal Profile|equilibrium profiles]].[[Image:nourish grain size.jpg|300px|thumb|right|Fig. 2. Relation between Nourishment Efficiency and the Grain Size Ratio for Nourishment<ref name="Vestkysten"/>]]<br />
<br />
If the borrow sand is finer than the native sand, it will tend to form a flatter profile than the natural one. The equilibrium reshaping of the nourished sand will reach out to the closure depth. If the objective of the nourishment is to obtain a wider beach, this will require very large volumes of sand, as illustrated in the upper part of Fig. 1. <br />
<br />
It is evident that the volume of sand needed to obtain a certain beach width increases drastically with the decreasing grain size of the nourished sand. Most coastal authorities realise this and some of them have introduced special bonuses for their nourishment contractors when they provide coarse sand. <br />
<br />
It is evident from this figure that if borrow sand with a larger grain size than that of the native sand is nourished into a coastal profile, it will tend to form a steeper profile than the natural profile. This means that a wider beach will tend to be formed, see the lower part Fig. 1. <br />
<br />
Furthermore, coarser sand will be more stable in terms of longshore loss. This nourishment efficiency of the nourished sand has been studied by the Danish Coastal Authority on basis of many years of nourishment along the Danish North Sea Coast<ref name="Vestkysten">Vestkysten 2000 (in Danish) (The West Coast or the Danish North Sea Coast 2000), The Danish Coastal Authority.</ref>. The nourishment efficiency is defined as the ratio between the erosion rate for the natural sand (theoretical) and that of the nourished sand. The nourishment efficiency has been analysed as function of the ratio between the mean grain size of the borrow sand and that of the native sand: <br />
<br />
:<math>GSR_{Nourishment}=d_{50,Borrow}/d_{50,Native}</math><br />
<br />
The analysis covers effects of cross shore as well as longshore effects. The results are expressed as a relation between the nourishment efficiency versus the grain size ratio <math>GSR_{Nourishment}</math>. It is evident from the relation shown in Fig. 2. that the Nourishment Efficiency increases considerably with increasing Grain Size Ratio for Nourishment.<br />
<br />
===Steepness of profile===<br />
Areas, which for a long time have been protected by hard coastal protection structures, have often developed steepened coastal profiles. Such areas are very far from their cross-shore equilibrium form. If nourishment is introduced in such areas it will require huge volumes of sand to restore the profile to the equilibrium profile, which is required to release the pressure on the coastal structures. In such cases, it is very important to find borrow sand, which is coarser than the native sand. <br />
<br />
<br />
<br />
==Methods, functional characteristics and applicability==<br />
The three different nourishment methods will be discussed briefly in the following.<br />
[[Image:backshore nourishment.jpg|250px|right|thumb|Fig. 3. Principles in backshore nourishment, beach nourishment and shoreface nourishment]]<br />
====Backshore nourishment==== <br />
Backshore nourishment is the strengthening of the upper part of the beach by placing nourishment on the backshore or at the foot of the dunes. <br />
The main objective of backshore nourishment is to strengthen the backshore/dune against erosion and breaching during extreme events. The material is stockpiled in front of the dunes and acts as a buffer, which is sacrificed during extreme events. This kind of nourishment works more by volume than by trying to restore the natural wide beach. The loss is normally large during extreme events, whereby steep scarps are formed. Backshore nourishment can be characterised as a kind of emergency measure against dune setback/breach; it cannot, therefore, be characterised as a sustainable way of performing nourishment and it does not normally look very natural. <br />
<br />
Backshore nourishment can be performed by hydraulic pumping sand through pipes discharging at the foot of the dunes and later adjusted using a bulldozer. The sand source can be either an offshore supply via a cross-profile pipeline, floating or buried, or it can be supplied along the shore from, for example, a sand bypassing plant. The sand can also be supplied via land transport by dumpers. <br />
<br />
====Beach nourishment==== <br />
Beach nourishment is the supply of sand to the shore to increase the recreational value and/or to secure the beach against shore erosion by feeding sand on the beach. It is not a coastal protection measure, as the beach will normally be flooded during extreme events, but it will support possible coastal protection measures. When performing beach nourishment, the borrow sand must be similar to the native sand to adjust smoothly to the natural profile. It may be an advantage to use slightly coarser sand than the natural beach sand, as this will enhance the stability of the resulting slightly steeper profile. Finer sand will very quickly be transferred to deeper water and will thus not contribute directly to a wider beach. However, the fine sand will help building up the outer part of the profile. See also [[experiences with beach nourishments in Portugal]].<br />
<br />
====Shoreface nourishment==== <br />
[[Image:nourishment methods.gif|thumb|Fig. 4. Nourishment methods in practice by the Danish Coastal Authority. Beach nourishment by pipe discharge on the beach and over the bow pumping and shoreface nourishment by split barge.]]<br />
Shoreface nourishment is the supply of sand to the outer part of the coastal profile, typically on the seaside of the bar. It will strengthen the coastal profile and add sediment to the littoral budget in general. This type of nourishment is used in areas where coastal protection measures have steepened the coastal profile or in areas with a long-term sediment deficit. Shoreface nourishment is sometimes used with beach nourishment in order to strengthen the entire coastal profile. It is recommended for obtaining a nourished profile close to the equilibrium profile. Stand-alone shoreface nourishment acts only indirectly as a shore protection measure through slightly decreased wave exposure and as a shore restoration measure with considerable delay and little efficiency. <br />
<br />
Shoreface nourishment is often performed using split barges. The unloading is fast and the unit price therefore low. Shoreface nourishment can profitably be used in connection with large beach nourishment schemes, in which borrow material, which does not fulfil the requirements for beach nourishment, can be used in the outer part of the profile where it naturally belongs.<br />
<br />
====Beach Scraping====<br />
<br />
=====Method===== <br />
A beach berm consisting of coarse sand or gravel is sometimes formed during relatively mild summer wave conditions, which tend to transport seabed material towards the beach. Beach scraping is normally performed using front loaders. <br />
<br />
=====Functional characteristics=====<br />
The purpose of beach scraping is to strengthen the upper part of the beach profile and the foot of the cliff. The material is placed in a position that reduces the erosion occurring during storm surge conditions. <br />
<br />
=====Applicability=====<br />
This method can be used for beaches, which are mainly exposed to seasonal erosion, whereas it is probably not feasible for locations, which are exposed to long-term erosion. One disadvantage of the method is that the material used for strengthening the upper part of the beach profile is taken from the lower part of the same profile, which means that the method only contributes insignificantly to the overall stability of the beach profile. Another issue is that equipment operated during late summer may disturb recreational activities.<br />
<br />
====Beach De-watering or Beach Drain====<br />
'''Definition'''<br />
A beach de-watering system or beach drain, is a shore protection system working on the basis of a drain in the beach. The drain runs parallel to the shoreline in the wave up-rush zone. The beach drain increases the level of the beach near the installation line, thus also increasing the width of the beach. The beach drain method is patented world wide by GEO, Denmark.<br />
<br />
=====Method=====<br />
The drain consists of a permeable plastic pipe installed 1.0 to 2.0 m below the beach surface in the wave up-rush zone. If there is a significant tide, the drain must be installed close to the MHWS line, i.e. near the shoreline. The drain is connected to a pumping well from which the drain water is pumped, either into a lagoon or back into the sea. The only visible part of the drain installation is the pumping well and a small control house. <br />
<br />
[[Image:beach drain fn.jpg|380px|center|thumb|Fig. 5. Principle of beach drain function]]<br />
<br />
=====Functional characteristics=====<br />
The conditions influencing the function of the drain are summarised in the following:<br />
*The site must have a sandy beach. The beach sediments must be sand, preferably with a mean grain diameter in the range of 0.1 mm < d<sub>50</sub> < 1.0 mm and preferably sorted to well sorted (C<sub>u</sub> = d<sub>60</sub>/d<sub>10</sub> < 3.5). These conditions give the permeability that provides optimal functionality of the beach drain. <br />
*The beach drain works by locally lowering the groundwater table in the uprush zone, which decreases the strength of the down-rush as a higher fraction of the water percolates into the beach. Furthermore, the physical strength properties of the beach sand is increased remarkably by the lowering of the water table in the wave up-rush zone thereby making the beach more resistant against erosion. The groundwater table in the beach is a function of several factors, the most important of which are: a) the groundwater table conditions in the coast and the hinterland, b) the groundwater table caused by tide and storm surge, and c) the groundwater table caused by waves. <br />
*A high groundwater table in the coast and the hinterland influences beach stability and beach formation. The hinterland-based groundwater table saturates a large portion of the beach, causing groundwater seepage through the foreshore. This seepage tends to destabilise (fluidise) the foreshore. The beach drain locally lowers the groundwater table to the level of the drain and counteracts the destabilisation.<br />
*The beach drain works well at locations with relatively high tide because the tide generates an elevated groundwater table in the beach, which can be lowered considerably by the drain. It can therefore be stated that the presence of high tide at a location enhances the functionality of the drain.<br />
*The presence of high storm surges will affect the functionality of the drain by moving the uprush zone landwards away from the drain. The function of the drain during high surge conditions will mainly be indirect; the previously accumulated sand will act as a buffer for the erosion during the storm. When the storm surge falls, the elevated groundwater-level in the beach will increase beach erosion if there is no beach drain to prevent it. <br />
*Waves on a beach increase the height of the local groundwater table in the beach, partly due to the wave run-up on the foreshore and partly due to the locally elevated water-level in the uprush zone called wave set-up. Once again, the beach drain counteracts this.<br />
*The beach drain requires some wave activity on the beach as the drain works by manipulating the downrush conditions on the foreshore. Too small and too high waves make the beach drain inefficient. It works best on moderately exposed coasts.<br />
*As the beach drain system functions only on the foreshore in the uprush zone, it does not directly protect the entire active profile against erosion. Consequently, it is best suited at locations with seasonal beach fluctuations or where the objective is a wider beach at an otherwise stable section of the shoreline. For locations that experience on-going recession of the entire active coastal profile, the beach drain is probably only suitable combined with other measures. The long-term capability of the beach drain under such circumstances remains to be tested.<br />
<br />
=====Applicability=====<br />
The beach drain is best suited for the management of beaches with the following characteristics:<br />
*Sandy beaches<br />
*Moderately exposed to waves<br />
*Exposed to tide<br />
*Suffering from high groundwater table on the coast and on the beach<br />
*Exposed to seasonal fluctuations of the shoreline<br />
*Exposed to minor long-term beach erosion<br />
*Locations with a narrow beach, where a wider beach is desired<br />
<br />
The beach drain is, however, not recommended as a primary shore or coastal protection at locations with the following characteristics:<br />
*Severely exposed locations<br />
*Protected locations<br />
*Locations exposed to severe long-term shore erosion and coast erosion<br />
<br />
==See also==<br />
[[Theme 5#Coastal protection techniques|Coastal protection techniques]]<br />
<br />
[[Experiences with beach nourishments in Portugal]]<br />
<br />
==References==<br />
<references/><br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
[[category:Theme 5]]<br />
[[category:Theme 8]]<br />
[[Category:Shoreline management]]<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal defence]]<br />
[[Category:Coastal flooding]]<br />
[[Category:Coastal erosion]]<br />
[[Category:Coastal flooding management]]<br />
[[Category:Coastal erosion management]]<br />
[[category:Coastal structures]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Shore_nourishment&diff=17166Shore nourishment2007-11-30T16:11:40Z<p>Juliettejackson: /* Beach Scraping */</p>
<hr />
<div>Nourishment can be divided into backshore nourishment, beach nourishment and shoreface nourishment. <br />
<br />
==What is nourishment?==<br />
Nourishment can be regarded as a very natural way of combating coastal erosion and shore erosion as it artificially replaces a deficit in the sediment budget over a certain stretch with a corresponding volume of sand. However, as the cause of the erosion is not eliminated, erosion will continue in the nourished sand. It is thus inherent in the nourishment concept that the nourished sand is gradually sacrificed. This means that nourishment as a stand-alone method normally requires a long-term maintenance effort. In general, nourishment is only suited for major sections of shoreline; otherwise the loss of sand to neighbouring sections will be too large. Regular nourishment requires a permanent well-functioning organisation, which makes nourishment as a stand-alone solution unsuitable for privately owned coastlines.<br />
<br />
The success of a nourishment scheme depends very much on the grain size of the nourished sand, the so-called borrow material, relative to the grain size of the native sand. As described in [[Coastal Hydrodynamics And Transport Processes#Onshore and Offshore Transport and Equilibrium Coastal Profile|Onshore and Offshore Transport and Equilibrium Coastal Profile]], the characteristics of the sand determine the overall shape of the coastal profile expressed in the equilibrium profile concept. Furthermore, in nature the hydrodynamic processes tend to sort the sediments in the profile so that the grain size decreases with increasing water depth.<br />
<br />
==Equilibrium conditions==<br />
When borrow sand is placed in a coastal profile, neither the profile nor the grain size distribution will match the equilibrium conditions. Nature will attempt to re-establish a new equilibrium profile so changes will always occur in the nourished profile. There will also be changes caused by the continued long-term erosion trend and the profile response to individual events. This means that in practice it is neither possible to perform a short-term nor a long-term stable nourishment at an eroding coast. It is inherently unstable on eroding shorelines. These are the basic realities, which the public, the politicians and those who fund the projects, find it hard to accept. On the other hand, as environmental concerns and requirements for sustainability are gaining in importance, nourishment has gradually increased its share of shoreline management schemes over the last decades.<br />
<br />
===Grain size===<br />
[[Image:nourished beaches equilibrium.jpg|350px|right|thumb|Fig. 1. Equilibrium conditions for nourished beaches required to obtain an additional beach width of &delta;w with borrow sand, which is finer and coarser than the native sand (upper and lower, respectively).]]<br />
As mentioned above, the performance of a nourishment scheme very much depends on the grain size of the borrow material relative to the grain size of the native material; see the discussion on [[Coastal Hydrodynamics And Transport Processes#Onshore and Offshore Transport and Equilibrium Coastal Profile|equilibrium profiles]].[[Image:nourish grain size.jpg|300px|thumb|right|Fig. 2. Relation between Nourishment Efficiency and the Grain Size Ratio for Nourishment<ref name="Vestkysten"/>]]<br />
<br />
If the borrow sand is finer than the native sand, it will tend to form a flatter profile than the natural one. The equilibrium reshaping of the nourished sand will reach out to the closure depth. If the objective of the nourishment is to obtain a wider beach, this will require very large volumes of sand, as illustrated in the upper part of Fig. 1. <br />
<br />
It is evident that the volume of sand needed to obtain a certain beach width increases drastically with the decreasing grain size of the nourished sand. Most coastal authorities realise this and some of them have introduced special bonuses for their nourishment contractors when they provide coarse sand. <br />
<br />
It is evident from this figure that if borrow sand with a larger grain size than that of the native sand is nourished into a coastal profile, it will tend to form a steeper profile than the natural profile. This means that a wider beach will tend to be formed, see the lower part Fig. 1. <br />
<br />
Furthermore, coarser sand will be more stable in terms of longshore loss. This nourishment efficiency of the nourished sand has been studied by the Danish Coastal Authority on basis of many years of nourishment along the Danish North Sea Coast<ref name="Vestkysten">Vestkysten 2000 (in Danish) (The West Coast or the Danish North Sea Coast 2000), The Danish Coastal Authority.</ref>. The nourishment efficiency is defined as the ratio between the erosion rate for the natural sand (theoretical) and that of the nourished sand. The nourishment efficiency has been analysed as function of the ratio between the mean grain size of the borrow sand and that of the native sand: <br />
<br />
:<math>GSR_{Nourishment}=d_{50,Borrow}/d_{50,Native}</math><br />
<br />
The analysis covers effects of cross shore as well as longshore effects. The results are expressed as a relation between the nourishment efficiency versus the grain size ratio <math>GSR_{Nourishment}</math>. It is evident from the relation shown in Fig. 2. that the Nourishment Efficiency increases considerably with increasing Grain Size Ratio for Nourishment.<br />
<br />
===Steepness of profile===<br />
Areas, which for a long time have been protected by hard coastal protection structures, have often developed steepened coastal profiles. Such areas are very far from their cross-shore equilibrium form. If nourishment is introduced in such areas it will require huge volumes of sand to restore the profile to the equilibrium profile, which is required to release the pressure on the coastal structures. In such cases, it is very important to find borrow sand, which is coarser than the native sand. <br />
<br />
<br />
<br />
==Methods, functional characteristics and applicability==<br />
The three different nourishment methods will be discussed briefly in the following.<br />
[[Image:backshore nourishment.jpg|250px|right|thumb|Fig. 3. Principles in backshore nourishment, beach nourishment and shoreface nourishment]]<br />
====Backshore nourishment==== <br />
Backshore nourishment is the strengthening of the upper part of the beach by placing nourishment on the backshore or at the foot of the dunes. <br />
The main objective of backshore nourishment is to strengthen the backshore/dune against erosion and breaching during extreme events. The material is stockpiled in front of the dunes and acts as a buffer, which is sacrificed during extreme events. This kind of nourishment works more by volume than by trying to restore the natural wide beach. The loss is normally large during extreme events, whereby steep scarps are formed. Backshore nourishment can be characterised as a kind of emergency measure against dune setback/breach; it cannot, therefore, be characterised as a sustainable way of performing nourishment and it does not normally look very natural. <br />
<br />
Backshore nourishment can be performed by hydraulic pumping sand through pipes discharging at the foot of the dunes and later adjusted using a bulldozer. The sand source can be either an offshore supply via a cross-profile pipeline, floating or buried, or it can be supplied along the shore from, for example, a sand bypassing plant. The sand can also be supplied via land transport by dumpers. <br />
<br />
====Beach nourishment==== <br />
Beach nourishment is the supply of sand to the shore to increase the recreational value and/or to secure the beach against shore erosion by feeding sand on the beach. It is not a coastal protection measure, as the beach will normally be flooded during extreme events, but it will support possible coastal protection measures. When performing beach nourishment, the borrow sand must be similar to the native sand to adjust smoothly to the natural profile. It may be an advantage to use slightly coarser sand than the natural beach sand, as this will enhance the stability of the resulting slightly steeper profile. Finer sand will very quickly be transferred to deeper water and will thus not contribute directly to a wider beach. However, the fine sand will help building up the outer part of the profile. See also [[experiences with beach nourishments in Portugal]].<br />
<br />
====Shoreface nourishment==== <br />
[[Image:nourishment methods.gif|thumb|Fig. 4. Nourishment methods in practice by the Danish Coastal Authority. Beach nourishment by pipe discharge on the beach and over the bow pumping and shoreface nourishment by split barge.]]<br />
Shoreface nourishment is the supply of sand to the outer part of the coastal profile, typically on the seaside of the bar. It will strengthen the coastal profile and add sediment to the littoral budget in general. This type of nourishment is used in areas where coastal protection measures have steepened the coastal profile or in areas with a long-term sediment deficit. Shoreface nourishment is sometimes used with beach nourishment in order to strengthen the entire coastal profile. It is recommended for obtaining a nourished profile close to the equilibrium profile. Stand-alone shoreface nourishment acts only indirectly as a shore protection measure through slightly decreased wave exposure and as a shore restoration measure with considerable delay and little efficiency. <br />
<br />
Shoreface nourishment is often performed using split barges. The unloading is fast and the unit price therefore low. Shoreface nourishment can profitably be used in connection with large beach nourishment schemes, in which borrow material, which does not fulfil the requirements for beach nourishment, can be used in the outer part of the profile where it naturally belongs.<br />
<br />
====Beach Scraping====<br />
<br />
=====Method===== <br />
A beach berm consisting of coarse sand or gravel is sometimes formed during relatively mild summer wave conditions, which tend to transport seabed material towards the beach. Beach scraping is normally performed using front loaders. <br />
<br />
=====Functional characteristics=====<br />
The purpose of beach scraping is to strengthen the upper part of the beach profile and the foot of the cliff. The material is placed in a position that reduces the erosion occurring during storm surge conditions. <br />
<br />
=====Applicability=====<br />
This method can be used for beaches, which are mainly exposed to seasonal erosion, whereas it is probably not feasible for locations, which are exposed to long-term erosion. One disadvantage of the method is that the material used for strengthening the upper part of the beach profile is taken from the lower part of the same profile, which means that the method only contributes insignificantly to the overall stability of the beach profile. Another issue is that equipment operated during late summer may disturb recreational activities.<br />
<br />
====Beach De-watering or Beach Drain====<br />
'''Definition'''<br />
A beach de-watering system or beach drain, is a shore protection system working on the basis of a drain in the beach. The drain runs parallel to the shoreline in the wave up-rush zone. The beach drain increases the level of the beach near the installation line, thus also increasing the width of the beach. The beach drain method is patented world wide by GEO, Denmark.<br />
<br />
=====Method=====<br />
The drain consists of a permeable plastic pipe installed 1.0 to 2.0 m below the beach surface in the wave up-rush zone. If there is a significant tide, the drain must be installed close to the MHWS line, i.e. near the shoreline. The drain is connected to a pumping well from which the drain water is pumped, either into a lagoon or back into the sea. The only visible part of the drain installation is the pumping well and a small control house. <br />
<br />
[[Image:beach drain fn.jpg|380px|center|thumb|Fig. 5. Principle of beach drain function]]<br />
<br />
=====Functional characteristics=====<br />
The conditions influencing the function of the drain are summarised in the following:<br />
*The site must have a sandy beach. The beach sediments must be sand, preferably with a mean grain diameter in the range of 0.1 mm < d<sub>50</sub> < 1.0 mm and preferably sorted to well sorted (C<sub>u</sub> = d<sub>60</sub>/d<sub>10</sub> < 3.5). These conditions give the permeability that provides optimal functionality of the beach drain. <br />
*The beach drain works by locally lowering the groundwater table in the uprush zone, which decreases the strength of the down-rush as a higher fraction of the water percolates into the beach. Furthermore, the physical strength properties of the beach sand is increased remarkably by the lowering of the water table in the wave up-rush zone thereby making the beach more resistant against erosion. The groundwater table in the beach is a function of several factors, the most important of which are: a) the groundwater table conditions in the coast and the hinterland, b) the groundwater table caused by tide and storm surge, and c) the groundwater table caused by waves. <br />
*A high groundwater table in the coast and the hinterland influences beach stability and beach formation. The hinterland-based groundwater table saturates a large portion of the beach, causing groundwater seepage through the foreshore. This seepage tends to destabilise (fluidise) the foreshore. The beach drain locally lowers the groundwater table to the level of the drain and counteracts the destabilisation.<br />
*The beach drain works well at locations with relatively high tide because the tide generates an elevated groundwater table in the beach, which can be lowered considerably by the drain. It can therefore be stated that the presence of high tide at a location enhances the functionality of the drain.<br />
*The presence of high storm surges will affect the functionality of the drain by moving the uprush zone landwards away from the drain. The function of the drain during high surge conditions will mainly be indirect; the previously accumulated sand will act as a buffer for the erosion during the storm. When the storm surge falls, the elevated groundwater-level in the beach will increase beach erosion if there is no beach drain to prevent it. <br />
*Waves on a beach increase the height of the local groundwater table in the beach, partly due to the wave run-up on the foreshore and partly due to the locally elevated water-level in the uprush zone called wave set-up. Once again, the beach drain counteracts this.<br />
*The beach drain requires some wave activity on the beach as the drain works by manipulating the downrush conditions on the foreshore. Too small and too high waves make the beach drain inefficient. It works best on moderately exposed coasts.<br />
*As the beach drain system functions only on the foreshore in the uprush zone, it does not directly protect the entire active profile against erosion. Consequently, it is best suited at locations with seasonal beach fluctuations or where the objective is a wider beach at an otherwise stable section of the shoreline. For locations that experience on-going recession of the entire active coastal profile, the beach drain is probably only suitable combined with other measures. The long-term capability of the beach drain under such circumstances remains to be tested.<br />
<br />
=====Applicability=====<br />
The beach drain is best suited for the management of beaches with the following characteristics:<br />
*Sandy beaches<br />
*Moderately exposed to waves<br />
*Exposed to tide<br />
*Suffering from high groundwater table on the coast and on the beach<br />
*Exposed to seasonal fluctuations of the shoreline<br />
*Exposed to minor long-term beach erosion<br />
*Locations with a narrow beach, where a wider beach is desired<br />
<br />
The beach drain is, however, not recommended as a primary shore or coastal protection at locations with the following characteristics:<br />
*Severely exposed locations<br />
*Protected locations<br />
*Locations exposed to severe long-term shore erosion and coast erosion<br />
<br />
==See also==<br />
[[Theme 5#Coastal protection techniques|Coastal protection techniques]]<br />
[[Experiences with beach nourishments in Portugal]]<br />
<br />
==References==<br />
<references/><br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
[[category:Theme 5]]<br />
[[category:Theme 8]]<br />
[[Category:Shoreline management]]<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal defence]]<br />
[[Category:Coastal flooding]]<br />
[[Category:Coastal erosion]]<br />
[[Category:Coastal flooding management]]<br />
[[Category:Coastal erosion management]]<br />
[[category:Coastal structures]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Shore_nourishment&diff=17165Shore nourishment2007-11-30T16:10:47Z<p>Juliettejackson: /* See also */</p>
<hr />
<div>Nourishment can be divided into backshore nourishment, beach nourishment and shoreface nourishment. <br />
<br />
==What is nourishment?==<br />
Nourishment can be regarded as a very natural way of combating coastal erosion and shore erosion as it artificially replaces a deficit in the sediment budget over a certain stretch with a corresponding volume of sand. However, as the cause of the erosion is not eliminated, erosion will continue in the nourished sand. It is thus inherent in the nourishment concept that the nourished sand is gradually sacrificed. This means that nourishment as a stand-alone method normally requires a long-term maintenance effort. In general, nourishment is only suited for major sections of shoreline; otherwise the loss of sand to neighbouring sections will be too large. Regular nourishment requires a permanent well-functioning organisation, which makes nourishment as a stand-alone solution unsuitable for privately owned coastlines.<br />
<br />
The success of a nourishment scheme depends very much on the grain size of the nourished sand, the so-called borrow material, relative to the grain size of the native sand. As described in [[Coastal Hydrodynamics And Transport Processes#Onshore and Offshore Transport and Equilibrium Coastal Profile|Onshore and Offshore Transport and Equilibrium Coastal Profile]], the characteristics of the sand determine the overall shape of the coastal profile expressed in the equilibrium profile concept. Furthermore, in nature the hydrodynamic processes tend to sort the sediments in the profile so that the grain size decreases with increasing water depth.<br />
<br />
==Equilibrium conditions==<br />
When borrow sand is placed in a coastal profile, neither the profile nor the grain size distribution will match the equilibrium conditions. Nature will attempt to re-establish a new equilibrium profile so changes will always occur in the nourished profile. There will also be changes caused by the continued long-term erosion trend and the profile response to individual events. This means that in practice it is neither possible to perform a short-term nor a long-term stable nourishment at an eroding coast. It is inherently unstable on eroding shorelines. These are the basic realities, which the public, the politicians and those who fund the projects, find it hard to accept. On the other hand, as environmental concerns and requirements for sustainability are gaining in importance, nourishment has gradually increased its share of shoreline management schemes over the last decades.<br />
<br />
===Grain size===<br />
[[Image:nourished beaches equilibrium.jpg|350px|right|thumb|Fig. 1. Equilibrium conditions for nourished beaches required to obtain an additional beach width of &delta;w with borrow sand, which is finer and coarser than the native sand (upper and lower, respectively).]]<br />
As mentioned above, the performance of a nourishment scheme very much depends on the grain size of the borrow material relative to the grain size of the native material; see the discussion on [[Coastal Hydrodynamics And Transport Processes#Onshore and Offshore Transport and Equilibrium Coastal Profile|equilibrium profiles]].[[Image:nourish grain size.jpg|300px|thumb|right|Fig. 2. Relation between Nourishment Efficiency and the Grain Size Ratio for Nourishment<ref name="Vestkysten"/>]]<br />
<br />
If the borrow sand is finer than the native sand, it will tend to form a flatter profile than the natural one. The equilibrium reshaping of the nourished sand will reach out to the closure depth. If the objective of the nourishment is to obtain a wider beach, this will require very large volumes of sand, as illustrated in the upper part of Fig. 1. <br />
<br />
It is evident that the volume of sand needed to obtain a certain beach width increases drastically with the decreasing grain size of the nourished sand. Most coastal authorities realise this and some of them have introduced special bonuses for their nourishment contractors when they provide coarse sand. <br />
<br />
It is evident from this figure that if borrow sand with a larger grain size than that of the native sand is nourished into a coastal profile, it will tend to form a steeper profile than the natural profile. This means that a wider beach will tend to be formed, see the lower part Fig. 1. <br />
<br />
Furthermore, coarser sand will be more stable in terms of longshore loss. This nourishment efficiency of the nourished sand has been studied by the Danish Coastal Authority on basis of many years of nourishment along the Danish North Sea Coast<ref name="Vestkysten">Vestkysten 2000 (in Danish) (The West Coast or the Danish North Sea Coast 2000), The Danish Coastal Authority.</ref>. The nourishment efficiency is defined as the ratio between the erosion rate for the natural sand (theoretical) and that of the nourished sand. The nourishment efficiency has been analysed as function of the ratio between the mean grain size of the borrow sand and that of the native sand: <br />
<br />
:<math>GSR_{Nourishment}=d_{50,Borrow}/d_{50,Native}</math><br />
<br />
The analysis covers effects of cross shore as well as longshore effects. The results are expressed as a relation between the nourishment efficiency versus the grain size ratio <math>GSR_{Nourishment}</math>. It is evident from the relation shown in Fig. 2. that the Nourishment Efficiency increases considerably with increasing Grain Size Ratio for Nourishment.<br />
<br />
===Steepness of profile===<br />
Areas, which for a long time have been protected by hard coastal protection structures, have often developed steepened coastal profiles. Such areas are very far from their cross-shore equilibrium form. If nourishment is introduced in such areas it will require huge volumes of sand to restore the profile to the equilibrium profile, which is required to release the pressure on the coastal structures. In such cases, it is very important to find borrow sand, which is coarser than the native sand. <br />
<br />
<br />
<br />
==Methods, functional characteristics and applicability==<br />
The three different nourishment methods will be discussed briefly in the following.<br />
[[Image:backshore nourishment.jpg|250px|right|thumb|Fig. 3. Principles in backshore nourishment, beach nourishment and shoreface nourishment]]<br />
====Backshore nourishment==== <br />
Backshore nourishment is the strengthening of the upper part of the beach by placing nourishment on the backshore or at the foot of the dunes. <br />
The main objective of backshore nourishment is to strengthen the backshore/dune against erosion and breaching during extreme events. The material is stockpiled in front of the dunes and acts as a buffer, which is sacrificed during extreme events. This kind of nourishment works more by volume than by trying to restore the natural wide beach. The loss is normally large during extreme events, whereby steep scarps are formed. Backshore nourishment can be characterised as a kind of emergency measure against dune setback/breach; it cannot, therefore, be characterised as a sustainable way of performing nourishment and it does not normally look very natural. <br />
<br />
Backshore nourishment can be performed by hydraulic pumping sand through pipes discharging at the foot of the dunes and later adjusted using a bulldozer. The sand source can be either an offshore supply via a cross-profile pipeline, floating or buried, or it can be supplied along the shore from, for example, a sand bypassing plant. The sand can also be supplied via land transport by dumpers. <br />
<br />
====Beach nourishment==== <br />
Beach nourishment is the supply of sand to the shore to increase the recreational value and/or to secure the beach against shore erosion by feeding sand on the beach. It is not a coastal protection measure, as the beach will normally be flooded during extreme events, but it will support possible coastal protection measures. When performing beach nourishment, the borrow sand must be similar to the native sand to adjust smoothly to the natural profile. It may be an advantage to use slightly coarser sand than the natural beach sand, as this will enhance the stability of the resulting slightly steeper profile. Finer sand will very quickly be transferred to deeper water and will thus not contribute directly to a wider beach. However, the fine sand will help building up the outer part of the profile. See also [[experiences with beach nourishments in Portugal]].<br />
<br />
====Shoreface nourishment==== <br />
[[Image:nourishment methods.gif|thumb|Fig. 4. Nourishment methods in practice by the Danish Coastal Authority. Beach nourishment by pipe discharge on the beach and over the bow pumping and shoreface nourishment by split barge.]]<br />
Shoreface nourishment is the supply of sand to the outer part of the coastal profile, typically on the seaside of the bar. It will strengthen the coastal profile and add sediment to the littoral budget in general. This type of nourishment is used in areas where coastal protection measures have steepened the coastal profile or in areas with a long-term sediment deficit. Shoreface nourishment is sometimes used with beach nourishment in order to strengthen the entire coastal profile. It is recommended for obtaining a nourished profile close to the equilibrium profile. Stand-alone shoreface nourishment acts only indirectly as a shore protection measure through slightly decreased wave exposure and as a shore restoration measure with considerable delay and little efficiency. <br />
<br />
Shoreface nourishment is often performed using split barges. The unloading is fast and the unit price therefore low. Shoreface nourishment can profitably be used in connection with large beach nourishment schemes, in which borrow material, which does not fulfil the requirements for beach nourishment, can be used in the outer part of the profile where it naturally belongs.<br />
<br />
====Beach Scraping====<br />
{{Definition|title=Beach scraping <br />
|definition=Beach scraping is recovering material from the berm at the foreshore and placing it on the backshore at the foot of the dunes or the cliff.<br />
}}<br />
<br />
=====Method===== <br />
A beach berm consisting of coarse sand or gravel is sometimes formed during relatively mild summer wave conditions, which tend to transport seabed material towards the beach. Beach scraping is normally performed using front loaders. <br />
<br />
=====Functional characteristics=====<br />
The purpose of beach scraping is to strengthen the upper part of the beach profile and the foot of the cliff. The material is placed in a position that reduces the erosion occurring during storm surge conditions. <br />
<br />
=====Applicability=====<br />
This method can be used for beaches, which are mainly exposed to seasonal erosion, whereas it is probably not feasible for locations, which are exposed to long-term erosion. One disadvantage of the method is that the material used for strengthening the upper part of the beach profile is taken from the lower part of the same profile, which means that the method only contributes insignificantly to the overall stability of the beach profile. Another issue is that equipment operated during late summer may disturb recreational activities.<br />
<br />
====Beach De-watering or Beach Drain====<br />
'''Definition'''<br />
A beach de-watering system or beach drain, is a shore protection system working on the basis of a drain in the beach. The drain runs parallel to the shoreline in the wave up-rush zone. The beach drain increases the level of the beach near the installation line, thus also increasing the width of the beach. The beach drain method is patented world wide by GEO, Denmark.<br />
<br />
=====Method=====<br />
The drain consists of a permeable plastic pipe installed 1.0 to 2.0 m below the beach surface in the wave up-rush zone. If there is a significant tide, the drain must be installed close to the MHWS line, i.e. near the shoreline. The drain is connected to a pumping well from which the drain water is pumped, either into a lagoon or back into the sea. The only visible part of the drain installation is the pumping well and a small control house. <br />
<br />
[[Image:beach drain fn.jpg|380px|center|thumb|Fig. 5. Principle of beach drain function]]<br />
<br />
=====Functional characteristics=====<br />
The conditions influencing the function of the drain are summarised in the following:<br />
*The site must have a sandy beach. The beach sediments must be sand, preferably with a mean grain diameter in the range of 0.1 mm < d<sub>50</sub> < 1.0 mm and preferably sorted to well sorted (C<sub>u</sub> = d<sub>60</sub>/d<sub>10</sub> < 3.5). These conditions give the permeability that provides optimal functionality of the beach drain. <br />
*The beach drain works by locally lowering the groundwater table in the uprush zone, which decreases the strength of the down-rush as a higher fraction of the water percolates into the beach. Furthermore, the physical strength properties of the beach sand is increased remarkably by the lowering of the water table in the wave up-rush zone thereby making the beach more resistant against erosion. The groundwater table in the beach is a function of several factors, the most important of which are: a) the groundwater table conditions in the coast and the hinterland, b) the groundwater table caused by tide and storm surge, and c) the groundwater table caused by waves. <br />
*A high groundwater table in the coast and the hinterland influences beach stability and beach formation. The hinterland-based groundwater table saturates a large portion of the beach, causing groundwater seepage through the foreshore. This seepage tends to destabilise (fluidise) the foreshore. The beach drain locally lowers the groundwater table to the level of the drain and counteracts the destabilisation.<br />
*The beach drain works well at locations with relatively high tide because the tide generates an elevated groundwater table in the beach, which can be lowered considerably by the drain. It can therefore be stated that the presence of high tide at a location enhances the functionality of the drain.<br />
*The presence of high storm surges will affect the functionality of the drain by moving the uprush zone landwards away from the drain. The function of the drain during high surge conditions will mainly be indirect; the previously accumulated sand will act as a buffer for the erosion during the storm. When the storm surge falls, the elevated groundwater-level in the beach will increase beach erosion if there is no beach drain to prevent it. <br />
*Waves on a beach increase the height of the local groundwater table in the beach, partly due to the wave run-up on the foreshore and partly due to the locally elevated water-level in the uprush zone called wave set-up. Once again, the beach drain counteracts this.<br />
*The beach drain requires some wave activity on the beach as the drain works by manipulating the downrush conditions on the foreshore. Too small and too high waves make the beach drain inefficient. It works best on moderately exposed coasts.<br />
*As the beach drain system functions only on the foreshore in the uprush zone, it does not directly protect the entire active profile against erosion. Consequently, it is best suited at locations with seasonal beach fluctuations or where the objective is a wider beach at an otherwise stable section of the shoreline. For locations that experience on-going recession of the entire active coastal profile, the beach drain is probably only suitable combined with other measures. The long-term capability of the beach drain under such circumstances remains to be tested.<br />
<br />
=====Applicability=====<br />
The beach drain is best suited for the management of beaches with the following characteristics:<br />
*Sandy beaches<br />
*Moderately exposed to waves<br />
*Exposed to tide<br />
*Suffering from high groundwater table on the coast and on the beach<br />
*Exposed to seasonal fluctuations of the shoreline<br />
*Exposed to minor long-term beach erosion<br />
*Locations with a narrow beach, where a wider beach is desired<br />
<br />
The beach drain is, however, not recommended as a primary shore or coastal protection at locations with the following characteristics:<br />
*Severely exposed locations<br />
*Protected locations<br />
*Locations exposed to severe long-term shore erosion and coast erosion<br />
<br />
==See also==<br />
[[Theme 5#Coastal protection techniques|Coastal protection techniques]]<br />
[[Experiences with beach nourishments in Portugal]]<br />
<br />
==References==<br />
<references/><br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
[[category:Theme 5]]<br />
[[category:Theme 8]]<br />
[[Category:Shoreline management]]<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal defence]]<br />
[[Category:Coastal flooding]]<br />
[[Category:Coastal erosion]]<br />
[[Category:Coastal flooding management]]<br />
[[Category:Coastal erosion management]]<br />
[[category:Coastal structures]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Coves_-_artificial_formation_and_use&diff=17164Coves - artificial formation and use2007-11-30T16:09:43Z<p>Juliettejackson: </p>
<hr />
<div>The formation of a [[cove]] can occur naturally (between to headlands) or artificially (between two coastal structures) and has specific functional characteristics. The [[cove]] concept is similar to the formation of pocket beaches, where ideally there is little or no exchange of sediment between the pocket beach and the adjacent shorelines. The cove concept is suited to coastlines that have an angle of incidence 10–50 degrees from the shore (0 degrees=shore normal). The formation of a cove can benefit an area by creating a recreational environment for both swimming and the beach-landing of small boats. There are however things to consider, if the cove opening is too narrow circulation will be reduced and the quality of the water will decrease and secondly the cove may trap debris and seaweed. <br />
<br />
<br />
==Method== <br />
A [[pocket beach]] is normally a small [[beach]] between two headlands with little or no exchange of sediment with the adjacent shorelines. Many natural pocket beaches exist throughout the world and artificial pocket beaches are usually constructed in areas where natural beaches are fairly narrow or absent. These artificial pocket beaches will begin to form by themselves as soon as the structures have been built, however it is recommended to include initial beach fill in the design.<br />
<br />
==Functional characteristics==<br />
The formation and shape of a [[cove]] is fairly independent of the wave climate due to the relatively narrow opening. The [[cove]] concept is very similar to the pocket beaches, which are formed in the gaps of segmented [[breakwaters]], with narrow gaps. It is important that there is a suitable distance from the coastline to the head of the structures in order to avoid dangerous currents, this distance should ideally be larger than the width of the [[littoral zone]].<br />
<br />
==Applicability==<br />
The [[cove]] concept is especially suited for coasts with a very oblique wave attack, i.e. type 3M and 3E coasts (see:[[Classification of Coastlines]]), and for locations with steep coastal profiles. This type of coast has a large [[littoral transport]] potential and is often exposed to [[erosion]] and will therefore in many cases already have been protected. Traditionally, this type of coast is protected by [[revetment|revetments]], as it is difficult to apply an optimal shore protection system. Long sections of accumulated material upstream of protruding coastal structures cannot be used in this situation due to the oblique wave attack. This type of coast is normally unsuitable for artificial nourishment as a stand-alone measure, as this will result in large maintenance requirements. Nor will beach fill in connection with structures work, as the structures can only hold a short beach section due to the oblique wave exposure.<br />
<br />
The small [[cove]] shown in Fig. 1 may change a small coastal section protected by a [[revetment]] into an attractive recreational environment. It is especially suited to provide a semi-protected beach at locations, which are too exposed for safe swimming. Such a semi-protected bay is also very suitable for beach-landing small fishing boats. <br />
<br />
[[Image:cove layout.jpg|350px|center|thumb| Fig. 1 Principle layout of a cove and photo of a cove constructed at the SW coast of Sri Lanka.]]<br />
<br />
Although the [[cove]] concept is the optimal solution for [[Classification of Coastlines|type 3 coasts]], the concept can also be used for all other types of coasts. It is also important to note that the opening to the [[cove]] should not be made too narrow, as this will reduce circulation and degrade the water quality. And that the [[cove]] may trap seaweed and debris, but the smooth shape of the structures will normally reduce this problem.<br />
<br />
==See also==<br />
[[Theme 5#Coastal protection techniques|Coastal protection techniques]]<br />
<br />
==References==<br />
Mangor, K. (2004). ''Shoreline Management Guidelines''. DHI Water and Environment, 294pg.<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
[[category:Theme 5]]<br />
[[Category:Shoreline management]]<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal defence]]<br />
[[Category:Coastal flooding]]<br />
[[Category:Coastal erosion]]<br />
[[Category:Coastal flooding management]]<br />
[[Category:Coastal erosion management]]<br />
[[category:Coastal structures]]<br />
[[Category:Theme 6]]<br />
[[Category:Geomorphological processes and natural coastal features]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Applicability_of_detached_breakwaters&diff=17163Applicability of detached breakwaters2007-11-30T16:08:53Z<p>Juliettejackson: /* See also */</p>
<hr />
<div>It is evident in the article [[Detached breakwaters]] that breakwaters are able to protect sections of shoreline in a more diversified and less harmful way than groynes, but some of the disadvantages of groyne schemes also characterise breakwater schemes. It has been demonstrated that, on some types of coast, breakwaters function differently to groynes. A breakwater can, for example, trap sand on a coastline with a perpendicular wave approach, which is hardly the case for a groyne. The applicability of breakwaters to different types of coasts is discussed in the following.<br />
<br />
===Type 1 coasts===<br />
Breakwaters can be used on 1M and 1E type coasts in a way similar to groynes, namely to prevent the loss of sand into adjacent sections. When groynes are used in this way, they will hardly trap any sand and there will hardly be any lee-side erosion. When breakwaters are used on type 1 coasts, they will trap sand in their lee areas, which causes local erosion on both sides of the breakwaters. This can be avoided if the sheltered areas behind the breakwaters are filled in as part of the project. In addition to this local erosion, adjacent to the breakwaters, a little additional erosion occurs on either side of the protected section, due to the lack of sand from the protected section.<br />
<br />
Single breakwaters or segmented breakwaters can both be used as direct protection of specific sections against shore erosion and against coast erosion if they are combined with initial sand fill.<br />
<br />
===Type 2 and the most perpendicular part of type 3 coasts=== <br />
Like groynes, breakwaters are applicable on coasts of type 2M and 2E and on the parts of the 3M and 3E coasts, which have an angle of incidence close to that of the type 2 coast. The breakwaters accumulate sand both on their upstream side and in their sheltered zones as a tombolo or as a salient, depending on their characteristics. The upstream accumulation will be very similar to that of a groyne if the breakwater is so long that it forms a tombolo. If this is not the case, the upstream accumulation will be smaller. Both the sand accumulation in the sheltered zone and the upstream accumulation will take place at the expense of lee side erosion on the downstream side of the breakwater scheme. In order to minimise the downstream erosion within the scheme, it is important to include the initial filling of the scheme in the construction project.<br />
<br />
Local erosion and scour will occur near the breakwater heads, and the outer part of the coastal profile will continue to erode unless the breakwaters cover the entire littoral zone. Beach breakwaters will eventually collapse if their sea sides are not strengthened, whereas coastal breakwaters constructed at a distance greater than say x*>1.2 will not be exposed to an eroding seabed.<br />
<br />
Lee side erosion and profile steepening effects of segmented breakwater schemes can be mitigated by regular nourishment, but regulating the length of the individual breakwaters in the scheme can also partly mitigate the extent of the lee side erosion. The combined use of segmented breakwaters and nourishment secures against erosion of the beach in the gaps, but also to some extent against coastal erosion in the areas directly protected by the breakwaters. This means that a segmented breakwater scheme combined with nourishment has to be further supplemented with revetment protection in the gaps in order to provide complete coastal protection. <br />
<br />
It is recommended to use few large structures instead of many small structures in order to enhance the aesthetic appearance of a segmented breakwater scheme. This is especially true for type 2 coasts, as the individual breakwaters are capable of suspending long upstream sand filets (Fig. 1.).<br />
<br />
[[Image:upgrade protection scheme.jpg|500px|thumb|center|Fig. 1. Upgrading of a worn-out protection scheme at a type 2E coast (upper part) with a breakwater scheme consisting of a series of singular large coastal breakwaters with long spacing combined with initial nourishment (lower part). ]]<br />
<br />
===Type 4 and 5 coasts and the most oblique part of type 3 coasts===<br />
A single structure parallel to the coast is not very efficient when the waves are fairly oblique to the shoreline, as the effective length of the structure perpendicular to the wave direction is relatively small is this case. As the sand filet accumulation for these types of coasts is very short, single breakwaters cannot be recommended.<br />
<br />
Segmented breakwater schemes with relatively short gaps can be used as a combined shore and coastal protection measure on all types of coasts because the formation of pocket beaches in the gaps does not depend very much on the wave direction when the gaps are small.<br />
<br />
===General comments on emerged breakwaters=== <br />
#Breakwaters tend to trap seaweed and floating debris in the bays, which are formed on the upstream side as well as on the lee side. <br />
#A breakwater does not constitute an obstruction for the passage along the beach. <br />
#Breakwaters are dangerous to walk on and should be constructed so that only a salient is formed. <br />
#The lee zone eddies are dangerous for bathers.<br />
#Breakwaters constitute a foreign element in the coastal landscape as they obscure the view of the sea, but if their number is minimised, and they are built relatively far from land, this problem is minimised. <br />
#The disadvantages of breakwaters and groynes can, to some extent, be avoided by optimising the shape of the structures. This is the subject of the next subsection.<br />
<br />
==See also==<br />
[[Theme 5#Coastal protection techniques|Coastal protection techniques]]<br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
[[Category:Theme 5]]<br />
[[Category:Shoreline management]]<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal defence]]<br />
[[Category:Coastal flooding]]<br />
[[Category:Coastal erosion]]<br />
[[Category:Coastal flooding management]]<br />
[[Category:Coastal erosion management]]<br />
[[category:Coastal structures]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Application_of_breakwaters&diff=17161Application of breakwaters2007-11-30T16:08:05Z<p>Juliettejackson: </p>
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<div>While detached breakwaters are the most common type of breakwater, many other types of breakwaters exist. These include submerged or low-crested breakwaters, floating breakwaters and special type breakwaters and will be described in this article. <br />
<br />
==Submerged or low-crested breakwaters==<br />
<br />
Submerged or low-crested breakwaters function by provoking wave-breaking and by allowing some wave transmission so that a milder wave climate is obtained in lee of the submerged structure, although it is not as mild as if the structure was emerged. The sediment transport capacity behind the breakwater will also decrease, which means that sand will accumulate in a manner similar to an emerged structure with a slightly smaller length. The wave-breaking and overtopping also mean mass transport of water over the structure. There is a close relationship between, on one hand, wave transmission and mass transport over the structure and reduction in the sand transport and, on the other hand, crest freeboard, crest width and wave height and steepness. <br />
<br />
===Reasons for selecting a submerged/low breakwater===<br />
*The visual impact of a submerged/low structure is less damaging<br />
*A submerged or low-crested structure is less expensive <br />
*The impact on the transport climate and on the sand accumulation is smoother<br />
*The overtopping water generates good water circulation behind the breakwater<br />
*Submerged breakwaters are very similar to natural reefs. They attract fish and are therefore popular among fishermen<br />
<br />
===Disadvantages of submerged/low crested breakwater===<br />
*A submerged structure can be dangerous for small craft navigation<br />
*The overtopping water initiates local currents, which can be dangerous for swimmers<br />
*A submerged or low-crested structure provides only partial attenuation of the wave action as well as partial shore protection and coast protection<br />
*The efficiency of a submerged structure with respect to the attenuation of both waves and littoral transport and with respect to shore protection very much depends on the crest freeboard of the design. If there is considerable tide and storm surge at the location in the design situation, even a submerged or low crested structure will end up being rather high relative to the normal water-level. This means that one of the advantages of a low-crested structure, namely that it is not visible most of the time, cannot be obtained<br />
*The design is very difficult and challenging because the proper function of a submerged or low-crested structure depends on both water-level and wave conditions as well as on the specific structure.<br />
<br />
==Floating breakwaters==<br />
<br />
[[Image:transmission rationame.jpg|300px|thumb|Fig. 1. Rough relation the between the transmission coefficient Ht/Hi and the ratio L/w between the wavelength L and the width of the floating structure w.]]<br />
<br />
Floating breakwaters work by dissipating and reflecting part of the wave energy. No surplus water is brought into the sheltered area in this situation. <br />
Floating breakwaters are normally used as piers in marinas, but they are also used as protective structures for marinas in semi-protected areas. They are especially suited for areas where the tidal range is high, as they follow the water-level. Floating breakwaters are seldom used as shoreline management structures because they are not suitable for installation in the open sea. <br />
<br />
The wave transmission coefficient H<sub>t</sub>/H<sub>i</sub>, i.e. the ratio between the height of the transmitted wave and the height of the incoming wave, depends very much on the ratio L/w between the wavelength L and the width of the floating structure w. As a rule-of-thumb the transmission varies between H<sub>t</sub>/H<sub>i</sub> = 0.3 for L/w = 3 and H<sub>t</sub>/H<sub>i</sub> = 0.9 – 1.0 for L/w = 8, see Fig. 1. below. <br />
<br />
<br />
Consider the example of a pontoon width of w = 3 m, and a requirement of a wave transmission of min. H<sub>t</sub>/H<sub>i</sub> = 0.5. In this case the wavelength should be smaller than L < 4.2w = 12.6 m, which corresponds to an approximate wave period of T = 2.9 seconds. Floating breakwaters can only be used in waters of very limited fetch.<br />
<br />
Floating breakwaters thus cannot be used as shoreline management structures at moderately exposed and exposed locations.<br />
<br />
==Modified breakwaters and headlands==<br />
===Background:===<br />
[[Image:artifical headland.jpg|300px|thumb|Fig. 2. Optimisation of coastal breakwater to artificial headland, applicable for moderately exposed to exposed coasts for small angles of incidence.]]<br />
:The supporting structures for coastal restoration schemes have, in the preceding discussions, mainly been traditional groynes and breakwaters. Various unwanted effects associated with these structures have also been highlighted. This subsection discusses possible modifications to the layout of traditional structures to minimise these unwanted effects, see also Fig. 2.<br />
<br />
The philosophy behind the optimisation of the traditional, coast-parallel breakwater into an artificial headland (Fig. 2.) is as follows:<br />
#To improve the bypass and to minimise offshore loss as well as lee side erosion <br />
#To eliminate dangerous rip currents as well as lee areas, which may otherwise trap debris<br />
#To enhance the aesthetic appearance and to gain some useful land.<br />
<br />
===Functional characteristics:===<br />
The initial current patterns for these structures are illustrated in Fig. 3.<br />
<br />
<br />
[[Image:current paterns A.jpg|300px|thumb]]<br />
[[Image:current paterns B.jpg|300px|thumb|Fig. 3. Current patterns for: a) A coast-parallel breakwater, b) A curved optimised breakwater, c) A Shore-connected smooth breakwater and d) An artificial headland.]]<br />
<br />
These current patterns only show the initial situation without nourishment so they are not fully applicable in evaluating the conditions after initial fill or trapping of sand has taken place. However, they are useful as indicators of how the different schemes will function. The following characteristic conditions are important.<br />
<br />
===Important characteristics===<br />
*Comparing a and b: The curved breakwater guides a larger amount of longshore current (and the littoral transport) around the structure on the offshore side, but turning the flow slightly towards the shore, it provides better bypass and less lee side erosion. The eddy in the lee area downstream of the structure is smaller for the curved structure, but it is still there. The scour hole at the lee end is evaluated to be smaller.<br />
*Comparing b and c: The breakwater connected to the shore provides a very smooth passage for the longshore current (and the littoral transport) around the structure, thereby providing optimal bypass and less lee side erosion. The eddy at the downstream side no longer exists, which improves the safety for swimmers and reduces the trapping of seaweed and debris. The internal current pattern of the area sheltered by the structure is not relevant because this area will be filled with sand. This sandy area will become very attractive for recreational activities.<br />
*Comparing c and d: The headland acts more or less like the breakwater connected to the shore; the only difference is a smoother transition of the longshore current (and the littoral transport) at the upstream end of the structure. This is due to the smooth transition between the coast and the structure in the case of the headland. The transition will be even more smooth if sand fill is introduced upstream of the headland. There is no eddy at the downstream side, which improves the safety for swimmers and reduces the trapping of seaweed and debris. If the reclaimed area is sufficiently elevated, it can be used for permanent recreational installations. <br />
<br />
The headland can also be partly submerged, whereby it will act as a headland continuing into a reef. Careful design will make this type of headland appear almost natural.<br />
<br />
===Applicability=== <br />
The curved breakwater, the breakwater connected to the shore and the headland are useful substitutes for traditional groynes and breakwaters on coastal types 1 and 2. <br />
<br />
The curved breakwater can be used for all types of coasts, where traditional breakwaters can be used, see above. <br />
<br />
Neither the breakwater connected to the shore nor the headland can be used as replacements for traditional segmented breakwaters with small gaps and pocket beaches.<br />
<br />
==See also==<br />
[[Theme 5#Coastal protection techniques|Coastal protection techniques]]<br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
[[Category:Theme 5]]<br />
[[Category:Shoreline management]]<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal defence]]<br />
[[Category:Coastal flooding]]<br />
[[Category:Coastal erosion]]<br />
[[Category:Coastal flooding management]]<br />
[[Category:Coastal erosion management]]<br />
[[category:Coastal structures]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Detached_breakwaters&diff=17160Detached breakwaters2007-11-30T16:06:41Z<p>Juliettejackson: /* See also */</p>
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<div><br />
[[Detached breakwater]]s are used as shore and coast protection measures. In general terms, a detached breakwater is a coast-parallel structure located inside or close to the surf-zone. This article discusses aspects of breakwaters as coastal protection, as well as their impacts in causing erosion. <br />
<br />
==Variable of breakwater schemes==<br />
Breakwater schemes have many variables, which determine the impact on the shoreline. The variable parameters are outlined in the following:<br />
<br />
*Emerged, submerged or floating<br />
*Distance from shoreline and location relative to the surf-zone<br />
*Length and orientation<br />
*Single or segmented<br />
*Special shapes<br />
<br />
There are further descriptions later in the article of combinations of all these parameters.<br />
<br />
==Purpose of a breakwater==<br />
A shoreline management breakwater serves two purposes:<br />
<br />
*To provide shelter from the waves<br />
*Through this shelter, to manipulate the littoral transport conditions and thereby to trap some sand<br />
<br />
Some breakwaters, offshore breakwaters, are constructed in deep water as a special type of port, providing shelter for a piled berth connected to the coast by a piled jetty. This kind of port is normally used when the slope of the coastal profile is very gentle, which makes the construction of a traditional port very costly. The philosophy is that the distance to the shoreline behind this kind of port will minimise the impact on the shoreline. However, in practice, it is very difficult to construct such a zero impact port.<br />
<br />
==Defining a detached breakwater==<br />
A detached breakwater can be characterised by several parameters as shown in Fig. 1. <br />
<br />
[[Image:parameter det breakwaters.jpg|400px|thumb|Fig. 1. Definition of parameters characterising detached breakwaters and accumulation forms.]]<br />
<br />
<br />
The most important parameters are:<br />
*L<sub>B</sub> Length of the breakwater<br />
*x Breakwater distance to shoreline<br />
*x<sub>80</sub> Surf-zone width, approximately 80 % of the littoral transport takes place landwards of this line<br />
<br />
Dimensionless length and distance:<br />
*L<sub>B</sub>* = L<sub>B</sub>/x Breakwater length relative to breakwater distance to shoreline<br />
*x* = x/x<sub>80</sub> Breakwater distance relative to surf-zone width<br />
<br />
Accumulation forms:<br />
*Salient: When the dimensionless breakwater length L<sub>B</sub>* is less than approx. 0.6 to 0.7, a bell-shaped salient in the shoreline will form in the lee of the breakwater. However, parameters other than the breakwater length and distance also influence the accumulation pattern.<br />
*Tombolo: When the dimensionless breakwater length L<sub>B</sub>* is greater than approx. 0.9 to 1.0, the sand accumulation behind the breakwater will connect the beach to the breakwater in a tombolo formation. But again, parameters other than the breakwater length and distance influence the accumulation pattern<br />
<br />
If there are several breakwaters in a series, this is referred to as a ''segmented breakwater'', where the length of the gap between the breakwaters is denoted:<br />
*L<sub>G</sub> Length of gap between breakwaters in a segmented breakwater<br />
<br />
==Functional characteristic of emerged detached single breakwaters==<br />
[[Image:types detached breakwater.jpg|300px|thumb|left|Fig. 2. Types of detached breakwaters.]]<br />
A detached breakwater provides shelter from the waves, whereby the littoral transport behind the breakwater is decreased and the transport pattern adjacent to the breakwater is modified. These characteristics of a breakwater are utilised in different ways for various types of breakwaters by varying relevant parameters. There are three different types of breakwaters, as can be seen in Fig. 2. below. <br />
<br />
A detailed description of the following three types of detached breakwaters can be found below.<br />
*Offshore breakwaters<br />
*Coastal breakwaters<br />
*Beach breakwaters<br />
<br />
<br />
<br />
===Offshore breakwaters===<br />
Offshore breakwaters are located far outside the surf-zone, x<sup>*</sup> > 3. The purpose of an offshore breakwater is normally to protect an offshore ship wharf against wave action, which means that an offshore breakwater is a special type of port. The offshore breakwater is used when the coastal profile is very flat. At such locations, a traditional port would have to extend far from the shoreline or extensive dredging works would have to be carried out to provide access to the port. In order to minimise the works and to avoid the associated impact, an offshore breakwater may be the answer. The offshore breakwater is normally located at a slightly greater water depth than is required for navigation, thus avoiding/minimising capital dredging and minimising sedimentation. The philosophy is thus threefold: <br />
<br />
[[Image:offshore breakwater.jpg|250px|thumb|Fig. 3. Sand accumulation forming a salient in the shoreline behind an offshore breakwater, Sergipe, Brazil. X* = 3.6.]]<br />
[[Image:coastal breakwater.jpg|250px|thumb|Fig 4. Coastal breakwater, which has <br />
formed a tombolo, Covis Place, Sri Lanka. X* = 1.0.]]<br />
[[Image:beach breakwater.jpg|250px|thumb|Fig. 5. Beach breakwaters, The Danish West Coast. X* = 0.14.]]<br />
#First, to provide shelter for a wharf, <br />
#Secondly, to minimise sedimentation, and <br />
#Thirdly, to minimise the impact on the coastline. <br />
<br />
The philosophy behind an offshore breakwater, in relation to its influence on transport conditions, is thus to place it as far away from the surf-zone as possible and make it as short as possible so that its impact on the coastal morphology is negligible. However, experience shows that this is very difficult to obtain in practice; offshore-detached breakwaters often cause accumulation in their lee zone resulting in erosion effects in adjacent areas. It is thus important that the coastal impact of offshore breakwaters is taken into account when carrying out an environmental impact assessment for a port. Offshore breakwaters will not be treated further in these Guidelines as offshore breakwaters are not used as shoreline management structures.<br />
<br />
The following two types of breakwaters are mainly used for shoreline management, where the inherent capability of a breakwater to manipulate the littoral transport is utilised. <br />
<br />
<br />
===Coastal breakwaters===<br />
Coastal breakwaters are located within a distance from the shoreline of half the width of the surf-zone, up to twice the width of the surf-zone, 2>x<sup>*</sup>> 0.5. Such breakwaters trap sand within the part of the littoral zone they cover, thus securing that part of the coastal profile against erosion.<br />
<br />
<br />
===Beach breakwaters===<br />
Beach breakwaters are located within less than half the width of the surf-zone from the shoreline, x<sup>*</sup>>0.5. Beach breakwaters trap sand on the foreshore without interfering significantly with the overall transport pattern.<br />
<br />
<br />
<br />
<br />
===Further notes===<br />
The philosophy behind coastal and beach breakwaters is as follows:<br />
*To partly provide wave shelter for a certain part of the shore and the coast<br />
*To modify the littoral transport in predictable way, so that the combined shore-restoration and coastal protection function can be properly designed. The extent of the sand trapping, or the ability to establish the desired sand accumulation pattern, is mainly regulated by choosing the length of the breakwater, the distance of the breakwater to the shore and the number of single breakwaters in a segmented breakwater and, finally, the length of the gaps. <br />
*The advantage of a breakwater in comparison to a groyne is that it is possible to modify the littoral transport in a smoother manner than for a groyne. In this way there will be less lee side erosion on the downstream shoreline. This applies especially to breakwaters, which are so short that a permanent tombolo does not develop.<br />
<br />
Sometimes coastal breakwaters are planned so as to provide shelter for the beach landing of small fishing boats or sheltered water for swimming. This is difficult as sufficient shelter for the boats and the swimmers requires such a long breakwater that a tombolo develops. When the tombolo has developed there is no sheltered water area left for boat landing and swimming, only a semi-protected bay downstream of the tombolo is left. This bay may be suitable for landing small boats, but it is certainly not suitable for swimming during rough wave conditions due to the eddy, which will always be present during such conditions. <br />
<br />
Sand will be trapped behind a breakwater if initial sand fill is not performed. The trapped sand comes from the adjacent beaches, which means that both the upstream and downstream beach will suffer from erosion during the development of a salient or a tombolo. When a tombolo has been formed, the adjacent beaches are influenced in a way similar to that of a groyne with upstream accretion and lee side erosion. The influence of a salient will be smoother.<br />
<br />
==Impacts of breakwaters==<br />
<br />
===Hydrodynamic impacts===<br />
A detached breakwater has the following impact on the hydrodynamic conditions in the adjacent area:<br />
<br />
*The breakwater shelters partly from the waves; however, as the waves diffract into the sheltered area, a complete shelter cannot be obtained. The longer the breakwater, the better the shelter. Submerged and floating breakwaters provide less shelter.<br />
*Wave-overtopping of submerged or low breakwaters will cause an additional supply of water in the area behind the breakwater, and consequently some compensation currents running out the sheltered area.<br />
*The wave set-up on the foreshore is less in the sheltered area than outside, which generates local currents towards the sheltered area along the foreshore from both sides of the breakwater so that two eddies develop. These eddies also develop in the case of oblique wave approach.<br />
*The longshore current is partially blocked by the circulation currents; this causes some of the longshore currents to be diverted outside the breakwater.<br />
<br />
[[Image:detatched breakwater_the one before_new.jpg|240px]]<br />
[[Image:detatched breakwater_new.jpg|230px|Wave and current conditions at a detached breakwater with a perpendicular wave approach.]]<br />
<br />
:<small>Fig. 6. Wave and current conditions at a detached breakwater with a perpendicular wave approach.</small><br />
<br />
===Morphological impact===<br />
And the following impact on the morphological conditions:<br />
<br />
*The littoral transport in lee of the breakwater decreases due to the attenuated wave and longshore currents in the area sheltered by the breakwater. This causes the trapping of sand behind the breakwater depending on the conditions. As a rule-of- thumb, the trapping of sand will develop into a tombolo formation connecting the breakwater and the shore by sand deposits, if the length of the breakwater is equal to or longer than 0.8 times the distance between the shore and the breakwater. For shorter breakwaters, only a salient in the shoreline will develop.<br />
*The diversion of the longshore currents will cause the development of local erosion close to the heads of the breakwater.<br />
*The trapping of sand, especially if a tombolo has developed, will cause lee side erosion downstream of the breakwater. This downstream erosion is very similar to what is developed for groynes. However, there are more parameters involved in breakwaters, so it is possible to manipulate the transport in a more refined manner.<br />
*A breakwater traps sand under all circumstances, even if the net transport is zero. This means also that there will be a coastal impact in any case. <br />
<br />
[[Image:initial sed trans_the one before_new.jpg|240px|Initial sediment transport pattern and initial morphological impact caused by a detached breakwater for perpendicular wave approach.]]<br />
[[Image:initial sed trans_new.jpg|230px|Initial sediment transport pattern and initial morphological impact caused by a detached breakwater for perpendicular wave approach.]]<br />
<br />
:<small>Fig. 7. Initial sediment transport pattern and initial morphological impact caused by a detached breakwater for perpendicular wave approach.</small><br />
<br />
When several breakwaters are constructed in a row, the system is referred to as a segmented breakwater. A segmented breakwater is used to protect long sections of shoreline; the downstream coastal impact will be correspondingly larger than for a single breakwater.<br />
<br />
===Other impacts===<br />
Other impacts by breakwaters:<br />
*If segmented breakwaters are constructed with too small gaps, the water exchange in the embayments between the breakwaters may be poor. <br />
*Breakwaters normally obstruct part of the view over the sea, which means that the visual impact can be unacceptable.<br />
*Swimmers may feel tempted to use the sheltered area in connection with detached breakwaters, but the circulation currents can be dangerous.<br />
<br />
<br />
<br />
<br />
==Influence on flow patterns==<br />
The influence of various coastal breakwaters on the flow pattern of the longshore current is discussed in the following. Two breakwaters, one relatively long and the other one relatively short, are considered. Immediately after the construction of the breakwaters, the flow pattern is simulated by applying DHI’s numerical model system MIKE 21. Two modules have been applied, the parabolic mild slope wave module and the depth-integrated hydrodynamic module, MIKE 21 PMS and HD, respectively. The dimensions of the breakwaters are:<br />
<br />
<br />
{| cellpadding="2" align="center" cellspacing="4"<br />
|- <br />
| || L<sub>B</sub> || x || x<sub>80</sub> || L<sub>B</sub><sup>*</sup> || x<sup>*</sup> <br />
|-<br />
|The long breakwater || 312 m || 240 m || 250 m || 1.3 || 0.96<br />
|-<br />
|The short breakwater || 144 m || 240 m || 250 m || 0.6 || 0.96<br />
|}<br />
<br />
<br />
The bathymetry was a plane beach with constant slope of 1:50<br />
Irregular and directional waves with: H<sub>s</sub> = 2.8 m, Ts = 8.0 s and &alpha;<sub>0</sub> = 12<sup>o</sup> were used. <br />
<br />
<br />
The flow patterns for these two breakwaters are presented in Fig. 8. below. <br />
<br />
[[Image:flow pattern breakwater.jpg|250px|thumb|right|Fig. 8. Flow pattern for a “short” and a “long” breakwater, LB* =0.6 and LB* =1.3, respectively.]]<br />
<br />
<br />
===Long breakwater===<br />
The long breakwater causes major changes in the flow pattern. There are increasing current speeds towards the lee zone, high current speeds close to the breakwater heads and circulation in the downstream end of the lee zone. This current pattern and the corresponding sediment transport pattern cause the formation of a tombolo. There will also be local scour close to the breakwater heads. The eddy in the lee zone will remain, even after the tombolo has formed, and the current around the upstream head of the breakwater will increase with a higher offshore component. The current eddy and the scour holes will be dangerous for swimmers, and the jet directed offshore will cause a loss of sand and decreased bypass, resulting in relatively large lee side erosion. The local lee bay will tend to collect seaweed and debris. Furthermore, the tombolo provides easy access to the breakwater, which is not practical, as walking on the structure and jumping from it can be dangerous.<br />
<br />
===Short breakwater===<br />
If the breakwater is fairly short, the flow pattern will be quite different, as seen in the above figure. There will be no high currents at the breakwater heads; there will be only a minor tendency for eddy formation and there will be only slight changes in the general pattern of the longshore current. The morphological response to such a breakwater will be a smooth salient in the shoreline; a tombolo will not form and there will be no scour holes at the end of the breakwater heads. There will be no offshore loss of material, and the lee side erosion will be mild. These are all positive effects; the negative effect is that the protection, provided by a short breakwater, is limited. It will secure a wider beach locally, but it will not be able to stabilise a long section of shoreline by a stable sand filet. If the sand filet consists of nourished sand, frequent re-nourishment will be necessary. <br />
<br />
===Optimality===<br />
The optimal solution is probably somewhere between a long and a short breakwater. It must form a solid salient, but not a tombolo. This solution is balanced between the requirements for coast and shore protection, the minimisation of downstream impacts and seaweed trapping, and the optimisation of safety. Other modifications to the breakwater can also be considered. This will be discussed under the heading: Modified breakwaters and headlands.<br />
<br />
==Functional characteristics of emerged detached segmented breakwaters==<br />
[[Image:segmented breakwater characteristicse.jpg|400px|thumb|right|Fig. 9. Characteristics for various segmented breakwater schemes.]] <br />
The above discussion was related to single breakwaters, but shoreline management schemes often utilise segmented breakwaters. A segmented breakwater scheme provides many possibilities, ranging from total coastal protection to mild shore protection. Fig. 9. below shows the characteristic shoreline development, which can be obtained with segmented breakwaters with various combinations of breakwater lengths and gap widths.<br />
<br />
A mixture of seawalls, revetments, groynes and breakwaters has, in the past, often been used in densely populated areas as coastal protection against long-term erosion. In some places the protection was co-ordinated, but most often it was everybody’s individual fight against the sea. Such areas are characterised by lost beaches, poor passage along the coastline and poor aesthetic appearance. The natural beauty of the coastal landscape is lost due to coastal protection measures and erosion continues. Such areas require an urgent upgrade to secure the values behind the coastline, as required by the landowners, and to re-establish the shore to the highest possible level, as required by the public and the authorities. This calls for a well co-ordinated shoreline management scheme. With modern techniques and sufficient funds, it will be possible to upgrade the spoiled coastline to a nearly natural condition. However, it is almost impossible to re-establish the active coastal cliff, which is also a valuable coastal resource. The only way to achieve this is to allow the natural coastal erosion to continue. However, this requires that the authorities purchase the coastal land and allow it to develop naturally. In most cases this is unrealistic, but one solution may be to leave public-owned sections without protection, provided they are of a suitable length. <br />
<br />
In such heavily populated and mostly privately owned areas, it will normally not be practical to choose a solution using nourishment alone, as it will not provide sufficient safety against coastal erosion, and as it is also difficult to manage. In this case a segmented breakwater scheme is a good choice, and it can be tailored to suit the requirements agreed upon by the interested parties. A pure nourishment solution requires substantial public involvement.<br />
<br />
The numerous possibilities are illustrated by the three very different schemes sketched in Fig. 9. above. It is evident that nearly all combinations of requirements can be met. The most difficult aspect of such projects is often the public and political process, which has to be carried through to reach consensus. The importance of this process must not be underestimated in the planning process.<br />
<br />
==Applicability of detached breakwaters==<br />
It is evident from the above that breakwaters are able to protect sections of shoreline in a more diversified and less harmful way than groynes, but some of the disadvantages of groyne schemes also characterise breakwater schemes. It has been demonstrated that, on some types of coast, breakwaters function differently to groynes. A breakwater can, for example, trap sand on a coastline with a perpendicular wave approach, which is hardly the case for a groyne. The applicability of breakwaters to different types of coasts (see:[[Classification of Coastlines|coast types]]) is discussed in the following:<br />
<br />
===Type 1 coasts===<br />
Breakwaters can be used on 1M and 1E type coasts in a way similar to groynes, namely to prevent the loss of sand into adjacent sections. When groynes are used in this way, they will hardly trap any sand and there will hardly be any lee-side erosion. When breakwaters are used on type 1 coasts, they will trap sand in their lee areas, which causes local erosion on both sides of the breakwaters. This can be avoided if the sheltered areas behind the breakwaters are filled in as part of the project. In addition to this local erosion, adjacent to the breakwaters, a little additional erosion occurs on either side of the protected section, due to the lack of sand from the protected section.<br />
<br />
Single breakwaters or segmented breakwaters can both be used as direct protection of specific sections against shore erosion and against coast erosion if they are combined with initial sand fill.<br />
<br />
===Type 2 and the most perpendicular part of type 3 coasts=== <br />
Like groynes, breakwaters are applicable on coasts of type 2M and 2E and on the parts of the 3M and 3E coasts, which have an angle of incidence close to that of the type 2 coast. The breakwaters accumulate sand both on their upstream side and in their sheltered zones as a tombolo or as a salient, depending on their characteristics. The upstream accumulation will be very similar to that of a groyne if the breakwater is so long that it forms a tombolo. If this is not the case, the upstream accumulation will be smaller. Both the sand accumulation in the sheltered zone and the upstream accumulation will take place at the expense of lee side erosion on the downstream side of the breakwater scheme. In order to minimise the downstream erosion within the scheme, it is important to include the initial filling of the scheme in the construction project.<br />
<br />
Local erosion and scour will occur near the breakwater heads, and the outer part of the coastal profile will continue to erode unless the breakwaters cover the entire littoral zone. Beach breakwaters will eventually collapse if their sea sides are not strengthened, whereas coastal breakwaters constructed at a distance greater than say x*>1.2 will not be exposed to an eroding seabed.<br />
<br />
Lee side erosion and profile steepening effects of segmented breakwater schemes can be mitigated by regular nourishment, but regulating the length of the individual breakwaters in the scheme can also partly mitigate the extent of the lee side erosion. The combined use of segmented breakwaters and nourishment secures against erosion of the beach in the gaps, but also to some extent against coastal erosion in the areas directly protected by the breakwaters. This means that a segmented breakwater scheme combined with nourishment has to be further supplemented with revetment protection in the gaps in order to provide complete coastal protection. <br />
<br />
It is recommended to use few large structures instead of many small structures in order to enhance the aesthetic appearance of a segmented breakwater scheme. This is especially true for type 2 coasts, as the individual breakwaters are capable of suspending long upstream sand filets - see Fig. 10. <br />
<br />
[[Image:upgrade protection scheme.jpg|400px|thumb|right|Fig. 10. Upgrading of a worn-out protection scheme at a type 2E coast (upper part) with a breakwater scheme consisting of a series of singular large coastal breakwaters with long spacing combined with initial nourishment (lower part). ]]<br />
<br />
<br />
===Type 4 and 5 coasts and the most oblique part of type 3 coasts===<br />
A single structure parallel to the coast is not very efficient when the waves are fairly oblique to the shoreline, as the effective length of the structure perpendicular to the wave direction is relatively small is this case. As the sand filet accumulation for these types of coasts is very short, single breakwaters cannot be recommended.<br />
<br />
Segmented breakwater schemes with relatively short gaps can be used as a combined shore and coastal protection measure on all types of coasts because the formation of pocket beaches in the gaps does not depend very much on the wave direction when the gaps are small.<br />
<br />
===General comments on emerged breakwaters=== <br />
#Breakwaters tend to trap seaweed and floating debris in the bays, which are formed on the upstream side as well as on the lee side. <br />
#A breakwater does not constitute an obstruction for the passage along the beach. <br />
#Breakwaters are dangerous to walk on and should be constructed so that only a salient is formed. <br />
#The lee zone eddies are dangerous for bathers.<br />
#Breakwaters constitute a foreign element in the coastal landscape as they obscure the view of the sea, but if their number is minimised, and they are built relatively far from land, this problem is minimised. <br />
#The disadvantages of breakwaters and groynes can, to some extent, be avoided by optimising the shape of the structures. This is the subject of the next subsection.<br />
<br />
<br />
<br />
==See also==<br />
[[Applicability of detached breakwaters]]<br />
<br />
[[Effects of breakwaters of port]]<br />
<br />
[[Detached shore parallel breakwaters]]<br />
<br />
[[Detached breakwaters for coastal protection]]<br />
<br />
[[Other types of breakwaters]]<br />
<br />
[[Theme 5#Coastal protection techniques|Coastal protection techniques]]<br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
<br />
[[category:Theme 5]]<br />
[[Category:Shoreline management]]<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal defence]]<br />
[[Category:Coastal flooding]]<br />
[[Category:Coastal erosion]]<br />
[[Category:Coastal flooding management]]<br />
[[Category:Coastal erosion management]]<br />
[[category:Coastal structures]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Cliff_stabilisation&diff=17159Cliff stabilisation2007-11-30T16:04:43Z<p>Juliettejackson: /* See also */</p>
<hr />
<div>Coastal cliffs can be unstable due to the combined effect of several factors, discussed in this article along with methods to stabilise them. <br />
<br />
==Background==<br />
Coastal cliffs can be unstable due to the combined effect of several factors, such as:<br />
*Erosion of the foot of the cliff caused by wave action and storm surge<br />
*Sliding or weathering of the slope due to geo-technical instability. The erosion of the foot of the cliff normally initiates geotechnical instability, but the sliding/collapse can be of different nature depending on the geo-technical conditions of the slope. There are basically three different situations:<br />
#If the material is non-cohesive material, the weathering of the cliff ill normally occur simultaneously with the erosion of the foot as a talus formation, which is the collection of fallen material forming a slope at the foot of the cliff.<br />
#If the material is a mixture of clay, silt, sand and boulders, such as in the case of moraine till, the slope can be very steep for a period due to the cohesive forces, but the slope will eventually collapse. Smaller or bigger fractions of the cliff will fall in connection with groundwater pressure, frost impact or general weathering, or by sliding. Sliding will especially occur in connection with groundwater pressure. <br />
#If the material consists of plastic clay or silty clay, the collapse of the cliff will be in the form of slides, which can go far behind the top of the cliff. <br />
*Weathering of the cliff by wind transport of sand. This will be most pronounced if the cliff material is sand; however, also exposed cliffs consisting of other types of material can be eroded by sand blown over the cliff from the beach.<br />
<br />
==Method==<br />
The basic cause of cliff instability is normally the marine erosion of the foot of the cliff, mitigation of this is covered under the protection method: Revetment. Installing the revetment will exclude further erosion of the foot, but at that stage the slope of the cliff may very well be so steep that weathering and sliding may still occur. This can be counteracted by the following means:<br />
*Artificial smoothing of the slope, if there is enough space at the foot as well as at top of the cliff for this. This will counteract future uncontrolled weathering and sliding.<br />
*Smoothing of the slope by filling with granular material at the foot of the cliff. This requires that there is sufficient space at the foot of the cliff for the filling.<br />
*Establish a vegetation cover on the cliff. This can best be done by following the above-mentioned smoothing of the slope. Good vegetation protects against weathering and groundwater seepage, and thereby to some extent against sliding<br />
*Drainage of groundwater. This can be used if the cliff suffers from sliding due to high groundwater pressure and poor drainage conditions. Horizontal and vertical drains can be used as well as the regulation of the surface runoff.<br />
<br />
Cliff slopes are often “protected” by dumping assorted rubbish, such as branches etc., over the cliff. It is a bad “solution” because it does not stop the risk of sliding. On the contrary, it spoils the vegetation and thereby increases the risk of sliding. <br />
<br />
==Functional characteristic==<br />
Cliff stabilisation presupposes that the foot of the cliff has been stabilised. Stabilisation counteracts the natural behaviour of cliffs to slide and weather. Such an active cliff is part of the dynamic coastal landscape and should therefore in principle be maintained as an integrated part of this landscape. <br />
<br />
==Applicability==<br />
Cliff stabilisation can be applied at all moderately exposed to exposed coasts; however, in order to preserve the dynamic coastal landscape cliff stabilisation should only be used sparingly. Preserving the active cliff at densely populated coasts is normally not feasible due to the limited space. Consequently, cliff stabilisation is normally only used when there is sufficient space in the backland to allow some smoothing.<br />
<br />
==References==<br />
<references/><br />
<br />
==See also==<br />
[[Theme 5#Coastal protection techniques|Coastal protection techniques]]<br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
<br />
[[category:Theme 5]]<br />
[[Category:Shoreline management]]<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal defence]]<br />
[[Category:Coastal flooding]]<br />
[[Category:Coastal erosion]]<br />
[[Category:Coastal flooding management]]<br />
[[Category:Coastal erosion management]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Dune_stabilisation&diff=17158Dune stabilisation2007-11-30T16:02:04Z<p>Juliettejackson: </p>
<hr />
<div>Dunes are a natural coastal feature on moderately exposed and exposed coasts. Dunes are formed by the sand, which blows inland from the beach and is deposited in the area behind the coastline. <br />
<br />
==Background==<br />
During storm surge events, the foot of the dunes can be eroded, but the dunes act as a very flexible buffer zone, which protects the hinterland from erosion and flooding. The eroded material supplies material to the littoral budget minimising the general erosion along the entire section of shoreline. During the storm and also during more normal events, sand will be transported inland, sometimes in connection with the formation of wind alleys in the dune row. After the storm, the damaged dune will gradually be built up again, maybe slightly more inland. This means that a dune acts as a natural flexible coast protection and sea defence measures. It moves backwards parallel with the eroding coastline and at the same time it maintains its form and volume as well as a wide beach. This is a natural quasi-equilibrium situation.<br />
<br />
However, the natural balance will shift if the dune vegetation is damaged by grazing or if beach-users, etc. generate too much traffic, etc. This may cause the dunes to degrade resulting in loss of the protection provided by the natural dunes. At the same time the sand blowing inland causes various kinds of damage. Consequently, authorities normally tend to protect dunes by regulating their use.<br />
<br />
In some cases authorities have been very eager to protect the dunes by planting marram grass and placing fascines in the wind alleys to trap the sand. (Fascines are the placing of pine or spruce branches). This has, in some cases, resulted in a complete fixing of the dune position and an unnatural growth in height. Consequently, the flexibility of the natural dune is lost resulting in a gradual disappearance of the dune due to erosion, whereby the protection, provided by the natural dune system, is lost. <br />
<br />
==Method==<br />
[[Image:Marram planting.jpg|thumb|Fig. 1. Marram planting and the placing of spruce fascines in wind alleys (Danish Coastal Authority<ref>Danish Coastal Authority, 1998. "Menneske, Hav, Kyst og Sand". (in Danish), (Man, Sea Coast and Sand in English). Kystinspektoratet 1973-1998.</ref>).]]<br />
Planting marram grass and setting up spruce fascines for trapping of sand and enhancement of dune build up. Larger wind alleys can also be filled artificially prior to planting. However, as mentioned above, the protection should not be so comprehensive that it completely fixes the dunes.<br />
<br />
Newly planted vegetation in particular can be strengthened by using fertiliser.<br />
<br />
Restrictions for their use can also protect the dunes. Grazing in dune areas is prohibited in most countries, and authorities often limit public access. Such restrictions may regulate the traffic in the dunes, e.g. by prohibiting motor traffic. Different options are paved walking passages in areas near parking lots and fencing fragile newly planted areas.<br />
<br />
==Functional characteristic==<br />
Dune stabilisation is a sustainable protection measure, enhancing the natural protection ability of dune areas. It protects against wave and storm surge attack and at the same time it preserves the natural coastal landscape, if performed moderately. Dune stabilisation requires a planned and co-ordinated effort. <br />
<br />
==Applicability==<br />
Dune stabilisation is applicable on all coastal types where natural dunes occur. This is especially the case on moderately exposed to exposed coasts with perpendicular to very oblique wave (wind) attacks, types 1M to4M and 1E to 4E. <br />
<br />
Artificial dunes are also used as a sea defence structure.<br />
<br />
==See also==<br />
[[Theme 5#Coastal protection techniques|Coastal protection techniques]]<br />
<br />
<br />
==References==<br />
<references/><br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
<br />
[[category:Theme 5]]<br />
[[Category:Shoreline management]]<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal defence]]<br />
[[Category:Coastal flooding]]<br />
[[Category:Coastal erosion]]<br />
[[Category:Coastal flooding management]]<br />
[[Category:Coastal erosion management]]<br />
<br />
<br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Risk_and_coastal_zone_policy:_example_from_the_Netherlands&diff=17151Risk and coastal zone policy: example from the Netherlands2007-11-30T15:54:45Z<p>Juliettejackson: </p>
<hr />
<div>'''Risk and coastal zone policy: example from the Netherlands'''<br />
<br />
<br />
==Definitions of risk==<br />
<br />
Various definitions of risk exist which can be applied to different situations and which often involve a degree of subjectivity. Roughly, divisions can be made between individual, societal, economic, environmental and technical risks (Bickerstaff, 2004). All known definitions of risk however have one element in common: the distinction between reality and possibility (Renn, 1998). <br />
A definition of risk contains three components, viz. (1) an outcome that has an impact on what humans value, (2) the possibility of occurrence (uncertainty) of an event and (3) a formula to combine both elements (Renn, 1998).<br />
In other words, risk can be described by a mathematical function of the probability of an event and the consequences of that event (Jonkman et al. 2003). In many cases, risk is defined as the probability of occurrence of a disaster (natural or man-induced) times the economic damage as a result of the disaster. Often, a cost-benefit analysis is carried out to determine the ‘acceptable’ risk level (Vrijling et al., 1998). The optimal risk level in a purely economic sense is determined by implementing measures in such a way that the total sum of the costs of risk reducing measures and the expected damage is minimized (RWS, 2005).<br />
<br />
<br />
Often, the terms risk and uncertainty are used in the same context. There is however a clear distinction between risk and uncertainty. The term risk can be applied to describe situations for which probabilities are available to describe the likelihood of various events or outcomes.<br />
If probabilities of various events or outcomes cannot be quantified however, or if the events themselves are unpredictable, the term uncertainty can be applied (Loucks & Van Beek, 2005).<br />
<br />
<br />
==Risk in the coastal zone==<br />
<br />
In most coastal zones around the world, the risk of flooding poses a threat to present and future socio-economic activities. In this case risk can be defined as the probability of occurrence of an extreme event (storms, tsunamis) leading to erosion and flooding multiplied by the (socio-) economic damage caused by the storm event. <br />
It is important to note that in this case the probability itself does not necessarily involve risk. If the area prone to the effects of extreme events is free from any socio-economic activities, risk will be reduced to zero. <br />
<br />
<br />
==Risk and probability of extreme storms in the Netherlands==<br />
<br />
Dunes and dikes protect parts of the Netherlands which are situated below sea-level (see figure 1). The design levels of these flood defense structures are related to extreme storm surge water levels (which are related to the frequency of occurrence of an extreme storm event) (Loucks & Van Beek, 2005). Dunes and dikes along the Mainland coast (i.e. the provinces of Noord-Holland and Zuid-Holland) (figure 1) should be able to withstand the effects of a storm which has a probability of occurrence of once per 10.000 years. This roughly corresponds to a storm surge level of + 5 m Dutch Ordnance Level (NAP), which varies slightly along the coast. These storm surge water levels are called base levels. The base level is the general standard used to determine the minimum requirements which should be met by the flood defenses. <br />
The actual dimensions of the flood defenses however, also depend on hinterland characteristics. If a flood defense structure protects an area with a large economic value the design level of the flood defense should be higher than the base level. For the less dense populated parts along the Dutch coast, i.e. the province of Zeeland and the Wadden Islands (see figure 1), the design level is based on the frequency of occurrence of an extreme storm event of respectively once per 4000 years and once per 2000 years. <br />
<br />
[[Image:Nederland.PNG]] <br />
<br />
Figure 1. Flood defenses along the Dutch coast <br />
''(Modified after:CPD, 2000)''<br />
<br />
Real estate landward of the sea defenses are protected by law via the so-called Flood Defense Act (Wet op de Waterkering) (1996). Those who work and live seaward of the sea defenses (in the ‘erosion zone’) do so at their own risk (RIKZ, 2002).<br />
<br />
<br />
==‘Soft’ sea defenses: dunes==<br />
<br />
The former Technical Advisory Committee for Flood Defenses (TAW, 1995) introduced a method to calculate dune erosion as a result of an extreme storm event and offered criteria to test safety provided by the dunes. If a storm event occurs which reaches the design level, complete safety against failure of the dunes should still be guaranteed (TAW, 1995). <br />
The concept of this method is illustrated in figure 2. After the occurrence of an extreme storm event which reaches base level, the part of the coastal profile above water level will be eroded and deposited under water. The erosion profile assumes a shape which is known in advance. The water level during storm surge determines the vertical position of the erosion profile, whereas the horizontal position is determined in such a way that erosion above base level equals deposition beneath water level.<br />
<br />
TAW (1995) argued that the probability of failure of the sea defense should even be ten times lower (i.e. 10-5 along the Mainland coast), than the probability that the design level is reached (which has a probability of occurrence of 10-4 along the Mainland coast). The predefined probability of failure for an arbitrary dune enables calculation of the minimum dune dimensions needed.<br />
<br />
[[Image:erosionprofile.PNG]] <br />
<br />
Figure 2. Erosion profile after storm surge <br />
''(Modified from: TAW, 2002)'' <br />
<br />
<br />
From the dune erosion calculations, erosion lines or setback lines can be determined, which indicate how far erosion might reach landward due to the occurrence of an extreme event (see figure 3). <br />
These erosion lines are based on interpolation between different positions of R alongshore (see figure 2).<br />
<br />
[[Image:fictious_erosionlines.PNG]] <br />
<br />
Figure 3. Current and future erosion line positions for a fictitious coastal town <br />
''(RIKZ, 2002)''<br />
<br />
==Coastal zone policy in the Netherlands==<br />
<br />
The primary concern of the Dutch government is to preserve the ‘rest strength’ of the dunes to provide safety against flooding (see figure 5). The rest strength can be defined as the minimum dune volume needed to withstand the design storm conditions (Mulder et al., 2006).<br />
<br />
In order to stop the structural erosion of the Dutch coast and hence to provide a sustainable safety level of the dunes, Dutch government decided in 1990 to maintain the coastline at its 1990 position. Around this coastline some natural fluctuations were allowed. The ‘Dynamic Preservation’ policy (CPD 1, 1990) was adopted in which a ‘base coastline’ (BCL) was defined for each 250 m wide coastal section. This BCL relates the coastline position to the ten-year trends in sand volumes in the upper part of the coastal profile, i.e. between dune foot – which is located at around + 3 m Dutch Ordnance Level- and – 5 m Dutch Ordnance Level.<br />
<br />
The risk of coastal flooding is expected to increase in the future, because the probability of occurrence of flooding and the socio-economic value in coastal zone are both expected to increase. <br />
<br />
Without management interventions, the erosion lines will continue to shift in a landward direction due to the rise in sea level (see figure 2). In addition, in the Netherlands, relative sea level will also rise without climate change-related sea level rise. This is due to the postglacial subsidence of the North Sea basin. Furthermore, climate change may lead to more frequent and more intense storm conditions, which jeopardize dune safety. The effects in terms of socio-economic damage will increase as well, not only as a result of the increased probability of a storm event, but also due to ongoing demographic and socio-economic developments, which will also demand more and more space in the coastal zone. Therefore, it is of great importance to gain more insight in how different socio-economic activities can adapt to the long-term and large-scale natural coastal dynamics. In the Dynamic Preservation policy, large-scale and long-term morphological developments are neglected. Therefore, in 2000, Dutch government decided to adopt a more large-scale approach (CPD 3, 2000), wherein besides maintaining safety on longer temporal scales, spatial quality in the coastal zone has to preserved and -where possible- improved (RIKZ, 2002). Coastal safety policy and safety measures should involve a time horizon of 200 years.<br />
<br />
This long-term and large-scale approach was elucidated in NSS (2004), in which the Coastal Foundation Zone concept was introduced, the Coastal Foundation Zone being defined as the area between the dunes and the – 20 m depth contour (figure 4). In this area, the sediment budget should be maintained. The primary method to maintain the sediment budget is by means of sand nourishments. <br />
<br />
Preservation of the Coastal Foundation Zone (CFZ) at the largest scale, provides the boundary conditions for preservation of the BCL on smaller temporal and spatial scales. The BCL in turn, provides the boundary conditions for safety of the dunes, i.e. maintaining the rest strength. Therefore, preservation of the CFZ will provide long-term safety of the dune area. These three scale levels in coastal management practices are indicated in figure 5.<br />
<br />
For all dunes along the Dutch coast erosion line positions for the next 50, 100, 150 and 200 years have been determined (RIKZ, 2002) (figure 3).<br />
<br />
[[Image:CFZ.PNG]] <br />
Figure 4. The coastal foundation zone<br />
''(Mulder et al., 2006)''<br />
<br />
[[Image:scalesCZM.PNG]] <br />
<br />
Figure 5. Scales in coastal zone management <br />
''(Modified after: Mulder et al., 2006)''<br />
<br />
In addition to maintenance of the BCL, different measures can be taken to maintain a predefined dune safety level. These measures include planting of grasses and/or placement of sand fences to stabilize the dunes and even complete fixation or remodeling of dunes (Arens and Wiersma, 1994).<br />
<br />
All sea defense structures in the Netherlands currently meet the safety standards. Sea defense structures which must be reinforced in the coming 200 years are called weak links. In these areas, a ‘reservation zone’ must be retained, to limit further socio-economic developments. <br />
<br />
<br />
'''References:'''<br />
<br />
Bickerstaff, K. (2004). Risk perception research: socio-cultural perspectives on the public experience of air pollution, Environment International, 30, 827-840<br />
<br />
CPD 1 (1990). Kustverdediging na 1990, 1st Coastal Policy Document, Ministry of Transport, Public Works and Water Management, 65 p.<br />
<br />
CPD 3 (2000). Traditie, Trends en Toekomst, 3rd Coastal Policy Document, Ministry of Transport, Public Works and Water Management, pp. 122<br />
<br />
Jonkman, S. N., van Gelder, P. H. A. J. M., Vrijling, J. K. (2003). An overview of quantitative risk measures for loss of life and economic damage, Journal of Hazardous Materials, A99, 1-30<br />
<br />
Loucks, D. P. and van Beek, E. (2005). Water Resources Systems Planning and Management An Introduction to Methods, Models and Applications, UNESCO and WL|Delft Hydraulics, ISBN 92-3-103998-9<br />
<br />
Mulder, J.P.M., Nederbragt, G., Steetzel, H.J., van Koningsveld, M., Wang, Z.B. (2006). Different implementation scenarios for the large-scale coastal policy of the Netherlands, proceedings ICCE2006<br />
<br />
NSS (2004). National Spatial Strategy; creating space for development. Interdepartementale Projectgroep Nota Ruimte, Ministeries van VROM, LNV, VenW en EZ, The Hague, The Netherlands<br />
<br />
Renn, O. (1998). The role of risk perception for risk management, Reliability Engineering and System Safety, 59, 49-62<br />
<br />
RIKZ (2002). Towards an Integrated Coastal Zone Policy, Policy agenda for the coast, Rijkswaterstaat, National Institute for Coastal and Marine Management/RIKZ, pp. 48<br />
<br />
RWS (2005). Veiligheid Nederland in Kaart, Hoofdrapport onderzoek overstromingsrisico’s, of Transport, Public Works and Water Management<br />
<br />
Taal, M. Mulder, J., Cleveringa, J., Dunsbergen, D. (2006). 15 years of coastal management in the Netherlands, Policy; Implementation and Knowledge Framework, Rijkswaterstaat, National Institute for Coastal and Marine Management/RIKZ,<br />
<br />
TAW (1995). Basisrapport Zandige Kust, Behorende bij de Leidraad Zandige Kust, ISBN 90-36-93-704-3, pp. 507<br />
TAW (2002). Leidraad Zandige Kust, ISBN 90-369-5541-6, pp. 224<br />
<br />
Vrijling, J.K., Van Hengel, W., Houben, R.J. (1998). Acceptable risk as a basis for design, Reliability Engineering and Systems Safety, 59, 141-150<br />
<br />
{{author<br />
|AuthorID=12933<br />
|AuthorName=LisettevanderBurgh<br />
|AuthorFullName=van der Burgh, Lisette}}<br />
<br />
<br />
[[category:Theme 5]]<br />
[[Category:Coastal risk management]]<br />
[[Category:Coastal flooding management]]<br />
[[Category:Coastal erosion management]]<br />
[[Category:Shoreline management]]<br />
[[Category:Coastal defence]]<br />
[[Category:Coastal erosion]]<br />
[[Category:Coastal flooding]]<br />
[[Category:North Sea]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Revetment&diff=17148Revetment2007-11-30T15:50:18Z<p>Juliettejackson: </p>
<hr />
<div>{{Definition|title=Revetment<br />
|definition= A revetment is a facing of stone, concrete units or slabs, etc., built to protect a scarp, the foot of a cliff or a dune, a dike or a seawall against erosion by wave action, storm surge and currents. This definition is very similar to the definition of a seawall, however a revetment does not protect against flooding. Furthermore, a revetment is often a supplement to other types of protection such as seawalls and dikes. <br />
}}<br />
<br />
<br />
==See also==<br />
[[Revetment]]<br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Revetments&diff=17147Revetments2007-11-30T15:49:10Z<p>Juliettejackson: </p>
<hr />
<div>The following article discusses the fixing of the coastline by revetments.<br />
<br />
==Method==<br />
[[Revetment|Revetments]] can be an exposed structure as well as a buried structure. <br />
<br />
===Exposed revetments===<br />
Revetments are always made as sloping structures and are very often constructed as permeable structures using natural stones or concrete blocks, thereby enhancing wave energy absorption and minimising reflection and wave run-up. <br />
<br />
However, revetments can also consist of different kinds of concrete slabs, some of them permeable and interlocking. In this way their functionality is increased in terms of absorption and strength. An example of a permeable and interlocking concrete slap is the so-called Flex Slap.<br />
<br />
Net mesh stone-filled mattresses, such as Gabions, are also used; however, they are only recommended for use at fairly protected locations. <br />
<br />
Revetments can also consist of sand-filled geotextile fabric bags, mattresses and tubes. Such structures must be protected against UV-light to avoid weathering of the fabric. Sand-bagging is often used as emergency protection. Geotextile fabric revetments are fragile against mechanical impact and vandalism, and their appearance is not natural. <br />
[[Image:Revetments.jpg|400px|thumb|center|Fig. 1. Examples of revetments.]]<br />
<br />
<br />
===Buried revetments===<br />
[[Image: Emergency revetments.jpg|thumb| Fig. 2. An example of an emergency revetment constructed in concrete blocks, the revetment will later be buried into an artificial dune. (Danish Coastal Authority<ref>Danish Coastal Authority, 1998. "Menneske, Hav, Kyst og Sand". (in Danish), (Man, Sea Coast and Sand in English). Kystinspektoratet 1973-1998.</ref>)]]<br />
<br />
A buried revetment can be constructed as part of a soft protection, e.g. as a hard emergency protection built into a strengthened dune which acts as shore protection and/or sea defence.<br />
<br />
==Functional characteristics==<br />
All types of revetments have the inherent function of beach degradation as they are used at locations where the coast is exposed to erosion. A revetment will fix the location of the coastline, but it will not arrest the ongoing erosion in the coastal profile, and the beach in front of the revetment will gradually disappear. However, as a revetment is often made as a permeable, sloping structure, it will normally not accelerate the erosion, as did seawalls; on the contrary, rubble revetments are often used as reinforcement for seawalls which have been exposed due to the disappearance of the beach. Such reinforcement protects the foot of the seawall and minimises the reflection. <br />
<br />
A revetment, like a seawall, will decrease the release of sediments from the section it protects, for which reason it will have a negative impact on the sediment budget along adjacent shorelines.<br />
<br />
==Applicability==<br />
A revetment is a passive structure, which protects against erosion caused by wave action, storm surge and currents. The main difference in the function of a seawall and a revetment is that a seawall protects against erosion and flooding, whereas a revetment only protects against erosion. A revetment is thus a passive coastal protection measure and is used at locations exposed to erosion or as a supplement to seawalls or dikes at locations exposed to both erosion and flooding. Revetments are used on all types of coasts, however mainly types 1 - 4.<br />
<br />
Rubble revetments and similar structures have a permeable and fairly steep slope; normally a 1:2 slope is used. This slope is suitable neither for recreational use nor for the landing or hauling of small fishing boats. Consequently, this kind of structure should not be used at locations, where the beach is used for recreation or fishing activities. For such locations, other types of protection measures must be considered, but if a revetment is required, a more gently sloping structure with a smooth surface is recommended.<br />
<br />
==References==<br />
<references/><br />
<br />
==See also==<br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
[[Category:Theme 5]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Revetments&diff=17146Revetments2007-11-30T15:48:19Z<p>Juliettejackson: </p>
<hr />
<div>The following article discusses the fixing of the coastline by revetments.<br />
<br />
==Method==<br />
Revetments can be an exposed structure as well as a buried structure. <br />
<br />
===Exposed revetments===<br />
Revetments are always made as sloping structures and are very often constructed as permeable structures using natural stones or concrete blocks, thereby enhancing wave energy absorption and minimising reflection and wave run-up. <br />
<br />
However, revetments can also consist of different kinds of concrete slabs, some of them permeable and interlocking. In this way their functionality is increased in terms of absorption and strength. An example of a permeable and interlocking concrete slap is the so-called Flex Slap.<br />
<br />
Net mesh stone-filled mattresses, such as Gabions, are also used; however, they are only recommended for use at fairly protected locations. <br />
<br />
Revetments can also consist of sand-filled geotextile fabric bags, mattresses and tubes. Such structures must be protected against UV-light to avoid weathering of the fabric. Sand-bagging is often used as emergency protection. Geotextile fabric revetments are fragile against mechanical impact and vandalism, and their appearance is not natural. <br />
[[Image:Revetments.jpg|400px|thumb|center|Fig. 1. Examples of revetments.]]<br />
<br />
<br />
===Buried revetments===<br />
[[Image: Emergency revetments.jpg|thumb| Fig. 2. An example of an emergency revetment constructed in concrete blocks, the revetment will later be buried into an artificial dune. (Danish Coastal Authority<ref>Danish Coastal Authority, 1998. "Menneske, Hav, Kyst og Sand". (in Danish), (Man, Sea Coast and Sand in English). Kystinspektoratet 1973-1998.</ref>)]]<br />
<br />
A buried revetment can be constructed as part of a soft protection, e.g. as a hard emergency protection built into a strengthened dune which acts as shore protection and/or sea defence.<br />
<br />
==Functional characteristics==<br />
All types of revetments have the inherent function of beach degradation as they are used at locations where the coast is exposed to erosion. A revetment will fix the location of the coastline, but it will not arrest the ongoing erosion in the coastal profile, and the beach in front of the revetment will gradually disappear. However, as a revetment is often made as a permeable, sloping structure, it will normally not accelerate the erosion, as did seawalls; on the contrary, rubble revetments are often used as reinforcement for seawalls which have been exposed due to the disappearance of the beach. Such reinforcement protects the foot of the seawall and minimises the reflection. <br />
<br />
A revetment, like a seawall, will decrease the release of sediments from the section it protects, for which reason it will have a negative impact on the sediment budget along adjacent shorelines.<br />
<br />
==Applicability==<br />
A revetment is a passive structure, which protects against erosion caused by wave action, storm surge and currents. The main difference in the function of a seawall and a revetment is that a seawall protects against erosion and flooding, whereas a revetment only protects against erosion. A revetment is thus a passive coastal protection measure and is used at locations exposed to erosion or as a supplement to seawalls or dikes at locations exposed to both erosion and flooding. Revetments are used on all types of coasts, however mainly types 1 - 4.<br />
<br />
Rubble revetments and similar structures have a permeable and fairly steep slope; normally a 1:2 slope is used. This slope is suitable neither for recreational use nor for the landing or hauling of small fishing boats. Consequently, this kind of structure should not be used at locations, where the beach is used for recreation or fishing activities. For such locations, other types of protection measures must be considered, but if a revetment is required, a more gently sloping structure with a smooth surface is recommended.<br />
<br />
==References==<br />
<references/><br />
<br />
==See also==<br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
[[Category:Theme 5]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Application_of_breakwaters&diff=17145Application of breakwaters2007-11-30T15:43:03Z<p>Juliettejackson: </p>
<hr />
<div>While detached breakwaters are the most common type of breakwater, many other types of breakwaters exist. These include submerged or low-crested breakwaters, floating breakwaters and special type breakwaters and will be described in this article. <br />
<br />
==Submerged or low-crested breakwaters==<br />
<br />
Submerged or low-crested breakwaters function by provoking wave-breaking and by allowing some wave transmission so that a milder wave climate is obtained in lee of the submerged structure, although it is not as mild as if the structure was emerged. The sediment transport capacity behind the breakwater will also decrease, which means that sand will accumulate in a manner similar to an emerged structure with a slightly smaller length. The wave-breaking and overtopping also mean mass transport of water over the structure. There is a close relationship between, on one hand, wave transmission and mass transport over the structure and reduction in the sand transport and, on the other hand, crest freeboard, crest width and wave height and steepness. <br />
<br />
===Reasons for selecting a submerged/low breakwater===<br />
*The visual impact of a submerged/low structure is less damaging<br />
*A submerged or low-crested structure is less expensive <br />
*The impact on the transport climate and on the sand accumulation is smoother<br />
*The overtopping water generates good water circulation behind the breakwater<br />
*Submerged breakwaters are very similar to natural reefs. They attract fish and are therefore popular among fishermen<br />
<br />
===Disadvantages of submerged/low crested breakwater===<br />
*A submerged structure can be dangerous for small craft navigation<br />
*The overtopping water initiates local currents, which can be dangerous for swimmers<br />
*A submerged or low-crested structure provides only partial attenuation of the wave action as well as partial shore protection and coast protection<br />
*The efficiency of a submerged structure with respect to the attenuation of both waves and littoral transport and with respect to shore protection very much depends on the crest freeboard of the design. If there is considerable tide and storm surge at the location in the design situation, even a submerged or low crested structure will end up being rather high relative to the normal water-level. This means that one of the advantages of a low-crested structure, namely that it is not visible most of the time, cannot be obtained<br />
*The design is very difficult and challenging because the proper function of a submerged or low-crested structure depends on both water-level and wave conditions as well as on the specific structure.<br />
<br />
==Floating breakwaters==<br />
<br />
[[Image:transmission rationame.jpg|300px|thumb|Fig. 1. Rough relation the between the transmission coefficient Ht/Hi and the ratio L/w between the wavelength L and the width of the floating structure w.]]<br />
<br />
Floating breakwaters work by dissipating and reflecting part of the wave energy. No surplus water is brought into the sheltered area in this situation. <br />
Floating breakwaters are normally used as piers in marinas, but they are also used as protective structures for marinas in semi-protected areas. They are especially suited for areas where the tidal range is high, as they follow the water-level. Floating breakwaters are seldom used as shoreline management structures because they are not suitable for installation in the open sea. <br />
<br />
The wave transmission coefficient H<sub>t</sub>/H<sub>i</sub>, i.e. the ratio between the height of the transmitted wave and the height of the incoming wave, depends very much on the ratio L/w between the wavelength L and the width of the floating structure w. As a rule-of-thumb the transmission varies between H<sub>t</sub>/H<sub>i</sub> = 0.3 for L/w = 3 and H<sub>t</sub>/H<sub>i</sub> = 0.9 – 1.0 for L/w = 8, see Fig. 1. below. <br />
<br />
<br />
Consider the example of a pontoon width of w = 3 m, and a requirement of a wave transmission of min. H<sub>t</sub>/H<sub>i</sub> = 0.5. In this case the wavelength should be smaller than L < 4.2w = 12.6 m, which corresponds to an approximate wave period of T = 2.9 seconds. Floating breakwaters can only be used in waters of very limited fetch.<br />
<br />
Floating breakwaters thus cannot be used as shoreline management structures at moderately exposed and exposed locations.<br />
<br />
==Modified breakwaters and headlands==<br />
===Background:===<br />
[[Image:artifical headland.jpg|300px|thumb|Fig. 2. Optimisation of coastal breakwater to artificial headland, applicable for moderately exposed to exposed coasts for small angles of incidence.]]<br />
:The supporting structures for coastal restoration schemes have, in the preceding discussions, mainly been traditional groynes and breakwaters. Various unwanted effects associated with these structures have also been highlighted. This subsection discusses possible modifications to the layout of traditional structures to minimise these unwanted effects, see also Fig. 2.<br />
<br />
The philosophy behind the optimisation of the traditional, coast-parallel breakwater into an artificial headland (Fig. 2.) is as follows:<br />
#To improve the bypass and to minimise offshore loss as well as lee side erosion <br />
#To eliminate dangerous rip currents as well as lee areas, which may otherwise trap debris<br />
#To enhance the aesthetic appearance and to gain some useful land.<br />
<br />
===Functional characteristics:===<br />
The initial current patterns for these structures are illustrated in Fig. 3.<br />
<br />
<br />
[[Image:current paterns A.jpg|300px|thumb]]<br />
[[Image:current paterns B.jpg|300px|thumb|Fig. 3. Current patterns for: a) A coast-parallel breakwater, b) A curved optimised breakwater, c) A Shore-connected smooth breakwater and d) An artificial headland.]]<br />
<br />
These current patterns only show the initial situation without nourishment so they are not fully applicable in evaluating the conditions after initial fill or trapping of sand has taken place. However, they are useful as indicators of how the different schemes will function. The following characteristic conditions are important.<br />
<br />
===Important characteristics===<br />
*Comparing a and b: The curved breakwater guides a larger amount of longshore current (and the littoral transport) around the structure on the offshore side, but turning the flow slightly towards the shore, it provides better bypass and less lee side erosion. The eddy in the lee area downstream of the structure is smaller for the curved structure, but it is still there. The scour hole at the lee end is evaluated to be smaller.<br />
*Comparing b and c: The breakwater connected to the shore provides a very smooth passage for the longshore current (and the littoral transport) around the structure, thereby providing optimal bypass and less lee side erosion. The eddy at the downstream side no longer exists, which improves the safety for swimmers and reduces the trapping of seaweed and debris. The internal current pattern of the area sheltered by the structure is not relevant because this area will be filled with sand. This sandy area will become very attractive for recreational activities.<br />
*Comparing c and d: The headland acts more or less like the breakwater connected to the shore; the only difference is a smoother transition of the longshore current (and the littoral transport) at the upstream end of the structure. This is due to the smooth transition between the coast and the structure in the case of the headland. The transition will be even more smooth if sand fill is introduced upstream of the headland. There is no eddy at the downstream side, which improves the safety for swimmers and reduces the trapping of seaweed and debris. If the reclaimed area is sufficiently elevated, it can be used for permanent recreational installations. <br />
<br />
The headland can also be partly submerged, whereby it will act as a headland continuing into a reef. Careful design will make this type of headland appear almost natural.<br />
<br />
===Applicability=== <br />
The curved breakwater, the breakwater connected to the shore and the headland are useful substitutes for traditional groynes and breakwaters on coastal types 1 and 2. <br />
<br />
The curved breakwater can be used for all types of coasts, where traditional breakwaters can be used, see above. <br />
<br />
Neither the breakwater connected to the shore nor the headland can be used as replacements for traditional segmented breakwaters with small gaps and pocket beaches.<br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
[[Category:Theme 5]]<br />
[[Category:Shoreline management]]<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal defence]]<br />
[[Category:Coastal flooding]]<br />
[[Category:Coastal erosion]]<br />
[[Category:Coastal flooding management]]<br />
[[Category:Coastal erosion management]]<br />
[[category:Coastal structures]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Emergency_Protection&diff=17103Emergency Protection2007-11-30T14:51:19Z<p>Juliettejackson: </p>
<hr />
<div>The following article discusses emergency protection of coasts. Emergency protection measures are by nature quickly built and not well designed measures.<br />
<br />
==Method==<br />
Typical building methods and materials are the following:<br />
*Rock dumping. Without filter layers, often too steep and low, without proper toe protection, which means that they are unstable<br />
*Sand bagging, sometimes supported by wooden piles. Often too low and without toe protection etc. The fabric is not durable, which means that such protection will collapse after a very short period<br />
*Dumping of other kinds of material easily at hand, such as different kinds of concrete pieces, building materials, old tires etc.<br />
<br />
==Functional characteristics==<br />
Emergency protection measures are typically having the following characteristics:<br />
*They are unstable and thus not providing proper protection<br />
*They need constant maintenance and supply of new materials<br />
*They are always passive, and promotes further loss of beach<br />
*They are spoiling the natural beauty of the beach <br />
*They prevent passage of the beach<br />
*They pollute the beach with unnatural elements, such concrete debris, bricks, rubber and plastic<br />
<br />
==Applicability==<br />
Private and public land owners are sometimes forced to "construct" emergency protection at locations where "unexpected" erosion occurs. The emergency protection is installed in order to prevent further damage to coastal installations. "Unexpected" can have different causes as discussed in the following:<br />
*Unexpected can be in the form of a rare of extreme event, such as a tidal wave situation or the passage of cyclone <br />
*Unexpected can be the development of ongoing erosion at locations where it has not been possible to provide funds for a proper and timely protection<br />
*Unexpected can be due to lack of knowledge to coastal processes and/or data, whereby erosion seems to be unexpected despite the fact that it could have been foreseen if proper monitoring and coastal investigations had been practised<br />
Emergency protection can to a great extent be avoided by proper monitoring, planing and funding.<br />
<br />
<br />
==References==<br />
<references/><br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
[[Category:Theme 5]]<br />
[[Category:Techniques and methods in coastal management]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Emergency_Protection&diff=17099Emergency Protection2007-11-30T14:48:46Z<p>Juliettejackson: </p>
<hr />
<div>The following article discusses emergency protection of coasts. Emergency protection measures are by nature quickly built and not well designed measures.<br />
<br />
==Method==<br />
Typical building methods and materials are the following:<br />
*Rock dumping. Without filter layers, often too steep and low, without proper toe protection, which means that they are unstable<br />
*Sand bagging, sometimes supported by wooden piles. Often too low and without toe protection etc. The fabric is not durable, which means that such protection will collapse after a very short period<br />
*Dumping of other kinds of material easily at hand, such as different kinds of concrete pieces, building materials, old tires etc.<br />
<br />
==Functional characteristics==<br />
Emergency protection measures are typically having the following characteristics:<br />
*They are unstable and thus not providing proper protection<br />
*They need constant maintenance and supply of new materials<br />
*They are always passive, and promotes further loss of beach<br />
*They are spoiling the natural beauty of the beach <br />
*They prevent passage of the beach<br />
*They pollute the beach with unnatural elements, such concrete debris, bricks, rubber and plastic<br />
<br />
==Applicability==<br />
Private and public land owners are sometimes forced to "construct" emergency protection at locations where "unexpected" erosion occurs. The emergency protection is installed in order to prevent further damage to coastal installations. "Unexpected" can have different causes as discussed in the following:<br />
*Unexpected can be in the form of a rare of extreme event, such as a tidal wave situation or the passage of cyclone <br />
*Unexpected can be the development of ongoing erosion at locations where it has not been possible to provide funds for a proper and timely protection<br />
*Unexpected can be due to lack of knowledge to coastal processes and/or data, whereby erosion seems to be unexpected despite the fact that it could have been foreseen if proper monitoring and coastal investigations had been practised<br />
Emergency protection can to a great extent be avoided by proper monitoring, planing and funding.<br />
<br />
<br />
==References==<br />
<references/><br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
[[Category:Theme 5]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Emergency_Protection&diff=17097Emergency Protection2007-11-30T14:47:25Z<p>Juliettejackson: </p>
<hr />
<div><br />
The following article discusses emergency protection of coasts.<br />
<br />
==Method==<br />
Emergency protection measures are by nature quickly built and not well designed measures. Typical building methods and materials are the following:<br />
*Rock dumping. Without filter layers, often too steep and low, without proper toe protection, which means that they are unstable<br />
*Sand bagging, sometimes supported by wooden piles. Often too low and without toe protection etc. The fabric is not durable, which means that such protection will collapse after a very short period<br />
*Dumping of other kinds of material easily at hand, such as different kinds of concrete pieces, building materials, old tires etc.<br />
<br />
==Functional characteristics==<br />
Emergency protection measures are typically having the following characteristics:<br />
*They are unstable and thus not providing proper protection<br />
*They need constant maintenance and supply of new materials<br />
*They are always passive, and promotes further loss of beach<br />
*They are spoiling the natural beauty of the beach <br />
*They prevent passage of the beach<br />
*They pollute the beach with unnatural elements, such concrete debris, bricks, rubber and plastic<br />
<br />
==Applicability==<br />
Private and public land owners are sometimes forced to "construct" emergency protection at locations where "unexpected" erosion occurs. The emergency protection is installed in order to prevent further damage to coastal installations. "Unexpected" can have different causes as discussed in the following:<br />
*Unexpected can be in the form of a rare of extreme event, such as a tidal wave situation or the passage of cyclone <br />
*Unexpected can be the development of ongoing erosion at locations where it has not been possible to provide funds for a proper and timely protection<br />
*Unexpected can be due to lack of knowledge to coastal processes and/or data, whereby erosion seems to be unexpected despite the fact that it could have been foreseen if proper monitoring and coastal investigations had been practised<br />
Emergency protection can to a great extent be avoided by proper monitoring, planing and funding.<br />
<br />
<br />
==References==<br />
<references/><br />
<br />
==Further reading==<br />
:Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.<br />
<br />
<br />
{{author<br />
|AuthorID=13331<br />
|AuthorFullName=Mangor, Karsten<br />
|AuthorName=Karsten}}<br />
[[Category:Theme 5]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Protection_in_emergency&diff=17095Protection in emergency2007-11-30T14:44:15Z<p>Juliettejackson: </p>
<hr />
<div>{{Definition|title=Emergency protection<br />
|definition=Emergency protection is a quick installation of a temporary revetment-type structure made by available material as response to "unexpected" coastal erosion. It is normally applied for securing buildings or infrastructure against unexpected erosion.<br />
}}</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Effect_of_climate_change_on_coastline_evolution&diff=17093Effect of climate change on coastline evolution2007-11-30T14:40:13Z<p>Juliettejackson: </p>
<hr />
<div>Global warming causes sea-level rise as oceans expand, and makes storm patterns more energetic. Consequently it will affect most of the world’s coastlines through inundation and increased erosion. Sound predictions of the development of these hazards over the next century are needed in order to manage the resulting risks. Coastal flooding is somewhat easier to predict than erosion since inundation can be estimated using coastal contours. However its prediction is not trivial since inundation may be followed by rapid reshaping of the shoreline by, amongst other things, waves, tidal currents and human interventions.<br />
<br />
Understanding of coastal morphological response to climate change and sea-level rise is quite underdeveloped. This is partly because the timescales over which concern of its effects are greatest (annual to centennial) falls between the small scales addressed by most numerical models and the large sales described in the conceptual models of geomorphologists. An additional problem is that the type of model often used to bridge this gap, which is based on extrapolation of historic behaviour, is inappropriate if the climate changes. <br />
<br />
__TOC__<br />
<br />
==Coastline response to accelerated sea-level rise==<br />
The most widely cited method of quantifying the response of a shore to rising sea-levels is known as Bruun’s rule (see [[Parametric equilibrium models]]). This was developed to describe the behaviour of sandy coasts with no cliff or shore platform. It assumes that the wave climate is steady and consequently the (average equilibrium) beach profile does not change, but does translate up with the sea-level. This rise in beach surface requires sand, which is assumed to be eroded from the upper beach and deposited on the lower beach. Thus as the profile rises with sea level it also translates landward, causing shoreline retreat. Note that despite the erosion of the upper beach no sand is actually lost; it simply translates a small distance down the profile. The Bruun rule has been the subject of some debate and criticism, but is still generally supported (e.g. Stive, 2004<ref>Stive, M. 2004 How important is global warming for coastal erosion? Climatic change 64, 27-39</ref>) and a recent observational study by Zhang et al. (2004)<ref>Zhang, K., Douglas, B., and Leatherman, S. (2004). Global Warming and Coastal Erosion. Climatic Change 64, 41-58</ref> lends weight to it. They found that the Bruun rule modelled retreat of eastern USA shorelines well, although they recognised that it does not represent long-shore transport, and restricted their study to sites where this could be neglected. <br />
<br />
Another constraint on the range of applicability of the Bruun rule results from its assumptions that the shore profile is entirely beach and loses no sediment. Along most coastlines the beach is a surface deposit that can only be eroded by a limited amount before the land underlying it is exposed and attacked. Here the shore profile is composed of both beach and rock. The rock element of such composite shores complicates its behaviour because it can only erode (not accrete) and it is likely to contain material that is lost as fine sediment. In addition, being purely erosive and relatively hard, it will have a different equilibrium profile to that of the beach and will take longer to achieve it.<br />
<br />
Modifications to the Bruun rule can be used to account for the loss of fine sediment (cfi Bray & Hooke 1997<ref>Bray MJ, Hooke JM (1997) Prediction of coastal cliff erosion with accelerating sea-level rise. J Coast Res 13, 453–467</ref>) but not changes in profile form. Relatively little work has been done on the relationship between sea-level rise and the profiles of composite beach/rock shores. Recent results indicate that such profiles do change, becoming steeper as the rate of sea-level rise increases (Walkden & Hall, 2005<ref>Walkden M.J. and Hall J.W. (2005) A predictive mesoscale model of the erosion and profile development of soft rock shores. Coast Engineering 52, 535–563</ref>).<br />
<br />
The Bruun rule predicts that rates of increase of sea-level rise and shoreline recession will be the same, i.e. R2/R1 = S2/S1 where R and S are the rates of equilibrium recession and sea-level rise respectively and 1 and 2 indicate historic and future conditions. Walkden & Dickson (2006)<ref>Walkden M and Dickson M, (2006) The response of soft rock shore profiles to increased sea-level rise: Tyndall Centre for Climate Change Research Working Paper 105</ref> predicted that low beach volume composite shores are rather less sensitive and that, for them, R2/R1 = sqrt(S2/S1), although, like the Bruun rule, this equation does not account for longshore interactions.<br />
<br />
Dickson at al (in press) modelled alongshore interactions along a 50 km stretch of composite beach/ rock coast under a range of sea-level rise scenarios. They demonstrated a marked increase in complexity of shore response to sea-level rise in areas where alongshore sediment transport was important, even observing some shoreline advance.<br />
<br />
Shore wave heights are normally limited by water depth, so an increase in sea-level might be expected to increase waves at the shore. This appears to be true at composite beach/ rock shores, however it does not necessarily occur at beach shores. Bruun’s model describes beach profiles remaining constant as they translate up and landward. This means that although the sea-level rises the water depth across the surf zone does not increase, and so larger waves can not be accommodated.<br />
<br />
==Coastline response to changed storm patterns==<br />
The form of a shoreline depends strongly on the climate of wave conditions it is exposed to. Larger waves are better able to erode both beach and land. The angle at which waves arrive has a strong effect on the rate at which beach material is redistributed along the shore. A shoreline may therefore represent a dynamic balance between the wave climate, land erosion and the distribution of beach sediment. Changes to the wave climate, such as a shift in average direction or a general increase in height will disturb this balance, and a period of shoreline adjustment would be expected. <br />
<br />
Interaction of neighbouring coasts makes such shoreline adjustment complex and difficult to predict. Fortunately One Line morphological models are able to represent alongshore beach movement at large spatial and temporal scales. Studies that have used this approach to predict shore response to wave climate change have found differing shoreline sensitivity. Slott et al. (2006)<ref>Slott J.M. Murray, A.B., Ashton, A.D. and Crowley, T.J. (1996) Coastline responses to changing storm patterns. Geophysical Research Letters 33 (18)</ref> found such shoreline change could be an order of magnitude greater than those caused by rising sea levels. Conversely Dickson et al. (in press)<ref>Dickson, M.E., Walkden, M.J. and Hall, J.W. (in press) Systemic impacts of climate change on an eroding coastal region over the twenty-first century. Climatic Change</ref> found both smaller overall sensitivity and that sea-level rise had a stronger effect. This difference is unsurprising because the two studies examined coasts that are different in many ways; Slott et al. dealt with sandy cuspate shores exposed to high angle waves, whereas Dickson et al. modelled composite beach/ rock shores. It appears that the high dependency of cuspate shores on wave angle strongly increases their sensitivity to changes in wave climate, relative to composite beach/ rock shores.<br />
<br />
==References==<br />
<references/><br />
<br />
{{author<br />
|AuthorID=12939<br />
|AuthorName= M.Walkden<br />
|AuthorFullName= Mike, Walkden}}<br />
<br />
<br />
[[category:Theme 5]]<br />
[[Category:Coastal risk management]]<br />
[[Category:Coastal flooding management]]<br />
[[Category:Coastal erosion management]]<br />
[[Category:Shoreline management]]<br />
[[Category:Coastal defence]]<br />
[[Category:Coastal erosion]]<br />
[[Category:Coastal flooding]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=EUROSION_project&diff=17088EUROSION project2007-11-30T14:34:37Z<p>Juliettejackson: </p>
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<div>{{Incomplete}}<br />
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Eurosion (European Commission, 2004<ref> European Commission, 2004, ’Living with coastal erosion in Europe – Sediment and space for sustainability’, Luxembourg office for official publications of the European Commission. 40 pp ISBN 92-894-7496-3.</ref>) was a European study into coastal erosion at a European scale. Its outputs were:<br />
<br />
* A map-based assessment of European coasts exposure to coastal erosion;<br />
* A review of existing practices and experience of coastal erosion management;<br />
* Guidelines to incorporate coastal erosion into environmental assessment, spatial planning and hazard prevention; and<br />
* Policy recommendations to improve coastal erosion management.<br />
<br />
Eurosion’s maps can be used to assess the coastal typography, geology and coastal erosion trends of a region. The maps also include the location of engineering works (whether harbours, jetties groynes or breakwaters). There is an additional map for regional exposure to coastal erosion.<br />
<br />
Eurosion concluded that a more strategic and proactive approach to coastal erosion is needed for the sustained development of vulnerable coastal zones. It developed the concept of coastal resilience: the inherent ability of the coast to accommodate changes induced by sea level rise, extreme events and occasional human impacts, whilst maintaining the functions fulfilled by the coastal system in the longer term. To promote coastal resilience, Eurosion introduced the concept of favourable sediment status: the situation where the availability of coastal sediments support the objective of promoting coastal resilience in general and of preserving dynamic coastlines in particular. This should be achieved for each coastal sediment cell by designating strategic sediment reservoirs: supplies of sediment of appropriate characteristics that are available for replenishment of the coastal zone, either temporarily (to compensate for losses due to extreme storms) or in the long term (at least 100 years). They can be identified offshore, in the coastal zone (both above and below low water) and in the hinterland. <br />
<br />
A coastal sediment cell is a coastal compartment that contains a complete cycle of sedimentation including sources, transport paths, and sinks. The cell boundaries delineate the geographical area within which the budget of sediment is determined, providing the framework for the quantitative analysis of coastal erosion and accretion. Eurosion considered that coastal sediment cells constitute the most appropriate units for achieving the objective of favourable sediment status and hence coastal resilience (European Commission, 2004).<br />
<br />
[http://www.eurosion.org/ Eurosion portal]<br />
<br />
{{author<br />
|AuthorID=12932<br />
|AuthorName= J.Sutherland<br />
|AuthorFullName= James, Sutherland}}<br />
<br />
[[category:Theme 5]]<br />
[[category:case studies]]<br />
[[Category:Shoreline management]]<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal erosion]]<br />
[[Category:Coastal flooding]]<br />
[[Category:Coastal flooding management]]<br />
[[Category:Coastal erosion management]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=EUROSION_project&diff=17085EUROSION project2007-11-30T14:33:57Z<p>Juliettejackson: </p>
<hr />
<div>{Incomplete}<br />
Eurosion (European Commission, 2004<ref> European Commission, 2004, ’Living with coastal erosion in Europe – Sediment and space for sustainability’, Luxembourg office for official publications of the European Commission. 40 pp ISBN 92-894-7496-3.</ref>) was a European study into coastal erosion at a European scale. Its outputs were:<br />
<br />
* A map-based assessment of European coasts exposure to coastal erosion;<br />
* A review of existing practices and experience of coastal erosion management;<br />
* Guidelines to incorporate coastal erosion into environmental assessment, spatial planning and hazard prevention; and<br />
* Policy recommendations to improve coastal erosion management.<br />
<br />
Eurosion’s maps can be used to assess the coastal typography, geology and coastal erosion trends of a region. The maps also include the location of engineering works (whether harbours, jetties groynes or breakwaters). There is an additional map for regional exposure to coastal erosion.<br />
<br />
Eurosion concluded that a more strategic and proactive approach to coastal erosion is needed for the sustained development of vulnerable coastal zones. It developed the concept of coastal resilience: the inherent ability of the coast to accommodate changes induced by sea level rise, extreme events and occasional human impacts, whilst maintaining the functions fulfilled by the coastal system in the longer term. To promote coastal resilience, Eurosion introduced the concept of favourable sediment status: the situation where the availability of coastal sediments support the objective of promoting coastal resilience in general and of preserving dynamic coastlines in particular. This should be achieved for each coastal sediment cell by designating strategic sediment reservoirs: supplies of sediment of appropriate characteristics that are available for replenishment of the coastal zone, either temporarily (to compensate for losses due to extreme storms) or in the long term (at least 100 years). They can be identified offshore, in the coastal zone (both above and below low water) and in the hinterland. <br />
<br />
A coastal sediment cell is a coastal compartment that contains a complete cycle of sedimentation including sources, transport paths, and sinks. The cell boundaries delineate the geographical area within which the budget of sediment is determined, providing the framework for the quantitative analysis of coastal erosion and accretion. Eurosion considered that coastal sediment cells constitute the most appropriate units for achieving the objective of favourable sediment status and hence coastal resilience (European Commission, 2004).<br />
<br />
[http://www.eurosion.org/ Eurosion portal]<br />
<br />
{{author<br />
|AuthorID=12932<br />
|AuthorName= J.Sutherland<br />
|AuthorFullName= James, Sutherland}}<br />
<br />
[[category:Theme 5]]<br />
[[category:case studies]]<br />
[[Category:Shoreline management]]<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal erosion]]<br />
[[Category:Coastal flooding]]<br />
[[Category:Coastal flooding management]]<br />
[[Category:Coastal erosion management]]</div>Juliettejacksonhttps://www.coastalwiki.org/w/index.php?title=Case_study:_Applying_ASMITA_to_UK_estuaries&diff=17057Case study: Applying ASMITA to UK estuaries2007-11-30T13:53:51Z<p>Juliettejackson: </p>
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<div>There are over 170 estuaries with a variety of physical characteristics, spatial extents and management issues dissect the coastline of the United Kingdom (Pontee & Cooper, 2005). Many have some form of nature protection designation and intertidal areas are particularly important for numerous species, including migrating birds. Intertidal areas provide important natural coastal defences, protecting the low lying land surround estuaries from flooding. Estuaries may also be used for recreational activities such as sailing, fishing and walking and are economically important as ports, fishing grounds and for aggregate extraction (Townend, 2002).<br />
The diverse uses and morphologies of estuaries can lead to complex and sometimes conflicting management demands. In order to manage estuaries effectively it is important to be able to predict how they are likely to change in the future, both to natural and anthropogenic forcing. This article looks at historical morphological development of four UK estuaries and uses a model (ASMITA) to predict the maximum rate of sea-level rise each estuary can undergo before intertidal areas are lost completely.<br />
<br />
== Issues ==<br />
<br />
<br />
=== Sea-level rise ===<br />
Sea-level rise is predicted to accelerate over the 21st Century, with a global-mean rise of 9 to 88 cm, with the largest relative sea-level rise in the UK in the south (Hulme ''et al'', 2002). As sea-level rises, the water volume of the estuarine channels increases, while the intertidal sediment volume decreases (Fig.1). With high rates of sea-level rise major changes in the morphology of the estuary may occur, including loss of intertidal areas, leading to habitat loss, shoreline erosion and flooding of low lying areas around estuaries. <br />
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[[Image:slrcrit_chic.jpg|thumb|Figure 1: The predicted effect of sea-level rise on element volume]]<br />
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== The Model ==<br />
ASMITA (Aggregated Scale Morphological Interaction between Inlets and Adjacent coast, Stive ''et al'', 1998) represents the estuary as a series of morphological elements (Fig.2). Each element evolves towards an empirically derived equilibrium volume and interacts with adjacent elements by sediment exchange.<br />
ASMITA was calibrated to reproduce historic estuary evolution for four UK estuaries. Calibrated models were used to predict the maximum rate of sea-level rise (SLRCRIT) each estuary can undergo before intertidal areas are lost.<br />
<br />
[[Image:asmita_schematic.jpg|thumb|Figure 2: Estuary schematisation used in ASMITA]]<br />
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== The estuaries ==<br />
===Humber Estuary===<br />
*High sediment supply from Holderness cliff erosion<br />
*High capacity for sediment transport<br />
*Relatively large intertidal area<br />
*Morphology is in equilibrium with sea-level rise<br />
[[Image:humber.jpg|thumb|Comparison between observed volumes and ASMITA predictions in the Humber estuary]]<br />
<br />
===Dart Estuary===<br />
*Morphology dominated by hard rock geology of area<br />
*Limited sediment supply<br />
*Small areas of intertidal mudflats and salt marshes<br />
*In equilibrium with sea-level rise and sediment supply on time-scale of interest<br />
[[Image:Dart.jpg|thumb|Comparison between observed volumes and ASMITA predictions in the Dart estuary]]<br />
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===Langstone Harbour===<br />
*Fine sediment transported into harbour on flood tides, coarse sediment moves seaward to form ebb-tidal delta<br />
*Extensive salt marshes and mud flats<br />
*Element volumes varied over the study period, but tend towards equilibrium<br />
[[Image:Lang.jpg|thumb|Comparison between observed volumes and ASMITA predictions in Langstone Harbour]]<br />
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===Chichester Harbour===<br />
*Sediment regime similar to Langstone Harbour<br />
*Extensive intertidal areas<br />
*Ebb-tidal delta is dredged for gravel<br />
*Element volumes tend towards equilibrium<br />
[[Image:Chich.jpg|thumb|Comparison between observed volumes and ASMITA predictions in Chichester Harbour]]<br />
<br />
==Results==<br />
ASMITA was able to reproduce the general evolution of each study estuary to a satisfactory level. Allowing for uncertainty in the data and model parameter values, a range of maximum sea-level rise rates was produced for each estuary (Fig.3).<br />
Maximum rates of sea-level rise varied between estuaries. The Humber has the largest maximum rates, suggesting it will be resilient to morphological change driven by sea-level rise. The Dart estuary has the smallest maximum rates, indicating it is sensitive to sea-level rise. <br />
The Dart estuary, Langstone Harbour and Chichester Harbour all have the lower limits of the SLRCRIT range within predicted future rates of sea-level rise.<br />
<br />
[[Image:slrcrit.jpg|thumb|Figure 3: Summary of the rates of sea-level rise predicted to cause 25, 50, 75 and 100% intertidal loss]]<br />
<br />
==Conclusions==<br />
The maximum rate of sea-level rise an estuary can undergo before losing all intertidal area varies between estuaries. Estuaries with a large sediment supply and capacity to transport sediment can withstand greater rates of sea-level rise.<br />
The results suggest that some UK estuaries may experience rates of sea-level rise within their SLRCRIT range by 2080 (Fig. 3). If intertidal areas are to be maintained in these estuaries, estuary management must focus on increasing the sediment supply to the intertidal zone. <br />
Dredging or managed realignment may exacerbate the problem by increasing the sediment demand of the estuary. Future work will examine this in more detail.<br />
<br />
==References==<br />
Rossington, S K, Nicholls, R J, Knaapen, M A F and Wang, Z B (2007). Morphological Behaviour of Uk Estuaries under Conditions of Accelerating Sea Level Rise. River, Coastal and Estuarine Morphodynamics, University of Twente, Enschede.<br />
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{{author<br />
|AuthorID=12930<br />
|AuthorName= K. Rossington<br />
|AuthorFullName= Kate, Rossington}}<br />
<br />
<br />
[[category:Case studies]]<br />
[[category:Theme 5]]<br />
[[Category:Coastal risk management]]<br />
[[Category:Coastal flooding management]]<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Estuaries and tidal rivers]]<br />
[[Category:Coastal flooding]]</div>Juliettejackson