Difference between revisions of "Beach nourishment"

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{{Definition|title=Beach nourishment
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|definition=Artificial sand supply to the beach with sand imported from a source outside the [[active coastal zone]].}}
  
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<div style="border:1px solid #000000; text-align:center; align=center; padding=5px ; margin-right:20px; margin-left:20px">
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==Purpose of beach nourishment==
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The application of coastal nourishments requires insight into the processes that shape the coastal profile. For an introduction to these processes, the reader is referred to the article [[Shoreface profile]] and other articles mentioned herein.
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Shore nourishments can be applied in various cases; e.g.:
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</div>
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#to broaden the beach (recreation purposes);
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#to create entirely new beaches (recreation purposes);
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#to enhance the safety of the mainland or to enhance the safety of properties built rather close to the edge of the dunes;
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#to compensate losses because of [[structural erosion]].
  
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The first three cases can be regarded as applications to improve an existing undesirable situation that does not necessarily imply an ongoing supplementation programme.
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In the fourth case, coastal nourishments are used as an instrument in coastal protection projects. Due to [[structural erosion]], sediments are lost more or less continuously from a stretch of coast; regular nourishments are required to compensate for the losses that occur on average.
  
  
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{{Definition|title=Shore nourishment
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==Non-equilibrium coastal profile==
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|definition=Shore nourishments are projects where the coastal system is fed with sediments (sand) from a source (borrow area) at a sufficient distance from the project area so that coastal hydrodynamics remain unaffected.
 
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}}
 
  
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Other examples of projects where the coastal system is fed, are large scale land reclamation projects and the construction of an artificial island in the open sea. In these cases often huge volumes of sand are required; typically several hundreds of millions m<sup>3</sup> per project. Shore nourishment projects involve moderate volumes, typically in the order of 1-5 million m<sup>3</sup> and rarely more than 10 or 20 million m<sup>3</sup>.
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[[File:EquilibriumNourishment.jpg|thumb|right|400px|<small>Fig. 1. Morphological response to beach broadening, assuming initial equilibrium. After nourishment, the coastal profile is out of equilibrium unless the entire active coastal zone is nourished. Beach broadening implies seaward growth of the front dune that will trap part of the beach sand. So nourishment is still necessary.</small>]]
  
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When borrow sand is placed in a coastal profile, in general neither the profile nor the grain size distribution will be in equilibrium. Nature will try to restore an equilibrium profile, so that there will always be changes in the nourished profile, as illustrated in Fig. 1. There will also be changes caused by alongshore distribution of the nourishment, by a possible trend of sustained long-term erosion or by the response to extreme weather events. In practice it is not possible to carry out stable nourishments in the short or long term on an eroding coast. Nourishments are inherently unstable on eroding coastlines. These are basic realities that are often difficult for the public, politicians and those who fund the projects to accept. On the other hand, as environmental concerns and requirements for sustainability gain in importance, the share of nourishment in coastal management schemes has gradually increased in recent decades.
  
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When a beach suffers from structural erosion, artificial nourishments can be applied as a soft remedy. The losses occurring from a stretch of coast are replenished from time to time. The regular application of nourishments with borrow sand with the same grain size as the native beach material will not strongly disturb the existing longshore sediment transport and thus will not change the occurring losses. Therefore the erosion will not stop; this means that after a certain period of time (the nourishment lifetime) the nourishment must be repeated.
  
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==Nourishment concept==
 
  
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Shore nourishments can be regarded as a 'natural' way of combating dune erosion and beach erosion as it artificially replaces a deficit in the sediment budget over a certain coastal stretch with a corresponding volume of sand. However, as the cause of erosion is not eliminated, erosion will continue and carry away 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 an ongoing maintenance effort. In general, nourishment is only suited for shoreline sections of substantial length; 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.
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==Nourishment design: Experience from Denmark==
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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. The influence of grain size on cross-shore sediment transport processes is discussed in the article [[Coastal Hydrodynamics And Transport Processes]].  
  
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[[Image:nourishment methods.gif|thumb|left|300px|<small>Fig. 1. Generally practiced nourishment methods. Pipe discharge on the beach for beach nourishment, over the bow pumping (rainbowing) for nearshore nourishment and split barge for nourishment in the outer part of the upper shoreface. </small>]]
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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 large volumes of sand, as illustrated in the upper part of Fig. 2.  
  
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If the objective is to reduce the required nourishment volume while retarding subsequent erosion (increasing the nourishment lifetime) it is advantageous to apply sand with a larger grain size than that of the native sand into a coastal profile. This will produce a steeper profile than the natural profile. 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 based on 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:
  
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:<math>GSR_{Nourishment}=d_{50,Borrow}/d_{50,Native}</math>
  
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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. 3. that the Nourishment Efficiency increases considerably with increasing Grain Size Ratio for Nourishment.
  
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The design 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 [[Shoreface profile]], the characteristics of the sand determine the overall shape of the coastal profile expressed in the equilibrium profile concept. Furthermore, natural hydrodynamic processes tend to sort the sediments in the profile so that the grain size decreases with increasing water depth. Borrow material is generally mined from the seabed sufficiently far offshore to minimize any influence on the hydrodynamics of the coastal zone. Placement in onshore nourishment sites makes use of pipes; for offshore sites, sand is dumped by split barges or by rainbowing (Fig. 1). For small nourishments, land sources are sometimes used. 
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{| border="0" align="center"
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|-
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| valign="top"|
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[[File:nourished beaches equilibrium.jpg|450px|right|thumb|<small>Fig. 2. 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). </small>]]
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| valign="top"|
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[[Image:nourish grain size.jpg|400px|thumb|right|<small>Fig. 3. Relation between Nourishment Efficiency and the Grain Size Ratio for Nourishment<ref name="Vestkysten"/></small>]]
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|}
 
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==Nourishment applications==
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==Nourishment design: Experience from the Netherlands<ref name=B>Brand, E., Ramaekers, G. and Lodder, Q. 2022. Dutch experience with sand nourishments for dynamic coastline conservation – An operational overview. Ocean and Coastal Management 217, 106008</ref>==
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In the following, a short introduction is given to various different nourishment methods.
 
  
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====Inner dune nourishment====
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[[File:NourishmentLife.jpg|thumb|right|300px|Fig. 4. Lifespan of beach nourishments compared to the nourishment volume<ref name=B/>.]]
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[[File:DuneNourishment.jpg|thumb|right]]
 
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Inner dune nourishment is the strengthening of the dune belt to prevent breaching by an extreme storm surge; it is not intended to prevent shoreline retreat. This measure can be applied when a single dune row that protects low-lying hinterland from flooding is not strong enough to withstand extreme storm surges. Sand is placed at the landward side of the front dune to increase the dune volume and/or to raise the dune crest. Beach morphodynamic processes are not affected by this measure. The amount of sand needed to achieve the desired protection level can be estimated with the methods described in [[Dune erosion]]. Inner dune nourishment is particularly effective if the dune foot location is stable i.e., no structural retreat due to ongoing erosion. In this case, a one-off intervention is sufficient, so that the additional costs for applying the sand on the land side are also a one-off investment. Recovery of dune vegetation can be promoted by depositing the top layer of the area to be nourished and later applying it as a cover layer over the nourishment.
 
  
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Beach nourishments are regularly carried out along the Dutch coast in order to maintain the coastline in all the sections that are subject to erosion. In total 258 beach nourishments have been carried out since 1990. From the experience of these nourishments some rule of thumbs have been derived to optimize their effectiveness.  
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====Backshore nourishment====
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Sand is borrowed from the seabed of the southern North Sea which mainly consists of thick sand deposits. Sand is mined within a distance of 12 miles from the coast to limit shipping distances, but below the 20 m depth contour to avoid an influence on coastal processes. Borrow sites are selected for grain sizes similar to the sand of the beach where the nourishment is to be applied.  
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[[File:BackshoreNourishment.jpg|thumb|right]]
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Beach nourishments are placed against the dune foot which is approximately at NAP +3 m. After a short beach platform the sand placement descends with a slope around 1:30 that is as similar to the natural beach profile. The average volume of beach nourishments is 200 m<sup>3</sup>/m and the average length is 2.3 km with a gradual decrease in volume towards both ends of the nourishment in the alongshore direction to minimize side effects. With this design, the average lifetime of the nourishment is about 3 years; in the first year approximately 40–50% of the nourished volume is lost from the zone which is considered in the Netherlands for the definition of the coastline position (see [[Coastline]]). There is no clear relationship between nourishment volume and nourishment lifetime (Fig. 4).
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[[Backshore]] nourishment is the strengthening of the upper part of the beach by placing the nourishment on the [[backshore]] or at the foot of the dunes. The material is stockpiled in front of the dunes and acts as a buffer, which is sacrificed during extreme events.
 
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The main objective of [[backshore]] nourishment is to strengthen the [[backshore]]/dune against erosion and [[breaching]] during extreme events. This kind of nourishment works more by volume than by trying to restore a 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.  
 
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[[Backshore]] nourishment can be performed by hydraulic pumping sand through pipes discharging at the foot of the dunes and later adjusted using a bulldozer (Fig. 1). 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.  
 
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====Beach nourishment====
 
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[[File:BeachNourishment.jpg|thumb|right]]
 
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[[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 protection measure against flooding, as the beach is normally submerged during extreme events. However, it enhances the protection function of the dune belt or other backshore flood defences. The sand is usually supplied via a cross-profile pipeline and adjusted with bulldozers to the natural beach profile. It is therefore necessary that the borrow sand has grain size similar to the native sand. 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 [[Beach nourishment]] and  [[Experiences with beach nourishments in Portugal]].
 
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====Shoreface nourishment====
 
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[[File:OuterBarNourishment.jpg|thumb|right]]
 
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[[Shoreface nourishment]] is the supply of sand to the subtidal part of the coastal profile. 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 in combination with beach nourishment, thus creating a nourished profile close to the equilibrium over the entire [[active coastal zone]]. See [[Shoreface nourishment]] for further details.
 
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Shoreface nourishment is often performed using split barges (Fig. 1). 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.
 
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====Beach Scraping====
 
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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. This coarse material can be brought to the upper part of the beach to protect the dune foot or the foot of the cliff against erosion by storm surges. This practice is called beach scraping and is normally performed using front loaders.
 
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This method can be used for beaches which are mainly exposed to seasonal erosion, whereas it is probably not feasible for locations 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 contributes insignificantly to the overall stability of the beach profile. Another issue is that equipment operated during late summer may disturb recreational activities.
 
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====Creation of a new beach====
 
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[[Image:BeachSlopeGrainSizeData.jpg|thumb|350px|right|<small>Figure 2. Correlation between sediment grainsize and related beach slope from 78 field studies, adapted from Bujan et al. (2019)<ref name=Bu>Bujan, N., Cox, R. and Masselink, G. 2019. From fine sand to boulders: Examining the relationship between beach-face slope and sediment size. Marine Geology 417, 106012</ref>. The violet area indicates the scatter of the data points.</small>]]
 
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In many places (e.g. along the Mediterranean coast) the coast is rocky with few wide sandy beaches. A beach can be created by artificial sand nourishment if this is desired for recreational purposes. There are two main differences with maintenance nourishment of an existing beach: (1) an entire equilibrium beach profile has to be created, up to the [[closure depth]], to minimize sand loss to deep water; (2) the new beach is subject to strong gradients in longshore sand transport that redistribute the nourishment alongshore. Additional measures are necessary to keep the nourished sand in place, such as confining the artificial beach between groynes or breakwaters. However, such structures must be carefully designed to ensure proper functioning, as explained in the articles [[Groynes]] and [[Detached breakwaters]]. The sand volume needed to create a stable beach strongly depends on the grain size. The equilibrium profile of a coarse sandy beach is much steeper than the equilibrium profile of a fine sandy beach (Fig. 2). With coarse sand smaller nourishment volumes are needed and longshore sand losses will be smaller.
 
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====Channel wall nourishment====
 
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[[File:ChannelWallNourishment.jpg|thumb|right]]
 
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At places where the coast is interrupted by tidal inlets, channels of the ebb tidal delta may run some distance along the adjacent shores. The landward migration of these tidal channels can cause erosion of the beaches along these shores. In this case channel wall nourishments can be applied to counteract this landward migration, even when large nourishment volumes are required. However, because lateral channel migration is usually a rather slow process, the lifetime of the nourishment is generally longer than the lifetime of beach or shoreface nourishments. Channel wall nourishments therefore can be a good option to prevent beach erosion by onshore migrating channels. Extensive experience with channel wall nourishments has been gained at tidal inlets along the Dutch coast <ref>Brand, E., Ramaekers, G. and Lodder, Q. 2022. Dutch experience with sand nourishments for dynamic coastline conservation – An operational overview. Ocean and Coastal Management 217, 106008</ref>.
 
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==Related articles==
 
==Related articles==
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:[[Beach nourishment]]
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* [[Experiences with beach nourishments in Portugal]]
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:[[Shoreface nourishment]]
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* [[Shore nourishment]]
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:[[Shoreface profile]]
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* [[Shoreface nourishment]]
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:[[Experiences with beach nourishments in Portugal]]
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* [[Coastal Hydrodynamics And Transport Processes]]
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:[[Dealing with coastal erosion]]
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* [[Shoreface profile]]
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:[[Nearshore sandbars]]
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* [[Coastline]]
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==Further reading==
 
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*Mangor, K., Drønen, N. K., Kaergaard, K.H. and Kristensen, N.E. 2017. Shoreline management guidelines. DHI https://www.dhigroup.com/marine-water/ebook-shoreline-management-guidelines
 
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*Reeve, D. E., Chadwick, A. C. and Fleming, C.A. 2018. Coastal Engineering: Processes, Theory and Design Practice. 3rd edition. Boca Raton, Florida, USA: CRC Press (Taylor & Francis Group), 512p.
 
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*CIRIA, 2010. Beach Management Manual, 2nd Edition, Publication no. RP787, London
 
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*Coastal Engineering Manual 2006. part V Ch. 4 Beach fill design.
 
  
  
 
==References==
 
==References==
 
<references/>
 
<references/>
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Revision as of 16:25, 18 January 2022

Definition of Beach nourishment:
Artificial sand supply to the beach with sand imported from a source outside the active coastal zone.
This is the common definition for Beach nourishment, other definitions can be discussed in the article

Purpose of beach nourishment

Shore nourishments can be applied in various cases; e.g.:

  1. to broaden the beach (recreation purposes);
  2. to create entirely new beaches (recreation purposes);
  3. to enhance the safety of the mainland or to enhance the safety of properties built rather close to the edge of the dunes;
  4. to compensate losses because of structural erosion.

The first three cases can be regarded as applications to improve an existing undesirable situation that does not necessarily imply an ongoing supplementation programme. In the fourth case, coastal nourishments are used as an instrument in coastal protection projects. Due to structural erosion, sediments are lost more or less continuously from a stretch of coast; regular nourishments are required to compensate for the losses that occur on average.


Non-equilibrium coastal profile

Fig. 1. Morphological response to beach broadening, assuming initial equilibrium. After nourishment, the coastal profile is out of equilibrium unless the entire active coastal zone is nourished. Beach broadening implies seaward growth of the front dune that will trap part of the beach sand. So nourishment is still necessary.

When borrow sand is placed in a coastal profile, in general neither the profile nor the grain size distribution will be in equilibrium. Nature will try to restore an equilibrium profile, so that there will always be changes in the nourished profile, as illustrated in Fig. 1. There will also be changes caused by alongshore distribution of the nourishment, by a possible trend of sustained long-term erosion or by the response to extreme weather events. In practice it is not possible to carry out stable nourishments in the short or long term on an eroding coast. Nourishments are inherently unstable on eroding coastlines. These are basic realities that are often difficult for the public, politicians and those who fund the projects to accept. On the other hand, as environmental concerns and requirements for sustainability gain in importance, the share of nourishment in coastal management schemes has gradually increased in recent decades.

When a beach suffers from structural erosion, artificial nourishments can be applied as a soft remedy. The losses occurring from a stretch of coast are replenished from time to time. The regular application of nourishments with borrow sand with the same grain size as the native beach material will not strongly disturb the existing longshore sediment transport and thus will not change the occurring losses. Therefore the erosion will not stop; this means that after a certain period of time (the nourishment lifetime) the nourishment must be repeated.


Nourishment design: Experience from Denmark

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. The influence of grain size on cross-shore sediment transport processes is discussed in the article Coastal Hydrodynamics And Transport Processes.

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 large volumes of sand, as illustrated in the upper part of Fig. 2.

If the objective is to reduce the required nourishment volume while retarding subsequent erosion (increasing the nourishment lifetime) it is advantageous to apply sand with a larger grain size than that of the native sand into a coastal profile. This will produce a steeper profile than the natural profile. 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 based on many years of nourishment along the Danish North Sea Coast[1]. 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:

[math]GSR_{Nourishment}=d_{50,Borrow}/d_{50,Native}[/math]

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. 3. that the Nourishment Efficiency increases considerably with increasing Grain Size Ratio for Nourishment.

Fig. 2. Equilibrium conditions for nourished beaches required to obtain an additional beach width of δw with borrow sand, which is finer and coarser than the native sand (upper and lower, respectively).
Fig. 3. Relation between Nourishment Efficiency and the Grain Size Ratio for Nourishment[1]



Nourishment design: Experience from the Netherlands[2]

Fig. 4. Lifespan of beach nourishments compared to the nourishment volume[2].

Beach nourishments are regularly carried out along the Dutch coast in order to maintain the coastline in all the sections that are subject to erosion. In total 258 beach nourishments have been carried out since 1990. From the experience of these nourishments some rule of thumbs have been derived to optimize their effectiveness. Sand is borrowed from the seabed of the southern North Sea which mainly consists of thick sand deposits. Sand is mined within a distance of 12 miles from the coast to limit shipping distances, but below the 20 m depth contour to avoid an influence on coastal processes. Borrow sites are selected for grain sizes similar to the sand of the beach where the nourishment is to be applied. Beach nourishments are placed against the dune foot which is approximately at NAP +3 m. After a short beach platform the sand placement descends with a slope around 1:30 that is as similar to the natural beach profile. The average volume of beach nourishments is 200 m3/m and the average length is 2.3 km with a gradual decrease in volume towards both ends of the nourishment in the alongshore direction to minimize side effects. With this design, the average lifetime of the nourishment is about 3 years; in the first year approximately 40–50% of the nourished volume is lost from the zone which is considered in the Netherlands for the definition of the coastline position (see Coastline). There is no clear relationship between nourishment volume and nourishment lifetime (Fig. 4).


Related articles


References

  1. ↑ 1.0 1.1 Vestkysten 2000 (in Danish) (The West Coast or the Danish North Sea Coast 2000), The Danish Coastal Authority.
  2. ↑ 2.0 2.1 Brand, E., Ramaekers, G. and Lodder, Q. 2022. Dutch experience with sand nourishments for dynamic coastline conservation – An operational overview. Ocean and Coastal Management 217, 106008


The main authors of this article are Mangor, Karsten, Jan van de Graaff, Anna Kroon and Job Dronkers
Please note that others may also have edited the contents of this article.
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