Coastline
Definition of Coastline:
A line adopted as a representative boundary between land and sea, generally defined from a measurable shoreline indicator for purposes such as monitoring coastal erosion or accretion and shoreline management.
This is the common definition for Coastline, other definitions can be discussed in the article
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In the Coastal Wiki, a distinction is made between the coastline and the shoreline. The latter is generally defined as the physical transition between land and water (instantaneous or at a given tide level and averaged over a given period, see the article Shoreline). The coastline is a representative boundary between land and sea adopted for shoreline management. The distinction between coastline and shoreline is not universal; in much of the literature the term shoreline is also used for the coastline.
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Coastline definition
There is no unambiguous and generally adopted definition of the coastline. Different coastline definitions are used in the literature and in coastal practice depending on the purpose, coastal setting and available observations.
Criteria for defining a coastline include:
- providing a representative estimate of trends in coastal erosion or coastal accretion;
- the accuracy with which the location of the coastline can be determined;
- robustness (repeatability) of the corresponding measuring technique (e.g. not influenced by tide or wave run-up);
- the effort required to determine the coastline position.
Monitoring of an adequately defined coastline should inform about trends in coastal erosion or accretion and about risks to coastal assets and interests. A reference coastline can then be adopted as a criterion for coastal management.
Several coastline choices are possible:
- A coastline based on fixed points (for example a cliff or seawall at the upper end of the beach). Disadvantage for sedimentary coasts: it does not provide insight into beach erosion or accretion.
- A coastline based on the instantaneous waterline. Disadvantage: the great variability, in particular the effect of tides.
- A coastline based on a tidal datum, for example the intersection of mean high water or mean low water with the coastal profile. Satisfactory for rocky coasts, but disadvantage for sandy dune coasts: coastline variability is still considerable due to the influence of storms and the influence of seasonal variability in the wave climate.
- A coastline based on easily identifiable beach features, for example the dune foot, the most seaward vegetation line, the crest of the most landward beach berm. Advantages: beach erosion or accretion is reflected in retreat or advance of these coastal features, while the variability is much less than for the intersection of a tidal datum with the coastal profile. Disadvantage: these coastal features can retreat substantially during occasional storms, although much of this retreat may be temporary and can be restored by natural processes under a calmer wave climate (see Dune erosion). Moreover, these beach features are not present everywhere.
As these different coastline indicators may respond differently to the same redistribution of sediment, their displacement does not necessarily represent a corresponding loss or gain of coastal sediment.
Shoreline detection techniques
An overview of shoreline detection techniques is given in the article Shoreline.
A volume-based coastline
Shoreline management is primarily interested in long-term erosion trends, also called structural coastal erosion. Long-term coastal erosion reflects a persistent negative sediment balance of the considered coastal section. The position of the instantaneous shoreline is not an optimal indicator of long-term erosion or accretion trends due to the strong variability caused by tides, wind setup and wave setup. Medium-term fluctuations of the shoreline related to post storm shoreline recovery, propagating shoreline sand waves and sandbar migration are also masking the long-term trend. A more appropriate indicator of long-term change in the shoreline position is provided by the sand volume in the active zone, or in the most dynamic portion of the active zone over which seasonal sand redistribution mainly occurs. This sand volume is much less sensitive to short-term and medium-term fluctuations than the instantaneous shoreline. A virtual volume-based shoreline position can be defined as indicated in Fig. 1.
The increasing availability of satellite-derived shorelines does not remove the need to define an appropriate coastline indicator: the detected waterline may contain substantial apparent displacement caused by tidal level, wave setup and intertidal beach morphology. For detecting long-term erosion or accretion trends, a volume-based coastline is more reliable than the instantaneous shoreline. Because of its greater stability, less frequent measurements are required to identify long-term trends in beach condition. However, its determination requires substantially greater effort, as the elevations of both the subaerial and submarine parts of the beach must be monitored. These surveys are typically carried out using airborne LiDAR for the subaerial beach and echo sounding from survey vessels for the submerged profile (see Fig. 2).
Shoreline management in the Netherlands
In the Netherlands, because large parts of the country are lying below mean sea level, the dune belt is of crucial importance to protect the hinterland against flooding. Structural beach erosion increases the vulnerability of the dune coast and is therefore systematically redressed along the entire coast by means of sand nourishments (see Beach nourishment and Shoreface nourishment). To determine where structural coastal erosion occurs, the coast has been divided into coastal sections with transects typically spaced a few hundred meters apart. For each coastal section a volume-based coastline, the Momentane Kustlijn (MKL), is calculated from the measured coastal profile. Its position is compared with the Basiskustlijn (BKL), the reference coastline adopted for coastal maintenance. The BKL provides the operational criterion for local coastline maintenance. In addition, Dutch nourishment policy considers the sediment balance of the wider coastal system, the so-called 'coastal foundation', which extends offshore to approximately the NAP −20 m depth contour.
For the MKL, both the volume above the waterline and below the waterline are taken into account. In the Netherlands, the vertical range [math]h[/math] in Fig. 1 is taken as twice the elevation difference between the dune-foot proxy and mean low water. The corresponding sand volume is determined annually through a national coastal monitoring program. Experience has shown that trends in the position of this volume-based coastline provide a reliable estimate of structural coastal erosion or accretion[1]. For further information, see Risk and coastal zone policy: example from the Netherlands.
Related articles
- Shoreline
- Shoreline retreat and recovery
- Risk and coastal zone policy: example from the Netherlands
- Dune erosion
- Dealing with coastal erosion
- Integrated Coastal Zone Management (ICZM)
- Shoreline management
- Shoreface profile
- Active coastal zone
References
- ↑ RIKZ 1996. Coastline management, from coastal monitoring to sand nourishment. Ministry of Transport, Public Works and Water Management, Directorate-General for Public Works and Water Management, National Institute for Coastal and Marine Management/RIKZ
Please note that others may also have edited the contents of this article.
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