The coastline is a line that is considered the boundary between sea and land.
There is no unambiguous and generally adopted definition of the coastline. Each country has its own definition.
Criteria for defining a coastline include:
- determining whether there is coastal erosion or coastal accretion;
- determining whether intervention is necessary if the coastline retreats (assets and interests at stake);
- the accuracy with which the location of the coastline can be determined;
- the effort required to determine the location of the coastline.
Several coastline choices are possible:
- A coastline based on fixed points (for example a 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 the average waterline at high tide or low tide. 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 the transition from beach to dune. This is a more usual choice. Advantages: It can be easily determined; beach erosion or accretion is reflected in retreat or advance of the coastline, while the variability is much less than for the average LW or HW lines.
Shoreline management based on structural erosion
However, the latter choice of coastline is sensitive to coastline retreat caused by occasional storms, which often has a temporary character and can be restored by natural processes when a calmer wave climate has established (see Dune erosion). In this case, interventions to restore the beach are not necessary. Shoreline management is therefore often more interested in structural coastal erosion, which is the result of a structural deficit in the balance of sand input and sand output for a particular coastal section. To gain insight into this sand balance, it is necessary to monitor the sand volume of the active zone for sections along the entire coast.
In the Netherlands, because it is lying for a large part 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. To determine where structural coastal erosion occurs, the coast has been divided into coastal sections of approximately 200m longitudinal length. A fictitious coastline, the so-called base coastline, has been defined for each section, which is based on the sand volume in this coastal section. Both the volume above the waterline and below the waterline are taken into account. The considered volume below the waterline is related to a water depth equal to the height of the dunefoot above the low waterline, see Fig. 1. This volume is determined annually by coastal monitoring. Experience shows that the variation of this fictitious base coastline provides a reliable estimate of the structural erosion or accretion of the coast.
- Risk and coastal zone policy: example from the Netherlands
- Dune erosion
- Typical examples of structural erosion
- Dealing with coastal erosion
- Integrated Coastal Zone Management (ICZM)
- Shoreline management
- 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
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