Coastline

Definition of Coastline: The coastline is a line that is considered the boundary between sea and land. This is the common definition for Coastline, other definitions can be discussed in the article

The terms coastline and shoreline are often used indiscriminately as synonyms. However, it is generally preferable to define the shoreline as the physical transition between land and water and the coastline as a proxy of the shoreline position that designates the boundary between land and sea for use in shoreline management, see the article Shoreline.

Coastline definition

There is no unambiguous and generally adopted definition of the coastline. Each country has its own definition. Because of the highly dynamic nature of the shoreline, the chosen definition of the coastline has to be defined on appropriate space and time scales.

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;
• determining whether intervention is necessary if the coastline retreats (taking in account assets and interests at stake).

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 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 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 average LW or HW lines. Disadvantage: this coastline choice 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). Besides, the mentioned beach features are not present everywhere.

Shoreline detection techniques

An overview of shoreline detection technique is given in the article Shoreline.

Fig. 1. Fig. 1. Coastline: a volume-based virtual shoreline that is more stable than the instantaneous shoreline. It is based on the sand volume within the most active part of the coastal zone, where sediment exchange occurs frequently under alternating storm and mild-wave conditions. This zone is bounded landward and seaward by elevations differing by a height [math]h[/math]. The beach volume [math]V [m³/m][/math] is defined as the sand volume between these boundaries over the height [math]h[/math]. The corresponding volume-based shoreline position lies at a distance [math]V/h[/math] from the landward boundary. If the landward boundary (e.g. the dune foot) is located at a fixed distance [math]c[/math] from a reference line (e.g. a line of beach poles), the virtual shoreline position [math]x[/math] relative to the reference line is given by [math]x=c+V/h[/math].

A volume-based coastline

Shoreline management is primarily interested in long-term erosion trends, also called structural coastal erosion. Long-term erosion trends are the result of a structural deficit in the balance of sand input and sand output along a 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, longshore propagating 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.

Fig. 2. Monitoring of the subaerial and submarine parts of the beach.

The volume-based coastline is a more reliable indicator of beach state 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 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 and vessel-mounted multibeam echosounders (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 of approximately 200m longitudinal length. A fictitious volume-based coastline has been defined for each section, which is related to the sand volume in this coastal section. Both the volume above the waterline and below the waterline are taken into account. In the Netherlands, the height [math]h[/math] in Fig. 1 is taken as twice the height difference between the dune foot and mean low tidal level. The corresponding sand volume is determined annually through a national coastal monitoring programme. Experience has showed that trends in the variation of this fictitious coastline provide a reliable estimate of the structural erosion or accretion of the coast. 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

The main author of this article is Job Dronkers Please note that others may also have edited the contents of this article. Citation: Job Dronkers (2026): Coastline. Available from http://www.coastalwiki.org/wiki/Coastline [accessed on 12-02-2026] For other articles by this author see Category:Articles by Job Dronkers For an overview of contributions by this author see Special:Contributions/Dronkers J