Types and background of coastal erosion
This article introduces what 'coastal erosion' involves. Two different types of coastal erosion are distinguished, dune erosion and structural erosion. It is also possible to distinguish between different causes of erosion: natural factors which cause erosion and erosion due to human intervention in transport processes. These two causes of erosion are described in the articles on human causes of coastal erosion and natural causes of coastal erosion. Erosion develops differently for different types of coasts. For more detailed information, see the articles on accretion and erosion for different coastal types and classification of coastlines. The rate of erosion is correctly expressed in volume/length/time, e.g. in m3/m/year, but erosion rate is often used synonymously with coastline retreat, and thus expressed in m/year.
What is coastal erosion?
Consider a (still) more or less natural stretch of a sandy coast (so without any hard structures). A cross-shore profile of that stretch of coast, seen in seaward direction, consists out of the mainland or dunes at a level well above mean sea level (MSL), an often rather steep mainland or dune face, a beach and a shoreface. See Figure 1 for a plan view picture and a schematic cross-shore profile.
In Figure 1 also two vertical lines and a horizontal line have been indicatively sketched in the cross-shore profile; these lines and the position of the actual cross-shore profile enclose a so-called control volume area. The developments of the volume V (in m3/m) with time of this control volume area, is further used to explain some typical features which might occur along a coast. (Mainland or dunes is further written in this contribution as mainland only.)
Let us look at the erosion of the mainland (including dunes). This is often the most valuable part of a cross-shore profile to mankind. The very seaward part of the mainland sometimes carries valuable properties like houses and hotels or infrastructure like roads and car park areas. Also coastal erosion forms an important element in the process of coastal squeeze .
In fact two entirely different processes might cause loss of mainland:
The basic principles of both types of erosion are discussed in the sections below.
Dune erosion is a typical example of cross-shore sediment transport process and results from sediment transport during a severe storm. Often a storm is accompanied with much higher water levels at sea than usual (storm surge) and also much higher waves do occur. Depending on the characteristics of the storm, erosion rates of several metres per event (say per day) have to be considered. These episodic events do occur along all types of coasts (along structural eroding, stable and even along accreting coasts). After the storm generally a recovery towards the original situation will occur due to the occurring processes under normal conditions.
While under ordinary conditions the shape of a cross-shore profile might be considered to be in accordance with these normal conditions ('dynamic equilibrium' shape), under storm (surge) conditions the initial shape before the storm can be considered to be far out of the equilibrium shape which belongs to the severe storm conditions. Profile re-shaping processes will occur, causing erosion of sediments from the mainland and the settlement of these sediments at deeper water again (see Figure 2). Notice in Figure 2 that the control volume does not change by the storm surge processes.
Although this dune erosion process will cause loss of mainland, to a first approximation no loss of sediments out of the cross-section occurs; only a redistribution of sediments over the cross-shore profile takes place during the storm.
For more information on the background of coastal erosion, see also the article Coastal Hydrodynamics And Transport Processes.
Many coasts all over the world suffer from structural erosion. Seen over a number of years one might observe that the position of e.g. the waterline is shifting in landward direction. Often a gradient in the (natural) occurring longshore sediment transports is the reason of structural erosion. Structural erosion is quite different from dune erosion. This implies that resolving a structural erosion 'problem' in coastal engineering practice calls for a quite different approach compared to the solution to a dune erosion 'problem'.
For people living along the coast the distinction between both causes for erosion is often not so clear. In both cases it looks like that the storm is the malefactor. During a storm actual damage at the mainland will occur. In the structural erosion problem the storm is only a link in the various coastal processes which are explained in Par. Morphological background below.
More information about structural erosion can also be found in the article Typical examples of structural erosion.
Structural erosion rates are often in the order of magnitude of a few metres per year. To overcome erosion problems related to either dune erosion or structural erosion, calls for quite different counter-measures. A coastal zone manager must be aware of this. See also the article Dealing with coastal erosion.
In the article Hard coastal protection structures and the articles listed herein some basic notions related to the use of hard structures in coastal engineering are dealt with.
Structural, long-term erosion yields a gradual loss of sediments out of a cross-shore profile. Looking at the volume of sediments in the control volume area as a function of time, we will see a diminishing tendency with time.
A gradient in the longshore sediment transport is often the reason of structural erosion. See Figure 3 for the development with time of an eroding cross-shore profile.
Notice the change with time of the control volume. Notice also that the mainland is eroding as well, although the longshore sediment transports, and so the gradients in the longshore sediment transport, do not occur at the mainland level. Events with some dune erosion redistribute sediments from the dunes to the beach and foreshore. In this case the recovery of the dunes is only partly; at the end of the day also the mainland retreats.
Figure 4 shows the loss of volume out of a control volume as a function of time.
The Example: Rate of recession versus volume rate of erosion explains the relationship between the annual loss of volume out of a cross-section and the annual rate of recession of the waterline.
Possible causes of structural erosion
In order to select a proper scheme to protect a structural eroding stretch of coast, it is necessary to understand the cause of the erosion problem. Principally pure natural and man-induced causes are to be distinguished. A pure natural morphological development of a part of the coast is often called autonomous behaviour. What looks like an autonomous behaviour at present, is in some cases in fact (the tail of) a man-induced development started a long time ago. So the distinction between both is not very clear in many cases.
A convex stretch of coast under wave action is a typical example of a natural, structural eroding coast. See Example: Erosion because of a convex coast.
- For more information on coastal processes and coastal types: Coastal Hydrodynamics And Transport Processes, Characteristics of sedimentary shores, Classification of coastlines and Accretion and erosion for different coastal types
- For more information on coastal protection see: Dealing with coastal erosion which describes the background of coastal protection and different types of shore protection measures.
- Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pp.
- Doody, J.P. (2004) 'Coastal squeeze' - an historical perspective. Journal of Coastal Conservation, 10/1-2, 129-138.
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