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 Human Causes of Coastal Erosion and Natural Causes of Coastal Erosion. Erosion develops differently for different types of coasts. For more information on this, see the articles Accumulation and erosion for different coastal types, Coastal zone characteristics 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 schematic cross-shore profile.
In Figure 1 also two vertical lines and a horizontal line have been indicatively sketched; 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 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'. In the articles Hard shoreline protection structures, Port breakwaters and coastal erosion and Hard structures and structural erosion some basic notions related to the use of structures in coastal engineering are dealt with. 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 articles Protection against coastal erosion and Shore protection, coast protection and sea defence methods.
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, as said, only a link in the various coastal processes which are explained below.
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. The example below explains the relationship between the annual loss of volume out of a cross-section and the annual rate of recession of the waterline.
Example: Rate of recession versus volume rate of erosion
Figure 3 sketches a typical structural erosion case. Two cross-shore profiles have been indicated, representing the position of the profiles at two different moments within time. To a first approximation it can be assumed that the shape of the profiles are identical (the boundary conditions; e.g. wave climate and tides are the same indeed). The one profile can be found from the other by a horizontal shift. It is assumed that not only for example the waterline shifts in landward direction with a certain speed, but that this holds in fact for all depth contours.
If on an average the waterline shows a recession of r m/year, the annual loss of volume out of the control volume area DV (m3/m.year) is r times (h + d); see Figure 3 for the definition of h and d. The dune height d with respect to MSL is easy to estimate. The under water part of the cross-shore profile h which has to be taken into account, is more difficult to determine. Often the depth of the so-called active part of the profile is taken as representative. The depth belonging to the active part of the profile is related to the annual wave climate. A first approximation for depth h can be found by multiplying the significant wave height which is exceeded for one day a year, with 2 - 3. With actual profile measurements often a more accurate estimate of h can be determined.
Figure 4 gives the development of the volume V of the control volume area with time of cross-shore profile. The slope m = dV/dt (m3/m.year) is in this case a measure of the gravity of the erosion problem
A similar figure like Figure 4 can be made of the recession of the position of the waterline (in m with respect to a reference point) with time of an eroding profile in a structural eroding part of the coast. The slope r (m/year) is a measure for the erosion rate. From a morphological point of view representing erosion rates according to volumes is preferred above a representation according to distances.
While the associated longshore sediment transports take place in the 'wet' part of the cross-shore profile, at the end of the day, also the mainland will permanently lose sediments. This can be understood by taking the erosion processes during a(n even moderate) storm into account. In a (seen over a number of years) stable condition the erosion of mainland is in fact only a temporary loss. In a structural erosion case this is (partly) a permanent loss. Sediments eroded from the mainland during the storm and settled at deeper water are eroded (by the gradient in the longshore sediment transport), before they have the chance and time to be transported back to the mainland. Although it looks like that the storm is the reason of the (permanent) erosion problem of the mainland, in this case the actual reason is the gradient in the longshore sediment transport. The storm is only to be considered as a necessary link in a chain of processes.
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 the example of a convex coast below).
Example: Convex coast
Consider a part of a sandy coast; a few kilometres long. In plan view it refers to a convex coast (see sketch). In the sketch only the waterline is indicated, but it is assumed that the depth contours are more or less parallel to the waterline. Waves approach the coast as indicated in the sketch. Along the part of the coast as has been plotted, the angle between the wave crests and the orientation of the coast is ever changing. Longshore currents and longshore sediment transports [S: e.g. in m3/year] are generated. In the sketch the magnitude and direction of the longshore sediment transport rates are schematically indicated. Gradients in the longshore sediment transport rates seem to occur [dS/dx is not equal to 0 in m3/m.year]. Due to the gradients loss of sediments out of the control volume area (see also Figure 1) occur; volume V (m3/m) is diminishing [dV/dt = dS/dx].
- For more information on coastal processes and coastal types: Coastal Hydrodynamics And Transport Processes, Coastal zone characteristics, Classification of Coastlines and Accumulation and erosion for different coastal types
- For more information on coastal protection see: Shore protection, coast protection and sea defence methods (which describes different types of shore protection) and Protection against coastal erosion (which describes the background of coastal protection and protection against structural erosion and dune erosion).
- 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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