Structural erosion
Contents
Structural erosion
Definition of Structural erosion:
Structural erosion is long-term erosion of a stretch of coast. Also looking at an individual cross-shore profile, gradual losses of sediments with time might be noticed.
This is the common definition for Structural erosion, other definitions can be discussed in the article
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Introduction
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 article Application of structures some basic notions related to the use of structures in coastal engineering are dealt with.
Development of structural erosion
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 Fig.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.
Fig.4 shows the loss of volume out of a control volume.
In Example 1 the relationship between the annual loss of volume out of a cross-section and the annual rate of recession of the waterline is discussed.
| Example 1 Rate of recession versus volume rate of erosion
In Fig.3 a typical structural erosion case has been sketched. 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 Fig.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. In Fig.4 the development of the volume V of the control volume area with time of cross-shore profile has been given. 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 Fig.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.
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.
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 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 Example 2.)
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Example 2 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 Fig.1) occur; volume V (m3/m) is diminishing [dV/dt = dS/dx]. |
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[[Category:Land and ocean interactions
