Difference between revisions of "Types and background of coastal erosion"
Revision as of 10:47, 28 March 2007
What is coastal erosion?
Considering a (still) more or less natural stretch of a sandy coast (so without any hard structures) a cross-shore profile, 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 Fig.1 for a schematic cross-shore profile. In Fig.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 chapter as mainland only.) Let us look at the erosion of the mainland, so 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. In fact two entirely different processes might cause loss of mainland, viz. dune erosion during a severe storm (surge) and structural erosion.
Figure 1 Control volume
During a severe storm sediments from the mainland and upper parts of the beach are eroded and settled at deeper water within a short time period; this is a typical cross-shore sediment transport process. Often a storm is accompanied with much higher water levels at sea than usual (storm surge) and also much higher waves do occur. 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 Fig.2). Notice in Fig.2 that the control volume does not change by the storm surge processes. Although this phenomenon 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. After the storm generally a recovery towards the original situation will occur due to the processes under normal conditions. Of course 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).
Figure 2 Dune erosion due to storm
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. Fig.4 shows the loss out of a control volume. (See Example 1 for an explanation of the relationship between the annual loss of volume out of a cross-section and the annual rate of recession of the coastline.)
Figure 3 Structural erosion
Figure 4 Volume loss out of control volume due to structural 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. 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 determine. 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.
Example 1 Rate of recession versus volume rate of erosion
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.)
Classical examples Three classical examples of man-induced structural erosion problems are mentioned here viz.:
- building a new port along a sandy coast;
- stabilization of a river mouth / tidal inlet;
- coastal erosion due to changes in river characteristics.
Building a port with two long breakwaters along a sandy coast with a net longshore sediment transport in a given direction (e.g. in m3/year), induces two typical morphological features at the updrift and the downdrift side of the port respectively. The downdrift side is relevant for the structural erosion discussion.
Example 2 Consider a part of a sandy coast; a few kilometres long. In plan view it refers to a convex coast (see sketch below). 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 ¹ 0 in m3/myear]. 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].
Example 2 Convex coast
A net longshore sediment transport is assumed to occur in this discussion. For understanding the morphological response of the coastal system to the construction of the breakwaters, it makes sense to make a distinction between cases without and with serious tidal currents. Without tidal currents effects, the net longshore sediment transport is mainly confined to the surf zone (sediment transports because of wave driven longshore currents). With long breakwaters the sediment transport might be totally interrupted. With serious tidal current effects, the (net) longshore sediment transport is spread over a much wider part of a cross-shore profile than only the surf zone. Even rather long breakwaters will not totally interrupt the sediment transport. In the next discussion a case without serious tidal effects is the starting point. Updrift side The updrift breakwater interrupts the longshore sediment transport; accreting of the beaches and mainland will occur at the updrift side of the new port. In many cases this gain of land is welcomed; after some time after the construction of the port this new land can be used for example for an extension of the port. On a long run, however, also the accreting side of a port will create problems as well. As soon as the accretion of the position of the waterline approaches the end of the updrift breakwater, sedimentation of the approach channel to the port will occur. A safe entrance to the port will be hindered. See Fig.5 for a sketch of the developments of the coastline near a port. Downdrift side At least in the first years after the construction of the port, no (or hardly any) sediments will pass the breakwaters. Because at the downdrift side of the port the original (undisturbed) longshore sediment transport takes place again, while no sediment is passing the breakwaters, large gradients in longshore sediment transport do occur at the downdrift side of the port. Massive erosion occurs at this side; the so-called lee-side erosion. This downdrift erosion is a typical example of a structural erosion process. Year after year the volume of sediments in the control volume area in a cross-shore profile is decreasing. This diminishing tendency of the control volume, which is typical for structural erosion, is expressed in m3/m per year. Sooner or later the lee-side erosion will harm interests located at the downdrift side of a port. Building a new port at the given location, undoubtedly will serve an important goal for the socio-economic development of a country. In the ultimate decision making process, the adverse effects of the port to the downdrift side have to be taken fully into account. If the unavoidable lee-side erosion is unwanted, adequate counter-measures (preferable at the spent of the port project) have to be taken. A proper system of artificially sand by-passing has to be considered as a serious option.
Figure 5 Developments of coastlines near port
Stabilization of a river mouth / tidal inlet
Similar accretion and erosion features like in the previous example, are to be expected if one likes to stabilize a natural river mouth with two jetties at both sides of the mouth. In the following description mainly river mouths are considered, but some similar processes occur and similar effects are to be expected if one likes to stabilize a tidal inlet. Let us assume a modest river flowing out in open sea. The position of a fully natural river mouth is often unstable. Accumulation of sediments at one side of the mouth, while forming a growing spit with time, together with the discharges through the mouth, cause erosion of the other side of the mouth. The position of the mouth shifts with time along the coast in the direction of the net longshore sediment transport along the coast. The river flows, landward of the spit, more or less parallel to the coast for some distance. By the growing spit, the length of the river becomes longer and longer with time (see Fig.6).
Figure 6 Spit and river mouth
When a rather long spit has been formed, often a new break-through occurs somewhere at the updrift side of the spit. Due to the increased length of the river, the water levels in the river behind the spit increase, and with a rather large discharge in the river (e.g. during a wet season), such a break-through of the slender spit is to be expected. This is a periodic process for a fully natural river mouth. Existing interests at the eroding side of the mouth (roads, buildings), and the desire to have a fixed entrance to the river for shipping (yachts; fishery vessels) call for stabilization. So two jetties at both sides of the inlet will serve that goal (see Fig.7). We assume that the position of the river mouth at the moment of stabilization is at a 'pleasant' position out of the many possible positions of the river mouth during a full natural cycle. Without an adequate sand by-pass system, the induced erosion at the downdrift side of the jettied inlet is also a typical man-induced structural erosion problem. At a first glance the (yearly) growth of the spit might be considered as useful measure for the annual net sediment transport. Be aware, however, that in natural systems often only a part of the annual net sediment transport is 'used' for the growth of the spit; the other part by-passes at a natural manner. If these features are not taken properly into account, the rates of accretion and erosion after stabilization, might become surprisingly large. Also in the case of stabilization of a river mouth or tidal inlet, a comprehensive decision making process has to be carried out, taking primary aims, but taking also the possible adverse consequences into account.
Figure 7 Stabilized river mouth
Changes in river characteristics
In a still fully natural situation where a (small) river brings year after year sediments into the sea, the coast in the vicinity of the river outlet is ever growing in seaward direction. The sediments from the river are distributed by the longshore sediment transport processes along the coast, while the coast is ever growing. In plan view such a river-sea system is often to be noticed as a crack in the orientation of the coastlines at both sides of the river outlet. (See Fig.8 for a sketch of the development with time.) Man-induced changes in the characteristics of the river (e.g. serious sand-mining in the river-bed, or damming of the river for irrigation or hydro-power purposes), might change the natural accreting tendency of the coast in an eroding tendency of the coast in the vicinity of the river outlet. When this occurs, it is also to be considered as a typical structural erosion problem for some stretches of the coast. In all cases as were mentioned (natural and man-induced), the control volume in a cross-shore profile is reducing with time; sooner or later also the interests at the mainland will be endangered. Coastal erosion due to a severe storm and/or structural erosion calls often for some counter-measures (coastal protection). The possibilities will be briefly discussed in the following section.
Figure 8 Developments of coastline near river outflow