Typical examples of structural erosion
This article mentions three classical examples of man-induced structural coastal erosion problems:
- building a new port along a sandy coast;
- stabilization of a river mouth / tidal inlet;
- coastal erosion due to changes in river characteristics.
We will see that in all cases (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. Therefore, coastal erosion due to a severe storm (dune erosion) and/or structural erosion calls often for some counter-measures to protect the shoreline.
Contents
Building a new port
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.
Longshore sediment transport
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 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 Figure 1 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. Serious 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.
See also Port breakwaters and coastal erosion.
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 Figure 2).
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 Figure 3).
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.
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.
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 Figure 4 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.
See also
- Coastal erosion
- Hard structures and structural erosion
- Structural erosion
- Coastal protection
- Port breakwaters and coastal erosion
References
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