10.4: Modification of longshore transport processes
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In most cases, the use of structures for coastal protection relies on the ability of such structures to interfere with the existing sediment transport processes. The main reason for structural erosion is often a gradient in the net longshore sediment transport along a stretch of coast. Structures counteracting this erosion are designed to change the curve of the longshore transport \(S\) versus distance along the shore \(x\), such that the transport gradients become zero and hence erosion is stopped. This situation is discussed in more detail below.
In Fig. 10.7, line \(a\) indicates the net yearly longshore sediment transport \(S\) along a coast (\(S\): expressed in \(m^3/yr\)). The \(x\)-axis is the alongshore coordinate, with the positive transport direction coinciding with the positive \(x\)-direction. The increasing transport from A to B (difference \(V\)) causes the erosion problem in stretch A-B (along A-B \(dS/dx \ne 0\)). Such a situation may occur for a straight coastline that is subject to waves that increase in height in the positive \(x\)-direction along the coast.
Assume that one wishes to protect only stretch A-B of the coast, for instance because important investments have been made in section A-B, which are at stake due to structural erosion. The erosion can be stopped by ensuring that the existing sediment trans- port distribution line \(a\) in Fig. 10.7 be changed to line \(b\) (along section A-B \(dS/dx = 0\) in that case). At least in section A-B the erosion would stop if distribution \(b\) could be achieved. In the left-hand section from A the erosion will just continue; the sediment transports have not changed. In the right-hand section from B the existing erosion continues; at an even higher rate since the throughput of sediment through the cross-section in point B has been reduced, yielding steep gradients in the longshore sediment transport distribution. At the right-hand side of B consequently lee-side erosion occurs.
The formulation of the requirements of the sediment transport distribution in section A-B is rather simple; however, it is rather difficult to acquire curve \(b\). Structures that, in principle, interfere with the process of longshore sediment transport rates can be used. Series of groynes or series of breakwaters with a crest either above (emerged) or below sea level (submerged) will undoubtedly affect the existing longshore sediment transport, but it is very difficult to ensure that these countermeasures are appropriate for specific cases. If the effectiveness of the countermeasure does not come up to expectations, the erosion will be reduced but not entirely stopped. If the countermeasure is too effective in reducing the sediment transport, or in other words if the reduction of the sediment transport in stretch A-B is too great (see line \(c\) in Fig. 10.7), accretion in stretch A-B will be unavoidable. This might be beneficial for section A-B (accretion instead of the desired stabilization), but the lee-side erosion in the section at the right-hand side of B will grow worse. In fact, achieving line \(c\) in Fig. 10.7 represents an ‘over-kill’ operation.
Even if the countermeasures in section A-B are well tuned, lee-side erosion is usually unavoidable. In fact, the solution of the erosion problem in section A-B with the help of structures that decrease the sediment transport along A-B always comes at the expense of the stretch of coast beyond B; the problem has simply been shifted. If the extra lee-side erosion beyond B would become unacceptable, countermeasures in this section are called for as well. The lee-side erosion is then shifted further down the coast. Only if there is an accreting stretch of coast beyond B, or if position B represents the very end of a stretch of coast (e.g. a tidal inlet beyond B), lee-side erosion is less obvious.
Alternatively, artificial nourishment could be selected as countermeasure in section A-B. The erosion problem is not permanently solved by artificial nourishment; the nourishment does not reduce the sediment transport involved, but eliminates the effects of the erosion instead. Since the erosion continues, the nourishment has to be repeated on a regular basis. In the example of Fig. 10.7, a volume \(V\) (in \(m^3/yr\)) must be nourished. It is not practical or economical to nourish every year, so usually a nourishment lifetime of 5 to 10 years is chosen.
At first sight, the limited lifetime of a nourishment may seem to be a drawback. In many cases, however, artificial nourishment is more cost-effective than solutions with structures. This is certainly the case, when the present-day value of the measures is calculated (the cost of future nourishment contributes little to the present-day cost). An additional advantage of artificial nourishment is that extra lee-side erosion does not take place.