# Hard structures and structural erosion

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This article describes the possible impacts of hard structures on the structural erosion of a stretch of coast.

## Description of the situation

Figure 1 - Sketch Sediment transport distribution

Consider a stretch of coast of several km including a part A - B. The coast shows structural erosion. Year after year the coastline retreats. Stretch A - B is considered as a very important part of the coast; e.g. houses, hotels and infrastructure are present there. Without counter-measures sooner or later these valuable properties will be destroyed because of the continuous erosion.

One likes to resolve the erosion problem in stretch A - B with the help of 'hard' structures. N.B.: Left from A and right from B it is not (yet) necessary to combat the erosion problem.

An extended description of a similar example can be found in Littoral drift in relation to shoreline protection (1983)[1] and an example can be found in the section about Use of structures in coastal protection in d’Angremond and Pluim-Van der Velden (2001)[2].

## Impacts on sediment transport

Figure 1 sketches various distributions of the (longshore) sediment transport along the coast.

• distribution (1): Going from left to right the transport increases, yielding a positive gradient in the longshore transport. [dS(x) / dx = positive.] This gradient in the sediment transport distribution is the reason that gradual erosion occurs in stretch A -B (but also along the other parts of the coasts in the sketch.
• distribution (2): With distribution (2) a 'better' sediment transport distribution (at least 'better' for part A - B) would be achieved. In part A - B the sediment transport is constant [dS(x) / dx = 0]. With distribution (2) no erosion would occur in part A - B anymore. In order to achieve the 'better' distribution, one has to interfere in the occurring transport processes. With the help of 'hard' protection tools (e.g. groynes; detached shore parallel offshore breakwaters) one is indeed able to interfere in the sediment transport processes, but from the sketch it will become clear that tuning of a proper system is quite difficult. If along part A - B indeed distribution (2) is achieved, it is also clear from the sketch that the 'solution' for A - B is at the spent of the part of the coast right from B. (Increased) lee-side erosion has to be expected right from B.
• distribution (3): Distribution (3) reflects a situation where the tuning of the protection system (probably) failed. Even accretion in part A - B occurs, but more lee-side erosion is a consequence.
• distribution (4): This distribution reflects a case where the interference of the 'hard' protection system was not large enough. The erosion rate in stretch A - B has been reduced, but still some erosion will occur in this stretch.