# Hard structures and structural erosion

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The basis of this article is especially written for the Coastal Wiki by the main author referred to at the bottom of this page.

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.