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Preview of the article Predicting morphological changes in rivers, estuaries and coasts - Executive Summary, published by the FLOODSite Consortium in partnership with the University of Plymouth. March 2009

Lead Author: Reeve Dominic
Report: T05-07-03. March 2009.

Flooding occurs when there is a failure of a defence. This can arise from a functional failure (the conditions exceed those for which the defence was designed) or a structural failure (where some element or components of the defence do not perform as intended under the design conditions). The former arise from society’s need  to find a compromise between the cost of the defence and the consequences of a flood. Structural failures are generally more dangerous, as they are unexpected, and have been the source of recent notable flooding events. Two major causes of structural failure are: low freeboard leading to excessive wave overtopping of the defence leading to erosion of the back and crest of the defence, or even damage to the armour layers; and toe failure, where erosion of the foreshore at the base of the defence occurs to such an extent that the structure is undermined and collapses.

In coastal areas changes in beach morphology that can affect the propagation of waves up to a sea wall, and run-up and overwash of dunes. The assessment and design of sea defences, (including dune systems), often relies on a qualitative review of the morphological variability of the bed levels and slopes in front of the sea defence. During large storms it is not uncommon for beach levels to lower by several metres or more, with beach material being moved tens or hundreds of metres offshore. Such beach lowering will lead to an increase in the still water depth at a structure for a fixed tide level, and thus enable larger waves to reach the structure before breaking. This is a positive feedback mechanism, with beach lowering allowing larger waves to reach the structure. In turn larger waves create more lowering and the process continues. Storm surge is also important in determining water depth and depth-limited wave breaking in front of the structure. Wave set-up serves to amplify the effect. Furthermore, equations used to predict suspended load and bedload sediment transport under breaking waves are only approximate.

Dunes are particularly vulnerable to erosion, and if there are fixed assets (such as sites of special scientific interest) being defended by a dune system, active management may be required. Figure 1.2 shows an example of a dune system which has been subject to recent erosion due to storm waves during a high tide and surge event. A reliable means of predicting the rate of dune erosion under wave attack would be a valuable tool for coastal managers, as would a means of quantifying the amount of overwash to expect should the dunes be breached. Both these questions are addressed in the following sections.