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Sea-dikes differ from revetments in the sense that a beach in front of the structure is absent (see Fig. 10.28). Just like dikes along e.g. a river, sea-dikes are often meant to prevent flooding.
The design crest height of a sea-dike has to be carefully chosen in order to prevent too much overtopping. In the Netherlands the so-called Hondsbossche and Pettemer sea defence is a good example (see Fig. 10.29). Because of the flat slope the wave run-up under the Dutch design conditions reaches rather high levels (about 8 m above design storm surge level). New (preliminary) insights in likely wave characteristics during design conditions showed that the crest level of the existing sea defence was too low. As a ‘no-regret’ measure for the time being the sea defence is partly supplemented with an additional sheet piling (see Fig. 10.30). Note that the additional measures only prevent wave overtopping; the sheet piling cannot withstand the hydraulic pressures in the case of high surge levels.
An additional point of concern is the position of the (sandy) bottom just in front of the sea defence. The wave run-up depends on the local wave height at the toe of the sea defence that is limited by the local depth. Therefore, when during design conditions erosion of the sandy bottom just in front of the sea defence is expected, the water depth and hence the characteristic wave height might be higher than expected.