Coastal flooding initiated by wave overtopping at sea defences

Analysis of coastal flooding events in The Netherlands and Germany shows that most dike breaching events were initiated by failure of the inner slopes of dikes. These failures occurred due to wave overtopping which resulted in slip failure of the inner slope or erosion at the inner slope and crest, or, most likely, a combination of both. In particular, saturation of the dike lowers the resistance to slip failure. Therefore, it is believed that this resistance is strongly affected by wave overtopping. Wave overtopping discharges depend, of course, strongly on the wave conditions that occur at the toe of the dike. Most existing prediction formulas for wave overtopping were derived from physical model tests in which the wave energy is distributed over the wave frequencies following a standard shape, i.e., single-peaked wave energy spectra with a constant shape. However, in practice the foreshore often affects these wave energy spectra such that they become double or multi-peaked. To study how this important effect can be accounted for in prediction formulas for mean wave overtopping discharges, studies presented in this paper were carried out. The applied methods, which include numerical and physical modelling, are similar to those used in Van Gent (2001-b), in which this topic is studied for wave run-up. It was found that the effects of wave energy spectra on wave overtopping can be taken into account by using one particular characteristic wave period in the wave overtopping predictions.