Crown walls or superstructures on top of permeable breakwaters are often used as a measure in existing structures to counteract insufficient design protection against overtopping. These crest elements, generally located well above the design water level, have been found to effectively decrease the mean overtopping discharge over a dike or rubble mound structure. The current study focusses at investigating the reduction in overtopping which can be expected from crest elements on a permeable breakwater.

The aim of the study was to quantify the effects of the wave period on dune erosion. Attention was focussed on 2D cross-shore effects in a situation with sandy dunes and extreme wave conditions. Large-scale physical model tests were set up to provide the necessary data for the study on a scale as close to prototype as possible. It was concluded that a longer wave period leads to a larger dune erosion volume and to a larger landward retreat of the dune face.