Analysis of wave reflection at coastal defence structures

The aim of this contribution is to develop a simple model for predicting the reflection coefficient at coastal defence structures. Wave reflection is examined on a database of more than 2’000 data collected on a wide variety of structures, both permeable and impermeable, showing that Postma formula (1989) is limited to a narrow range of the surf similarity parameter. Two approaches are presented for developing a prediction formula for low-crested structures. The first is to derive a reduction coefficient for Postma formula to represent the effect of low crest level; in order to achieve this objective, two ways are proposed, one based on grouping data by crest height and one based on linear wave theory. Both these ways give formulations that are limited by the too many parameters involved in explaining the physical process. The second approach consists of a simple regression model including all the most significant process parameters - wave height and length, water depth, berm width, structure permeability- and allows to describe the reflection coefficient for low-crested structures with an error of about 6%. This model and observations performed on the database allow finally to derive a prediction model for the wave reflection coefficient that is valid from submerged till non-overtopped structures.