Nearshore Topography and Bather Risk

Abstract

This document describes a study into the influence of alongshore bathymetric variability on currents which form a risk to bathers. Changes to coastal zone management along the Delfland Coast have given rise to questions on the influence of alongshore bathymetric variability on bather risk. Extensive measurement data is available from two measurement projects, mapping the entire Delfland Coast. This data has been used to study the presence and development of alongshore bathymetric variability along the Delfland Coast. Subtidal variability was found to generally increase during the winter months, in accordance with literature. In the intertidal area variability was found to decrease in the winter months. Both intertidal and subtidal variability were found to be relatively low near the breakwater at Hoek van Holland. The range in measured variability along the Sand Motor was found to be very wide. Using a parameterization of the alongshore variability, subsections of the Delfland Coast have been selected for a modelling study. Artificial bathymetries have been created, using the selected measurement data. These artificial bathymetries have been used as the basis of numerical models. These have been forced with linearized wave conditions, in order to investigate the relation between different levels of alongshore bathymetric variability and wave induced currents which might form a risk to bathers. Good correlations were found between the alongshore variability and the wave induced currents, especially for the intertidal area. Alongshore bathymetric variability seems to be an important factor in generating wave induced currents which might form a risk to bathers. In order to be able to fully assess the influence of the bathymetry on such nearshore currents, the influence of the depth of the variable bar patterns, the distance between the subtidal bar and the intertidal bar and the phase differences between the subtidal and the intertidal bar need to be investigated further.