Impact of sand nourishments on hydrodynamics and swimmer safety

Abstract

Artificial sand nourishments are a common measure to mitigate coastal erosion problems. Such nourishments can have an impact on currents and waves near the beach, especially when the nourishment has a large size. As nourished beaches often have a recreational function, these altered wave and current patterns may pose a threat to swimmers. This study has investigated the impact of nourishments on currents, waves and swimmer safety. De Sand Motor, an experimental large-scale nourishment at the Dutch coastline south of The Hague, served as a central case study. Using a combination of current measurements at sea and computer models, this study has revealed several interesting flow patterns around the Sand Motor, amongst others the presence of large eddies in the tidal flow. To determine the impact of such flow patterns on swimmer safety, the presence and spatial spreading of beach users at the Sand Motor was monitored with a set of cameras. Although the tidal eddies have a clear influence on currents and sand transport around the Sand Motor, their impact on swimmer safety remains limited. At the part of the Sand Motor where hazardous currents due to tidal eddies may occur, hardly any beach users are present due to the large distance from beach entrances, parking lots and restaurants. The most significant hazard is formed by tidal currents in the artificial lagoon, which has been incorporated in the initial design of the Sand Motor. Especially in the first years after construction of the nourishment, currents in the channel connecting the lagoon to the North Sea were quite strong, while that part of the Sand Motor can be crowded on nice summer days. The findings of this study enable engineers to incorporate swimmer safety considerations in the design of future nourishments. Furthermore, more fundamental insights into waves and currents around the Sand Motor contribute to the understanding of sediment transport, coastal erosion and eventually prevention of coastal flooding.