Evolution of alongshore bathymetric variability around a mega-scale beach nourishment

The presence of complex nearshore sand bar patterns (i.e. alongshore bathymetric variability) has an impact on local currents, affecting recreational safety and nearshore mixing processes. This study assesses the evolution of alongshore bathymetric variability along the Delfland coast in The Netherlands, over the first 5 years after construction of a mega-scale beach nourishment (the Sand Motor) in the central part of the coastal cell. A total of 38 bathymetric surveys was conducted over this period. Alongshore variability was quantified by subtracting an alongshore averaged bathymetry from the actual surveyed bed levels for both the intertidal and subtidal zone. From 2 years after construction onwards, the subtidal nearshore bathymetry at the Sand Motor is considerably more alongshore variable than the adjacent parts of the Delfland coast. Intertidal variability tends to be high in areas where beach groynes are present.