Large-scale laboratory observations of beach morphodynamics and turbulence beneath shoaling and breaking waves

Abstract

In 2012, large-scale laboratory experiments were carried out in the Deltagoot in the framework of the Hydralab IV-funded BARDEXII project. The overall project aims were to examine the effect of swash/groundwater interactions to sand transport and morphological development in the swash zone and, additionally, to investigate the sediment exchange between the swash and surf zone. Work package 5 in the BARDEXII project focused on this second aim and, therefore, paid particular attention to the effect of surface-generated turbulence on sand suspension and on the direction and magnitude of the subsequent sand transport. We know from earlier, predominantly small-scale laboratory experiments that breaking-induced surface-generated eddies can penetrate through the water column to hit the sea bed and suspend sand. The inherently intermittent nature of this turbulence and hence suspension is strikingly at odd with sand transport equations that assume sand transport to be related to the orbital flow to some power. Although attempts to include breaking-induced turbulence into sand-transport equations have been performed to improve the modeling of swash-surf sand exchange, our quantitative understanding of the effects of breaking-induced turbulence on sand suspension is limited because of a lack of simultaneous and accurate measurements of turbulence and sand suspension under breaking waves. This paper will describe the experimental design for this work package and present preliminary results on (1) the bathymetric evolution during the series of tests and (2) the vertical turbulence structure beneath shoaling and breaking waves.