Near-Bed Sediment Transport Processes During Onshore Bar Migration in Large-Scale Experiments: Comparison With Offshore Bar Migration

Abstract

Detailed information on nearshore sediment transport processes during onshore bar migration were obtained from large-scale laboratory experiments with bichromatic wave groups on a relatively steep initial beach slope (1:15). Detailed measurements of velocity and sand concentration near the bed from shoaling up to the outer breaking zone including suspended sediment and sheet flow transport are presented. The analysis focuses on onshore migration under an accretive wave condition but comparison to an erosive condition highlights important differences. Decomposition shows that total transport mainly results from a balance of short wave-related, bedload onshore transport and current-related, suspended offshore transport. When comparing the accretive to the more energetic erosive condition, the balance shifts toward onshore transport, and onshore migration, because the short wave-related transport does not decrease as much as the current-related transport. This is related to the effects of skewness and asymmetry combined with less sediment suspension in the water column and undertow magnitude under the accretive condition. Transports from streaming in the wave boundary layer and from infragravity waves become visible but only play a subordinate role. Identified priorities for numerical model development include parametrization of wave nonlinearity effects and better description of wave breaking and its influences on sediment suspension.