Bed stress estimation in the swash zone of a proto-type laboratory beach

A proto-type laboratory experiment examining swash dynamics on a coarse sandy beach was carried out in a large-scale wave flume. This paper addresses boundary layer dynamics using near-bed velocity measurements collected in the swash zone under irregular wave conditions. Bed shear stress estimates were obtained from the velocity measurements using two models: (1) the logarithmic model and (2) the momentum integral model. The logarithmic model provides the most reliable bed shear stress estimates. In general terms, the momentum integral model underestimates bed shear stress during the backwash and performs poorly during the uprush when positive (onshore directed) stresses are hardly observed even though the velocity is onshore. Numerical results suggest that this drawback of the momentum integral method results is likely to be the consequence of the omission of the upper part of the boundary layer thickness in the momentum integration.