Numerical simulation of swash zone fluid accelerations

A volume-of-fluid Navier-Stokes solver incorporating k-ε closure model, NumErical Water FLUME (NEWFLUME) (Lin and Xu, 2005) is used to investigate the temporal and spatial structure of accelerations in the swash zone of steep beaches for surging, plunging, and nearly spilling waves. Simulations for idealized swash over planar slopes show that shoreward directed local accelerations exist only for up to 22% of the swash cycle depending on wave type. In general, shoreward directed local accelerations were predicted to occur only near the wave run down limit (Baldock and Holmes, 1997). Similarly, convective accelerations had the largest magnitudes during this time and tended to be either near zero or shoreward directed for the remainder of the swash cycle. This finding is in direct contrast to a modified ballistic theory for swash motion arising as a particular solution to the depth-averaged nonlinear shallow water equations (Peregrine and Williams, 2001) that predicts the convective acceleration maintains the sign of the fluid velocity. Near-bed pressure gradients are found to be poorly correlated to both the depth-averaged and near-bed local acceleration. These numerical findings indicate that local fluid accelerations are not a proxy for pressure gradients in the swash zone for enhanced sediment transporting mechanisms or parameterizations. However, the numerical results require corroboration with highly resolved swash zone data.