Infragravity Driven Suspended Sediment Transport in the Swash, Inner and Outer-Surf Zone

Abstract

Nearbed suspended sediment concentration and velocity measurements from 3 field sites in the mid-outer surf zone (low energy), inner surf zone (high energy) and swash (high energy) are discussed. Calculation of net sediment particle flux at a single elevation is decomposed into steady and fluctuating components. The cospectrum of concentration and velocity is used to calculate the direction and magnitude of the fluctuating component of sediment flux as a function of frequency. Preliminary results indicate that the incident band contribution to the net flux of sediment is primarily directed onshore. The infragravity band contribution, however, can have both offshore and onshore directed fluxes depending on the relative phase between the velocity and sediment concentration. The net sediment flux due to the fluctuating components, the integral of the co-spectrum over all frequencies, is of the same order as the contribution due to the drift, or mean, velocities. In the swash and inner surf zone, particle flux due to the steady and the fluctuating components may act in opposite directions, and imply that net sediment transport in the surf zone is the result of several competing processes of comparable magnitude whose direction of transport can change with both cross-shore position and the overall energetics of the nearshore.