Velocity Parameters for Predicting Cross-Shore Sediment Movement

Abstract

This paper discusses a method to estimate the tendency of sediment to move onshore or offshore under the influence of waves in shallow water. Motivation for this study is provided by the need for conceptually simple models to help explain the movement of noncohesive sediments under waves, especially cross-shore transport. Two dimensionless velocity parameters are developed to predict the tendency of sediment to move onshore under wave crests or offshore under troughs. The parameters are the ratio of the instantaneous, maximum near-bottom velocity under a wave crest or trough to the critical velocity for the initiation of sediment movement. Near-bottom velocities are estimated using stream function wave theory. One parameter shows considerable skill in predicting net cross-shore sediment movement for a large number of observations of laboratory and field beaches. Taken together, the two parameters provide a qualitative explanation for observed movement of a submerged gravel mound. Previous studies indicate that wave asymmetry can cause net onshore sediment movement and undertow can cause net offshore movement in the surf zone. The utility of these parameters also is examined for mounds composed of dredged material.