Threshold Effects on Sediment Transport by Waves

Abstract

An analytical technique is developed for correcting the time‐averaged net sediment transport rate under oscillatory flows for the effects of the cessation of transport whenever the magnitude of the near‐bottom velocity is less than some threshold value. This estimation scheme eliminates the analytical complexity of incorporating the threshold effect directly into the transport model. The threshold speeds are calculated using an empirical formulation. Using a simple model of longshore transport across the surf zone and employing some recent findings on velocity distributions, the magnitudes of threshold effects are explored for both broadband and monochromatic waves. An explanation for the observed difference in longshore transport efficiency for laboratory and field observations is suggested. A simple model for cross‐shore transport is used to explore the effects of threshold of motion on this transport mechanism. [Graphical presentations are used to show under what combinations of wave height, wave period, and sand size threshold effects are significant for various transport regimes.