Laboratory investigations into wave period effects on sand bed erodibility, under the combined action of waves and currents

Abstract

Laboratory experimental data on sediment threshold conditions for fine to coarse sands, under the combined action of waves and currents are presented. Higher flows are needed to satisfy threshold under short period waves than for longer period waves. The combined threshold velocities indicate that under the shorter (5 s) period waves, the oscillatory and steady flows are independent of each other, but interact to a greater extent as the wave period increases. The derived threshold flow conditions are converted into threshold shear stress values using existing linear and non-linear models. The performance of the models for various periods of oscillation is then examined in view of the assumption that a unique threshold value exists for a particular grain size. All models suggest that the critical stress of a particular grain is higher under shorter period waves, for the range of grain sizes presented. Using different models for waves, of different periods, appears to remove the effect of wave period upon the data. It is suggested that theories used by the existing models are insufficient to fully describe the interaction of the steady and oscillatory flows during the experiments.