Onset of sediment motion under waves and currents

Abstract

Experiments have been made to study the resultant flow field and bed shear stress produced by the combination of surface waves and steady currents. Comparison has been made between the sediment threshold flow conditions for steady flow, for waves alone, and for combined waves and currents. Experiments, using a laser Doppler anemometer and sediments sieved into narrow size ranges, show that a similar maximum velocity condition, close to the bed, causes onset of motion for all the conditions tested. This maximum velocity can be calculated from the Manning equation, the log velocity law, and second-order wave theory, if it is assumed that the near-bed maximum wave and average current velocity at one roughness height above the bed combine linearly. It is demonstrated that the Shields threshold criterion can be extended to both the wave and combined wave plus current situations. The maximum velocity at approximately one roughness height above the bed is therefore shown to be a good general criterion, which can also be used to estimate the probability of sediment motion. Key words: waves and currents, sediment transport, coastal erosion.