Incipient motion of sediments in shallow water waves

Modelling of coastal sediment transport processes in laboratories requires appropriate scaling of the sediments. The current practice in the U.K. is to scale the model sediments according to three criteria, i.e., the threshold of motion, the percolation slope and the onshore/offshore criterion. The equation defining the threshold of motion was derived based on experimental results under sinusoidal oscillatory flow conditions. In coastal waters, waves arc highly non-linear and asymmetric, thus the threshold equation based on sinusoidal flows may be an inaccurate representation of the real situation. This paper describes an experimental investigation into the threshold of the motion of natural sediments under asymmetrical flow conditions. It will be shown that the threshold of motion equation currently in use is not appropriate under coastal wave conditions. A new empirical equation will be presented.