Incipient motion of coarse particles on a slope by regular or irregular waves

The initiation of motion of coarse particles in the region of wave breaking is studied. The nondimensional sediment and wave characteristics that are significant in this phenomenon are determined. Experiments are performed with regular and irregular waves to investigate the initiation of motion of gravel on a straight slope. Effects of variables such as wave height and period, water depth, particle size, and beach slope on the incipient depth of motion are clarified. The experimental results are used to obtain regression equations to predict the incipient depth of motion for the cases of regular and irregular waves. These equations relate the nondimensional threshold motion depth to the stability number, surf similarity parameter, and beach slope. The formulas are compared with the results of an earlier study. The discrepancies are explained as being caused by the longer duration of the earlier experiments where the purpose was to determine the equilibrium depth of the coastal slope.