Initial Motion of Sediment in Oscillatory Flow

A new model is developed to study the initial motion of sediment in oscillatory flow over a flat bottom on the basis of available experimental data. It is found that the initial motion of sediment in oscillatory flow is uniquely defined by the simple model of A/d=KA2ω/ν+B, where A= semiexcursion of wave orbital motion near the bed; ω= angular frequency; d= grain diameter; ν= kinematic viscosity; and K and B= dimensionless coefficients determined by the immersed sediment weight. For a given sediment, the onset velocity derived from the model is found to initially increase with the wave period T and then approach a constant. A practical formula is also presented to calculate the onset velocity of sediment in the coastal zone.