Initial motion of sediment under waves and wave-current combined motions

Recently, one of the authors proposed a very convenient formula to evaluate the friction coefficient induced by wave-current combined motion, which spans all flow regimes including the transitional regime and requires no iteration. This formula is applied to an analysis of existing experimental data regarding initial motion of sediment particles under pure waves and wave-current combined motion. It was found that most of the data analyzed here were located in the transitional regime. Thus, the full-range friction coefficient formula can be conveniently applied as compared with previous implicit formulae and graphical friction laws. The formula of general motion under pure wave motion can be modified to compute the critical water depth of sediment movement. Some discontinuities in the former work in the transitional regime now can be overcome in the present study.