ABSTRACT

As the velocity of the to-and-fro water motion near the bottom under oscillatory waves is increased, there comes a stage when the water exerts a stress on the particles sufficient to cause them to move. This study reviews the analyses and available data on this threshold of sediment motion under wave action.

For grain diameters less than about 0.05 cm (medium sands and finer) the threshold is reached while the flow in the boundary layer is still laminar and the threshold is best related by the equation

u_m²/(_s-) gD = 0.30 (d_o/D),

where u_m and d_o are the near-bottom velocity and orbital diameter of the wave motion, is the density of water, and _s and D are respectively the density and diameter of the sediment grains. This relationship is modified after an empirical equation deduced by Bagnold but has a theoretical basis.

For grain diameters greater than 0.05 cm (coarse sands and coarser) the threshold occurs after the boundary layer has become turbulent and is best predicted with an empirical curve relating d_o/D to u_m/(_s - ) g T where T is the wave period. This latter dimensionless number represents the ratio of the acceleration forces to the effective gravity force acting on the grains.