Influence of cnoidal wave-induced seepage force on shields number in shallow water

Abstract

Sediment incipient motion in shallow water is a major concern for coastal engineers. Wave-induced seepage force has reportedly had an important impact on the sediment incipience. The effect of wave asymmetry and skewness is important for defining nearshore wave motions and the resulting sediment movement, but it has been discounted in most previous studies considering seepage. Using analytical and numerical models, this study examines the influence of two-dimensional nonlinear wave-induced seepage forces on sediment incipient motions in shallow water. A new modified Shields number was derived, which better addresses the problem of seepage. The results indicate that seepage force could stabilize the sand particles by lowering the maximum Shields number by up to 38.0%; in the linear wave cases, the opposite is true. This discrepancy is subject to the nonlinearity and asymmetry of shallow wave motions, as well as the decreasing “suction” force and increasing “injection” force. Besides, disregarding horizontal seepage would greatly underestimate the contribution of seepage to sediment incipient motions, especially in situations with steeper waves or an unsaturated and less permeable seabed. A lower Young's modulus, larger saturation, permeability, and water depth would all considerably decrease the maximum Shields number.