Sediment eddy diffusivity and selective suspension under waves and currents on the inner shelf

Abstract

Sediment diffusivity and effective settling velocity, we, (or equivalently grain size) of near-bed suspended sand was inferred from observed concentration profiles. Concentration data were obtained at 20-m depth off Dounreay, Scotland, and at 13-m depth off Duck, North Carolina, USA. These data accommodate different dynamic conditions (from wave-dominated at Dounreay to wind-driven current-dominated at Duck) and different sediment properties (median size of bed sediment ranging from 120 to 290 μm). Regression of observed concentration profiles using the Rouse-type diffusion equation yielded the Rouse parameter P=we/κu*, where κ is von Karman’s constant, u* is the characteristic shear velocity, and we is the effective settling velocity. Linearly increasing eddy diffusivity extended several times higher than the classical wave boundary layer and was not observed to strongly change slope at the top of the wave boundary layer. Instead, u* within several 10s of cm of the bed was nearly equal to wave-current shear velocity (u*cw) or current shear velocity (u*c) depending on whether conditions were wave- or current-dominated. For the Rouse parameter, it was found that u* ≈ u*c for u*c > ws and u* ≈ u*cw for u*c < ws, where ws is the median settling velocity of the bed sediment. The effective settling velocity in suspension (we) was, in turn, evaluated from P using independent estimates of u*cw and u*c. The inferred size of suspended sediment varied in response to forcing conditions. For an inverse Rouse number u*sf/ws near one, where u*sf is the skin-friction shear velocity, the representative grain size of suspended sediment approached the median size of bed sediment. A similar trend was seen for u*cw/ws or u*c/ws near two, depending on whether conditions were wave or current dominated. With increasing (or decreasing) u*/ws, the grain size of suspended sediment also increased (or decreased), suggesting selective suspension of sediment particles as a function of flow strength.