Bottom-boundary-layer processes associated with fine sediment accumulation in coastal seas and bays

Abstract

Bottom-boundary-layer velocity profiles, bed stresses and suspended sediment concentration profiles were measured with instrumented tripods in four contrasting shelf and semi-enclosed bay environments that are presently accumulating fine sediments. The sites were: the northern California shelf off the mouth of the Eel River; Eckernförde Bay, southern Baltic Sea; the York River estuary, lower Chesapeake Bay; and the Louisiana shelf to the west of the Mississippi River mouth. Bed micromorphology was often biogenic and generally smooth. Suspended sediment concentration within the mean current boundary layer was attributable to local resuspension from underlying, very easily eroded pools and to the horizontal advection of near bottom turbid layers (benthic plumes). The presence of density stratification caused simple fits of log profiles to velocity observations over the lowest meter to overestimate bottom stress. Observed semi-log velocity profiles were (1) concave downward, (2) straight, or (3) concave upward depending on whether the density anomaly decreased with height above the bed (1) much more slowly than, (2) at roughly the same rate as, or (3) much more rapidly than z −1. These patterns were shown to be consistent with analytical relations between velocity and density stratification within the near-bed constant stress layer. Stable stratification was attributable to a combination of locally resuspended sediment and thermohaline effects, with the former and latter dominating under high- and low-energy conditions, respectively. Near-bed thermohaline stratification was observed to increase z oc , the elevation of the zero intercept of the logarithmic velocity profile. The effects of sediment-induced stratification on z oc were less clear. At all sites the near-bed gradient Richardson number ( Ri) approached or exceeded the critical value of 1 4 implying that turbulence was damped by stable stratification. Results point to the likelihood that stable near-bed stratification, which is often associated with fine sediment accumulation, plays a role in accelerating accumulation by reducing near-bed turbulence. Results further indicate that the benthic sediment plumes and/or mud pools which result in Ri⩾ 1 4 cannot be maintained simply by vertical suspension within the current boundary layer. Rather, horizontal advection, flux convergence and/or high concentration sediment suspension within the wave boundary layer is also required.