Across-shelf variations in bed roughness, bed stress and sediment suspension on the northern California shelf

Field measurements of bed micromorphology (roughness), benthic flow, bed stress, and suspended-sediment flux were made on the northern California continental shelf in connection with the STRATAFORM program. A sediment profiling camera and side-scan sonar were used to observe bed features in December 1995. Biogenic roughness with similar to 2 cm relief prevailed at both sites. Bottom-boundary-layer tripods were deployed on the 60-m and 70-m isobaths over the period 5 January to 7 March 1996, during which time two high-energy events occurred. Skin-friction shear stresses were subcritical under 'average' conditions but appreciably exceeded the threshold for sediment suspension during storms. For those high-energy events, near-bed suspended-sediment concentrations reached 2 g/l at 60 m (15 cm above bed) and lg/l at 70 m (27 cm above bed) and suspended-sediment-induced stratification significantly affected bed stress estimates. At those times, increases in current shear were accompanied by increases in suspended-sediment-concentration gradients, causing the gradient Richardson number within the log layer to remain near the critical value of 1/4. This suggests suppression of turbulence by sediment-induced stratification. The abundance of under-consolidated fine sediment on the shelf to the north of the Eel River mouth presumably allows increases in stress to be accompanied by progressive increases in suspended-sediment concentration within the log layer to maintain the critical Richardson number. This contrasts to situations where bed armoring limits the total amount of fine sediment in suspension. Applications of a wave-current boundary-layer model, with stratification effects included, reduces estimates of current friction velocities by (u(*c)) about 24% relative to results from fitting the von Karman-Prantdl equation without stratification. The model suggests that the total wave-current friction velocity (u(*cw)) reached 3.0 cm/s at 70 m and 3.5 cm/s at 60 m. Depth-integrated across-shelf suspended-sediment fluxes were seaward at the 60-m isobath and near zero to weakly landward at the 70-m isobath during high-energy periods, implying flux convergence. This is consistent with the conclusions of other STRATAFORM investigators that rapid long-term accumulation of mud is occurring on the mid-shelf. (C) 1999 Elsevier Science B.V. All rights reserved.