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 st ress, and suspended-sediment flux were made on the northern California cont inental shelf in connection with the STRATAFORM program. A sediment profili ng 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 sit es. Bottom-boundary-layer tripods were deployed on the 60-m and 70-m isobat hs over the period 5 January to 7 March 1996, during which time two high-en ergy events occurred. Skin-friction shear stresses were subcritical under ' average' conditions but appreciably exceeded the threshold for sediment sus pension during storms. For those high-energy events, near-bed suspended-sed iment 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 signific antly affected bed stress estimates. At those times, increases in current s hear were accompanied by increases in suspended-sediment-concentration grad ients, causing the gradient Richardson number within the log layer to remai n near the critical value of 1/4. This suggests suppression of turbulence b y sediment-induced stratification. The abundance of under-consolidated fine sediment on the shelf to the north of the Eel River mouth presumably allow s increases in stress to be accompanied by progressive increases in suspend ed-sediment concentration within the log layer to maintain the critical Ric hardson 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 estima tes of current friction velocities by (u(*c)) about 24% relative to results from fitting the von Karman-Prantdl equation without stratification. The m odel suggests that the total wave-current friction velocity (u(*cw)) reache d 3.0 cm/s at 70 m and 3.5 cm/s at 60 m. Depth-integrated across-shelf susp ended-sediment fluxes were seaward at the 60-m isobath and near zero to wea kly landward at the 70-m isobath during high-energy periods, implying flux convergence. This is consistent with the conclusions of other STRATAFORM in vestigators that rapid long-term accumulation of mud is occurring on the mi d-shelf. (C) 1999 Elsevier Science B.V. All rights reserved.