Observations of storm and river flood-driven sediment transport on the northern California continental shelf

In the winter of 1996-1997, three bottom-boundary layer tripods were placed in an alongshelf array on the northern California continental shelf off Eu reka, CA in 60-65-m water depth. During the observation period, multiple st orms and river discharge events occurred, as well as the largest hood on re cord since 1964. Suspended-sediment concentration at all three sites fluctu ated in response to both wave resuspension and advection of river-derived s ediments. However, considerable spatial differences in low-frequency curren ts and suspended-sediment concentration were observed at the three sites. S ediment Aux vectors calculated during periods of high suspended-sediment co ncentration suggest a convergence of sediment flux coincident with the cent er of recent flood deposits. Suspended-sediment concentrations observed at the two northern tripod sites following the large flood reached magnitudes typical of fluid mud (> 10 g/l) in a thin near-bed layer. The net sediment flux during the single three-day event was two orders of magnitude larger t han any other event during the winter, and accounted for seven times the fl ux observed over an entire year (1995-1996), A conceptual model for the adv ection of sediment to the mid shelf is proposed in which river plume sedime nts are trapped on the inner shelf either due to a weak front or the rapid input of sediment from a confined plume, and form a thin layer of fluid mud . The fluid mud subsequently is transported seaward due to gravitational fo rcing. The measured sediment concentration and velocity profiles on the she lf provide strong evidence to support this conclusion. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.