Flood dispersal and deposition by near‐bed gravitational sediment flows and oceanographic transport: A numerical modeling study of the Eel River shelf, northern California

Abstract

A large flood of the Eel River, northern California, created a thick sediment deposit between water depths of 50 and 70 m in January 1997. The freshwater plume, however, confined sediment delivery to water depths shallower than 30 m. Mechanisms proposed to explain the apparent cross‐shelf transport include dispersal by oceanographic currents, resuspension by energetic waves, and gravitationally forced transport of a thin layer of fluidized mud. Field observations indicate that these processes were all active but cannot determine their relative significance or whether these mechanisms alone explain the location, size, and timing of deposition. Approximately 30% of the sediment delivered by the Eel River is accounted for in the midshelf mud bed and inner shelf, but the fate of the remaining 70% is uncertain. A three‐dimensional, hydrodynamic model was used to examine potential mechanisms of sediment transport on the Eel River shelf. The model includes suspended sediment transport and was modified to account for a thin, near‐bed layer of fluidized mud. It was used to simulate flood dispersal on the Eel River shelf, to compare the relative importance of transport within the near‐bed fluid mud layer to suspended sediment transport, and to evaluate sediment budgets for floods. Settling properties of fine‐grained sediment, both within the flood plume and the fluid mud layer, critically impact depositional patterns. To a lesser degree, wind‐driven ocean currents influence the volume of sediment that escapes the shelf, and wave magnitude affects the cross‐shelf location of flood deposits. Though dilute suspension accounts for a large fraction of total flux, cross‐shelf transport by gravitational forcing appears necessary to produce a midshelf mud deposit similar in volume, location, and timing to those seen offshore of the Eel River.