The role of wave-induced density-driven fluid mud flows for cross-shelf transport on the Eel River continental shelf

Observations of cross-shelf sediment transport conducted in the winter of 1997–1998 as part of the STRATAFORM program reveal that gravitationally forced density flows of fluid mud trapped within the thin wave bottom boundary layer provide a mechanism for forming flood deposits on the Eel river continental shelf. The data from two moored tripods located on the 20 and 60 m isobaths combined with “rapid response” hydrographic surveys, indicate a process whereby the Eel River delivers sediment on to the inner shelf faster than dispersal and transport processes are able to move it offshore. The river does not deliver sediment beyond the inner shelf because the plume is trapped along the coast due to onshore surface flow associated with downwelling favorable winds. However, the final flood deposition region is located seaward of the 50-m isobath. Acoustic backscattering data taken on the 60-m isobath (in the historic flood deposit region) show two depositional events of 6 and 13 cm during a period of high river discharge and high waves in January of 1998. These depositional events are associated with fluid mud layers that scale in thickness with the wave boundary layer. Velocity profiles from a vertical array of current meters spanning the bottom 2 m of the water column show that the current meter closest to the seafloor has the largest offshore velocity during the depositional events, indicating an offshore flow of the fluid mud from the inner shelf to the flood deposit region. During periods of low concentration suspended sediment transport without fluid mud layers present, either no deposition or erosion was found indicating that the offshore flow of the fluid mud is the dominant depositional mechanism.