Interannual variability of nearbed sediment flux and associated physical processes on the Eel River shelf, northern California, United States.

摘要

A five-year long data set of nearbed current and suspended-sediment concentration are analyzed to evaluate the interannual variability of physical processes and sediment transport events on the Eel River shelf, northern California. This data set encompasses a wide range of shelf conditions due to the occurrence of major floods (1996/97), strong El Nino (1997/98), strong La Nina (1998/99), and major storm (1999/00) events. Data were collected within 2m of the seabed in 60-m water depth, at a site located 25km north of the Eel River mouth. During the winter months sediment resuspension is forced primarily by nearbed oscillatory flows, and sediment transport occurs both as suspended load and as fluid mud. Over the five-year period sediment flux past the study site is almost zero in the alongshelf direction, being primarily driven by winds and very low frequency oscillations; in the across-shelf direction, sediment flux is always directed offshore and driven by annual variations in the across-shelf currents.; Yearly comparisons of winter physical conditions and sediment transport shows that: periods of increased sediment transport averaged 140d, ranging between 67 (1996/97) and 170d (1997/98, 1998/99); hourly averaged values of significant wave height varied between 0.5 and 10.67m; hourly averaged values of nearbed orbital velocities ranged between zero and 124 cms-1 ; over the five-year period, sediment threshold conditions were exceeded 35% of the time by winter waves, ranging between 19% (1996/97) and 50% (1998/99); mean concentration of suspended sediment ranged from close to zero to values above 4gl-1; each winter major flux events exhibit different patterns due to combinations of physical processes as river floods, waves and shelf circulation.; On the Eel River shelf, the mid-shelf mud deposit is apparently emplaced by secondary processes such as offshelf transport of sediment suspensions and fluid mud, not directly from river plumes traversing the shelf. Thus, these inner shelf processes redistribute sediment supplied by the Eel River (a point source) making the inner shelf a line source of sediment that nourishes the mid-shelf. Large scale shelf circulation patterns are also important in determining the locus of the mid-shelf deposit, being influenced by interannual climate variations.