Suspended-solids flux at a shallow-water site in south San Francisco Bay, California

Abstract

Time series measurements of current velocity and suspended solids-concentration (SSC) made during December 1993 and March 1994 at a shallow-water site in South San Francisco Bay were used to estimate and compare suspended-solids flux during the two periods. In December, the average residual flux at the site was 2.88 g/m/s, to the northeast, whereas in March the average residual flux was four times greater, 12.2 g/m/s, and was directed to the southeast, the direction of flood tide. Residual flux was decomposed and the three components that accounted for most of the flux were analyzed: residual advective flux (Stokes drift flux ( u ' h ' c ), and dispersive flux, which is the tidal cycle correlation between velocity and SSC ( u ' h'c ). During both periods, the Stokes drift flux was to the north, and the dispersive flux was to the southeast. In December, these two components, with nearly opposite directions, had the greatest magnitudes, resulting in a lower total residual flux. In March, the residual advective flux was greater than the Stokes drift flux and was in the same direction as the dispersive flux because of a southeasterly residual current. Wind data indicate that the residual current in March was induced by persistent northwest winds. The southeasterly dispersive flux in March and December was due to generally higher SSC on flood than ebb tides. Increases in SSC frequently occurred at low water (before flood tides). Comparison of calculated bottom orbital velocities to SSC identified wind waves as a mechanism of resuspension.