SEDIMENT RESUSPENSION AND BED ARMORING DURING HIGH BOTTOM STRESS EVENTS ON THE NORTHERN CALIFORNIA INNER CONTINENTAL-SHELF - MEASUREMENTS AND PREDICTIONS

Geoprobe bottom tripods were deployed during the winter of 1990-1991 o n the northern California inner continental shelf as part of the STRES S field experiment. Transmissometer measurements of light beam attenua tion were made at two levels and current velocity was measured at four levels in the bottom 1.2 m of water. Intervals of high measured botto m wave velocity were generally correlated with times of both high atte nuation and high attenuation gradient in the bottom meter of the water column. Measured time series of light attenuation and attenuation gra dient are compared to values computed using a modified version of the SMITH [(1977) The sea, Vol. 6, Wiley-Interscience, New York, pp. 539-5 771 steady wave-current bottom-boundary-layer model. Size-dependent tr ansmissometer calibrations, which show significantly enhanced attenuat ion with decreasing grain size, are used to convert calculated suspend ed sediment concentration to light attenuation. The finest fractions o f the bed, which are the most easily suspended and attenuate the most light, dominate the computed attenuation signal although they comprise only about 5-7% of the bed sediment. The calculations indicate that a djusting the value of the coefficient gamma0 in the expression for nea r-bed sediment concentration cannot in itself give both the correct ma gnitudes of light attenuation and attenuation gradient. To supply the volumes of fine sediment computed to be in suspension during peak even ts, even with values of gamma0 as low as 5 x 10(-5), requires suspensi on of particles from unreasonably large depths in the bed. A limit on the depth of sediment availability is proposed as a correction to susp ended sediment calculations. With such a limit, reasonable attenuation values are computed with gamma0 almost-equal-to 0.002. The effects of limiting availability and employing a higher gamma0 are to reduce the volume of the finest sediment in suspension and to increase the suspe nded volumes of the coarser fractions. As a consequence, the average s ize and settling velocity of suspended sediment increases as bottom sh ear stress increases, with accompanying increases in near-bed concentr ation gradients. Higher concentration gradients produce larger stratif ication effects, particularly near the top of the wave boundary layer at times when wave shear velocities are high and current shear velocit ies are low. These are the conditions under which maximum attenuation gradients are observed.