SEDIMENT RESUSPENSION AND BED ARMORING DURING HIGH BOTTOM STRESS EVENTS ON THE NORTHERN CALIFORNIA INNER CONTINENTAL-SHELF - MEASUREMENTS AND PREDICTIONS
Pl. Wiberg et al., SEDIMENT RESUSPENSION AND BED ARMORING DURING HIGH BOTTOM STRESS EVENTS ON THE NORTHERN CALIFORNIA INNER CONTINENTAL-SHELF - MEASUREMENTS AND PREDICTIONS, Continental shelf research, 14(10-11), 1994, pp. 1191-1219
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.