WIND STRESS, BED ROUGHNESS AND SEDIMENT SUSPENSION ON THE INNER SHELF DURING AN EXTREME STORM EVENT

Instrumented bottom boundary layer tripods were deployed on the inner shelf at depths of 13 and 8 m off the U.S. Army Corps of Engineers Field Research Facility at Duck, NC, U.S.A., over a 2-week period that included the severe and prolonged ''Halloween Storm'' of late October 1991. The storm persisted for 5 days and generated waves with heights and periods of up to 6 m and 22 s. Although the instrumentation was destroyed, current profile and suspended sediment concentration profile data were recovered from the 13 m site. Mean currents attained speeds of nearly 0.5 m s-1 at 0.29 m above the bed and were directed about 10-degrees offshore from shore-parallel. These strong currents are shown to be wind driven and result in predictions of a wind-drag coefficient, C(a) = 4.7 x 10-3. The currents were recorded simultaneously with root-mean-square (rms) wave orbital velocity amplitudes in the 0.6-1.0 m s-1 range. During the peak of the storm suspended sediment concentrations exceeded 1 kg m-3 throughout the lower 1 m of the water column. Analysis of current profiles, accounting for the presence of waves, is performed to obtain an equivalent bottom roughness, k(n), of approximately 15 times the median sediment diameter, i.e. k(n) congruent-to 15 d50. Analysis of the suspended sediment concentration profiles, using the experimentally obtained hydrodynamic characteristics, results in a value of 4 x 10(-4) for the resuspension parameter, gamma0, with the reference concentration taken 7 d50 above the bed. From the severity of the storm condition it is inferred that our estimates of k(n) and gamma0 correspond to sheet flow conditions.