WIND STRESS, BED ROUGHNESS AND SEDIMENT SUSPENSION ON THE INNER SHELFDURING 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 Fie ld Research Facility at Duck, NC, U.S.A., over a 2-week period that in cluded the severe and prolonged ''Halloween Storm'' of late October 19 91. The storm persisted for 5 days and generated waves with heights an d periods of up to 6 m and 22 s. Although the instrumentation was dest royed, current profile and suspended sediment concentration profile da ta 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-d egrees offshore from shore-parallel. These strong currents are shown t o 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 roo t-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 concentrati ons exceeded 1 kg m-3 throughout the lower 1 m of the water column. An alysis of current profiles, accounting for the presence of waves, is p erformed to obtain an equivalent bottom roughness, k(n), of approximat ely 15 times the median sediment diameter, i.e. k(n) congruent-to 15 d 50. Analysis of the suspended sediment concentration profiles, using t he experimentally obtained hydrodynamic characteristics, results in a value of 4 x 10(-4) for the resuspension parameter, gamma0, with the r eference concentration taken 7 d50 above the bed. From the severity of the storm condition it is inferred that our estimates of k(n) and gam ma0 correspond to sheet flow conditions.