Measurements of storm-generated bottom stresses on the continental shelf

Abstract

Large values of bottom friction velocity, u *, and roughness length, z 0 , determined from burst-averaged speed data taken on the continental shelf in outer Norton Sound, Alaska, with the GEOPROBE tripod during a storm in September 1977 are correlated with extremely large values of near-bottom concentration of total suspended particulate matter (TSM). Combined wind-driven and tidal currents exceeding 30 cm/s at 1 m above the bottom and intense oscillatory bottom currents with maxima above 45 cm/s were associated with the largest measured values of TSM at 2 m above the sea floor. The values of u * and z 0 obtained from the ‘law of the wall’ velocity-depth relationship are diminished substantially throughout the storm period (average reduction of 44%) when the turbulence reducing effects of the vertical concentration gradient of TSM are considered. The form of the latter correction was adapted from Smith and McLean (1977 a ). Values of the mean u * computed from the theory of Grant and Madsen (1979), which predicts an enhanced shear stress due to nonlinear wave-current interactions, compare favorably with the u * values determined from the measured velocity profiles. The measured values of z 0 , however, are considerably larger than any of the estimates based on (1) the actual scales of the physical roughness elements; (2) the apparent roughness of Grant and Madsen (1979); or (3) the thickness of the bed-load layer as formulated by Smith and McLean (1977 a ).