Measurements of storm‐generated bottom stresses on the continental shelf

Abstract

Large values of bottom friction velocity, u*, and roughness length, z0, 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 z0 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 (1977a). 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 z0, 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 (1977a).