Benthic Boundary-Layer Velocity Profiles: Dependence on Averaging Period

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Abstract
The relationship between benthic boundary-layer velocity profiles and current meter averaging time is investigated using detailed (0.61 Hz) current measurements recorded within 1 m of the bottom on the inner continental shelf. The percentage of velocity profiles which correspond to the von Kármán-Prandtl model of the neutrally stratified turbulent boundary layer increases rapidly as the averaging period is lengthened from a few seconds. When the averaging period exceeds ∼20 times the characteristic time scale of the flow, the percentage of logarithmic profiles becomes independent of averaging period. This relationship is found for both wave and turbulence dominated flows. Increasing the averaging period beyond the minimum required to define stable mean values does not significantly change either estimates of mean friction velocity or the statistical distribution of the drag coefficient C100 computed from the observations.

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