Estimating Bottom Stress in Tidal Boundary Layer from Acoustic Doppler Velocimeter Data

Abstract

Bed stresses in the bottom boundary layer of the York River estuary, Virginia, were estimated from three-dimensional near-bottom velocities measured by Acoustic Doppler Velocimeters (ADVs) and also by a profiling array of electromagnetic current meters. By assuming the measurements were made in a constant stress layer, four methods of stress estimation were evaluated using ADVs: direct covariance (COV) measurement, turbulent kinetic energy, inertial dissipation utilizing the Kolmogorov spectrum, and log profile. The four methods yielded similar estimates of frictional velocity (U*) based on ADV output from both 14 cm and 44 cm above bed. All eight estimates of average U* were consistent with the overall mean of 1.10 cm/s to within the 95% confidence interval for individual burst estimates. The COV method worked slightly better nearer the bed, possibly because of the sensitivity of COV to the upper limit of the constant stress layer. The inertial dissipation method performed marginally well at 14 cm above bed, likely because of sediment induced stratification and insufficient separation of turbulent production and dissipation scales. The log profile method was the most variable and appeared most sensitive to stratification and to the thickness of the constant stress layer. The turbulent kinetic energy method was the most consistent at both heights and appears most promising for further development. Results encourage future use of the ADV in estuarine environments but also favor the simultaneous use of several methods to estimate bottom stress.