Direct Measurement of Turbulence Properties by BB-ADCP in Bottom Boundary Layer

Direct Measurement of Turbulence Properties by a BB-ADCP

in Bottom Boundary Layer

R.T. Cheng, C.-H. Ling and J.W. Gartner

Water Resources Division, U.S. Geological Survey, Menlo Park, CA 94025 USGS

Abstract. Broad-band acoustic Doppler current profilers (BB-ADCPs), in high resolution modes, have been shown to be very effective for studies of turbulent mean flow properties in the marine bottom boundary layer. The BB-ADCP is capable of measuring the velocity distribution in the bottom boundary layer, providing about 20-30 velocity points within 1.5 m from the sediment-water interface at a sampling rate slightly above 1 Hz. In this study, the potential of the high frequency sampling scheme of the BB-ADCP is explored. By saving single-ping velocity measurements, the turbulence properties of the bottom boundary layer can be extracted from these measurements. Specifically, the high frequency velocity profiles are analyzed leading to direct measurements of turbulence properties of a bottom boundary layer in South San Francisco Bay, California. The objectives of this study are: 1) to explore techniques of measuring turbulence properties by a BB-ADCP; 2) to determine the Reynolds stress distribution in the bottom boundary layer from correlation of high frequency velocity components; 3) to compare the Reynolds stress with the bottom boundary layer properties deduced by conventional methods; and 4) to characterize the rate of vertical mixing in the water column based on these measurements. This investigation is moderately successful; some limitations of this approach have been identified for future consideration of improvements.