Using a Broadband ADCP in a Tidal Channel. Part II: Turbulence

Abstract

A four-transducer, 600-kHz, broadband acoustic Dopple current profiler (ADCP) was rigidly mounted to the bottom of a fully turbulent tidal channel with peak flows of 1 m s?1. Rapid samples of velocity data are used to estimate various parameters of turbulence with the covariance technique. The questions of bias and error sources, statistical uncertainty, and spectra are addressed. Estimates of the Reynolds stress are biased by the misalignment of the instrument axis with respect to vertical. This bias can be eliminated by a fifth transducer directed along the instrument axis. The estimates of turbulent kinetic energy (TKE) density have a systematic bias of 5 ? 10?4 m2 s?2 due to Doppler noise, and the relative statistical uncertainty of the 20-min averages is usually less than 20%?95% confidence. The bias in the Reynolds stress due to Doppler noise is less than ±4 ? 10?5 m2. The band of zero significance is never less than 1.5 ? 10?5 m2 s?2 due to Doppler noise, and this band increases with increasing TKE density. Velocity fluctuations with periods longer than 20 min contribute little to either the stress or the TKE density. The rate of production of TKE density and the vertical eddy viscosity are derived and in agreement with expectations for a tidal channel.