Turbulence Characteristics in a Tidal Channel

Abstract

A broadband ADCP and a moored microstructure instrument (TAMI) were deployed in a tidal channel of 30-m depth and with peak speeds of 1 m s?1. The measurements enable us to derive profiles of stress, turbulent kinetic energy (TKE), the rate of production and dissipation of TKE, eddy viscosity, diffusivity, as well as mixing length, and to test the parameterization of dissipation rate in the model of Mellor and Yamada. At middepth in the channel where the influence of stratification was present, the Ellison length agrees with the Ozmidov length. The measured mixing length is smaller than the simple z-dependence formulation proposed for unstratified turbulence. The diffusivity of density and heat, and the viscosity for momentum, are correlated and comparable in magnitudes. The 20-min averaged production rate deduced from the ADCP agrees with the dissipation rate estimated from microstructure measurements. The dissipation rate calculated with the Mellor?Yamada model agrees with the measured values with TAMI, but the empirical constant B1 derived from the data is larger than that conventionally used in the model. In the near-bottom layer, there is a tight correlation between the production rate and the closure-based dissipation rate. The Reynolds stress at 3.6 m above the bottom is consistently 2.5 times smaller than the shear velocity squared (u2?), which is inferred from fitting the velocity profiles to a logarithmic form. A logarithmic velocity profile almost always exists and reaches heights of 5.6 to 20 m, but the Reynolds stress is seldom constant in any part of the logarithmic layer.