Microstructure Observations of Turbulent Mixing in a Partially Mixed Estuary. Part I: Dissipation Rate

Abstract

Variations of turbulent mixing in the water column and in the benthic boundary layer were observed with a microstructure profiler in the Hudson River estuary during two cruises in summer and fall of 1995. Variability patterns of stratification, shear, Richardson number (Ri), and turbulent dissipation rates (?) were similar to those of earlier observations, with strong turbulence in the weakly stratified bottom layer, weak turbulence in the halocline during neap tides, and low Ri and high ? spanning the water column during spring ebbs. Depth-integrated turbulent dissipation rates ???? dz approximately equaled the work done by the tidal pressure gradient force adjusted for the change in tidal kinetic energy. Alternatively, a suitable scaling for ???? dz is also provided by the product of bottom shear stress and depth-average velocity τb?, a relationship that fails at slack tide, however. At heights above bottom of z ? 0.3 m, and again excepting slack tides, observed ? were highly correlated with a law-of-the-wall dissipation rate ?b = u3?/(?z). Ratios ?/?b were close to 1. Here, the friction velocity is u?, and von Kármán?s constant is ?. Most profiles of the normalized dissipation rate ?/?b showed a weak increase with z in the lowest 1.5 m, a departure from law of the wall scaling attributed to stable stratification. Such deviations from the law of the wall were smallest during spring floods when the near-bottom stratification was weak or unstable. Turbulence in the stratified water column well above the bottom appeared to be locally generated by shear instability even though ?b and ? were correlated throughout the water column. During spring ebbs, ? exceeded ?b by almost an order of magnitude at z ? 2 m.