Broadly Distributed and Locally Enhanced Turbulent Mixing in a Tidal Estuary

Abstract

Spatial variations of vertical turbulent mixing along a stretch of the Hudson River estuary are examined with focus on the vicinity of a “hydraulic control point” at the George Washington Bridge at which the cross section narrows and the thalweg takes a bend. Richardson numbers are lowered and mixing is enhanced downstream of this “modest” morphological feature, qualitatively following predictions based on hydraulic theory by Chant and Wilson. The enhancement in the viscous dissipation rate ε is only a modest factor of 2–3, however, extending over 2 km along the river. Upon averaging over tidal and fortnightly cycles and multiple cruises, streamwise variations of ε along the probed 15-km stretch of the estuary are surprisingly small, given that individual depth averages of ε across the halocline vary by over three orders of magnitude. The principal result is that the observed part of the Hudson River showed strong broadly distributed mixing everywhere, with little local concentration.