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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 ℇ 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 ℇ along the probed 15-km stretch of the estuary are surprisingly small, given that individual depth averages of ℇ 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. [ABSTRACT FROM AUTHOR]

ABSTRACT Turbulent mixing in the water column was observed with a microstructure profiler in the Hudson River estuary during two cruises in summer and fall 1995. The focus is on the estimation of turbulent salt flux and turbulent stress from measured viscous dissipation rates (e), and on the tidal and fortnightly variability of these fluxes. In estimating eddy viscosity (K[sub m]) and eddy diffusivity (K[sub p]), the authors follow measurement/modeling techniques of Busch and Osborn, while prescribing a variable flux Richardson number (R[sub j]) dependent upon the gradient Richardson number (Ri). It is argued that a steady-state production-dissipation balance holds in the turbulent kinetic energy budget. All turbulence characteristics varied strongly over semidiurnal tidal cycles and over the fortnightly cycle. Subject to complications arising from nontidal flows, the "strongest" mixing occurred during flood on neap tides, and during ebb on spring tides. In the lower part of the water column during floods and spring ebb K[sub m] and K[sub p] reached maxima of 1-5 (x 10[sup -2] m[sup 2] s[sup -1]) and decreased roughly exponentially with increasing height by 1-3 decades. The smallest eddy coefficients occurred in the halocline during neap tide with K[sub m] Is equivalent to 10[sup 4] m[sup 2] s[sup -1] and K[sub p] Is approximately equal to 10[sup -5] m[sup 2] s[sup -1]. Mostly, the internal turbulent stress (tau[sub y]) was close to 0 in the upper third of the water column and approached the bottom shear stress with decreasing height. Neap ebb had small |tau[sub y]| even close to... [ABSTRACT FROM AUTHOR]