The Dynamics of a Partially Mixed Estuary*

Abstract

Measurements of velocity, density, and pressure gradient in the lower Hudson River estuary were used to quantify the dominant terms in the momentum equation and to characterize their variations at tidal and springneap timescales. The vertical momentum flux (assumed to be due mainly to turbulent shear stress) was estimated indirectly, based on the residual from the acceleration and pressure gradient terms. The indirect estimates of stress compared favorably to bottom stress estimates using a quadratic drag law, supporting the hypothesis that the tidal momentum equation involves a local balance between tidal acceleration, pressure gradient, and stress divergence.Estimates of eddy viscosity indicated that there was significant tidal asymmetry, with flood tide values exceeding ebb values by a factor of 2. As a consequence of the asymmetry, the vertical structure of the tidally averaged stress bore no resemblance to the tidally averaged shear. In spite of the asymmetry of vertical mixing, the tidally averaged, estuarine circulation was found to depend simply on the intensity of bottom turbulence, which could be parameterized by a Rayleigh drag formulation based on the tidal velocity magnitude and the tidally averaged near-bottom flow. This seemingly paradoxical result indicates that the estuarine circulation can be modeled without detailed knowledge of the effective eddy viscosity, only requiring an estimate of the bottom drag coefficient, the tidal forcing conditions, and the baroclinic pressure gradient. A notable characteristic of this solution is an inverse dependence of the estuarine circulation on the amplitude of the tides.

Behavior and mixing of a cold intermediate layer near a sloping boundary

“…In these estuaries, the stratification varies within a broad range of values between ebb and flood conditions with often very weak stratification during strongest tidal flow (e.g. Nepf and Geyer, 1996;Peters, 1997;Geyer et al, 2000;Kay, 2003). The LSLE does not behave such as these estuaries since even in the near bottom part of the water column the stratification remains nearly constant between the ebb and flood (Fig.…”

“…For certain shallow highly stratified or partially mixed estuaries, one may expect that most of the dissipation is determined by bottom stress and stress in the pycnocline (e.g. Geyer and Smith, 1987;Geyer et al, 2000Geyer et al, , 2010. In these estuaries, the stratification varies within a broad range of values between ebb and flood conditions with often very weak stratification during strongest tidal flow (e.g.…”

Tidal and spring‐neap variations in horizontal dispersion in a partially mixed estuary

“…[41] For the ebb data, the vertical spreading of the dye patch provides a more reliable estimate of the vertical mixing rate, because the vertical mixing rate is expected to be more uniform through the water column during the ebb [Peters and Bokhorst, 2000;Geyer et al, 2000]. Also, the entrainment model of Chant et al [2007] was not applicable to the more vertically continuous vertical gradients observed during the ebb.…”

Using a composite grid approach in a complex coastal domain to estimate estuarine residence time

“…Interestingly, these processes tend to vary inversely with each other. As tidal amplitude increases, the mixing increases, and the strength of the estuarine exchange flow decreases (Geyer and Cannon, 1982;Geyer et al, 2000), thus potentially increasing the residence time. Conversely, stronger tidal velocities cause greater horizontal dispersion which can decrease residence time (Zimmerman, 1986).…”