The dynamics of a partially mixed estuary

Measurements of velocity. density, and pressure gradient in the lower Hudso n River estuary were used to quantify the dominant terms in the momentum eq uation and to characterize their variations at tidal and spring-neap timesc ales. The vertical momentum flux (assumed to be due mainly to turbulent she ar stress) was estimated indirectly, based on the residual from the acceler ation and pressure gradient terms. The indirect estimates of stress compare d favorably to bottom stress estimates using a quadratic drag law, supporti ng 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 asym metry, with food tide values exceeding ebb values by a factor of 2. As a co nsequence of the asymmetry, the vertical structure of the tidally averaged stress bore no resemblance to the tidally averaged shear. In spite of the a symmetry of vertical mixing, the tidally averaged, estuarine circulation wa s found to depend simply on the intensify of bottom turbulence, which could be parameterized by a Rayleigh drag formulation based on the tidal velocit y magnitude and the tidally averaged near-bottom flow. This seemingly parad oxical result indicates that the estuarine circulation can be modeled witho ut detailed knowledge of the effective eddy viscosity, only requiring an es timate of the bottom drag coefficient. the tidal forcing conditions, and th e baroclinic pressure gradient. A notable characteristic of this solution i s an inverse dependence of the estuarine circulation on the amplitude of th e tides.