Recent advances in the understanding of the physics of estuarine fronts (small-scale regions of significant changes in hydrographic variables) are reviewed. These fronts may be classified in three groups—tidal mixing fronts, plume fronts, and shear fronts—on the basis of the mechanism dominating the local rate of change in the horizontal density gradient. These classes are not independent, however, since it appears that both tidal mixing fronts and shear fronts can produce buoyancy-driven flow structures that propagate away from the generation region as plume fronts. Field observations of typical examples of each class are presented and laboratory experiments and theoretical models which isolate the important physical processes are summarized. Some important unresolved questions are identified and suggestions for further research are offered.

With an analytic model, this paper describes the subtidal circulation in tidally dominated channels of different lengths, with arbitrary lateral depth variations. The focus is on an important parameter associated with the reversal of the exchange flows. This parameter (δ) is defined as the ratio between the channel length and one-quarter of the tidal wavelength, which is determined by water depth and tidal frequency. In this study, a standard bottom drag coefficient, CD = 0.0025, is used. For a channel with δ smaller than 0.6–0.7 (short channels), the exchange flow at the open end has an inward transport in deep water and an outward transport in shallow water. This situation is just the opposite of channels with a δ value larger than 0.6–0.7 (long channels). For a channel with a δ value of about 0.35–0.5, the exchange flow at the open end reaches the maximum of a short channel. For a channel with a δ value of about 0.85–1.0, the exchange flow at the open end reaches the maximum of a long channel, with the inward flux of water occurring over the shoal area and the outward flow in the deep-water area. However, near the closed end of a long channel, the exchange flow appears as that in a short channel—that is, the exchange flow changes direction along the channel from the head to the open end of the channel. For a channel with a δ value of about 0.6–0.7, the tidally induced subtidal exchange flow at the open end reaches its minimum when there is little flow across the open end and the water residence time reaches its maximum. The mean sea level increases toward the closed end for all δ values. However, the spatial gradient of the mean sea level in a short channel is much smaller than that of a long channel. The differences between short and long channels are caused by a shift in dynamical balance of momentum or, equivalently, a change in tidal wave characteristics from a progressive wave to a standing wave.

A mathematical model that describes the formation and dilution of a frontally bounded river plume is presented. Such features were first studied at the mouth of the Connecticut River during periods of high discharge and have subsequently been reported elsewhere. The model incorporates the effects of nonlinear advection, Coriolis acceleration, time dependency, mixing, friction, and a free frontal boundary. A numerical solution technique is employed to obtain approximate solutions to several problems which are interpreted to yield new insights to the dynamics of these phenomena. In particular, computed solutions for the growth of a plume discharged from a radial source into a steady crossflow are presented for a variety of crossflow velocities and physical scales. These demonstrate that the stability of the layer to vertical shear in the horizontal velocity is sensitive to the relative directions of the crossflow current and that of a free Kelvin wave. The effect of a reversing crossflow, represent... Abstract A mathematical model that describes the formation and dilution of a frontally bounded river plume is presented. Such features were first studied at the mouth of the Connecticut River during periods of high discharge and have subsequently been reported elsewhere. The model incorporates the effects of nonlinear advection, Coriolis acceleration, time dependency, mixing, friction, and a free frontal boundary. A numerical solution technique is employed to obtain approximate solutions to several problems which are interpreted to yield new insights to the dynamics of these phenomena. In particular, computed solutions for the growth of a plume discharged from a radial source into a steady crossflow are presented for a variety of crossflow velocities and physical scales. These demonstrate that the stability of the layer to vertical shear in the horizontal velocity is sensitive to the relative directions of the crossflow current and that of a free Kelvin wave. The effect of a reversing crossflow, represent...