T

Lt — — UT n where T is the tidal period (in seconds) and uT is the magnitude of the tidal velocity. For the tidal currents in the Hudson of 0.7-1 m s-1, the excursion is 10-14km.The reason that tides are not dominantin

Figure 3.10. An eddy in the tidal stream due to deflection of the ebbing flow by the headland at the George Washington Bridge (from Chant and Wilson, 1997). The sticks indicate the direction and magnitude of the depth-averaged current (with dots at the origin). The eddy results in a salinity anomaly of 3 psu due to trapping in the core of the eddy.

Figure 3.10. An eddy in the tidal stream due to deflection of the ebbing flow by the headland at the George Washington Bridge (from Chant and Wilson, 1997). The sticks indicate the direction and magnitude of the depth-averaged current (with dots at the origin). The eddy results in a salinity anomaly of 3 psu due to trapping in the core of the eddy.

the horizontal exchange in the estuary is that in the other half of the tidal cycle, the water parcel will be transported back roughly the same distance. What makes tides important is their net influence over a tidal cycle, which is due to nonlinearities (i.e., processes that depend on uT2).

Tidal dispersion is the net transport accomplished by the asymmetry between the flood and ebb motions that results in net displacement of water parcels over a tidal cycle. Tidal dispersion arises from a number of different mechanisms, most of which are associated with differences in the strength of the tidal current across the estuary. These processes can collectively be regarded as shear dispersion. Shear dispersion occurs both due to lateral and vertical variations in tidal velocity. Its magnitude is dependent not only on the cross-sectional variations of velocity; it is also dependent on the rate of mixing either in the vertical or transverse direction. The flow around headlands can produce eddies that enhance the transverse shears and thus increase the tidal dispersion (Fig. 3.10). Rarely, however, does tidal shear dispersion reach the magnitude of exchange induced by the estuar-ine circulation (Zimmerman, 1986).

Other, more complicated types of dispersion can occur due to interactions between the tides and the estuarine circulation. The estuarine circulation and its associated salt flux are defined based on tidal averages of the flow and the salinity, but there can be correlation between variations in velocity and salinity that lead to net salt transport. In regions of irregular topography, these transports can exceed the strength of the estuarine circulation (Geyer and Nepf, 1996).

tide-induced mixing

As discussed in context with the estuarine circulation, one of the most important nonlinear processes accomplished by the tides is the generation of turbulence. The generation of turbulence at the bottom of the estuary is well understood: the flow over the rough bottom produces eddies that diffuse momentum and water properties in the vertical dimension. The turbulence problem becomes more complicated farther up in the water column, where stratification is stronger. Stratification tends to suppress turbulence associated with bottom-generated turbulence, but as that turbulence is suppressed, the shears tend to increase. Once the shears get high enough relative to the strength of the stratification, a new source of turbulence, shear instability, can start mixing within the stratified water column (Peters, 1997). Shear-induced mixing is important in the Hudson during neap tides and times of high flow, when stratification is strong. The complex interactions between tidal currents, shear-induced mixing, and internal waves are not yet fully understood, and these interactions represent an important aspect of es-tuarine dynamics that limits our ability to model estuarine physical processes.

new york harbor

The character of the estuary changes at the Battery, where the Hudson River joins the East River at New York Harbor. In contrast to the simple morphology of the Hudson, the Harbor has a complex geometry, with interconnections between several adjacent embayments through a series of tidal straits (Fig. 3.11). The flow in these straits, among the swiftest in the harbor complex, are driven primarily

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