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Figure 3.7. Vertical profiles of net estuarine velocity, during neap and spring tides, observed at the Battery in the spring of 1999 (from data presented in Geyer et al., 2001). Stronger estuarine currents occur during neap tides, when tidal mixing is weaker.

Figure 3.7. Vertical profiles of net estuarine velocity, during neap and spring tides, observed at the Battery in the spring of 1999 (from data presented in Geyer et al., 2001). Stronger estuarine currents occur during neap tides, when tidal mixing is weaker.

density contrast between seawater and fresh water, which yields a landward-directed force at the bottom of the estuary. The tilt of the water surface toward the sea provides a driving force for the surface outflow, but the density gradient is strong enough to reverse the direction of that force at the bottom.

The estuarine circulation is the mechanism that transports salt into the estuary against the outward motion of the river flow. This is accomplished by carrying high-salinity water in at the bottom and carrying out low-salinity water at the surface, resulting in a net inward motion of salt. The estu-arine circulation is driven by the density gradient between fresh and salt water, thus the stronger the gradient, the stronger the estuarine circulation. The salinity distribution along the estuary is like a spring: when it is compressed during high river-flow conditions (Fig. 3.5), it exerts a greater force, driving a more vigorous estuarine circulation (Fig. 3.8). During high flow conditions, the seaward transport of salt due to the river is greater; thus a stronger estuarine circulation is required to keep salt in the estuary. When the river flow decreases, the spring relaxes, and the forcing of the estuarine circulation decreases.

stratification

The estuarine circulation is not the only factor responsible for the salt transport in the estuary; the vertical salinity stratification is also key. The amount of salt that is transported by the two-way flow depends on the salinity difference between the surface and bottom waters. As that salinity difference increases, the amount of salt that is transported increases proportionately. Perhaps more importantly, the stratification is closely related to the amount of vertical mixing that occurs in the estuary, which in turn regulates not only most of the physical exchange processes in the estuary but also its ecology and biogeochemistry. Thus, stratification is generally considered the most important variable for the classification of estuaries.

Stratification originates from the interaction of the estuarine circulation and salinity gradient. The estuarine circulation always increases the salinity of the deep water and decreases the salinity of the surface water due to horizontal advection (Figs. 3.6 and 3.7). If there were no mixing, eventually the near-bottom water would be purely ocean water and the near-surface water just riverine, with a very strong halocline, or salinity gradient, between the two layers. Vertical mixing, due mainly to tidal currents, partially counteracts the stratifying tendency of the estuarine circulation. As tidal currents increase, there is greater vertical mixing andless stratification for a given amount of estuarine circulation (Fig. 3.8). Tidal mixing also has a direct influence on estuarine circulation by increasing the momentum exchange (or drag) between the incoming and outgoing water. Thus, tidal mixing affects the stratification directly, by producing vertical exchange between the upper and lower layers, and indirectly, by influencing the strength of the estuarine circulation (Fig. 3.8), which provides the source of stratification.

the spring-neap cycle

The sensitive dependence of the stratification on the tides leads to large spring-neap changes in stratification in the Hudson (Figs. 3.6 and 3.8). These changes in stratification indicate large variations in vertical exchange in the estuary. Whereas stratification provides an indicator of the amount of vertical mixing, it also exerts a direct dynamical influence on turbulent motions. The vertical

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Figure 3.8. Time-series of discharge (top panel), tidal velocity amplitude (2nd panel), stratification (3rd panel), and estuarine circulation (bottom panel) from observations near the Battery in 1999 (from Geyer et al., 2001). Stratification reaches its maximum value during neap tides and its minimum during springs. The estuarine circulation also varies with the spring-neap cycle, but not as distinctly as stratification. Note that the freshwater inflow only has a modest influence on stratification.

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Year day (1999)

Figure 3.8. Time-series of discharge (top panel), tidal velocity amplitude (2nd panel), stratification (3rd panel), and estuarine circulation (bottom panel) from observations near the Battery in 1999 (from Geyer et al., 2001). Stratification reaches its maximum value during neap tides and its minimum during springs. The estuarine circulation also varies with the spring-neap cycle, but not as distinctly as stratification. Note that the freshwater inflow only has a modest influence on stratification.

density gradient (due to salinity stratification) acts to suppress turbulence, thus preventing the influence of tide-induced mixing from reaching the upper part of the water column during neap tides. This vertical barrier of stratification that occurs during neap tides affects the vertical transport of nutrients and oxygen, with important ecological implications.

These spring-neap variations in stratification also have important implications for horizontal transport of salt. During neap tides, vertical gradients are strong, and there is minimal vertical

Figure 3.9. Salt flux in the Hudson estuary, during observations in 1995 (from Bowen and Geyer, 2003). The upper panel shows the landward salt flux due to the sum of the estuarine and tidal pumping transport. The lower panel indicates the net transport, including the river outflow and all of the other contributors. Large peaks in landward salt transport occur during weak neap tides, when stratification is maximal. Strong river outflow at the end of the observation period is responsible for the large negative value in the total transport.

Figure 3.9. Salt flux in the Hudson estuary, during observations in 1995 (from Bowen and Geyer, 2003). The upper panel shows the landward salt flux due to the sum of the estuarine and tidal pumping transport. The lower panel indicates the net transport, including the river outflow and all of the other contributors. Large peaks in landward salt transport occur during weak neap tides, when stratification is maximal. Strong river outflow at the end of the observation period is responsible for the large negative value in the total transport.

exchange of either momentum or salt between the upper and lower layers. Thus, both the estuarine circulation and the stratification are enhanced, and the salt transport due to the estuarine circulation is maximal (Fig. 3.9). This causes salt to advance into the estuary during neap tides and to retreat during spring tides. Whereas the large variation in horizontal salt transport due to the spring-neap cycle is clearly evident, the changes in position of the salinity intrusion are not as obvious. The salinity intrusion is usually long enough that these spring-neap changes in salinity are small relative to the total length of the salt intrusion (Bowen and Geyer, 2003). In addition, variations in stratification may overwhelm the signal of the changes in the horizontal position of the salt front.

tidal dispersion

It is surprising that the estuarine circulation and river flow would have such important effects on the Hudson estuary, when the tidal currents are so much stronger. The energy provided by the tides far exceeds that provided by any other source in the estuary, and the velocities due to the tides are 5 to 10 times as great as the estuarine circulation and as much as 100 times as great as the river flow. The reason the tides do not totally dominate over these other motions with respect to the salt balance and exchange within the estuary is because of the oscillatory nature of the tidal flow. The tidal excursion is the distance that a parcel of water is transported by the tide in one-half cycle. It is calculated by the formula

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