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Abstract The salt balance in estuaries is maintained by the outflow from the river, which removes salt from the estuary, and dispersive processes, which drive downgradient fluxes bringing salt into the estuary. We analyzed the salt fluxes in a realistic model of the North River, a tidal salt marsh estuary, using a quasi-Lagrangian moving plane reference based on the theory of Dronkers and van de Kreeke. Our study confirms their theoretical finding that in a plane moving with the tides, all landward salt flux results directly from shear dispersion, that is, the spatial correlation between cross-sectional variations in velocity and salinity. We separated cross-sectional variations in velocity and salinity not only based on their lateral and vertical components but also by distinct regions of the cross section: the main channel and the marsh. In this way, we quantified the salt flux contributions from vertical and lateral shear dispersion, as well as from trapping—the salt flux due to the difference between the mean velocity and salinity of the main channel compared to the marsh. Trapping accounted for up to half of the total landward salt flux in the estuary during spring tides but decreased to about one-quarter during neap tides. Within the channel, the primary mode of dispersion shifted from lateral shear dispersion due to flow separation during spring tides to vertical shear dispersion due to tidal straining during neap tides. These results demonstrate the important role of topographically induced dispersion on maintaining the salt balance, particularly in tidally dominated estuaries.
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Curves, Coriolis, and Cross-Channel Circulation in the Hudson River Estuary
Abstract Despite its relatively small magnitude, cross-channel circulation in estuaries can influence the along-channel momentum balance, dispersion, and transport. We investigate spatial and temporal variation in cross-channel circulation at two contrasting sites in the Hudson River estuary. The two sites differ in the relative strength and direction of Coriolis and curvature forcing. We contrast the patterns and magnitudes of flow at the two sites during varying conditions in stratification driven by tidal amplitude and river discharge. We found well-defined flows during flood tides at both sites, characterized by mainly two-layer structures when the water column was more homogeneous and structures with three or more layers when the water column was more stratified. Ebb tides had generally weaker and less definite flows, except at one site where curvature and Coriolis reinforced each other during spring tide ebbs. Cross-channel currents had similar patterns, but were oppositely directed at the two sites, demonstrating the importance of curvature even in channels with relatively gradual curves. Coriolis and curvature dominated the measured terms in the cross-channel momentum balance. Their combination was generally consistent with driving the observed patterns and directions of flow, but local acceleration and cross-channel advection made some notable contributions. A large residual in the momentum balance indicates that some combination of vertical stress divergence, baroclinic pressure gradients, and along-channel and vertical advection must play an essential role, but data limitations prevented an accurate estimation of these terms. Cross-channel advection affected the along-channel momentum balance at times, with implications for the exchange flow’s strength. Significance StatementCurrents that flow across the channel in an estuary move slower than those flowing along the channel, but they can transport materials and change water properties in important ways, affecting human uses of estuaries such as shipping, aquaculture, and recreation. We wanted to better understand cross-channel currents in the Hudson River estuary. We found that larger tides produced the strongest cross-channel currents with a two-layer pattern, compared to weaker currents with three layers during smaller tides. Higher or lower river flow also affected current strength. Comparing two locations, we saw cross-channel currents moving in opposite directions because of differences in the curvature of the river channel. Our results show how channel curvature and Earth’s rotation combine to produce cross-channel currents.
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- PAR ID:
- 10499107
- Editor(s):
- Maarten Buisman
- Publisher / Repository:
- Journal of Physical Oceanography
- Date Published:
- Journal Name:
- Journal of Physical Oceanography
- Volume:
- 54
- Issue:
- 3
- ISSN:
- 0022-3670
- Page Range / eLocation ID:
- 967 to 984
- Subject(s) / Keyword(s):
- Estuaries Currents Momentum Secondary circulation Subseasonal variability
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1175/JPO-D-23-0093.1
