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Abstract The salinity distribution of an estuary depends on the balance between the river outflow, which is seaward, and a dispersive salt flux, which is landward. The dispersive salt flux at a fixed cross‐section can be divided into shear dispersion, which is caused by spatial correlations of the cross‐sectionally varying velocity and salinity, and the tidal oscillatory salt flux, which results from the tidal correlation between the cross‐section averaged, tidally varying components of velocity and salinity. The theoretical moving plane analysis of Dronkers and van de Kreeke (1986) indicates that the oscillatory salt flux is exactly equal to the difference between the “local” shear dispersion at a fixed location and the shear dispersion which occurred elsewhere within a tidal excursion; therefore, they refer to the oscillatory salt flux as “nonlocal” dispersion. We apply their moving plane analysis to a numerical model of a short, tidally dominated estuary and provide the first quantitative confirmation of the theoretical result that the spatiotemporal variability of shear dispersion accounts for the oscillatory salt flux. Shear dispersion is localized in space and time due to the tidal variation of currents and the position of the along‐channel salinity distribution with respect to topographic features. We find that dispersion near the mouth contributes strongly to the salt balance, especially under strong river and tidal forcing. Additionally, while vertical shear dispersion produces the majority of dispersive salt flux during neap tide and high flow, lateral mechanisms provide the dominant mode of dispersion during spring tide and low flow.
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This content will become publicly available on July 1, 2026
Physics of Transport in the Oligohaline Reach of the Delaware Estuary
Abstract A study of transport mechanisms in the oligohaline reach of the Delaware Estuary examines several locations with rapid changes in cross-sectional area that increase the horizontal salinity gradient, the exchange flow, and the upstream salt flux. This study is motivated by the proximity of municipal drinking water intakes upstream of the oligohaline range of the estuary, which can advance to just below the City of Philadelphia, PA, during low flow events. A Regional Ocean Model System (ROMS) numerical model was used to analyze the potential mechanisms of dispersion in the tidal Delaware River. While the model domain is largely within the tidal-fresh upper estuary, the domain below Philadelphia becomes oligohaline (0.5–5 psu) during low-flow events. Results found that landward transport of salt is facilitated by a combination of steady vertical shear dispersion and tidal oscillatory salt flux, with the latter becoming increasingly important approaching the upstream extent of salt intrusion. Frontogenesis is also an important intrusion mechanism in the vicinity of specific bathymetric features between km 110 and 114 near the Delaware Memorial Bridge and near km 126 near Marcus Hook. Moreover, the tidal advection of these fronts near km 117 produces strong lateral gradients that drive secondary flows and produce stronger tidal oscillatory salt flux away from the regions of frontogenesis.
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- Award ID(s):
- 2318998
- PAR ID:
- 10631136
- Publisher / Repository:
- Springer
- Date Published:
- Journal Name:
- Estuaries and Coasts
- Volume:
- 48
- Issue:
- 4
- ISSN:
- 1559-2723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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