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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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Observations of Estuarine Salt Intrusion Dynamics During a Prolonged Drought Event in the Rhine‐Meuse Delta
Salt intrusion poses a global threat to estuaries and deltas, exacerbated by climate change, drought, and sea level rise. This observational study investigates the impact of river discharge, wind, and tidal variations on salt intrusion in a branching river delta during drought. The complexity and spatial extent of deltas make comprehensive measurements challenging and rare. In this paper, we present a 17‐week data set of a historic drought in the Rhine‐Meuse Delta, capturing dynamics in a multiple‐channel system in a wide range of conditions. Key characteristics of this low‐lying delta are its branching channel network and complicated, human‐controlled discharge. Despite the system's complexity, we found that the subtidal salt intrusion length, defined by the 2 PSU isohaline , follows a power law relationship with Rhine River discharge . Subtidal water level variations contribute to short‐term variations in intrusion length, shifting the limit of salt intrusion upstream and downstream with a distance similar to the tidal excursion length. This can be attributed to the up‐estuary transport of seawater, caused by the estuary adjusting to variations in water levels at its mouth. However, spring‐neap variation in the tidal range does not alter the subtidal salt intrusion length. Side branches exhibit distinct dynamics from the main river, and their most important control is the downstream salinity. We show that treating the side branches separately is crucial to incorporate the highly variable downstream boundary condition, and may apply in other deltas or complex estuaries.
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- Award ID(s):
- 2318999
- PAR ID:
- 10632163
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 130
- Issue:
- 1
- ISSN:
- 2169-9275
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1029/2024JC021655
