SEDHYD-2023, Sedimentation and Hydrologic Modeling Conference

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Dynamics of The Salt-Water Wedge In The Lowermost Mississippi River During The 2022 Low Flow Season.

During periods of low discharge (< 300,000 cfs), saline water from the Gulf of Mexico intrudes into the channel of the Lowermost Mississippi River (LMR). The density of the marine water relative to the fresh river water allows the marine water to overcome the momentum of the river discharge and flow upstream tens of river miles. While this behavior is typical of many low-slope, coastal rivers world-wide, it poses a management problem as the saline water introduces water quality issues to municipal and industrial water in-takes located along the LMR. As the U.S. Army Corps of Engineers (USACE) maintains river depths below natural bed levels within the navigation channel of the LWR to benefit national commerce, the USACE mission includes mitigating the derogatory effects of the salt-water wedge promoted by channel dredging. The mitigation efforts include forecasting the leading ‘toe’ of the wedge as it progresses upstream and by building a sill (i.e., a submarine dam constructed of sandy bed sediment) to arrest the upstream movement of the wedge if the location nears critical infrastructure. In this study, we present observations of the salt-water wedge dynamics during the ‘historic’ low flow 2022 season as well as results from numerical modeling tools in development to aid in forecasting wedge behavior.

Our study results show how the saline marine water preferentially advances up-channel by filling channel bed areas of low elevation and how the location of the toe can linger in deep pools while moving swiftly over more shallow reaches. Because of the dependence on river bathymetry, the rate at which the wedge advances upstream is highly variable for a given river discharge. While current forecasting methods reliant primarily on river discharge have successfully identified river conditions when sill construction was required, methods that incorporate the effects of river bathymetry will add improved precision to future salt-water wedge management decisions and reduce uncertainty related to the timing of sill construction.

Brendan Yuill
US Army Coprs of Engineers
United States

Gary Brown
US Army Coprs of Engineers

Eden Krolopp
US Army Coprs of Engineers

Hailey Laurent
US Army Coprs of Engineers

David Ramirez
US Army Coprs of Engineers

 



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