The University of Southern Mississippi (USM) modeling group has developed a daily instance of the 400m resolution, 24-layer circulation model of the Mississippi Sound and Bight region based on a regional application established during the GoMRI-funded CONCORDE consortium. Called msbCOAWST, this model uses NOAA National Water Model (NWM) as river forcing, NOAA High Resolution Rapid Refresh (HRRR) as atmospheric forcing and is downscaled from a regional application of Navy Coastal Ocean Model (NCOM) for American Seas (AMSEAS), used as open boundary conditions. The model forcing assimilates observed stream gage values (NWM) and radar data (HRRR). NCOM AMSEAS assimilates all quality-controlled observations in the region (satellite sea surface temperature, altimetry, surface and profile temperature and salinity). Daily msbCOAWST output is available through Coastal CUBEnet and the USM THREDDS server. The modeling system has been designed to incorporate openings of the Bonnet Carré Spillway and is being used to provide guidance on the impact that local rivers and freshwater diversions have on conditions in the Mississippi Sound. Currently a daily hindcast is being run, and future efforts plan to include a now-cast and a one-day forecast. The msbCOAWST model will support the coastal management community and stakeholders, providing the tools and resources needed to better evaluate complex scientific issues and inform natural resource management decisions in the study area. The model can also play a role in connecting sparse in situ observations, assimilating and synthesizing them, and assist in the design of more robust observation networks.

The Mississippi Sound, and adjoining Lake Pontchartrain, Louisiana, are complex fluvial dominated regions connected to the Mississippi River only during flooding periods through the Bonnet Carré Spillway. We utilize water isotopes (δ18O and δD) in conjunction with salinity to differentiate the sources of freshwater. During the Bonnet Carré Spillway opening of 2019, Mississippi River water was identified throughout the Mississippi Sound, with Mississippi River water being the dominant source of fresh water in many areas. Mississippi River water lingered in the Mississippi Sound until the wind and current patterns shifted and brought in saltier offshore water, flushing the area. Following the Bonnet Carré Spillway closure, a period of low river discharge persisted. During this time, we observed increased δ18O and barium concentrations that were not associated with the Mississippi River, local fluvial inputs, or particle barium desorption. It was determined that δ18O of groundwater has a unique signal of -3.0‰ which differs from the local rivers (-3.8‰). This more positive δ18O signal was observed in local waters of Chef Menteur Pass, the Rigolets, Lake Pontchartrain, as well as the Mississippi Sound, suggesting groundwater input was present. Furthermore, elevated barium concentrations that could not be accounted for by river sources enhances the argument that we are observing a groundwater signal through δ18O when local river discharge is low.

The Bonnet Carré Spillway is a large flood control structure that diverts Mississippi River floodwaters into Lake Pontchartrain and the Mississippi Sound to prevent flooding in southern Louisiana and New Orleans. When operating at full capacity, the spillway releases water at a rate of 7,080 m3/s. Spillway openings regularly last a month or more. The enormous amount of freshwater that is diverted through the spillway impacts salinity and nutrients in the Mississippi Sound.
The objective of this research is to use a hydrodynamic model to simulate the impact of Bonnet Carré Spillway openings on the salinity of the Mississippi Sound over multiple years. Specifically, four hypothetical simulations of spillway openings are compared to simulations during the same time when the spillway is closed.
The results show how much, how long, and where salinity is impacted. The maximum difference in salinity, at any given location over the mapped dates between the non-opening and hypothetical opening scenarios, vary between 22 and 30 in each year. Differences in salinity between the opening and non-opening scenarios begin to decline approximately 18 days after spillway closure. Decreases in salinity in Lake Borgne persist over a year. The Bonnet Carré Spillway affects salinity most in Lake Borgne and along an east/west ribbon that hugs the northern coastline. Decreases in salinity caused by spillway openings are seen up to 200 km east of the spillway. These results are important for planning the management of estuarine resources during spillway openings.

Maurepas swamp is a coastal forested wetland on the east bank of the Mississippi River between Baton Rouge and New Orleans and is connected to Lake Pontchartrain via Lake Maurepas. Due to the lack of the regular freshwater and nutrient supply from the seasonal Mississippi River floods after construction of the river levees, the swamp has been transitioning to marsh and open water.
A project is being designed to reintroduce up to 2,000 cfs river water into the swamp. The specific objectives of the project are to: restore natural swamp hydrology, increase sediment and nutrient loading to the project area, increase substrate accretion, retain and increase existing areas of swamp vegetation including overstory cover, and reduce salinity levels.
To assist in the evaluation of the water quality and hydrologic benefits, a two-dimensional numerical model was developed using Delft3D software. The model was calibrated and validated under normal tidal conditions as well as tropical storm conditions. The model was utilized to evaluate distribution of river water throughout the swamp, and to evaluate fate and transport of total nitrogen and total phosphorous. The model was also applied to evaluate potential freshening of the swamp following saline conditions after a tropical storm surge. The output data were used in a Wetland Valuation Assessment Model to estimate project benefits.
The analysis indicated that the proposed project can provide significant benefits to maintain a controlled supply freshwater and nutrient to the swamp. The project can be effectively operated to reduce salinity following a tropical storm surge.