Simulating How Freshwater Diversions Impact Salinity Regimes in an Estuarine System

Abstract

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 7080 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 by how much, for how long, and where salinity is impacted in the estuarine system. The maximum difference in salinity at any given location over the mapped dates between the non-opening and hypothetical opening scenarios varies between 22 and 30 in each year. Differences in salinity between the opening and non-opening scenarios begin to decline in the study area approximately 18 days after Spillway closure. Decreases in salinity in Lake Borgne persist over a year. The Bonnet Carré Spillway affects salinity mostly in Lake Borgne and along an east/west ribbon that embraces the Mississippi coastline. Decreases in salinity caused by Spillway openings are seen up to 200 km east of the Spillway. These results are important for planning management strategies for estuarine resources during Spillway openings.

Keywords

• hydrodynamic model
• Bonnet Carré Spillway
• diversion
• salinity
• Mississippi Sound
• EFDC