Quantifying the Flux of Water Exchange Between the Mississippi Sound and Bight Through the Cat Island Channel Under the Influence of Bonnet Carré Opening

The Great Mississippi Flood of 1927 was a major natural disaster in American history, resulting in extensive damage and the displacement of hundreds of thousands of people. As a response, engineers and policymakers sought to find a solution to effectively manage the river's flow during floods. They ultimately devised the Bonnet Carré Spillway system as a means of achieving this goal. The Bonnet Carré Spillway is a control structure designed to prevent flooding in New Orleans because of high Mississippi River flows during the spring freshet. Opening this structure modifies the dynamism of freshwater by diverting Mississippi River waters into Lake Pontchartrain and subsequently into the Mississippi Sound and Bight. The Bonnet Carré Spillway was opened twice in 2019 (February 27th and April 11th) with each opening lasting for more than 40 days. These two diversions significantly impacted the salinity environment of the MS Sound, resulting in catastrophic die-offs of the extant oyster population. The Mississippi Sound and Bight is a point of interest to researchers and coastal managers due to the various fisheries and recreational economic engines housed in this region. Protecting these resources from potential hazards has been a top priority for the stakeholders involved. Understanding the northern Gulf of Mexico's complex circulation pattern and exploring the dynamism and the impact of water exchange between the Mississippi Sound and Bight is an important step in predicting and mitigating future harmful impacts. For this reason, among others, a 400 m resolution, 24-layer circulation model of the Mississippi Sound and Bight was developed. This model is based on a regional application established during the GoMRI-funded CONCORDE consortium (msbCOAWST) and has most recently been applied to assess proposed additional freshwater diversion infrastructure [1]. For the study presented here, our numerical modeling application is used in conducting a twin experiment with the aim of quantifying the changes in volume fluxes of riverine freshwater flowing into the western Mississippi Sound and how this modified advective patterns and material exchanges with the Mississippi Bight through the Cat Island Channel. The first experiment establishes the state of the Cat Island Channel in terms of calculating the volume fluxes of freshwater from the various regional river sources, such as the Pearl River, entering the Mississippi Bight through the Cat Island Channel. The second experiment seeks to understand the changes in the freshwater volume flux when the Bonnet Carré Spillway is opened. Cat Island Channel is purposely chosen because of its role as an exchange pathway for estuarine (fresh) waters of Mississippi Sound and oceanic (saline) waters of Mississippi Bight; therefore, having a direct effect on the oyster reefs sites adjacent to the Cat Island Channel. Previous studies have involved studying the variability of salinity and temperature state of water mass in this same domain under the influence of Bonnet Carré and diurnal land-sea breeze circulation [2], [3]. These studies have shown that the 2019 diversions have to some great extent impacted the habitat suitability of most oyster beds in the domain of study. In support of the previous studies, this investigation aims at quantifying the freshwater fluxes which were not explicitly determined in earlier research. Having run a long-time simulation of the 2019 model, it was observed that during the first opening, the Bonnet Carré water pushes water eastward and subsequently southward through the Cat Island Channel. For this study, we focus more on the second opening of the Bonnet Carré spillway to examine the impact of the combined 2019 openings of the spillway on regional circulation patterns.