Investigation of the Bonnet Carré Spillway Impact on Water Exchange Process Through the Main Pass of Mobile Bay, Alabama

The diversion of Mississippi River water through the Bonnet Carré Spillway (BCS) into Lake Pontchartrain and its subsequent eastward propagation, has triggered harmful ecosystem impacts to the Mississippi Sound and adjacent waters [1]–[4]. A previous study has shown that the BCS flow may reach as far as Mobile Bay [5]. The Main Pass, which serves as a prominent channel for Mobile Bay allows the water exchange process between the bay and the inner shelf of Mississippi Bight. Examining the water circulation system through this inlet is important to gain more understanding of the interactions of BCS operation with the ecosystem function in Mobile Bay. For this purpose, a 400 m resolution, 24-layer Coupled Ocean Atmosphere Wave Sediment Transport (COAWST)-based circulation model of the MS Sound/Bight region (msbCOAWST), which resolves the complex estuarine/inner shelf exchanges that are prevalent throughout the region, is employed. Specifically designed bathymetry for the studies in the Gulf of Mexico is used as model input along with atmospheric and river forcing. Furthermore, lateral forcing is employed from the regional Gulf of Mexico application of the Navy Coastal Ocean Model (NCOM). Model solutions show a consistent result with the water level and temperature observation from NOAA Stations. The impact of BCS openings to Main Pass exchange system is investigated through the parameter of salt and volume transport. Twin experiment is designed to include both BCS and river forcing and river forcing alone into the hydrodynamic model. In general, the volume transport pattern in the Main Pass shows an association with the meridional wind (north-south wind) and river discharge pattern. Net water volume export from Mobile Bay through Main Pass is observed, however, the estuarine circulation return flow at depth through Main Pass acts to accumulate salt content inside Mobile Bay. The western shoal and deep ship channel aspects of Main Pass have unique, circulation characteristics. Intermittent extreme events like hurricanes may impact the salt and volume flux through the Main Pass. The BCS operation in the twin experiment designed in this study appears to impose a subtle impact on the transport pattern through Main Pass, with an amplification of oceanward volume transport and a dampening of salt flux entering Mobile Bay.