Modeling the impact of the 2019 Bonnet Carré Spillway opening and local river flooding on the Mississippi Sound

The Bonnet Carré Spillway (BCS) is the final flood defense structure on the Mississippi River protecting the levee system of New Orleans from high pressure and overtopping due to elevated river stage. This protection is accomplished by diverting Mississippi River flow into Lake Pontchartrain (LP), which then progresses into the Western Mississippi Sound (MSS). Given typical seasonal wind patterns, spring and summer openings of the BCS are more likely to bring diverted waters into the MSS [1]. A recent increase in the frequency of BCS openings coincides with increasing trends in annual precipitation near the south coastal area of the Lower Mississippi River Alluvial valley and moist subsurface conditions that make the system prone to flooding events [2] [3]. This increase includes the first time the BCS opened 3 years consecutively (2018, 2019, & 2020) and the first time the BCS opened twice in one year (2019). The unprecedented 2019 double opening which released flood waters throughout the spring (February 27 – April 11) and summer (May 10 – July 27) resulted in a toxic algal bloom closing of 25 Gulf Coast beaches, a dolphin mortality event, and 100% mortality of MSS shellfisheries at many locations [4] [5].In this study, we use a Coupled Ocean Atmosphere Wave Sediment Transport (COAWST) [6] modeling system application to the Mississippi Sound and Bight (msb-COAWST) with improved river forcing to study the impact of freshwater diversion and local river runoff on the estuarine systems of MSS and LP. River forcing for the model is provided from USGS gauges that measure river discharge flowing into the study area. Hourly atmospheric forcing is provided by NOAA’s High-Resolution Rapid Refresh (HRRR) model [7]. BCS computed discharge available from the US Army Corps of Engineers is provided as forcing for the 2019 openings. Implementation of the BCS in the msb-COAWST domain allows us to run a series of scenarios to isolate processes within model simulations and identify the importance of spatial and temporal impacts of spillway and riverine forcing.We conduct a twin experiment on the 2019 double spillway opening, the largest combined Mississippi river water volume diverted through the BCS (hereinafter referred to as BCS freshwater) since its construction. In order to assess the extent of the BCS freshwater influence two scenarios were run with improved river forcing, one with realistically hindcast 2019 BCS openings and a hypothetical scenario with no BCS openings in 2019. During both the first and second opening, spillway waters flow through Chef Menteur Pass and the Rigolets into and around the Southern portion of Lake Borgne, then south of Grand Island, into Biloxi Marsh and into the Western MSS (Fig. 1). Bottom salinity is one of the most important factors considered in oyster habitat suitability and so we use it here as a measure of BCS freshwater influence. The increase in consecutive daily average bottom salinity less than 2 due to the BCS opening is as high as 173 days in the Southeastern part of Lake Borgne, and remains greater than 30 days throughout Western Mississippi Sound, Bay St. Louis and Biloxi Marsh. The difference between the two scenarios’ cumulative count of model bottom cells where daily average salinity is below 2 illustrates how the inclusion of BCS freshwater diversion impacts the duration of low salinity for the back to back spillway openings of 2019. The model results demonstrate that local river discharge has a strong influence on salinity in the Rigolets as well as in Northwest portions of Lake Borgne and MSS during the spring opening. Local river discharge has a lesser influence during the summer opening and BCS waters extend as far North as Bay St. Louis and as far East as Mobile Bay.