Mid-Breton Sediment Diversion (MBrSD) Assessment – Final Report

The purpose of this project is to provide managers at the Mississippi Department of Marine Resources (MDMR) with the scientific information needed to accurately address public concerns regarding the potential effects of the Louisiana Coastal Master Plan / Coastal Protection and Restoration Authority (CPRA) Mid-Breton Sediment Diversion (MBrSD) on the jurisdictional waters and resources of Mississippi. The stated design purpose of the MBrSD is to reconnect and re-establish the deltaic sediment deposition process between the Mississippi River and the Breton Sound Basin through a diversion that will deliver up to 75,000 cfs of sediment-laden freshwater. The report presented herein provides model-based guidance on the impact that the introduction of the MBrSD will have on salinity conditions in the Mississippi Sound (MSS) and Mississippi's jurisdictional waters that encompass oyster reef locations. Oysters are key ecosystem health indicators and economic drivers for the State of Mississippi and freshwater diversions into the western MS Sound (WMSS) have recently led to significant, unprecedented environmental impacts resulting in oyster mortality. The potential addition of a new pathway for additional freshwater to be introduced into the MSS requires careful assessment of the potential impacts that may be incurred. This project is designed to assess the impact of implementing the MBrSD on the physical environment in the WMSS. The primary aim is to understand the connectivity between MBrSD-derived freshwater input to Breton Sound on the environmental conditions impacting the oyster reefs of the WMSS near Bay St. Louis. A physical ocean modeling system based on the Coupled Ocean Atmosphere Wave Sediment Transport (COAWST) has been used to simulate the circulation and dynamics over the entire MSS with the analysis presented herein focusing particularly on the western to central MSS. This project demonstrates the importance of applying modeling-based scientific research and the capability of physical ocean circulation models for assessing aquatic ecosystem health, particularly in key oyster reef areas.