Model Application For Design of The Mid-Breton Sediment Diversion Project

Since the Mississippi River was leveed in the 1930s, the Mississippi River Delta, which includes the Barataria and Breton Basins, has lost approximately 1,800 square kilometers (km) of land, representing one of the highest rates of land loss in the world. This is attributed to sediment deprivation, hydrologic alteration, subsidence, sea level rise, and saltwater intrusion. Recognizing the need for an innovative approach to restore and sustain land, the State of Louisiana’s Coastal Master Plan has proposed multiple sediment diversions along the Mississippi River as cornerstone projects essential to creating and sustaining coastal land. Controlled sediment diversions offer a unique opportunity to strategically re-establish hydrologic flows, carry land-building sediment, nourish marshes, and sustain land. CPRA decided in fall 2015 to recommend that the Mid-Breton and Mid-Barataria diversions move forward to preliminary engineering and design. Design and construction is being funded through a settlement from the 2010 BP Oil Spill, administered through the National Fish and Wildlife Foundation (NFWF). The discussion in this session will focus on the hydraulic design and supporting analyses for the Mid-Breton diversion. The Mid-Breton project will be located at River Mile 68.0 AHP on the left descending bank of the river near the community of Wills Point. It is being designed for a peak flow of approximately 1,415 cms (50,000 cfs) when the river is flowing at 28,316 cms (1,000,000 cfs). The main project features are an intake structure with three lift gates operated by a gantry crane and a conveyance channel to move water and sediment from the river to the basin. The conveyance channel will have a depth of -6.1 m (-20 ft) NAVD 88, a bottom width of 48.8 m (160 ft), side slopes of 3.5H:1V, and will be approximately 0.8 km (0.5 mile) long. Design and construction will be necessary to accommodate and/or modify the Mississippi River levee, a state highway, domestic utilities, and a hurricane protection levee and associated drainage.

The Mid-Breton project is currently in the 60% Design Phase. Design completion is scheduled for early 2024, with a Final EIS and permitting decision to follow in late 2024. Construction is expected to begin in early 2025. The design team has performed numerical model runs to evaluate design variations for the various components of the project, including the intake, diversion structure, conveyance channel and outfall transition to the Breton Sound. These model runs have been performed to optimize the hydrodynamic performance of the structure, establish erosion armoring requirements, estimate delta development to inform tailwater conditions and assess morphologic change in the river. The discussion in this session will focus on the application of the numerical and physical models described in previous sessions to support testing of various geometric variations and selection of the recommended design configuration.