Two-dimensional numerical modeling of a proposed freshwater diversion from the Bonnet Carre Spillway to the Labranche wetlands

Abstract

Engineered modifications to the lower Mississippi River within the past century have limited the magnitude and frequency of flood events in wetlands along the Louisiana coast. Without this natural delivery of freshwater, sediment, and nutrients, the ecological health of these wetlands are now degrading. A recent study within the Bonnet Carre Spillway has revealed that the 7,623 acres of floodway between the Mississippi River and Lake Pontchartrain is one of the few areas in Louisiana that is actively accreting land as a result of pulsed sediment-laden freshwater input during high discharge events in the Mississippi River. On the other hand, the productivity of the spillway region is geographically juxtaposed to the deterioration of the Labranche wetlands directly to the east, which have lost an extensive amount of marsh and swamp land to open water since becoming hydrologically disconnected from the river in the 1930’s. A two-dimensional finite-element numerical model of the Mississippi River, Bonnet Carre Spillway, Lake Pontchartrain, and Labranche wetlands is presented, which is used to examine the hydrodynamics of a freshwater input in the Labranche wetlands via a hypothetical diversion channel through the eastern guide levee of the Bonnet Carre Spillway. Flow velocities, water distribution patterns, and residence time distributions are used to highlight the potential for reintroducing river water and resources to these degrading wetlands.