Numerical modeling of the Lower Mississippi River-influence of forcings on flow distribution and impact of sea level rise on the system

The Mississippi River, with one of the world´s largest watersheds draining 41% of the continental U.S., is the seventh largest river in the world. The lower Mississippi River delta system along the northern Gulf of Mexico coast experiences annual coastal marsh losses between 25 and 35 square miles. The lower River is a highly complex system impacted by multiple forcings. An understanding of the hydrodynamics in this system will be important for understanding the potential outcomes of coastal restoration projects such as large-scale River diversions and for future management decisions. This study describes the development and application of a hydrodynamic model of a reach of the Lower Mississippi River from Carrolton (New Orleans) at RM 103 down to the Gulf of Mexico. The USACE Adaptive Hydraulics Model (ADH), an unstructured finite element model, is used to model the hydrodynamics. An unstructured mesh was developed for the study area, which includes detailed bathymetry and topography from the most recent available survey data. The mesh is fine enough to capture the changes in bathymetry and relies upon automated mesh refinement to capture flow details. Mississippi River stage data collected from thirteen stations between Carrolton and Port Eads for the water years between 1987 and 2008 is used in the model calibration. In addition, discharge data collected in various Lower River passes is used to examine the ability of the hydrodynamic model to properly simulate the flow distribution through different reaches and lower River passes. Steady state solutions for water surface elevations at gage locations match well with observational data and the distribution of river flow among the different sections of river channel and passes are consistent with the limited field data available. The effect of the sea level rise is most significant in the lower 20 miles of the river and it loses its effect with increasing flow rates