Numerical Experiments on Variation of Freshwater Plume and Leakage Effect From Mississippi River Diversion in the Lake Pontchartrain Estuary

Abstract

A Finite Volume Community Ocean Model is used to investigate how wind impacts the circulation and evolution of a freshwater plume from Mississippi River diversion in the Lake Pontchartrain Estuary. Results show that northerly and southerly winds tend to stretch the plume in the east-west directions, while easterly and westerly winds constrain the plume in the north-south directions. Increasing wind magnitude tends to increase the total salt content of the estuary except under weak westerly wind (<6 m/s) during which salt content decreases. A no-motion middepth interface is found (by the model and verified by the data), separating the top layer downwind flow and bottom layer upwind flow. Increasing wind magnitude can enhance the two-layered flows and lower the no-motion plane between the two opposite flows. Apparent small leakage of the river water through the diversion structure prior to its opening is found to impact the vertical structure of flows and salinity: Mixing is facilitated by the large amount of freshwater leaked into the lake prior to the opening of the diversion; wind-driven gyres are diminished; the average potential energy demand, a quantity used to measure the vertical stratification, is reduced to very low values; more deviation from the quasi-steady state balance tends to occur; and a total of 3.7 × 108 kg of salt is reduced during the opening period of the Bonnet Carré Spillway. The Lake Pontchartrain Estuary is completely dominated by the river water within about 25 days, when salinity drops from an average value of 4 g/kg to essentially zero.