Hydrodynamic Modeling of First-Order Transport Timescales in the St. Louis Bay Estuary, Mississippi

The transport timescales residence time, exposure time, and age of water are evaluated for the St. Louis Bay estuary, Mississippi, to investigate the impacts of freshwater inflows and tidal dynamics from Mississippi Sound on the estuary’s transport characteristics. The timescales are explicitly defined and computed using a hydrodynamic model and tracer experiments for a set of 11 hydrologic scenarios designed to represent permanent low, average, and high flow conditions. Results indicate that (1) the estuary’s residence time can vary between 2.0 and 134.5 days during high and low flow conditions, respectively; (2) under low flow conditions dispersive processes caused by the tidal dynamics at the estuary’s open boundary may be dominant, and the returning flows can increase the exposure times up to 30% in relation to the residence times; (3) during high flow conditions advective transport caused by the freshwater flows may be dominant resulting in exposure times similar to the residence times; and, (4) there are important spatial variations in the estuary's flushing characteristics as evaluated by the age of water.