Webb, B.M. and Marr, C., 2016. Spatial variability of hydrodynamic timescales in a broad and shallow estuary: Mobile Bay, Alabama.
Residence, exposure, and flushing times are examples of hydrodynamic timescales that describe the physical mass transport within a water body. The response and spatial variability of these measures to tides and discharge were investigated through hydrodynamic model simulations of Mobile Bay, Alabama using a two-dimensional depth-integrated circulation model coupled with a Lagrangian particle tracking model. Hydrodynamic timescales were estimated and analyzed using the particle tracking results. Flushing of the estuary was found to transition from tidally enhanced to river dominated for Q > 1715 m3 s−1. A simple power law regression was found to accurately represent the spatially averaged timescales (R2 > 0.99) for tidal and river forcing. Spatially averaged timescales generally ranged from 4 to 130 days, with large deviations related to particle initial position, magnitude of river discharge, and local winds. The longest timescales were found in areas where adverse pressure gradients contributed to water retention as a result of separated flow and closed-cell circulation. Increasing river discharge enhanced the barotropic pressure gradient, which dominated the along- and across-estuary momentum balances in all but the minimum flow scenario where the across-estuary balance was geostrophic. Meteorological forcing was found to increase the spatial variance in hydrodynamic timescales by as much as 60% as compared with cases with discharge and tides only. The increased spatial variability was a result of inhibited flushing due to enhanced northerly flows along the shorelines and expanded regions of closed-cell circulation in the eastern portion of the estuary.