Effects of Late Holocene Morphologic Evolution on Tidal Hydrodynamics in Mobile Bay, AL

Mobile Bay, Alabama is a microtidal estuary located in the northern Gulf of Mexico that has undergone significant morphologic changes over time. Presently, tidal exchange is primarily controlled by an inlet to the Gulf of Mexico, bounded by Dauphin Island to the west and the Morgan Peninsula to the east. A second inlet connects Mobile Bay to the Mississippi Sound. Sedimentological and geologic studies have shown there were two distinct Holocene environmental settings in Mobile Bay: a more marine-influenced, open embayment, and a fresher, more restricted embayment. Geologic data show that through the Holocene, spit elongation of the Morgan Peninsula reduced the width of the inlet to the Gulf of Mexico. Studies of beach ridges indicate there was a period when the Morgan Peninsula may have breached; a sediment core extracted in the bay behind the spit also shows increased marine influence dating 3500 -2200 years before present (yr BP). Using geologic data from previous studies including high-resolution seismic data, sediment cores and composite Holocene sea level rise curves, plausible shoreline configurations were reconstructed for Mobile Bay for the period of 3500 yr - 2200 yr BP. The historic shorelines provide initial conditions for a large-domain hydrodynamic model that was used to evaluate changes in tidal amplitudes, velocities and circulation patterns from present-day conditions. Scenarios include a wider inlet to the Gulf of Mexico, an addition inlet to the Gulf of Mexico resulting from breaching of the Morgan Peninsula, and closing of the inlet connecting to the Mississippi Sound. Results illustrate the geophysical influence of the number of inlets and inlet cross-sectional area on tidal exchange to Mobile Bay, and provide insight on the marine characteristics found in the sedimentary record.