HISTORICAL DEPOSITIONAL TRENDS IN MOBILE BAY, AL (USA), AND THEIR IMPLICATION FOR MODELING FUTURE COASTAL CHANGE

A common and well-recognized shortcoming in some sea-level rise (SLR) models is the assumption of a static topographic-bathymetric coastal surface. Changes in sediment supply to and sedimentation within bays due to both natural processes and anthropogenic activities can have a significant influence on this flooding surface. In an attempt to characterize long-term accretion rates within the Mobile Bay Estuarine System (MBES), seven box cores were collected from the bay, sectioned into 1-cm depth intervals, and analyzed for excess lead-210 (²¹⁰Pb_xs) and cesium-137 (¹³⁷Cs). MBES receives sediment and water from the Alabama and Tombigbee River watersheds that coalesce into the Mobile-Tensaw River (MTR) system prior to discharging into the bay. Riverine discharge from MTR to the bay is second only to the Mississippi River discharge to the Gulf of Mexico. Based on the bathymetry and morphology, the bay proper can be divided into three sub-basins: middle bay, lower bay, and Bon Secour Bay. To quantify long-term sedimentation in each sub-basin, downcore trends of ²¹⁰Pb_xs were modeled, assuming a constant supply rate of ²¹⁰Pb. Linear sedimentation rates over the last 120 yrs and across the various sub-basins average 0.4 cm y^-1, which is slightly higher than the relative SLR rate of 0.298 cm y^-1 recorded at Dauphin Island Tide Station (NOAA). However, differences in long-term sedimentation are observed within and among sub-basins, reiterating the need to understand the controls on spatially heterogeneous bay infilling. For example, sediment accumulation rates for the middle sub-basin east of the shipping channel are three to four times higher than rates west of the shipping channel. This difference may be in part controlled by the isolation of the western middle bay from the primary sediment supply from MTR due to the continual dredging of the shipping channel. The eastern Bon Secour sub-basin has the highest sediment accumulation rates of all the sub-basins and reflects the proximity of this sub-basin to sediment fluxes from the Bon Secour River and Intra-coastal Waterway. While we continue to examine factors contributing to spatial variation in sedimentation rates, the need to integrate dynamic bathymetric surfaces in SLR models is demonstrated with this data set.