Abstract
Shoreline erosion is a fundamental concern to residents and stakeholders along the Gulf Coast. While much attention has focused on barrier islands and marshes, less investigation has occurred along sandy, estuarine shorelines that support woodlands or residential areas. Properties along these shorelines are increasingly fortified by seawalls and rip-rap, whereas adjacent non-walled properties are rapidly eroding. This study investigates linkages between erosion and nearshore sedimentation along two shorelines at Bon Secour Bay, Alabama (Weeks Bay NERR), and Perdido Bay, Florida (Tarkiln Bayou Preserve S.P.). Both shorelines are similarly oriented (NW-SE), are not protected by structures, and are not buffered by marsh. Time-averaged erosion rates are 0.30 – 0.67 m/yr (1992 – 2018) and 0.55 m/yr (1994 – 2018) at study areas along Bon Secour Bay and Perdido Bay, respectively. Discriminant analysis of sedimentary parameters of five subaqueous sediment cores in the upper 1 meter of the nearshore substrate indicate that relatively fine-grained size fractions (median size and smaller) and organic matter content are predictor variables that distinguish a somewhat coarser and organic substrate at Bon Secour Bay relative to Perdido Bay, likely as a result of greater fetch (energy) and fluvial inputs. Wood fragments, shell hash, and coarse sand in the basal deposits of two cores possibly represent a transgressive ravinement surface. Nearshore cores are contextualized with sediments collected from discrete bottom samples and boreholes along the adjacent beach, indicating that sediment supply is from the eroding shoreline. As a secondary product of this research, shallow groundwater levels along the estuarine beach reveal that rainfall events increase the water table elevation for a sustained duration, which likely exacerbates shoreline erosion if compounded with wave energy.
