LONG-TERM AND SHORT-TERM SHORELINE CHANGE TRENDS OF COCKSPUR ISLAND IN THE SAVANNAH RIVER ESTUARY IN GEORGIA

Cockspur Island is managed by the National Park Service as a part of Fort Pulaski National Monument. The landmass is located in the Savannah River estuary. Originally just salt marsh, the island was continually expanded by dredge-spoil throughout the 18-20^th centuries. The island’s east and south shorelines remain salt marsh, but large sections of the north shoreline are now armored. Monitoring changing shorelines in reference to different wave forcings is of high importance for park managers to understand for conservation and protection of the park’s cultural resources. In 2015, beneficial-use dredge spoil was deposited as a defensive berm at a highly erosive site on the north shoreline in order to protect two historic structures, the North Pier and Battery Hambright, which are in danger of loss from continued salt marsh erosion. In order to determine the longevity of the defensive berm, we are evaluating rates of change and total loss/gain of elevation and volume. For this analysis, we compare three-dimensional surface models derived from monthly drone-based aerial surveys. To further understand the berm’s sediment displacement patterns, surface sediment samples are collected quarterly and analyzed for mean grain size. To evaluate long-term shoreline change on the island’s perimeter, we digitize shorelines from historic aerial imagery and analyze rates of lateral erosion or accretion using the R-based package AMBUR. Preliminary results have found that the island’s south and north shoreline both maintained erosional conditions over an 80-year analysis period, with the north shoreline experiencing higher rates of erosion. The defensive berm has experienced erosion along the channel-ward edge since July 2018.