A MODERN GEOLOGIC HISTORY OF COCKSPUR ISLAND AND FORT PULASKI, GA

Fort Pulaski is a Civil War era fort located on Cockspur, GA adjacent to the Savannah River shipping channel. The National Park Service is concerned about natural processes and anthropogenic impacts threatening natural and cultural resources. Shoreline change rates were assessed encompassing 1852-2015, using an archive of historic charts and aerial imagery. Major features examined include the island shoreline, a large, dynamic oyster shell ridge located along the north channel of the Savannah River, and a modern sand berm built to protect historic resources. These features were digitized in a GIS to determine shoreline change rates for time periods that bracket channel deepenings and average rates for the entire period of historic data using the AMBUR analysis package. The oyster shell ridge was surveyed with a GPS biweekly to analyze short-term change in length, width, distance from cultural resources, and progradation rate in order to document the ridge’s history.

Cockspur Island tripled in length from 1852 to 1905 due to jetty emplacement along the north shore in 1896 and dredge spoil deposition until 1942. Shoreline change rates varied from +0.7 to +3.5 m/yr prior to the first major channel deepening event in 1929. The average annual rate of change from 1933 to 2013 for the entire north shore was -0.37 m/y, with 86% of the shoreline eroding; for the southern shoreline the rate was -0.12 m/y, with 81% of the shoreline eroding. The eastern shoreline has experienced substantial accretion (at rates up to +1.16 m/yr). The oyster shell ridge along the north shore, sourced from an adjacent manmade island created in the late 1800’s, was first observed to be migrating in 1970 imagery. Oyster ridge migration onshore occurred at rates of +2.5 m/yr and alongshore at a rate of +41.7 m/yr for the period 1982-2004. The oyster ridge had migrated onshore and alongshore by 2009 and only remnants of the ridge now are found along unarmored shoreline adjacent to the Fort.