SEDIMENTATION AND LANDSCAPE CHANGES IN A DYNAMIC COASTAL SALT MARSH, FORT PULASKI NATIONAL MONUMENT, SAVANNAH, GEORGIA

Short-term processes of erosion and accretion were recorded for salt marsh near the Fort Pulaski National Monument (Savannah, Georgia). Vertical sediment accumulation was measured daily over a one week period and mass accumulation was measured once for a complete 12 hour tidal cycle. The study area was approximately 329 square meters. Environments studied included vegetated and de-vegetated high and low marsh, an oyster shell ridge, sand deposits, and areas of exposed peat and mud. A series of transects were established across different marsh sub-environments and were marked by flags. Vertical accretion and erosion were measured at each site by measuring flag heights relative to the sediment surface and mass accumulation was measured by deploying 9 cm diameter sediment traps. The rates of vertical change and mass of sediment accumulation varied considerably in the different marsh sub-environments. The two maximum daily losses of elevation, -4.4 and -5.2 cm, were recorded at sandy sites. The greatest net accretion of 8.6 cm over the week-long study was also recorded at a sandy site. On the other end of the spectrum, daily and week-long cumulative changes of marsh surface elevation of 0 cm were recorded for numerous sites. The minimum and maximum average mass accumulation of sediment on the sediment traps over one tidal cycle was 0.46 g and 56.20 g, respectively, with both occurring at sandy sites. This high variability in sedimentation and erosion within the sandy areas may result from exposure to wave energy and lack of protection by vegetation. A base map of the study area was created through ArcGIS using data collected by GPS surveys. The map displays topography and the extent of each of the marsh environments represented in the study. This will facilitate comparison to prior measured areas of shell ridge deposits in this dynamic system that has seen net marsh loss over recent years. This information is vital since nearby cultural resources are at risk if erosion and marsh loss continue.