ACCELERATION IN SHORELINE RETREAT AT ST. CATHERINES ISLAND, GEORGIA: POTENTIAL FORCING BY ANTHROPOGENIC MODIFICATIONS TO SEDIMENT SUPPLY

The current study has evaluated shoreline dynamics and environmental change at St. Catherines Island, Georgia, with attention to the major controls of barrier island formation and modification processes. These major controls include the rate of sea level rise for the Georgia Bight which has remained constant in 20th and 21st century tide gauge data and dynamically changing rates of sediment supply based on anthropogenic modifications to land cover that are reflected in sediment transport. The evaluation of anthropogenic modifications to the rate of sediment supply indicates that in spite of significant changes in sediment flux rates of +300% (pre-dam era) and -20% (post-dam era), shoreline retreat was continuous during the 1859-2012 study period with an acceleration noted in the rates of shoreline retreat associated with spit and berm landforms during the post-dam or modern era. The apparent acceleration in the shoreline retreat rates for the spit and berm landforms is indicated as an inflection point in time-series plots of shoreline displacement and the landforms result from the combination of erosion along the shoreface and deposition via washover processes. The change in shoreline retreat rates for the spits and berms appears to correlate with the timing of major disruptions in the rate of sediment supply to the coast of Georgia, specifically the placement of the initial impoundments on the major rivers of the southeastern US. A qualitative review of land cover and vegetation in the historical imagery revealed distinct differences in the types of vegetation associated with the spits and berms where mature vegetation is observed in historical imagery and the modern vegetation types appear to be dominated by grasses. This change in the maturity of vegetation is reflective of a less stable landform that may be retreating at a higher rate where the natural successions of plant types are no longer in balance with the rate of shoreline retreat. Results from the current study indicate that anthropogenic changes to land cover and sediment transport may have resulted in the acceleration of shoreline retreat associated with a specific shoreline landform at St. Catherines Island, Georgia.