Impact of Savannah Harbor Deep Draft Navigation Project on Tybee Island Shelf and Shoreline

The purpose of this study is to determine if the Savannah Harbor Deep Draft Navigation Project is adversely impacting the shores of Tybee Island (including sand lost from the beach and the Tybee shelf). The study methodology includes numerical modeling of waves, currents, water levels, and sediment transport rates and sediment budgets analysis for pre-project and post-project conditions. Sediment budgets were developed for the period 1854 to 1897 (pre-project) and 1897 to 2005/06/07 (post-project). The post-project bathymetry change shows a pattern of ebb shoal deflation on the Tybee shelf, which is a typical consequence of jetty construction and channel deepening. The ebb shoal deflation resulted from sediment pathways across the channel being disrupted. The major impact of the project is the loss of sand from the Tybee shelf. The ebb shoal deflation also resulted in shoreline change on Tybee Island, including erosion on the northern end of the island and accretion on the southern end of the island. The impact of the project is evaluated as the difference in volume loss rates (post-project minus pre-project) for the Tybee Island shelf cell of the sediment budget plus the estimated shoreline change rate (converted to a volume). The estimated combined shelf and shoreline impact at Tybee Island is 73.6 percent (including beach fill placement) or 78.5 percent (excluding beach fill placement) (±20 percent). This means that an estimated 73.6 percent (or 78.6 percent) of the reduction in sand volume on the Tybee shelf and shoreline is due to the project, with the remainder of the erosion attributed to the natural processes.