Large-scale sediment transport patterns on the continental shelf and influence on shoreline response: St. Andrew Sound, Georgia to Nassau Sound, Florida, USA

Abstract

Regional sediment transport patterns on the continental shelf seaward of Cumberland Island, Georgia and Amelia Island, Florida are documented using historical shoreline position and bathymetry data. Spatial variability in the net rate of shoreline change is considerable due to jetty construction at St. Marys Entrance in the early 1900s. Net average shoreline progradation is documented for both islands (1.5 m/yr for Cumberland and 0.4 m/yr for Amelia), however, localized areas of shoreline retreat are recorded along Amelia Island, especially for the southernmost 5 km of beach where erosion has been chronic since 1871. Rapid shoreline progradation adjacent to the jetties accompanied sediment deposition by longshore sediment transport. Simultaneously, a large quantity of sediment from the natural ebb-tidal delta was reworked and transported offshore in response to jetty construction and channel dredging, creating the modern ebb-tidal delta. Patterns of sediment movement at this inlet and throughout the study area indicate a dominant direction of drift to the south-southeast. Sediment losses and gains were quantified to evaluate long-term coastal change within the framework of a sediment budget. Qualitative descriptions of net sediment transport were integrated with quantitative results to produce a model of large-scale coastal evolution for the study area. From this analysis, net sediment transport in this coastal compartment is controlled by inlet and shelf hydraulics, and littoral zone processes have minimal impact on net long-term coastal change.