Shallow Water Estuarine Mapping in High-Tide-Range Environments: a Case Study from Georgia, USA

Efficient methods for shallow water data collection have been employed to create comprehensive bathymetric surfaces for 3 Georgia sounds. Survey areas were characterized using existing charted depths, aerial photographs, and lidar-derived elevation data to identify regions shallower than 3-m mean lower low water. Single-beam sonar missions were completed for these shallow areas using various marine platforms to constrain the existing 3-m isobath. Below 3 m, waters are deep enough, and swath width is wide enough, to use a larger vessel with an interferometric sonar system collecting swath bathymetry and sidescan imagery. The collected data were merged and gridded in a geographic information system environment to generate digital elevation models. The modern digital surfaces were compared with historic depth data to identify areas of shoaling and deepening. The mean overall change between the historic data and modern data was relatively small (0.7 m for Wassaw Sound, 0.4 m for Ossabaw Sound, and 0.1 m for Sapelo Sound); only the change in Wassaw was greater than the error in the analysis. However, each sound exhibited up to 20 m of shoaling and deepening associated with entrance channel migration, illustrating that a significant change occurs in specific geomorphic zones in each of these settings. Sediment grabs ground-truth sidescan sonar imagery to interpret environmental energy and bottom character. The mean sediment size for all sounds fell in the fine to very fine sand range (2–4 phi), reflecting the characteristics of the coastal sediment prism, although gravels and muds characterize areas of scour and quiescent conditions, respectively. These surveys led to the first identification of hard-bottom habitats of erosion-resistant, vertical cliffs up to 10 m high and 1000 m long in Southeastern US estuaries.