INTEGRATING NEW TECHNIQUES FOR SEDIMENTOLOGICAL AND MATERIAL TRANSPORT STUDIES IN ESTUARINE SETTINGS: HIGH-RESOLUTION GPS, LIDAR AND NUMERICAL MODELING IN GROVES CREEK, GEORGIA, USA

Studies of the marsh environment are particularly complex because of the need to embed observations and conduct numerical modeling within the context of fine-scale geomorphology, which is commonly unavailable. New techniques in the past decade (i.e., RTK GPS, VRS networks, multibeam sonars, high-precision echo sounders, high-resolution numerical models) now provide a means to gather detailed geomorphological observations and document fine-scale conditions within salt marsh environments, although questions remain about the utility of some of these methods (i.e., LiDAR).

Groves Creek marsh near Savannah, GA, was mapped during 2011-2012 at cm-scale resolution (2.7 M points), using the techniques described above. Comparison of RTK groundtruth and LiDAR performance in salt marshes demonstrates that the highest vertical errors are located in the non-platform, creek and levee zone where taller, more dense grass is observed (mean difference = 0.07 m, RMSE = 0.14 m). On the marsh platform where grasses are sparse, LiDAR-derived elevations were generally in good agreement with RTK-surveyed elevations (mean difference = 0.00 m, RMSE = 0.06 m), suggesting that LiDAR can be useful in salt marsh settings.

Groves Creek marsh is typical of Spartina alterniflora salt marshes of the southeastern US, with a main subtidal channel, numerous intertidal channels and broad, vegetated, intertidal marsh platforms. Dynamical processes move water and sediment within this system semidiurnally; dye studies and numerical modeling demonstrate that dissolved constituents are detained by the marsh on weekly timescales and follow flow paths that can only be reasonably delineated using realistic morphologic data. Geologic and microfossil evidence suggests that environmental conditions have changed significantly over the past few thousand years as the estuarine marshes have developed, creating a distinct fining-upward sequence from a combination of allochthonous and autochthonous materials. The upper meter of the marsh reflects conditions closer to those at present: textural composition of 5%, 25% and 70% sand, silt and clay, respectively, carbonate content of 10-20%, short-term (⁷Be) and long-term (²¹⁰Pb) accumulation rates of 0.1-1 cm/y and δ¹³C values (-17 to -20‰) reflecting the strong S. Alterniflora input.