ASSESSING LONG-TERM CHANGES TO GIANT SAND WAVES IN EASTERN LONG ISLAND SOUND, U.S.A

We used Arc8 Geographic Information Systems (GIS) software to produce imagery of and quantify 16 years of changes to a 1 km² region within a giant sand wave field in the eastern Long Island Sound estuary. Despite the previously reported stability of these large sand waves (Ht £ 17 m) over a 7 month period in 1987 (Fenster et al., 1990), the bedform asymmetry, nature of the surficial material, in situ submersible observations, and hydraulics suggest that these bedforms reside in a zone of long-term active sediment transport. We tested this hypothesis by producing a highly accurate, digital bathymetric map in June 2003 and comparing this map to two highly accurate, digital bathymetric maps dating to May and December, 1987. Arc8 provided an extraordinary tool for visualizing and analyzing volumetric, morphologic, and planimetric changes to the sand wave field. The results show migration into the estuary (southwest) at an average rate of 2.5 m/yr +/- 0.45 m for the 17 year period and a net migration distance across the sand wave field of 38.4 m +/- 6.4 m over the same period. In addition, the southern flanks of the bedforms migrated at faster rates than other portions of the bedforms. This differential migration along individual sand waves resulted in an overall westerly rotation in orientation and bedform coalescing. Volume estimates obtained at 5 m depth datum increments ranging from 55 m to 35 m showed an exponential reduction in surface area at all three survey dates as the depth decreases. We are currently determining if a power function equation generated from these data is capable of predicting volume changes at various locations across individual bedforms. Finally, cut and fill analyses produced maps revealing the areas of sediment losses (erosion primarily along the stoss slopes) and sediment gains (deposition along active slip faces).