Sand ridge morphology and bedform migration patterns derived from bathymetry and backscatter on the inner-continental shelf offshore of Assateague Island, USA

The U.S. Geological Survey and the National Oceanographic and Atmospheric Administration conducted
geophysical and hydrographic surveys, respectively, along the inner-continental shelf of Fenwick and
Assateague Islands, Maryland and Virginia over the last 40 years. High resolution bathymetry and backscatter
data derived from surveys over the last decade are used to describe the morphology and presence of sand ridges
on the inner-continental shelf and measure the change in the position of smaller-scale (10–100 s of meters)
seafloor features. Bathymetric surveys from the last 30 years link decadal-scale sand ridge migration patterns to
the high-resolution measurements of smaller-scale bedform features. Sand ridge morphology on the inner-shelf
changes across-shore and alongshore. Areas of similar sand ridge morphology are separated alongshore by
zones where ridges are less pronounced or completely transected by transverse dunes. Seafloor-change analyses
derived from backscatter data over a 4–7 year period show that southerly dune migration increases in
magnitude from north to south, and the east-west pattern of bedform migration changes ~ 10 km north of the
Maryland-Virginia state line. Sand ridge morphology and occurrence and bedform migration changes may be
connected to observed changes in geologic framework including topographic highs, deflated zones, and sand
availability. Additionally, changes in sand ridge occurrence and morphology may help explain changes in the
long-term shoreline trends along Fenwick and Assateague Islands. Although the data presented here cannot
quantitatively link sand ridges to sediment transport and shoreline change, it does present a compelling
relationship between inner-shelf sand availability and movement, sand ridge occurrence and morphology,
geologic framework, and shoreline behavior.