CONSTRAINING THE VERTICAL LIMIT OF BLUFF EROSION ON BLOCK ISLAND, RHODE ISLAND USING TOPOGRAPHIC PROFILES EXTRACTED FROM PRE AND POST STORM LIDAR

Superstorm Sandy made landfall in New Jersey 213 km (194 mi) south west of Block Island on October 29, 2012. Significant wave heights during Sandy exceeded 9 m) at a buoy southeast of Block Island (with the largest individual waves > 14 m (47 ft). Coupled with a 1.5 m (5 ft) storm surge, Sandy caused extensive erosion along parts of the island. The objective of this research is to analyze the erosion of the bluffs along the shorelines of Block Island, RI after Superstorm Sandy using cross-sectional profiles extracted from sequential LiDAR elevation models.

Cross-sectional profiles extracted from bare earth digital elevation models (1 m cell size) created from the 2011 U.S. Geological Survey (pre-Sandy), and 2012 U.S. Army Corps of Engineers LiDAR data collected two weeks after Sandy, were compared to examine the response of the bluff shoreline to the storm. Transects perpendicular to the shoreline were cast at a spacing of 100 m and topographic profiles were extracted using the Interpolate Line tool in ESRI ArcMap v. 10.1 Spatial Analysis extension.

166 transects featured a bluff backed shoreline, and we compared the pre-Sandy and post-Sandy profiles. The bluffs range in height from < 5 to >50 m above MLLW, and vary in composition from Cretaceous-Tertiary aged Coastal Plain strata to Late Wisconsinan glacial till and stratified deposits, producing a complex response to storm events. The vertical limit of bluff erosion averaged 7.5 m above MLLW for the 166 transects, although the elevation ranged from < 1 m to > 20 m. Where the elevation of the bluff crest was lower than 10 m, the entire bluff face retreated during the storms. Bluffs between 10 and 15 m above MLLW had a varied response to erosion; in some cases the entire bluff face retreated. The bluffs > 15 m above MLLW saw erosion limited to the toe of the bluff and lower bluff face, although significant variation in the vertical limit of erosion was observed.

While the varied response appears to be largely due to the heterogeneity of the bluff composition, nearshore bathymetry and shoreline orientation, bluff height provides a first-order control on the response to storms. Whether the segments of the bluff that show erosion at the toe, are more likely to fail in the future require continued observation, and quantifying the volume of sediment eroded at each transect will be examined in the future.