CENTURY-SCALE (1886 – 2018) BLUFF EROSION: BLOCK ISLAND, RHODE ISLAND, USA

Block Island, Rhode Island, a 25 km² island located 15 km south of the Rhode Island mainland and 22 km east of Montauk Point, Long Island is comprised of two sections of end moraine connected by barrier spits. Bluff erosion here has produced some large slump events and presents a management challenge particularly for residential properties along the edge of the bluff. The bluffs here vary in height from > 2.5 m above sea level to > 50 m in elevation. The complicated glaciotectonic formation of the island results in a variable stratigraphy, composed of a mixture of Cretaceous Coastal Plain Strata and Late Wisconsinan (and older) glacial deposits which range from (diamict (till) and ice-marginal sand and gravel to glacial lacustrine (silt and clay) deposits. The position of the bluff crest was mapped using an 1886 1:10,000 scale National Ocean Service Topographic map (T-sheet) and 2018 U.S. Army Corps of Engineers National Coastal Mapping Program Topobathymetric LiDAR derived digital elevation models. The 1886 survey was very detailed, known locally as the ‘stonewall map’ for the detailed mapping of the innumerous stonewalls on the island. The bluff edge was clearly delineated on the T-sheet along both the open coast and in the gullies along the southern shoreline. The stonewalls have largely been preserved, and the intersections of these walls provided targets for georeferencing, resulting in a rectifying error of 2.8 m and a total positional error of 10.4 m. The position change of the bluff crest was analyzed at transects cast using the U.S. Geological Survey Digital Shoreline Analysis System at a spacing of 25 m alongshore. The cast transects were manually adjusted to remain orthogonal to the bluff crest. Excluding the transects without a bluff resulted in a total analysis of 630 transects. The total positional uncertainty was exceeded at 86% of the transects. The average recession of the bluff crest was 28 m (annualized rate -0.21 m yr^-1) and the average bluff crest retreat varied between 20 to 32 m of retreat for different subsections of the island. The maximum retreat (>80 m) occurred along the southern (Mohegan Bluffs) shoreline. The bluff retreat was also compared to shoreline change rate and bluff crest elevation with mixed results.