COMPARISON OF CENTURY (1886 – 2018) AND QUASI-DECADAL SCALE (2011-2018) RATES OF COASTAL BLUFF EROSION: BLOCK ISLAND, RHODE ISLAND, USA

Block Island, a 25 km² island 15 km south of Rhode Island is comprised of two sections of interlobate end moraine connected by barrier spits. The moraine bluffs vary in relief from < 2.5 m to > 50 m. The complicated glaciotectonic formation of the island results in a variable stratigraphy, composed of a mixture of displaced Cretaceous Coastal Plain and glacial deposits which range from (diamict (till) and ice-marginal sand and gravel to glacial lacustrine (silt and clay) deposits. Bluff erosion here has produced large slump events and presents a management challenge particularly for residential properties along the edge of the bluff. This project examines the retreat of the bluff crest at the century (1886 to 2018) and quasi-decadal scale (2011-2018) using a National Ocean Service Topographic map (T-sheet) and aerial LiDAR derived digital elevation models (DEM) from April 2011, November 2012, November 2013, and June 2018. The period between the 2011 and 2012 surveys encompasses both Tropical Storm Irene (August 2011) and Hurricane Sandy (October 2012). The bluff crest retreated at an average of 0.23 m yr^-1 between 1886 and 2018 (end-point rate); the rate was more than double between 2011 and 2018 (average retreat 0.49 m yr^-1). On average, 13% of the total retreat occurred between 2011 and 2018 despite (< 5% of the total elapsed time), showing the importance of episodic storm events in the long-term retreat of the bluffs. Transects extracted from the LiDAR DEM shows the bluffs >15 m in relief were largely undercut during Sandy with no change in bluff crest position. Many of these transects had subsequently collapsed by November 2013, causing a retreat in the bluff crest. Comparing the oldest (2011) and most recent (2018) LiDAR DEM calculates that ~475,000 (+/-70,000) m³ of sediment have eroded from the bluffs over that period, and almost half of the sediment eroded between 2011 and 2012 (227,000 +/- 81,000 m³) due to the combined erosion from Tropical storm Irene and Hurricane Sandy. A back of envelope calculation suggests the total volume of sediment eroded from the bluffs is >3.5 million m³ between 1886 and 2018.