INITIAL STRUCTURE-FROM-MOTION (SFM) CHARACTERIZATION OF GEOMORPHIC CHANGE FOR 3 CONTRASTING COASTAL BLUFF SHORELINES, BLOCK ISLAND, RI

We use ground, drone, and helicopter SfM modelling to map, visualize and measure the geomorphic character and, for some locations, change at 3 contrasting stratigraphic settings on southwest (Graces Point, GP), south (Snake Hole, SH), and northeast (Clayhead, CH) Block Island. Ground-based models at CH (2017, 19, 21, 22, 23) are most detailed/precise for scaled measurements but cover small areas and are not all in real world coordinate space limiting use for change detection. Drone models (Mavic Pro 2 & Mavic 3 Enterprise RTK; 2021, ’22, ‘23) are detailed covering up to ≈500 m of shoreline in known coordinate space. Helicopter models encompass the entire Island and primarily are used to assess stratigraphy.

Metashape/Cloud Compare/ArcGIS analyses characterize laterally complex stratigraphy and geomorphic change at the 35-37 m tall GP, CH, and SH bluffs that expose glacial, pro-glacial and former coastal plain deposits. Curved shear-planes occur at all site, often vertically juxtaposing rheologically differing strata including stratified sandy meltwater deposits, boulder-rich diamict/till, finer silt-to-clay lacustrine sediments, and displaced Cretaceous boulder layers and fine-grained strata. Shoreline retreat at GR occurs within sand to gravel-rich stratified meltwater deposits. Change detection (Jan. – Jun. ‘23) illustrate progressive headward erosion and raveling of sands to ≈5 m above a Dec. ‘22 wave-cut notch resulting in a broad angle-of-repose colluvial apron with headward migrating slumps and sand flows. SH includes several steeply inclined ≈15-20 m thick sequences of desiccating lacustrine silty clay with ≈10 m of intervening stratified sand/cobble deposits, and ≈3 m of overlying horizontal stratified sands. Fine-grained strata likely contributed to a ≈150 m wide slump of overlying sands that occurred in 2000. Weathering promotes steep sided pinnacle-like exposures that periodically topple and/or spall significant boulder-sized silty-clay blocks to the foreshore. CH illustrates staggered development of rill/gully/fan complexes within sandy-gravelly stratified deposits that deliver sediments from bluff to foreshore. In contrast, slow undercutting of Cretaceous boulder layers periodically triggers failure and delivery of boulder masses to the foreshore for diminution.