APPLICATION OF DRONES IN MONITORING LIMESTONE BOULDER GENERATION AND TRANSPORT BY STORM WAVES ALONG ROCKY SHORELINES OF SAN SALVADOR ISLAND, BAHAMAS

To understand better the dynamics of coastal processes, and especially formation and movement of limestone boulders by storm waves, we have been monitoring two areas on San Salvador, Bahamas, since Jan. 2012: Singer Bar Point (SBP) on the gently-sloping, reef- and lagoon-protected northern coast, and The Gulf (TG) on the 3-5 m high cliff along the island’s high-energy southern coast. Initially, 27 large boulders were photographed, GPS-located, measured, and characterized by composition and morphology. Continuing monitoring from Jan. 2013, 2016, and 2017, revealed modifications by Hurricanes Sandy (Oct. 2012), Joaquin (Oct. 2015), and Matthew (Oct. 2016), respectively. Post-Joaquin, we could not relocate many of the boulders at TG, where we also documented 20-26 m landward transport of 1-3 ton boulders. Use of drones in Jan. 2016 and 2017 greatly helped document the heterogeneous impact of Hurricane Joaquin on the two study areas. In June 2019, we also initiated tagging of boulders using RFID (Radio Frequency Identification) to aid in their relocation after major storms. This technology allows tagging of smaller cobbles and pebbles, which necessitated need for even higher-resolution drone imaging, conducted in Jan. 2020. At that time, no major changes were noted at TG since no hurricane impacted the island during the 2019 season. In contrast, significant changes in the location of tagged cobbles and smaller boulders at SBP were documented between June 2019 and Jan. 2020. In conjunction with high-resolution drone imaging, this allowed us to evaluate impact of winter storms (cold fronts) on this more protected site. The presence of smaller and better-rounded pebbles and cobbles at SBP relative to generally larger and more angular boulders at TG indicates more frequent movement of clasts by waves at SBP, and the combined use of RFID and drone technologies allows quantification of amounts and direction of sediment movement and establishing relationships to coastal morphology and storm parameters. In conjunction with digital elevation maps (DEM) and digital transects created from drone-acquired data, this new database serves as baseline for future comparative analyses of these coastal areas and their response to sea-level rise and predicted increase in frequency and intensity of storms.