MONITORING TRANSPORT OF COASTAL CARBONATE BOULDERS AND COBBLES BY STORM WAVES USING RFID (RADIO FREQUENCY IDENTIFICATION) ON SAN SALVADOR ISLAND, BAHAMAS

Since 2012, teams from Smith College have monitored formation and transport of limestone boulders by storm waves at two study sites on San Salvador: 1) Singer Bar Point (SBP) along the reef- and lagoon-protected, gently seaward-sloping northern coast; and 2) The Gulf (TG) on a 3-5 m elevated cliff bench, along the high-energy southern coast. This long-term monitoring aims at documenting direction and amount of boulder movement to gain insights into effects and intensity of storms impacting this small, low relief, tropical carbonate island.

Observations after Hurricanes Sandy (2012), Joaquin (2015), and Matthew (2016), revealed only modest modifications at SBP, but major changes to TG, where 2 boulders were not relocated post-Sandy, and only 5 of 12 monitored boulders were relocated after Joaquin, which passed over the island as a Category 3 hurricane with 120-130 mph sustained winds. New boulders, as large as 3 m in diameter, were generated, and blocks from prior storms, weighing 1-3 tons, moved up to 26 m inland.

To aid in boulder relocation, in June 2019, >50 clasts were tagged at each study site for Radio Frequency Identification (RFID) utilizing small (up to 32 mm long, <4 mm diameter) Passive Integrated Transponder (PIT) tags. Tagged clasts were relocated in Jan. 2020. No hurricanes impacted the island in 2019, and coastal cliffs at TG were not overtopped by waves. Significant changes, however, were documented at SBP: 14 tagged clasts, weighing ~3 to 70 kg moved up to 13 m, with 2 missing due to potential offshore transport. Clast size does not correlate to transport distance. Most clasts moved inland and laterally along the coast, and some rolled down the boulder ridge towards the water. Calculated flow velocity needed to initiate transport ranged from 1.1 to 2.5 m/s according to Nandasena et al. 2011 equations.

Smaller size and better rounding of clasts at SBP (relative to TG) indicate more frequent movement by waves in this relatively protected site, but use of RFID in conjunction with high-resolution drone imaging from Jan. 2016 and 2020, allowed quantification of the amount and direction of sediment movement by winter storms from cold fronts. This information would otherwise not be readily available and as useful in evaluating coastline response to increasing storm impacts with ongoing climate change and sea-level rise.