Proceddings of the 3rd Joint Symposium on the Natural History and Geology of The Bahamas
Beginning in January 2012, we have monitored two boulder ridges on San Salvador: Singer Bar Point (SBP, length ~790 m) along the reef- and lagoon-protected northern coast and The Gulf (TG, length ~460 m) on the island’s high-energy southern coast. This long-term monitoring aims at documenting changes in ridge morphology and distribution, and the direction and amount of movement of individual boulders to gain insights into the intensity and effects of storms on these coastal areas.
In the initial stage of our investigations, the largest boulders from each site were photographed, GPS-located, measured, and characterized by composition and morphology. Boulders at SBP are generally smaller (15 total; ~150-4000 kg; with most <1500 kg) than those at TG (12 original; ~700-6500 kg; most >1000 kg). Our monitoring surveys from January 2013, 2016, and 2017, after Hurricanes Sandy (October 2012), Joaquin (October 2015), and Matthew (October 2016), respectively, indicated only modest modifications at SBP, and major changes to TG, where we were unable to relocate 2 boulders post- Sandy, and 5 of the 10 remaining original boulders after Joaquin. Two TG boulders, weighing ~1 and 3 tons, were transported inland to the NNW by 20 and 26 meters, respectively during Hurricane Joaquin, and there was significant movement inland of the entire boulder ridge.
Even though documentation of boulder movement allows calculation of minimum flow velocity needed to initiate transport, our experience indicated that lack of adequate tagging made it challenging or impossible to relocate individual boulders after major storms. This problem was addressed by the application of RFID (radio frequency identification) tagging in June 2019. With a larger cohort of boulders now tagged, our monitoring program is well established to continue into the future, as passive tags are inductively charged by the reader and can remain operational for decades. Drilling to insert small tags (23 and 32 mm long, and <4 mm in diameter) is minimally destructive and also allows tagging of pebbles and cobbles. This is especially important for monitoring at SBP where large boulders are not moved often or much by waves, but smaller-sized sediment movement is significant during storms.
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Glumac, Bosiljka; Miquel, Ursula; and Curran, H. Allen, "New Advances in Long-Term Monitoring of Storm-Deposited Boulder Ridges Along Rocky Shorelines of San Salvador Island, Bahamas" (2023). Geosciences: Faculty Publications, Smith College, Northampton, MA.