GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 202-5
Presentation Time: 9:00 AM-6:30 PM


MIGUEL, Ursula, GLUMAC, Bosiljka and CURRAN, H. Allen, Department of Geosciences, Smith College, Northampton, MA 01063

Since January 2012, we have been monitoring two boulder ridges on San Salvador Island, Bahamas: 1) Singer Bar Point (SBP, length ~790 m) along the reef- and lagoon-protected northern coast; and 2) The Gulf (TG, length ~460 m) on the high-energy southern coast. Our long-term monitoring is designed to document changes in ridge morphology and distribution, and the direction and amount of movement of individual boulders to gain insights into intensity and effects of storms.

Initially, the largest boulders (15 at SBP, 12 at TG) were photographed, GPS-located, measured, and characterized by composition and morphology. Boulders at SBP are generally smaller (~150-4000 kg; with most <1500 kg) than those at TG (~700-6500 kg; most >1000 kg). Monitoring 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 only 5 of 12 original boulders were relocated after Joaquin. Two of those, weighing ~1 and 3 tons, were transported inland to the NNW by 20 and 26 meters, respectively.

Even though documentation of boulder movement allows calculation of minimum flow velocity required to initiate their transport, the lack of adequate tagging made it challenging or impossible to relocate individual boulders after major storms. This problem is addressed by application of RFID (radio frequency identification) tagging in June 2019 when >50 boulders were tagged at each study site. Their locations will be checked in January 2020 after the 2019 hurricane season. More tagging is expected to be done at that time and in 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 invasive and also allows tagging of pebbles and cobbles. This is especially important for monitoring at SBP where large boulders have not been moved much by waves in the recent past, but smaller clasts do move actively. In conjunction with continuing high-resolution drone imaging, use of tagging that can uniquely identify an object within a large population allows significant increase in our database and improvement of the monitoring efforts.