MONITORING CHANGES IN BOULDER RIDGES ALONG ROCKY SHORELINES OF SAN SALVADOR ISLAND, BAHAMAS, AS INDICATORS OF STORM ACTIVITY (Invited Presentation)

Monitoring changes in morphology and distribution of coastal boulder ridges and the direction and amount of movement of individual large boulders can provide useful information about the intensity and effects of storms that impact Bahamian island coasts. We have been monitoring two boulder ridges on San Salvador island: one along the reef- and lagoon-protected northern coast aound Singer Bar Point (SBP, length ~790 m) and the other on the high-energy southern coast west of The Gulf (TG, length ~460 m).

Our initial work in January 2012 described the SBP ridge as wide (up to 14 m) and with a low crest (~1.5 m above mean sea level), whereas the TG ridge was generally narrower, with a sharp crest located on a cliff-bench 3-5 m above mean sea level. The largest boulders from each site were photographed, located with GPS coordinates, measured (length, width, thickness), and characterized by composition (subtidal calcarenite, coral rubblestone, eolianite, paleosol), shape (tabular or irregular), and degree of roundness.

Largest boulders at SBP are generally smaller (15 total; ~150-4000 kg; with most <1500 kg) and more rounded than those at TG (12 total; ~700-4500 kg; with all but one >1000 kg). Boulders are eroded from the seaward rocky coast, transported and deposited by high-energy storm waves. Smaller size and better rounding of clasts are more common at SBP than TG, indicating multiple events of milling in the surf prior to deposition along this low-profile coast as compared to the high cliff-profile TG coast. The presence of larger boulders and fossil coral rubblestone boulders at TG indicates that stronger storm waves were required to form and for and move them.

Our monitoring from January 2013 and 2016, after Hurricanes Sandy (October 2012) and Joaquin (October 2015), respectively, indicated very little modification to the SBP ridge along the protected northern shore. In contrast, the TG area along the high-energy southern shore was drastically modified: we were not able to relocate 2 boulders post-Sandy, and only 5 of the remaining boulders were relocated with certainty after Joaquin. Joaquin modified the formerly sharp-crested, narrow boulder ridge into a substantially larger and broader boulder field. These results illustrate the usefulness of continued monitoring of storm deposited coastal boulders throughout the Bahamas.