Documenting the Generation and Transport of Large Rock Boulders by Storm Waves Along the High-energy Southern Coast of San Salvador Island, Bahamas

Documenting changes in morphology and distribution of coastal boulder ridges and the direction and amount of transport of individual large boulders provide useful information about the intensity and effects of storms that impact Bahamian islands. We have been monitoring an almost 500 m long boulder ridge on a cliff-bench, 3-5 m above mean sea level, along the high-energy southern coast of San Salvador Island since January 2012. Twelve large boulders ( 700-4500 kg; with all but one >1000 kg) were initially photographed, located with GPS coordinates, measured, and characterized by composition (subtidal calcarenite, coral rubblestone, eolianite, lithified paleosol), shape, and degree of roundness. The boulders are eroded from the seaward rocky coast, transported and deposited by high-energy storm waves.

Our continuing monitoring in January 2013, 2016, and 2017, after Hurricanes Sandy (October 2012), Joaquin (October 2015), and Matthew (October 2016), documented drastic modifications to the boulder ridge. We were not able to relocate 2 boulders post-Sandy, and only 5 of the remaining boulders were relocated with certainty after Joaquin, which passed directly over the island as a high category 3 hurricane with 120-130 mph sustained winds. Storm waves overtopped the coastal cliffs, causing erosion at the leading edge and extensive landward movement of boulders. This modified the formerly sharp-crested, narrow boulder ridge into a larger, broad boulder field, 6.3 ha in area, stripped of vegetation. New boulders, as large as 3 m in diameter, were generated, and blocks from prior storms, estimated to weigh 1-3 tons, moved up to 26 m inland. The principal coastal road was damaged and inundated by debris. The southern edge of the boulder ridge moved landward by 4-5 m exposing an underlying Pleistocene/Holocene boundary terra rossa paleosol, which stands out in aerial images and marks the extent of storm erosion. Post-Hurricane Matthew, which passed too far from the island to have a major impact, 11 new boulders and high-resolution drone aerial imagery were added to our monitoring program to expand our database and allow continuing documentation and communication of information on vulnerability to hurricanes with local residents, developers, and other decision- and policy-makers on San Salvador and elsewhere in the Bahamas.