STORM-DEPOSITED COASTAL BOULDER RIDGES ON SAN SALVADOR ISLAND, BAHAMAS IN THE AFTERMATH OF HURRICANE JOAQUIN

This study focuses on storm-deposited boulder ridges in two localities on San Salvador Island, Bahamas, and how specific large boulders were moved and/or modified by hurricanes. Fifteen boulders from the Singer Bar Point site along the north coast and twelve boulders from The Gulf site on the island’s south shore were cataloged initially in January 2012, then in early 2013 following Hurricane Sandy in October 2012, and again in January 2016 after Hurricane Joaquin, which passed directly over the island in October 2015. Cataloging included photographing each selected boulder and documenting its GPS location, size, and composition. Aerial photos taken from a drone were also used for comparative analysis of overall erosional impact of Hurricane Joaquin.

All cataloged boulders at Singer Bar Point were relocated in 2013 and 2016 at or near their former positions, although surrounding smaller clasts were moved by storm waves. All but two boulders from The Gulf were relocated in 2013, post-Hurricane Sandy. In contrast, only five of the remaining cataloged boulders at The Gulf were relocated with certainty after Hurricane Joaquin. Two of those, weighing ~2 tons each, were transported as much as 20 m and 26 m inland to the NNW. 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 can be used to map the extent of storm erosion. The shape of this formerly sharp-crested boulder ridge was modified by storm waves into a larger, broad boulder field, stripped of vegetation, and partially covering the island’s main coastal road.

Results indicate that Hurricane Joaquin had much greater impact than Hurricane Sandy, which passed close but not over San Salvador. In contrast, the eye of Joaquin went directly over the island on October 1-3, 2015 as a high category 3 hurricane with winds of 120-130 mph. The northern coast of the island is more protected and underwent less modification than the high-energy southern coast where the storm caused substantial erosion. Distribution and morphology of boulder ridges and information about dynamics of their modification are long-term indicators of storm patterns and activity and should be used to inform coastal development on San Salvador and elsewhere in the Bahamas.