IMPACT OF HURRICANE MATTHEW (OCTOBER 2016) ON SEDIMENT-STARVED SOUTHERN COAST OF LITTLE EXUMA ISLAND, BAHAMAS

Conch House Beach (CHB), a leeward, sediment-starved setting on the south coast of Little Exuma Island, Bahamas, was impacted by Hurricane Matthew, a major category 3 hurricane whose eye passed ~65 km offshore southwest of the study site on October 6, 2016. Goals were to understand better storm impact and general depositional patterns and sediment composition in this remote carbonate coastal setting through field and petrographic observations and historical aerial imagery analysis.

Sand at CHB is predominantly composed of heavily micritized ooids and lithoclasts derived from weathering of bedrock (mainly Holocene beachrock and eolian ooid grainstones), with subordinate amounts of skeletal fragments. The unusual brown color of beach sand indicates subaerial weathering in the presence of iron oxides and supports low rates of sediment transport to and deposition in this setting. Historical Google Earth images (2005-2016) also indicate a consistently sand-starved beach dominated by exposures of dark, weathered bedrock.

Abundance of ooids in beach sand and bedrock at CHB indicates that leeward settings of Bahamian islands are conducive to ooid formation. Ooids are likely forming in wave-agitated shallow offshore waters <2 m deep. In deeper waters, sediment is being stabilized by an organic microbial film. In conjunction with relatively low fair-weather wave energy, this prevents large quantities of sand from being readily transported and deposited on the beach, leading to generally sediment-starved conditions.

Erosion by hurricanes also has a major impact on this coastal setting. Evidence of Hurricane Matthew on CHB, observed in January 2017, included the abundance of freshly exposed, light-colored bedrock with erosional scars reaching ~3.5 m above mean sea level, imbricated boulders encrusted with fresh bivalves found on the beach >10 m from the water’s edge, and deposition of beach and dune sand in a 1 m-thick washover fan into an adjacent mangrove swamp.

Our results indicate that storms have a major impact on style and rate of sedimentation and erosion in this otherwise relatively low-energy coastal setting. This should be taken into account, along with the documented low rates of sediment deposition and high rates of erosion, in any future plans for development along this reach of coast.