COMPARING HURRICANE MATTHEW (2016) AND HURRICANE JOAQUIN (2015) BOULDER TRANSPORT ON SAN SALVADOR ISLAND, THE BAHAMAS

Wave-generated energy of tropical storms is capable of moving boulders of various sizes across shorelines and inland thus shaping shore platforms and cliffs in coastal regions. Relatively little is currently known about the magnitude of the waves on shorelines during tropical storms. By utilizing long-term monitoring of boulders throughout multiple storm surge cycles, we can collect valuable information to help decipher storm-wave induced boulder movement and use it as a proxy for the past. Aerial images were collected at the Gulf site near French Bay and Green Cay on San Salvador island in The Bahamas in 2016 and 2017 following Hurricanes Joaquin and Matthew. In 2015, Hurricane Joaquin passed through the Bahamas and its eye passed directly over San Salvador as a Category 4 hurricane passing from the southwest toward the northeast (Berg, 2016). More recently, Hurricane Matthew passed through the Bahamas in the Fall of 2016 and had a very different storm track, passing west of San Salvador as a Category 3 storm along a northwest path. We utilized a DJI Phantom 3 Advanced drone to collect photographs of the rocky coastline with a 60% overlap from an altitude of 50 m. In addition, GPS-tagged digital photographs of specific very large boulders were collected in order to characterize the shape and volume of the rocks. The photographs were processed using Agisoft PhotoScan software to create orthomosaics as well as 3D renderings of the boulders. As part of the fieldwork, the boulders we photographed were also sampled to determine composition, porosity, and density. The samples were examined using a digital analysis of stained thin sections which were viewed with a petrographic microscope and used to determine rock composition and porosity, while density was determined by the displacement method. This data allow us to correlate boulder movement to a specific storm season by comparing boulder shape, composition, porosity, and density to storm parameters recorded by satellite imagery which will lead to a better understanding of the dynamics of storm surge transport.