HIGH-RESOLUTION UAV IMAGING AND MAPPING OF COASTAL EROSION AND BOULDER MOVEMENT PRODUCED BY THE 2015 HURRICANE JOAQUIN ON SAN SALVADOR, THE BAHAMAS

The Bahamian island of San Salvador was directly affected by Hurricane Joaquin over a two-day period in Oct. 2015 when the storm travelled SW from Bermuda, passed SE of the island then turned, intensified to a Category 4 hurricane, and then passed across the island from the S-SW. To map the hurricane related changes in the coastal environment we utilized a DJI Quadcopter 3 mounted with 12 Megapixel, 4K video camera to collect aerial data at four locations. We preprogrammed flight paths designed to acquire 70% image overlap using a smartphone app designed by Pix4D (in beta version for the DJI Phantom). The data were processed using Agisoft PhotoScan to render both a high-resolution digital orthomosaic, as well as a georeferenced digital elevation model (DEM). These datasets were then compared to pre-hurricane aerial kite imagery that we collected in March and June 2014 and to satellite imagery from Google Earth. These before and after images allow us to determine how the coastline changed. Our field research in both March and June 2015 and 2016 provide ground-based photographic and orientation data on boulder location, imbrication, and beach conditions. The high-resolution, low-altitude imagery allowed us to map the boulder field, measure the evidence of storm surge height by flotsam lines surrounding the boulder field, and calculate boulder movement by matching erosion scars to boulder position, and boulder size to position. We also noted where road construction debris provided boulders that had moved. It is clear that boulders that are exposed along the wave-cut platform at low tide of the Pleistocene Cockburn Town Reef have been transported up the cliff and inland in this storm event based on boulder identification. We have previously noted that coastal reentrants are the location of coves where wave action is focused and thus increase lift. Coastline retreat where cliffs collapse downward are located along these coves and provide large boulders which are available for transport upward. These coves become the staging ground for boulders to be elevated in extreme storm events. Our data show that the coves are locations of wave focusing and greater storm surge landward transport. Our aerial UAV imagery from Green Cay show that large boulders previously noted there did not move in this hurricane.