MAPPING SURFACE CHANGES ANNUALLY AND AFTER INDIVIDUAL STORM EVENTS IN A COASTAL DUNE COMPLEX USING UNMANNED AERIAL SYSTEMS

We used an unmanned aerial system (UAS) to measure surface change at Green Mountain Beach (GMB), along the southeastern shore of Lake Michigan. At GMB a 40 m tall establish foredune ridge contains multiple active blowouts. One large blowout (30 m tall and 400 m long) has a secondary blowout (10 m tall and 120 m long) migrating up its axis. We used imagery obtained by UAS to make digital elevation models (DEM) using photogrammetry. Maps of surface change over the complex were constructed by subtracting successive DEM. We evaluated the accuracy of this approach by comparing surface change measurements from the DEM to manual measurements made at 13 deposition pins located in active parts of the complex. The locations of the pins were marked during each flight so they could be identified in the imagery. This also gave fixed positions that we used to estimate the horizontal positioning error in the maps. Preliminary results indicate that the mean horizontal positioning error was 1.1 cm with a maximum horizontal error of 2.5 cm. The mean error in the vertical surface change measurements was 2.4 cm, and the maximum error was 6 cm. Surface change maps were used to explore patterns of deposition and erosion both annually and during selected storm events. For example, following an October 2019 storm with regional winds of 18 m/s and wave heights of 3.5 m, a series of small dunes (10 m long, 25 m wide) were formed by sand deposited (0.5 m deep) within the trough of the large blowout, up to 2.9 m of sand was deposited on the SE limb of the secondary blowout, and approximately 1m of sand was deposited along the blowout crest. There were patches of erosion and deposition throughout the main trough and erosion (up to 1 m) within the trough of the secondary blowout. Undercutting of the foredunes resulted in over 3 m of vertical erosion and 5.5 m of inland retreat in some areas along the shoreline.