MAPPING SURFACE CHANGES RESULTING FROM INDIVIDUAL STORM EVENTS IN A COASTAL DUNE COMPLEX USING AN UNMANNED AERIAL SYSTEM

We used an unmanned aerial system to measure surface change resulting from storm events at a dune complex along the southeastern shore of Lake Michigan. The complex consists of ephemeral foredunes and a foredune ridge with an active blowout, Green Mountain Beach dune (GMB), that is 30 m tall and 400 m long. A secondary blowout 10 m tall and 120 m long is migrating up the axis of GMB. We used imagery collected between storm events to construct digital elevation models (DEM) via photogrammetry. We evaluated accuracy by comparing surface change measurements from successive DEM to manual measurements at 13 pins located in active parts of the complex. Preliminary results from the accuracy study are based on measurements made on three successive images. The horizontal location of each pin was estimated using the average horizontal position of the pin on the DEM. Horizontal positioning errors were measured relative to these positions; the mean error was 1.1 cm with a maximum of 2.5 cm. Surface change measurements were obtained by extracting elevations from the DEM. The manually measured surface changes at pins ranged from -19 cm to 23 cm. The mean absolute error in the surface change measurements on the DEM was 2.4 cm, and the maximum error was 6 cm. Surface change maps were used to explore patterns of deposition and erosion throughout the complex during storm periods. 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 GMB. Up to 2.9 m of sand were deposited on the SE limb of the secondary blowout, and ~1 m of sand was deposited along the GMB crest. There were patches of erosion and deposition throughout the GMB trough and areas of erosion (up to 1 m) within the trough of the secondary blowout. Undercutting of the foredune ridge caused over 3 m of erosion and 5.5 m of inland retreat along the shoreline.