UAS-BASED QUANTIFICATION OF POST-STORM GEOMORPHIC CHANGE TO A NATURAL BEACH-DUNE SYSTEM

In the Northeast United States, extratropical cyclones (ETCs) can produce extreme wave and water levels that lead to geomorphic change along coastlines. Extreme weather events are projected to increase in frequency and size, creating a greater need for rapid, fine scale mapping of coastal areas, which provide pre-storm elevation baselines for quantifying post-storm change.

74 hectares of a natural beach-dune system was surveyed in Provincetown, Massachusetts before and after a series of 4 extratropical cyclones during March of 2018, using a DJI Phantom 4 Pro unmanned aircraft system (UAS). The UAS was flown at an altitude of 90 meters with 75% frontal and side overlap between images. Digital elevation models (DEMs) with a pixel resolution of 3 cm were derived from the UAS imagery using structure from motion (SfM) techniques. Uncertainty was assessed by comparing model output elevations to observed elevations collected with a Trimble RTK-GPS unit. To quantify volumetric change, a surface difference was performed using ArcMap GIS software. Comparison of pre- and post-storm elevation data show considerable dune erosion occurring over the 22 days between surveys. An Acoustic Doppler Current Profiler (ADCP) was deployed near the survey area, collecting waves and current data during the entire month of March. This data was used to examine the hydrodynamic conditions that led to the observed change.