UAS-Based Quantification of Post-Storm Geomorphic Change to a Natural Beach-Dune System

In the northeastern United States, extratropical cyclones (ETCs) can become high impact weather events by creating prolonged conditions of extreme wave and water levels that often lead to geomorphic change along coastlines. It has been well documented on natural beaches that the storm response of a barrier system is dependent on the pre-existing morphology of the barrier and that the morphodynamic state of the beach will determine how waves interact with the system. Unmanned aircraft systems (UASs) allow for quick, repeatable surveys before and after storm events, which allow for a more qualitative and quantitative description of pre-existing morphology and storm-related change. The primary goal of this study was to quantify the geomorphic change that occurred to Wood End, a natural beach-dune system in Provincetown, MA, following a series of ETCs that occurred in March of 2018 and to assess what water and wave conditions caused the observed change. The March storms were characterized based on observed hydrodynamic conditions measured with an Acoustic Doppler Current Profiler (ADCP). On March 1st, 2018, the study area was surveyed using a DJI Phantom 4 Pro quadcopter. Following the survey, four ETCs affected the area. The survey was repeated on March 23rd, 2018. Structure from motion techniques were used to create pre-storm and post- storm digital surface models (DSMs). The mean RMSE, between observed and modeled elevations, was less than 8 cm for both DSMs. Geomorphic features were extracted from the DSMs using a semi-automatic extraction technique that combines landscape position, slope and curvature surfaces in ArcMap. Extracted geomorphic features were used to create surface zones based on cross-shore position. The variable storm response in each zone was assessed. There were observed patterns of erosion and deposition based on cross-shore position. The foreshore and foredune experienced a negative net volume change. The backshore and back- barrier experienced a positive net volume change. Based on ADCP measurements, the March 2nd storm had the greatest impact on the area. This storm produced a high storm surge that lasted for four high tides and coincided with the spring tide