UAS Coastal Geomorphology Research

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. UAS-based methods were used to survey a natural beach-dune system within Cape Cod National Seashore before and after a series of 4 extratropical cyclones during the month of March 2018. 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.

The study area is in Provincetown, MA (town boundary shaded). The beach-dune system of interest is outlined in red and shown in the inset.

Structure from motion techniques were used to process UAS imagery and create pre-storm and post-storm digital surface models (DSMs) of the area. On average, the vertical displacement of the dune toe was 0.4 m. Overall, the 16-hectare area experienced a positive net volume change of 1,224 m³of deposited material.

Alongshore variation in dune toe displacement

Pre-storm position of the dune toe (red line in image) and the post-storms vertical displacement in meters (bottom). The alongshore position of areas that were overwashed are highlighted in gray. Portions of the dune base that showed slumping are shown in blue. Portions of the dune that were cut back are shown in red.

Volume change based on cross-shore position

Areas of erosion (red) and deposition (green) based on cross-shore position (top) and total volume change estimates in m3 for each cross-shore zone (bottom).

Surface comparison - volume change

Areas of high and low volume change on a cell by cell basis, overlaid on pre-storm UAS imagery. Areas that experienced the greatest amount of volumetric change are dark

Three beach profiles were extracted and compared from the pre-storm and post-storm DSMs.

Transect locations (red) within the study area boundary (white).

Transect 1 shows a cross-shore profile at the northernmost extent of the foredune. Between surveys the northern tip of the foredune was removed and material was deposited onto the back-barrier.

Transect 1: Cross-shore beach profiles from the pre-storm (blue) and post-storms (red) surveys, 5x vertical exaggeration. The MHW line is shown in black.

Transect 2 and 3 include elevation data that was collected in addition to the pre-storm and post-storm profiles. The additional elevation data allowed for the observed overwash event to be isolated and also showed how quickly beach aggradation can occur following a large erosive event. A layer of material, with a depth of roughly 0.5 m, was redeposited along the backshore and back-barrier.