UAS & SfM-based approach to Monitor Overwash Dynamics and Beach Evolution in a Sandy Spit

Abstract

The role of overwash processes is of great relevance in the long-term evolution of sandy barriers, and it can also affect coastal infrastructures in the short term. In this work, a 10-month monitoring program based on the use of Unmanned Aerial Systems (UAS) in combination with Structure from Motion (SfM) algorithms was performed in order to monitor morphological changes and ulterior evolution at Camposoto beach, an overwash-prone coastal spit located in SW Spain. This UAS&SfM approach allowed the reconstruction of 6 Digital Elevation Models (DEMs) of the area with high temporal and spatial resolutions (RMS vertical errors spanning from 5 to 8 cm). The morphological changes occurred were detected using Geomorphic Change Detection Software in ArcGIS, and they were correlated with the oceanographic conditions that prevailed during the analysed period. The results obtained provided insight into the response of the system against storm-induced overwash, which caused significant erosion/accretion patterns over a pre-existing washover fan, as well as the landward migration of the system. In addition, this methodology captured the progressive partial recovery of the system, which included onshore transport of sand, fore-beach accretion, and berm reconstruction. Despite the partial recovery observed, spring tides also triggered non-storm overwash in the area during fair-weather conditions, allowing to rethink the actual vulnerability state of the spit against rollover processes.