UAS-based High-resolution Record of the Response of a Seminatural Sandy Spit to a Severe Storm

Abstract

Human infrastructures in barrier islands restrict the natural storm-induced overwash needed by these systems toevolve, further increasing their vulnerability in the medium and long term. For this reason, accurate overwashmeasurements in coastal environments subject to varying degrees of occupation are needed. In this work, twoUnmanned Aerial System (UAS) flights were performed over two distinct sectors of a sandy spit located inCádiz (SW Spain) prior and after the landfall of storm Emma. This storm was a high-energy event that lasted153 hours,with an average significant wave height of 4 m and a storm peak of 6.81 m., and water levels (tideand surge) of up to 4 m. The images derived from the UAS flights were processed with Structure From Motion(SfM) algorithms using Pix4D software, resulting in high-resolution mosaics and Digital Elevation Models(DEMs) of the study area. These allowed (1) the digitization of the washover deposits in both sectors beforeand after the storm, and (2) the computation of DEMs of Difference (DoDs), which in turn allowed the analysisof volumetric estimates of erosion/accretion triggered by the significant overwash. The results revealed a non-uniform coastal response between both sectors. The area subject to higher degree of development showed aclear restriction in the onshore delivery of sediment during overwash due to the presence of a channel and aroad, the coalescence of the nine pre-existing washovers into a unique deposit of 77546 m 2, and much higherdamage extents and associated recovery costs. In the natural sector, the overwash processes reactivated thethree pre-existing washovers (which enlarged their areas up to 200%) with no significant damages reported.These findings serve as examples of coastal responses to future similar hydrodynamic conditions, and suggestthat this spit will likely evolve following different trajectories in the long-term.