Medium-term Analysis of Coastal Cliff Retreat Change at Pacheco Beach, Northeastern Brazil

Façanha, M.C.; Albuquerque, M.G.; Paula, D.P.; Leisner, M.M.; Espinoza, J.M.A.; Vasconcelos, Y.G.; Alves, D.C.L., and de Sá, L., 2024. Medium-term analysis of coastal cliff retreat change at Pacheco beach, northeastern Brazil. In: Phillips, M.R.; Al-Naemi, S., and Duarte, C.M. (eds.), Coastlines under Global Change: Proceedings from the International Coastal Symposium (ICS) 2024 (Doha, Qatar). Journal of Coastal Research, Special Issue No. 113, pp. 335-340. Charlotte (North Carolina), ISSN 0749-0208. The coastal zone has a variety of landforms, such as gently sloping beaches and cliffs. Erosion of the cliffs can threaten infrastructure, leisure resources, public safety and coastal transportation. In this sense, this study sought to use aerophotogrammetry techniques, based on Unmanned Aerial Vehicle (UAV) images, to understand the morphodynamic behavior of sedimentary cliffs located on an urban coastal stretch in northeastern Brazil (Pacheco beach, Caucaia-CE). The method applied was based on a protocol of techniques applied in the analysis of cliff morphodynamics using UAVs - (i) obtaining ground control points; (ii) programmed flights; (iii) orthorectification of photographs; (iv) topographic maps; (v) data integration and analysis using transects and the Digital Shoreline Analysis System (DSAS), which made it possible to delineate cliff top indicators and determine their evolution in the medium term (May/2021 - April/2024). Os resultados indicam mudanças morfológicas severas em um trecho de 700 m de falésias sedimentares, com padrões de erosão consistentes. The results indicate severe morphological changes in a 700m stretch of sedimentary cliffs, with consistent erosion patterns. The line defined for the top of the cliff showed erosion over 100% of its length, with a total retreat of -5.7m between May/21 and April/24, with an average monthly rate of -0.16m. The cliff retreat prognosis for 10 and 20 years shows an eroded land area of 1.22 ha and 2.63 ha, respectively. Pacheco's sedimentary cliffs are susceptible to marine (e.g. wave abrasion) and sub-aerial (e.g. rain, wind) processes that act on a friable sedimentary material (sand-clay), leading to episodes of mass movement. In this way, the cliff top indicator proves to be a reliable indicator of the retreat and evolutionary behavior of this type of coastal relief, highlighting the portions most vulnerable to erosion, i.e. areas where built infrastructure is impacted. Therefore, the study emphasizes the importance of studies that understand the evolution of cliffs, which helps in appropriate and sustainable coastal management measures that avoid disasters, devaluation of properties and threats to human life.