The flood protection benefits provided by beaches

Beaches and other coastal ecosystems provide protection services against flooding that can be as valuable as hard engineered structures. In economic terms, this has been evaluated for corals and mangroves but, in the case of beaches, most studies are limited to exploring the physical evidence of their effectiveness as natural flood defences. In this presentation, we will discuss the role of beaches in mitigating flood risk and show a novel way of quantifying this contribution. The proposed evaluation considers the dynamic interplay between storm-induced and sea-level rise-related erosion and flooding. For illustration purposes, we will present an application of the approach to the Narrabeen-Collaroy beach system located in Australia (Toimil et al., 2023a). We will speak about uncertainties related to climate aspects of shared socioeconomic pathways, sea-level rise projections, and initial beach conditions (both uneroded and eroded due to back-to-back storms without time for recovery) that can be deemed. The proposed method is a dynamic variant of the avoided damage cost approach. It is based on the assumption that flood protection is intrinsically linked to shoreline changes driven by climate forcing. This approach is dynamic in nature, capturing the complex interactions between coastal flooding and erosion across both short and long-term temporal scales (Toimil et al., 2023b). The method involves using process-based models for downscaling offshore waves, modelling of storm hydrodynamics and morphodynamics, and modelling of overland flooding. It incorporates shoreline changes into the flood modelling by updating the existing topo-bathymetry to include both sea-level rise-induced and storm-induced erosion. We will look at scenarios of coastal flooding considering both sea-level rise and storm erosion, as well as scenarios without erosion. In each scenario, flood damage is quantified by integrating flood maps with the valuation of buildings and vulnerability assessments. The flood protection value of the beach is determined by the differential in flood damage between scenarios with and without erosion. Besides, we will also discuss about other benefits of beaches such as recreation and show a first-pass benefit-cost ratio in Narrabeen-Collaroy, hypothesizing that the mean shoreline position can be maintained through beach nourishment under various scenarios. Toimil A, Losada IJ, Álvarez-Cuesta M, Le Cozannet G (2023a) Demonstrating the value of beaches for adaptation to future coastal flood risk. Nat. Comms., 3474. Toimil, A., Álvarez-Cuesta, M., Losada, I. J. (2023b). Neglecting the effect of long-and short-term erosion can lead to spurious coastal flood risk projections and maladaptation. Coast. Eng., 179, 104248. ACKNOWLEDGEMENTS. Alexandra Toimil is funded by the Ramon y Cajal Programme (RYC2021-030873-I with funding from MCIN/AEI and NextGenerationEU/PRTR). The authors also acknowledge the funding of the H2020 CoCliCo Project (grant agreement No 101003598) and COASTALfutures (PID2021-126506OB-100 with funding from MCIN/AEI/10.13039/501100011033/FEDER UE).