Three-dimensional amplification of storm-driven beach erosion: Implications for setback lines at Narrabeen-Collaroy Beach, Australia

Accurate estimation of short-term coastal erosion caused by storms (“storm demand”) is critical for determining setback lines for coastal planning purposes. Existing approaches typically rely on two-dimensional (2D) beach profiles analyses, which may underestimate erosion due to localized three-dimensional (3D) effects, particularly on wave-dominated beaches with intermediate morphodynamic beach states. This study presents an empirical analysis of storm erosion-frequency relationships at Narrabeen–Collaroy Beach, Australia, using nearly five decades of beach profile data complemented by targeted 3D storm response observations. Extreme value analysis (EVA) is applied to quantify short-term erosion volumes and shoreline change at five long-term beach profiles, while 3D storm response surveys are used to assess the contribution of localized processes. The results show that localized 3D processes, including rip currents and nearshore wave focusing, can substantially amplify storm demand. For a 100 year return period event, erosion volumes are estimated to be up to 46% greater than values derived from 2D profile data alone. Empirical design curves are developed for both typical (2D) and enhanced (3D) erosion scenarios to support coastal setback line estimation along this coastline. The analysis further shows that pre-storm beach morphology exerts a strong control on storm demand, suggesting that accreted conditions (at least 80 th percentile) should be used as the initial baseline in setback calculations. Overall, the findings highlight the value of combining long-term transect-based monitoring with rapid-response 3D surveys to improve storm erosion estimates and inform setback line planning in wave-dominated coastal environments. • Extreme value analysis performed on near-50 year dataset of short-term erosion • Significant localized amplification of erosion due to 3D effects • Accreted conditions recommended for baseline data in coastal setback line analysis