Demonstrating the value of beaches for adaptation to future coastal flood risk

The study addresses how to quantify the flood protection value of beaches under climate change, specifically sea-level rise (SLR) and storm-driven processes. Coastal zones face increasing inundation and extreme flood events, altering landscapes and exposure of communities and assets. Ecosystems such as mangroves, coral reefs, and beaches provide flood protection services that are declining without mitigation and adaptation. While mangroves and coral reefs have been economically assessed for flood protection, beaches—comprising roughly one-third of the world’s coasts—require improved valuation methods that account for their dynamic response to storms and rising sea levels. The research proposes a dynamic, process-based valuation framework that couples shoreline evolution (both storm-scale and long-term SLR-driven changes) with coastal flooding to estimate avoided damages. The purpose is to inform cost-effective adaptation, including nourishment decisions, by quantifying the benefits of maintaining beach width under uncertainty in emissions pathways, SLR projections, and beach conditions. The approach is demonstrated at Narrabeen-Collaroy, Australia.

Prior work has quantified the flood protection benefits of mangroves and coral reefs and highlighted their conservation and restoration value to agencies and insurers. For beaches, studies have examined their role as natural defences and monetised services such as recreation losses due to erosion, effects of erosion and nourishment on property values, taxation for adaptation projects, and willingness-to-pay for erosion prevention and flood protection. However, traditional coastal flood assessments often neglect dynamic shoreline changes, risking misestimation of flood risk. This study advances the literature by integrating storm-scale morphodynamics and long-term SLR-driven shoreline retreat directly into flood modelling and economic valuation, aligning with avoided damage cost methods used for other ecosystems while addressing the specific dynamics of sandy beaches.

The authors develop a dynamic valuation framework that couples erosion and flooding across time scales and compares it to a static (no-erosion) approach. Key steps (Fig. 1): (1) Nearshore wave downscaling from offshore hindcasts; (2) Integration of SLR-induced erosion via long-term topo-bathymetric updates; (3) Process-based surf-zone hydrodynamics and morphodynamics during a reference storm; (4) Integration of storm erosion on long-term profiles; (5) 2D inland flood propagation; (6) Flood damage estimation; (7) Calculation of avoided flood damage as the beach’s flood protection benefit. Study site and data: Narrabeen-Collaroy, Sydney, Australia. Hourly offshore waves from GOW2 (1980–2020), storm surges from GESLA (Port Jackson gauge), tides from TPXO7.2. SLR projections from IPCC AR6 for SSP2-4.5 and SSP5-8.5, medium and low confidence, percentiles P5, P50, P95 for 2050 and 2100. High-resolution topo-bathymetry from NSW Marine LiDAR Topo-Bathy 2018; deep water GEBCO. Land values (NSW Office of the Valuer General, July 2021), building values from real estate sources, land use (NSW Landuse 2017) for Manning roughness. Modelling chain: Offshore-to-nearshore wave transformation using SWAN with nested grids; selection of 500 representative sea states via maximum dissimilarity; nearshore reconstruction via radial basis functions. Long-term profile translation using ShoreTrans to apply SLR-driven upward/landward shifts preserving sediment budgets and considering non-erodible barriers. Storm definition: 30-year return period event derived from empirical TWL (tide + surge + wave setup + infragravity swash) using annual maxima fit; duration based on significant wave height threshold. Storm-scale hydro-morphodynamics simulated with XBeach (1D surfbeat) along 200 m-spaced transects, with morphodynamics on/off to distinguish dynamic vs static effects; initial conditions for storm (peak TWL) and poststorm (end of storm) conditions to represent storm clustering. Short-term erosion patterns transferred to 10 m alongshore transects and gridded surfaces for updated topo-bathymetry. Inland flooding simulated at 5 m resolution with RFSM-EDA forced by time-varying TWL and scenario-specific topography. Economic assessment: Flood exposure computed by intersecting flood maps with digitised land parcels, buildings, and roads, applying values per m² and vulnerability functions (land parcels 1% relative damage above 0.2 m; buildings depth-damage functions for residential/commercial in Oceania; roads 3.71 AUD/m² above 0.3 m plus 30% indirect damages). Beach flood protection value defined as avoided flood damage: difference between dynamic (with erosion) and static (without erosion) scenarios. Recreational value from the Sydney Beaches Valuation Project (118.1 million AUD per year consumer surplus), capitalised using 4% discount rate; value per m² applied to beach areas for scenarios with/without erosion; avoided loss of recreation calculated as benefit of maintaining present shoreline. Benefit-cost analysis compares benefits of holding the mean shoreline against nourishment costs computed from eroded volumes and a unit cost of 30 AUD/m³ (inflation-adjusted), assuming sediment availability and one nourishment per scenario. Variance partitioning: ANOVA-based decomposition attributing uncertainty fractions to approach (dynamic/static), SSP, and SLR percentile (with nesting), reported for TWL, flooded area, and flood damage, for storm and poststorm conditions and both SLR confidence levels.

- Total Water Level (TWL): Storm erosion redistributes profile sediment, reducing foreshore slope and wave setup, leading to TWL reductions up to ~18% (2020) and 15–20% (2100, SLR P5). The dissipative effect diminishes for higher SLR percentiles and low-confidence SLR scenarios. The TWL spread increases over time and with higher emissions/uncertainty. - Flooded area: Under storm conditions, SLR dominates flooded area changes; without-erosion TWLs yield larger flooded areas at present (+150%) but smaller areas in future scenarios (−10% to −22%) relative to with-erosion results. Under poststorm (storm-cluster) conditions, cumulative erosion makes flooded area with erosion larger; without-erosion flooded area is smaller by ~40% at present and up to 45% in future. - Flood damage: Differences between dynamic (with erosion) and static (without erosion) approaches are larger for damage than for TWL/area, especially by 2100. Assuming a fixed shoreline can underestimate flood damage by roughly a factor of 2 (60–100% lower) by 2100. Worst-case flood damage reaches ~7 or 34 million AUD in 2050 (storm vs poststorm) and ~553 or 880 million AUD in 2100 (storm vs poststorm). - Avoided flood damage (flood protection value) of maintaining the present shoreline: Present-day difference between storm and poststorm benefits is ~11 million AUD. For a single storm, avoided damage ranges ~1–7 million AUD (2050) and 22–456 million AUD (2100); for poststorm conditions, up to ~146 million AUD (2050) and ~785 million AUD (2100). By 2100, maintaining present beach width can avoid up to ~785 million AUD in assets from flood damage. - Recreational benefits (avoided loss of recreation): ~65–596 million AUD (2050) and ~355–2120 million AUD (2100), up to ~1877 and ~2300 million AUD for poststorm conditions. These represent upper bounds, as beaches may recover after storms. - Variance decomposition: The modelling approach (dynamic vs static) accounts for increasing shares down the modelling chain—TWL (7–51%), flooded area (9–49%), and flood damage (14–80%)—with especially large contributions under poststorm conditions (16–91%), often exceeding SLR uncertainty even in 2100. SLR percentile contributions are larger for low-confidence SLR (7–75%) than medium (4–59%). SSP contributions are negligible in 2050 but increase in 2100 (>16% medium-confidence; >18% low-confidence). - Benefit-cost analysis (holding the mean shoreline): Total benefit-cost ratio ~65–160 (2050) and ~75–230 (2100), dominated by recreational benefits. Flood-protection-only ratios indicate nourishment is cost-effective in all but the lowest SSP2-4.5 percentiles in 2050; avoided flood damage can be up to ~4.5× (2050) and nearly ~50× (2100) the nourishment cost. Abstract-level summary: by 2050, combined flood protection and recreation benefits can exceed nourishment costs by >150×.

The findings demonstrate that beaches provide substantial and growing flood protection benefits under SLR, particularly when their dynamic morphodynamic response to storms and long-term shoreline retreat is considered. Coupling erosion and flooding alters TWL, flooded areas, and especially damage estimates, showing that static approaches can materially underestimate future flood risk. The results clarify when storm-driven and chronic SLR erosion jointly produce the largest flood impacts (e.g., poststorm storm clusters on an SLR-retreated shoreline), highlighting the importance of timing, beach condition, and profile geometry. Beaches exhibit resilience at storm scales—with potential recovery during calm periods—but storm clustering and chronic SLR-driven retreat can push systems past resilience thresholds, potentially inducing tipping points that amplify flooding and damages. Incorporating dynamic behaviour improves adaptation planning by identifying when maintaining beach width (e.g., via nourishment) yields large avoided damages. The benefit-cost assessment suggests nourishment to hold the mean shoreline can be highly cost-effective, particularly due to recreational co-benefits, while also offering flexibility compared to grey infrastructure. However, potential feedbacks between protection policies and coastal development may create maladaptation risks without complementary land-use planning and regulation. The approach underscores the need to model and value beaches as natural defences within integrated adaptation strategies and financial instruments (e.g., parametric insurance, catastrophe and green bonds).

This study introduces a dynamic, process-based framework that couples shoreline morphodynamics and coastal flooding to quantify the flood protection value of beaches and associated economic benefits. Applied to Narrabeen-Collaroy, the approach shows that neglecting erosion can lead to significant underestimation of future flood damages, while maintaining present-day beach width can avoid substantial losses and deliver high benefit-cost ratios when recreational values are included. The methodology is transferable to other sandy coasts, supporting realistic valuation to guide beach conservation, nourishment, and adaptation financing. Future research should: (i) incorporate projections of changes in storm surges and wave climate; (ii) account for evolving exposure and vulnerability (land use, asset values, building typologies); (iii) expand to risk-aggregated metrics (e.g., avoided expected annual damages); and (iv) include broader ecosystem services, ancillary costs, and alternative adaptation strategies. Identifying resilience thresholds and tipping points for beach systems will further refine adaptation pathways and timing of interventions.

- Climate forcing: Did not include projections of future changes in storm surges and waves due to uncertainty in sign and magnitude for the region. - Asset and vulnerability assumptions: Assumed future asset values, land use, and vulnerability functions remain constant; building parameters calibrated uniformly alongshore for simplicity. - Event-based valuation: Focused on a single 30-year storm; broader risk metrics (e.g., EAD) were not computed, though the framework can be extended to multiple return periods. - Nourishment analysis: First-pass benefit-cost assessment assumes sediment availability, one intervention per scenario, constant unit costs over time, and addresses SLR-driven erosion only (storm effects limited to TWL dynamics). - Model and data constraints: Use of 1D XBeach for morphodynamics may miss 2DH processes (e.g., rips, tidal currents) in complex settings; topo-bathymetry and non-erodible barriers constrain landward translation; transfer of profile changes assumes spatial uniformity; recreational valuation assumes uniform value per m² and fixed 4% discount rate. - Upper-bound benefits: Recreational and flood protection benefits are upper bounds because beaches may partially or fully recover post-storm within their natural resilience limits.