The future impacts of climate change and of mean sea-level rise (SLR) are expected to have a profound effect on the coastal zone1,2, although the magnitude of these impacts is still uncertain3,4. Higher water levels will lead to the permanent inundation of some low-lying coastal zones5 and more frequent extreme flood events6,7,8, which will in turn modify coastal landscapes increasing the exposure of coastal communities and assets9,10. Essential features of coastal landscapes include ecosystems such as wetlands, mangroves, coral reefs, and beach and dune systems, which provide multiple benefits that have begun to decline and will continue to do so at rapid rates without climate change mitigation, risk management and adaptation 11,12,13,14. One of the coastal ecosystem services under threat is flood protection, which is particularly important along densely populated and developed coastlines15,16.

The flood protection services provided by mangroves and coral reefs have been economically assessed in the literature considering climate and degradation scenarios17,18,19. This has contributed to the benefits of conserving and restoring them being increasingly recognised by scientists, multilateral and governmental agencies, and the insurance industry20,21. As for beaches, there are several studies that analyse their effectiveness as natural flood defences22,23,24,25, however, additional work is needed to understand their benefits in flood risk reduction. Knowing this information can be of key importance as beaches represent one-third of the world’s coasts26, they are subject to the action of storms and rising mean sea levels27,28, and their maintenance has long been under discussion29,30. Cost-effective decision-making on beach conservation would benefit from assessing the trade-off between the cost of an action and the benefit, including the economic benefit, that would accrue from its implementation.

Previous efforts to monetise beach services have examined the loss of human recreation due to erosion31,32,33, the influence of erosion (and beach nourishment policies) on coastal property values34,35,36,37, the tax rates needed to fund adaptation projects38,39, and the willingness-to-pay for erosion prevention40,41,42 and flood protection 43,44,45. Here, our goal is to advance knowledge on how to quantify the flood protection value of beaches to deliver relevant information for adaptation decisions.

We propose a dynamic approach based on the avoided damage cost method, which is similar to that applied for mangroves and coral reefs (e.g., refs. 17,18), but considers the specificities of beaches. We acknowledge that coastal flood protection is strongly dependent on the shoreline response to coastal dynamics. Therefore, our approach accounts for the dynamic interaction of shoreline evolution and flooding by coupling both processes at different time scales. This allows us to evaluate how the shoreline evolves under extreme events and due to long-term changes and how this evolution affects the total water level (TWL) and the propagation of flooding inland, which are in turn influenced by shoreface geometry and terrain heights. We also assume that robust adaptation requires not only a good understanding of physical processes but also sufficient uncertainty sampling to accommodate decisions in different contexts and time horizons and for different levels of risk and uncertainty tolerance.

We use the widely studied Narrabeen-Collaroy beach system (Australia) as an illustration to showcase our approach. The need for coupling flooding and erosion in beach protection benefits assessment is highlighted by comparing flooding results with and without including coastal erosion at the storm scale and in the long term. The flooding scenarios are built for the present-day 30-year return period storm combined with AR6 SLR46 in 2050 and 2100. The scenarios with erosion consider the effect of storm and SLR erosion on the TWL, flooded area, and flood damage to assets. As represented in seven steps in Fig. 1, we use process-based models to downscale offshore waves, compute storm hydro- and morphodynamics and propagate flooding inland. Throughout this process, we update the present-day topo-bathymetry to first incorporate the action of SLR and then that of the storm. We quantify flood damage by combining our flood maps with data on spatially distributed land and buildings value. Further details on the approach can be found in the “Methods” section.

We applied the dynamic approach to obtain flood damage with erosion and the traditional static approach to obtain flood damage without erosion. The scenarios without erosion assume that the coastline and beach morphology are fixed over time and that changes in flood damage are caused by changes in the TWL and subsequent flooding. We define the beach flood protected area as the increase in the flooded area due to erosion. The beach flood protected value is the flood damage that occurs in the flood protected area and can be considered as the benefit we obtain in terms of avoided flood damage if the present shoreline is maintained in the face of storms and SLR. We provide estimates of TWL (dynamic wave setup and sea-level components), flooded area, flood damage, and avoided flood damage for 2020, 2050 and 2100 considering uncertainty in the choice of emissions scenarios (shared socioeconomic pathway—radiative forcing level SSP2-4.5 and SSP5-8.5), SLR driving processes of different confidence (medium and low), and SLR trajectories (associated with percentiles P5th, P50th and P95th). In addition, we assess the trade-off between the benefits and nourishment costs of maintaining present-day mean beach width.


The flood protection value concept

Figure 2 shows the flood protection value of Narrabeen-Collaroy (Fig. 3) for the impact of a 30-year storm now and in the future. This shows the key role played by the beach in protecting coastal assets. The interaction between storm hydrodynamics and morphodynamics results in an eroded shoreface and in an attenuated wave contribution to the TWL. At present, reduced TWL can result in lower or higher flooding than without erosion depending on whether storm erosion takes place on a beach uneroded (storm condition, Fig. 2a) or previously eroded due to a recent storm without having time to recover (poststorm condition, Fig. 2b), respectively. In the future, storm erosion compounded by SLR-driven chronic coastline retreat will further narrow the coastal landscape. Even if wave dissipation occurs, this narrowing combined with SLR-driven higher TWL will lead to greater flooding (Fig. 2c, d).