Cutting the costs of coastal protection by integrating vegetation in flood defences

The global population residing in coastal areas is steadily increasing, alongside economic activity concentrated in these regions. This growth unfortunately coincides with a decline and degradation of coastal ecosystems, despite their crucial role in buffering flood hazards. While the flood reduction capabilities of ecosystems have been established in various studies, the contribution of ecosystems within hybrid coastal protection (a blend of natural and engineered defenses) remains under-quantified at a global scale. This study addresses this gap by investigating the integration of coastal vegetation (mangroves and marshes) in front of levees to assess its impact on global coastal protection costs. The increasing exposure to coastal flooding necessitates exploration of cost-effective and sustainable adaptation strategies. Nature-based solutions (NbS), like the use of vegetated foreshores, are increasingly recognized as viable alternatives or complements to traditional 'grey' infrastructure (e.g., seawalls and levees). These ecosystems, such as mangroves and salt marshes, offer significant wave attenuation capabilities, potentially permitting the construction of lower levees. Furthermore, they provide valuable ecosystem services such as carbon storage, habitat provision, and improved water quality, highlighting their multifaceted benefits. The decreasing extent of these valuable ecosystems globally underscores the urgency of incorporating them into coastal protection planning. Existing studies often focus on individual aspects, either examining ecosystem services or evaluating flood risk reduction in isolation. A global-scale assessment integrating both aspects within hybrid coastal protection systems is lacking. This research uniquely bridges this gap and quantifies the economic and environmental benefits of combining natural and engineered defenses.

Numerous studies have highlighted the role of coastal vegetation in reducing coastal flood risk at various scales, from local to global. However, these studies often lack a process-based, global-scale assessment incorporating the interaction of waves and vegetation within hybrid protection schemes. Existing global-scale studies on wave attenuation by vegetation often focus exclusively on mangroves, neglecting other crucial types like salt marshes. The limited use of process-based wave modeling in previous global studies also necessitates further investigation using advanced methodologies. The potential cost-effectiveness of combining coastal vegetation with traditional flood defenses, creating hybrid systems, hasn’t been comprehensively explored on a global scale. Understanding this potential is crucial because hybrid protection offers advantages over solely relying on grey infrastructure or relying on ecosystems alone. Small to medium-sized vegetation belts alone are often insufficient to prevent flooding, while levees without protective vegetation require larger dimensions and greater investment. This necessitates a comprehensive, global-scale study that integrates wave-vegetation interactions to fully assess the potential of hybrid coastal protection.

This study employed a novel approach combining earth observation data and hydrodynamic modeling to assess the global cost-savings potential of integrating coastal vegetation into hybrid coastal protection. A new global data layer was created using time-ensemble average satellite images, offering high-resolution (20 m horizontal, 0.52 m RMSE vertical accuracy) intertidal elevation data. High-resolution coastal vegetation maps were constructed at 10 m resolution using Sentinel-2A and Landsat-8 images, combined with existing vegetation maps to identify vegetation types (primarily mangroves and marshes). Wave heights and periods were obtained from ERA-Interim reanalysis, and extreme water levels from a global tide and surge model. Coast-normal transects were defined along global coastlines (between 66°N and -60°S), with a distance of 1 km between each transect. For each transect, bathymetric profiles, vegetation cover, and hydrodynamic boundary conditions were determined. The wave propagation was then calculated, with and without vegetation, using a lookup table generated by XBeach model simulations. The lookup table provided wave heights at regular intervals along steady slopes under various conditions. This significantly sped up the process and allowed for iterative improvements compared to detailed modeling for each transect. The difference in required levee crest heights to prevent flooding for both scenarios (with and without vegetation) was then calculated. The economic benefits of incorporating vegetation were monetized using unit investment costs for levees, corrected for construction cost variations across countries. Finally, populated coastal areas susceptible to flooding were identified based on inundation maps and population density, with urban and rural areas distinguished. The study considered three different standard levee unit cost scenarios (low, mid, high) to account for uncertainty in construction costs. The study also conducted a validation of its key data layers and models, including comparison with local data and model outputs from the South-Western Netherlands, examining wave transmission and levee crest height reductions. A sensitivity analysis was performed to assess the impact of uncertainties in key parameters, such as levee costs, critical overtopping discharge, wave breaker index, topography, storm return period, and vegetation width, on the cost reduction estimates.

The study's key findings highlight the significant potential of integrating coastal vegetation into coastal protection strategies. Analysis revealed that 18.5% of the global coastline is vegetated (mostly with narrow belts of 25-250m), with wider belts ( > 1000m) occurring near the equator. For 11.5% of the global coastline, vegetation can reduce wave heights by more than 25%. Along 27.6% of populated, flood-susceptible coastlines, existing vegetation allows for lower levee crest heights. The mean crest height reduction for a 100-year return period was 96cm. The potential cost reduction globally, if levees were constructed along all populated susceptible coastlines, is estimated at 320.2 billion USD (67.5 billion USD in urban areas) for a 100-year protection standard. This potential cost saving varies across countries. The top ten countries with the highest potential cost reductions account for over 65% of the total potential, with the highest average reduction in Guinea-Bissau (0.77 m km⁻¹). Expressing cost savings as a percentage of GDP reveals the substantial benefits for small island developing states (SIDS). The average vegetation belt width decreases with increasing population density, possibly due to past land conversion. Sensitivity analysis indicates that the largest uncertainty in cost reduction estimates stems from uncertainty in levee construction costs, followed by other factors such as topography, critical overtopping discharge, and storm return period.

The findings of this global assessment strongly support the integration of coastal vegetation in hybrid coastal protection schemes. The substantial cost savings associated with utilizing existing coastal vegetation emphasize the economic benefits of preserving and managing these vital ecosystems. The identified locations where hybrid coastal protection offers the greatest potential highlight areas requiring focused attention and planning. The study underscores the significance of conserving existing vegetation, with its removal likely leading to increased erosion and heightened future coastal protection costs. Incorporating vegetation into coastal defense planning is particularly important for countries with extensive rural coastlines where conventional ‘grey’ infrastructure is economically unfeasible. The results provide crucial insights for policymakers and coastal engineers to design cost-effective and sustainable solutions to safeguard coastal communities and assets.

This global-scale study demonstrates the considerable economic benefits of integrating coastal vegetation into hybrid coastal protection systems. The substantial cost savings identified—estimated at hundreds of billions of dollars—highlight the importance of protecting and managing existing coastal ecosystems. The findings strongly advocate for incorporating Nature-based Solutions into coastal defense strategies, particularly in regions with extensive rural coastlines and vulnerable SIDS. Future research could focus on refining the models with improved resolution data and incorporating the dynamics of vegetation growth and resilience to sea-level rise and extreme events. Local-scale assessments are also crucial for optimizing the design and implementation of hybrid protection systems.

This study involved certain assumptions and simplifications inherent in global-scale analyses. The limited resolution of some datasets (e.g., hydrodynamic data and topography) can affect the accuracy of results. The study focused on current vegetation presence, neglecting the potential for restoration or expansion of vegetation. Furthermore, the model did not explicitly account for the effects of vegetation on storm surge reduction, focusing instead on wave attenuation, and the resilience of vegetation under severe conditions is uncertain. Lastly, uncertainties in levee construction costs and the assumed levee quality (overtopping discharge) contribute to the overall uncertainty of cost reduction estimates.