Coastal green belt policies informed by wave attenuation by mangroves are crucial for coastal flood risk reduction. The legislation mandating mangrove greenbelt widths varies globally, with widths ranging from 50 to 200 meters. A comprehensive analysis based on realistic conditions was conducted to quantify the confidence in the wave reduction capacity of mangroves for wind and swell waves. The study revealed that tidal flat areas in front of mangrove forests already contribute significantly to wave height reduction, accounting for up to 70%.
Within mangrove forests less than 500 meters wide, wave dissipation is highly influenced by local water levels, wave characteristics, and forest density. However, for forest widths exceeding 500 meters, which constitute a substantial portion of global coastal mangroves, over 75% of incoming wave energy is dissipated. This suggests that a new standard advocating for mangrove widths of 500 meters or more would be more effective in dampening shorter waves.
Globally, the increasing vulnerability of people and assets to coastal flooding necessitates the integration of mangroves in flood risk reduction and climate change adaptation strategies. Mangroves, covering approximately 10% of global coastlines, play a vital role in reducing coastal flood risk, with estimated annual savings of $250 million and an 8.5% decrease in future flood risk. The integration of mangroves into coastal protection strategies offers cost-effective solutions and contributes to coastline stabilization and erosion reduction.
Despite the significant coastal protection benefits provided by mangroves, these ecosystems face threats from activities such as fish and shrimp farming, urban development, and unsustainable exploitation for resources. Countries worldwide have implemented policies to protect mangroves and enforce greenbelt zones along coasts and rivers to safeguard against waves, erosion, and salinity intrusion. These policies recommend greenbelt widths ranging from 50 to 1500 meters, with approximately 100 meters considered minimally effective in limiting hydraulic forces.
Existing guidelines and policies for mangrove greenbelts often prescribe widths of 100-200 meters or less. However, the study underscores the importance of wider green belts, especially in combination with gradually sloping tidal flats, for more effective wave attenuation with greater confidence. The analysis also highlights the sensitivity of wave attenuation to various factors such as incoming wave heights, water levels, and mangrove densities, emphasizing the need for a systematic exploration of these factors to inform coastal protection policies and designs.
Furthermore, the study emphasizes the ecological resilience of mangroves and underscores the importance of considering ecosystem health and resilience in designing mangrove greenbelts. Mangroves, in combination with water retaining structures like levees, offer hybrid solutions for coastal protection, especially in mitigating flood risk. The study calls for a more comprehensive assessment of mangrove characteristics and their sensitivity to local conditions to enhance coastal protection efforts and promote sustainable coastal management.
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