Wave Reduction by Mangroves during Cyclones in Bangladesh: Implementing Nature-Based Solutions for Coastal Resilience

The effect of mangroves on surges has already been studied for the design conditions of Bangladesh. However, the impact of wave attenuation by mangroves on embankment designs is not known. A model is thus developed to estimate the wave height reduction by a mangrove forest, and how such wave attenuation would influence the design of a landward embankment. Model simulations suggest that mangrove belts with a width between 100 and 1,000 meters (perpendicular to the coast) could provide wave attenuation rates between 7 and 55 percent (compared to a situation without mangroves) at potential afforestation sites identified in previous studies. Such wave attenuation rates would reduce the embankment height by 0.09–0.30 meters, diminish the slope revetment thickness by 13–46 percent, and decrease the wave shear stresses at the embankment toe up to 25–70 percent. Relatively wider mangrove belts not only cause a larger reduction of the embankment design requirements, but also host larger biodiversity and are more resilient against pests and extreme events. The model results are highly sensitive to the mangrove properties, and collecting data on the local mangrove species is recommended to reduce uncertainty in the predictions. Moreover, the results also suggest that trees older than 10–20 years might collapse during storms. Expanding the mangrove stability model, including other pioneer species in the analysis, and exploring the option of canopy pruning are thus advised to ensure the integrity of any future afforestation efforts. Overall, this paper provides a methodology that could be applied to design nature-based solutions in Bangladesh.