Effectiveness of Mangroves in Flood Risk Reduction

Abstract

The risk of flood risk is increasing with global trends such as rising sea levels, land subsidence, increasing populations in coastal areas and economic growth. Traditional solutions to reduce flood risk are a levee or a dam; however, the research on hydraulic engineering is increasingly promoting nature-based solutions for flood protection. Salt marshes and mangroves can attenuate waves and could thus help to reduce failure probability. This research focuses on the attractiveness of mangroves in flood risk reduction. The essence of the reasoning followed in this research is that mangroves in front of a levee enable a lower levee and therefore reduce costs while maintaining the same failure probability. There are three aspects related to this assessment: the biological behavior of mangroves; the physical behavior of waves and storm surge in a mangrove forest; and the economics of flood risk, levees, and mangroves. Mangroves grow under specific conditions, such as high temperatures, saline water with fresh water input, between mean sea level and high tide, and with low wave impact. Mangroves attenuate waves, with the attenuation rates depending on mangrove height, stem diameter and density, in addition to the water depth, wave height and wave period. Lastly, the costs of restoring mangroves and building levees have been considered. In this thesis a method has been developed to define the optimal configuration of a levee-mangrove system. The goal is to reach a desired safety level while keeping costs as low as possible. Costs consist of building a levee, restoring mangroves, and the expected annual damage and maintenance. This depends on the ratio of mangrove and levee costs, the ability of mangroves to attenuate waves, and levee characteristics. Although all variables have equal significance in defining the mangrove forest width, the variety in levee and mangrove costs and significant wave height are the variables most likely to affect the optimal mangrove forest width. Mangroves are only useful to decrease the height of waves, thereby reducing wave run-up. This wave-attenuating effect decreases exponentially with the mangrove forest width. all Based on the literature, when considering common values for levee costs and mangrove restoration costs, levee characteristics and wave attenuation, wave attenuation is strong enough such that the optimal mangrove forest width is larger than zero. The costs of restoring mangroves depend on the required measures to enable mangroves to grow back. Hydrological restoration involves returning the mangrove area to the natural condition where mangroves are able to grow. This could, for example, be restoring a fresh water source that has been blocked by human intervention. If hydrological restoration is required and can be done cheaply, then this is the most effective option. Planting mangroves can also be a cost-effective option. Sheltering the mangrove area by using a permeable structure requires more financial resources. If large-scale filling or excavation is required, costs may increase significantly, and it is likely to be too expensive to use mangroves in flood-risk reduction. However, this cost assessment may change if the ecosystem services of mangroves are taken into account. As a rule of thumb, for mangroves to be economically effective, restoration costs [USD/m/m] in case of a 1 m wave height should remain below 0.003 times the variable levee costs [USD/m/m]. This value increases linearly with the wave height. For levee costs of 1 million USD/km/m, of which 0.6 million USD/km/m variable costs, the mangroves restoration costs should not exceed 0.18 USD/m/m, or 18,000 USD/ha. For Kaback, Guinea, the optimal mangrove forest width and height of the levee are assessed. These are 900 meter and 1.1 meter respectively, with the levee located just behind the mangroves. 900 meter is the maximum width possible, considering the physical conditions where mangroves grow. Since mangroves are growing back naturally already, costs for mangroves restoration is limited to fixed costs. The model developed in this study can be used to create a ``mangrove opportunity map”. This map can indicate—based on physical attributes—the costs, hydraulic conditions such as storm surge and wave height, and whether mangrove restoration for flood risk reduction is effective. Governments or organizations such as the World Bank can use this method to explore whether mangrove-based strategies are an option. Further research can focus on improving the cost estimates on levees and mangroves. In this study, costs are assumed to be linear with the levee height or mangrove area. In practice, levee costs might especially increase exponentially with the levee height, which would make mangroves more effective.