Quantifying the Engineering Function of Coastal Habitats in Flood Risk Reduction

The costs of defending increasingly risky coastlines are rising with rising sea – levels. Some of these costs may be offset by the use of coastal habitats for risk reduction. There is a growing body of evidence to support the claim that coastal habitats can help mitigate coastal risk. Recently, increasing attention is being paid to systematic and effective use and management of coastal habitats in risk reduction. The engineering function and effectiveness of coastal habitats can vary widely depending on multiple parameters including the type of habitat, the coastal setting and local environmental conditions. Including and utilising coastal habitats in risk reduction approaches and actions requires understanding this variation. This paper presents a quantitative synthesis of current knowledge on the engineering function of coastal habitats to provide guidance to engineers and coastal managers on effectively managing coastal risk.

83 studies of wave reduction across coastal habitats were analysed. The studies included direct field measurements and field-based estimates of wave reduction across habitats such as salt-marshes, coral reefs, mangroves. Using data from these studies, statistical analyses were performed to understand the engineering and cost effectiveness of the different habitats in risk reduction. Without exception, the presence of a habitat results in reduced wave heights at the coastline. Preliminary results demonstrate that the extent of wave reduction depends primarily on the type of habitat and the environmental conditions. The project data show that this wave reduction function is found to be robust under increasing water-levels. This can have significant implications when accounting for habitats in coastal risk modelling - models which assume that a habitat’s wave reduction function drops to zero once it is overtopped are considerably under-estimating its risk reduction potential and hence its cost-effectiveness. In fact, the robustness of their wave reduction functionality to changes in water depths implies that coastal habitats may be an effective risk reduction measure even under rising sea-levels. The project data also show that the use of coastal habitats for risk reduction is cost-effective in a majority of the cases. Variation in cost-effectiveness depends on the coastal setting, the number of people and assets at risk and the cost of alternate engineering measures. Further multivariate analyses are being conducted to assess the influence of different physical and risk parameters on the effectiveness and viability of these habitats in risk reduction. Ultimately this knowledge will be used to provide guidance to coastal engineers and managers world-wide on when, where and how to use and manage coastal habitats for flood risk reduction.