Nature-based flood risk mitigation in river deltas is most effective with upstream and channel-fringing mangroves

Abstract Global changes induce increased flood risks in river deltas, which concentrate a large portion of the global population and economic activity. Nature-based solutions, such as the conservation and restoration of mangroves in tropical river deltas, are considered a sustainable addition to expensive engineered flood defence systems. However, we currently lack science-based guidelines on where, in a river delta, mangrove conservation and restoration should be prioritised to attenuate extreme water levels most effectively and, conversely, where mangrove conversion to human land use would have limited impact on flood risks. Here, we apply state-of-the-art models to (1) an idealised generic geometry of a river delta and (2) a complex geometry of a real river delta case, which explicitly represents the complex pattern of mangroves, channels and human land use, to show that upstream-concentrated mangroves and channel-fringing mangroves are more effective in attenuating extreme water levels. Conversely, converting mangroves to human land use in zones located farther away from river channels and more downstream, has much less impact on extreme high water levels. Our study provides first step advice towards effective spatial planning that conciliates conservation of natural wetlands, economic development and flood risk mitigation.