Nature-based solution enhances resilience to saturation excess flooding in coastal cities in the Global South

Coastal cities are facing a rise in groundwater levels induced by sea level rise, further triggering saturation excess flooding where groundwater levels reach the topographic surface or reduce the storage capacity of the soil, thus putting stress on the existing infrastructure. Lowering groundwater levels is therefore a priority for sustaining the long-term livelihood and resilience of coastal cities. This project discusses the feasibility of using tree-planting as a Nature-based solution to alleviate saturation excess flooding as a result of rising groundwater levels in coastal cities in the Global South. In environments with shallow groundwater, trees uptake groundwater by intercepting water that percolates in the unsaturated zone or reduce groundwater recharge by canopy interception of rainwater. These contributions, in turn, lower groundwater levels and increase the unsaturated zone thickness, further mitigating the risk of saturation excess flooding. A case study was conducted in Nouakchott City (Mauritania) where rising groundwater levels has led to permanent saturation excess flooding for more than a decade, making parts of the city inhabitable and posing long-term health threats. Consequently, this work presents an interdisciplinary approach using both ecohydrogeology and plant physiology to model the dewatering capacity of five local tree species. These species were selected based on their tolerance to the exceptionally challenging conditions for vegetation posed by the hot desert climate and the shallow and brackish groundwater table. Preliminary results from a 3D groundwater model indicate that a city-scale tree-planting program could induce a groundwater drawdown of up to 70 cm within a 40-year horizon. Thus, a tree-planting program is anticipated to lower the groundwater levels, thereby reducing flooding during the wet season. Tree-planting programs constitute long-term solutions, sustained by environmental factors, that complement conventional engineering solutions. The multi benefits of such Nature-based solutions, as well as the expected positive environmental, economic, and social outcomes, makes them particularly promising for alleviation of saturation excess flooding.