EIRA – Enhancing Infrastructure Resilience to flooding using Afforestation: the case of Nouakchott city, Mauritania

Despite a warm and dry climate, the city of Nouakchott has been facing constant flooding for almost a decade, making part of the city inhabitable and posing long-term health threats. Groundwater levels are relatively constant over the year, except for October, when the groundwater table rises at the end of the rainy season, resulting in an almost doubled flooded area in the city compared to drier periods. Saltwater intrusion maintains a constant level in the water table beneath the city. The infiltration of most of Nouakchott’s wastewater acts as systematic artificial aquifer recharge, thus increasing the risk of groundwater saturation excess and flooding. Hence, in comparison to the driest decade (1971-1980), flooding in the city today cannot only be attributed to the slight increase in precipitation over the last decade.

The project objective is to assess if the resilience of the city of Nouakchott to flooding can be achieved by planting salt-tolerant trees to lower the water table level. This work presents a joined interdisciplinary ecohydrology and plant physiology approach for monitoring and modeling the transpiration and dewatering capacity of different local tree species. It is a collaborative project between the Laboratory of Hydraulic Constructions (LCH, EPFL), the plant ecology research laboratory (PERL, EPFL), the Région de Nouakchott (Nouakchott’s city hall), and the Centre National de la Ressource en Eau (CNRE, Mauritanian Water Resource Survey). The project aims to provide scenarios for an integrated and sustainable afforestation strategy for Nouakchott. In addition to increasing the city’s resilience to flooding, the role that afforestation could play in enhancing the provision of sustainable services for the local population and the economy (e.g., an increased amount of shade in the streets, potential fruit harvesting and wood production, etc.) will also be discussed. The first field campaign of the project in March 2023 allowed for the monitoring of five observation wells with automatic water depth measurements and 12 sap flow sensors on five tree species. Eventually, to reinforce the relatively scarce groundwater data, a spatiotemporal time series of the city’s flooded areas will also be developed using remote sensing data, and its reliability to calibrate an eco-hydrogeological model will be discussed.