Exploring how nature-based solutions can provide effective flood and drought mitigation for Somalia 

Over the past decades, Somalia has been experiencing increasingly frequent and severe floods. With environmental degradation being considered one of the main contributors to this change, nature-based solutions (NbS) have a high potential for mitigating flood risk in the area. Using hydrologic and hydraulic mathematical modelling to analyse options such as reforestation or sand dams, UNEP-DHI Centre on Water and Environment assessed this potential within the Shabelle River basin and found that a variety of NbS are applicable in Somalia. These findings will support the National Flood and Drought Task Force of Somalia in planning future flood and drought interventions in the basin. 

Challenge

Many ephemeral rivers, or ‘wadis’, in Somalia are prone to flash floods. Flooding occurs when heavy rains lead to a sudden increase in river flow, followed by a quick recession, and can cause loss of lives and property in downstream settlements. The severity and frequency of flooding from the Shabelle Rivers has been increasing in the past few decades, and it is thought that increased sediment deposition in rivers (brought about by activities such as deforestation which degrade soil structure), and subsequent raising of the riverbed, is the major contributor to this change. NbS, aimed at restoring degraded riverine ecosystems, have the potential to mitigate flooding in the area. With some of these solutions simultaneously enhancing groundwater recharge, their implementation could serve the dual purpose of flood and drought management, increasing community resilience to climate extremes. 

Solution

In the implementation of the project, UNEP undertook research into the potential of nature-based solutions for mitigating flood and drought hazards. The UNEP-DHI Centre team provided the technical inputs necessary to support this research, which can assist Somali decision-makers in planning and scaling up nature-based approaches to flood mitigation.

UNEP-DHI’s study identified flood-targeting NbS already tested in the Somali context and assessed their effectiveness using hydrological and hydraulic models developed with MIKE HYDRO River (now known as MIKE+ Rivers). For flash flood mitigation in ephemeral wadis, peak flow in response to heavy rainfall was compared between a baseline scenario and scenarios employing reforestation, terracing or water retention structures such as sand dams and weirs. For sediment-related riverine floods, a hydraulic river model with sediment transport components was used to assess the effectiveness of different NbS in decreasing the mass of sediment entering the river and in decreasing incidence of flooding.