For decades Somalia has been beset by illegal logging, deforestation and land degradation. These problems, worsened by overgrazing, conflict and climate change, have made many areas prone to devasting flash floods, caused by heavy rainfall that leads to a sudden increase in river flow followed by a quick recession.
This tends to happen during the Gu rainy season between March and June and the Deyr season from October to December. At the same time, Somalia also suffers frequent periods of water scarcity or drought.
Ahead of flash flood season in Somalia, the United Nations Environment Programme (UNEP) -DHI Centre on Water and Environment has released a study to help the country better adapt to flooding by tapping into nature.
The modelling study reveals the vulnerability of communities to short-lived, seasonal flooding in Beledweyne, Jalalaqsi, Bulo Burde, Mahaday Weyne, Jowhar in Hirshabelle State, and Qardho in Puntland. Experts say that storing water and managing water extremes is key to development and food security across the country.
The study finds that triangular, V-shaped or V-notch weirs (a concrete structure built through an open waterway) – along with with planting shrubs, grasses or trees – can reduce the impact of flash floods.
While dams impound water behind a wall; weirs change the flow characteristics of a river. Built into the riverbank, they help recharge groundwater, revitalize ecosystems, and can store water for later use when water is scarce.
“V-shaped or combined weirs can significantly reduce flood peaks and potentially reduce loss of lives and property in downstream settlements,” says Gareth Lloyd, an expert at the UNEP-DHI Centre on Water and Environment. “The highest efficiency can be expected at locations where floods are particularly intense, with high maximum flow values but limited total volume of run-off.”
Reducing peak flows
The study examines both “pure” and “hybrid” nature-based solutions – where “grey” construction materials are combined with “green” measures such as revegetation and reforestation. It concludes that hybrid nature-based solutions – being relatively low-cost and low tech – can be combined with green elements for additional protection and resilience. Many of these hybrid solutions are also rooted in traditional knowledge, and use locally available materials, hence can be upscaled.
Unlike V-shaped weirs, sand dams, or similar small dams across the river, were found to have a limited effect on flood peaks. Similarly, deforestation/reforestation was found to have little impact on mitigating flash floods.
According to the study, other effective options to reduce peak water flows include terracing and diversion. Constructing terraces along contours in the catchment area can considerably minimize soil loss due to erosion. Downstream structures can also help to divert excess water to areas where flooding will cause limited damage.
Early warning
Flash floods in Somalia are difficult to predict and generally occur when heavy rain causes large volumes of water to flow into dried-up riverbeds called “wadis,” which in some places have a spring – a valuable resource for people who live and travel in the desert.
Real-time monitoring of river water levels has been implemented in some mountainous areas around the world to automatically activate a siren that warns inhabitants further downstream.
The research found that while weather radars can do short-term warnings, most satellite-based rainfall data is only available several hours after measurement and is not a viable option for early warning on flash floods. It recommends expanding radar capabilities to improve flash flood forecasting. Collecting local observations and community knowledge on flood characteristics can also improve forecasting and nature-based solutions.
The study is part of a larger project with Somalia’s Ministry of Energy and Water Resources funded by the United Kingdom Foreign, Commonwealth and Development Office.
For more information, please contact Gareth Lloyd: Gareth.lloyd@un.org or Lis Mullin Bernhardt: lis.bernhardt@un.org
