Dry river valley degradation has been identified as one major challenge in the Sahel zone by German development cooperation to effectively adapt to climate change. As a result of agricultural over-use and climatic change, river valleys in the Sahel zone have increasingly experienced droughts in the past twenty years. This study provides a socio-economic and technical introduction to water-spreading weirs, which mark a cost-effective option for practical measures to rehabilitate dry river valleys through a diversion of water resources.
Water-spreading weirs are low retentions walls developed to reduce runoff and erosion, and regulate floodwater in medium-sized watercourses and in wider degraded valley bottoms with a pronounced low-water channel. It consists out of stonewalls (up to 1 m high) which are divided into a spillway in the actual riverbed, lateral abutments and wingwalls. They are often built in series behind each other to maximize their impact. They slow the flow of water in valleys and spread it over a wider area where it can infiltrate into the soil. In this way, they control river floodwater, and this reduces erosion and the loss of water. At the same time, sediments improve soil fertility and replenish the water table. Water-spreading weirs cause temporary flooding of the adjacent land area above and below the weir. This has the effect that the distributed water can be used for crop growing, livestock raising and forestry, water infiltration raises the groundwater table, the soil becomes more fertile and erosion is reduced so that overall the degraded parts of the valley are rehabilitating. The costs for water-spreading weirs, including labour, equipment and material costs are on average 2000 $ per ha. Besides the rehabilitation of dry river valleys in which severe gully-erosion prevents regular flooding, they are suitable for improving agricultural production in intact valley floors.
The study functions as an introduction into the topic and summarises the experience acquired with them since the late 1990s. It further provides case studies from three countries in the Sahel zone where more than 370 water spreading weirs have been implemented covering an improved cultivation area of more than 20,000 ha.
Download publication: GIZ,KfW (2012) Water-spreading weirs for the development of degraded dry river valleys.pdf
Mekdaschi Studer, R. and Liniger, H. 2013. Water Harvesting: Guidelines to Good Practice. Centre for Development and Environment (CDE), Bern; Rainwater Harvesting Implementation Network (RAIN), Amsterdam; MetaMeta, Wageningen; The International Fund for Agricultural Development (IFAD), Rome.
GIZ (2012): Good Practices in Soil and Water Conservation - a contribution to adaptation and farmer's resilience towards climate change in the Sahel