A sand dam is a reinforced-concrete wall built across seasonal river beds, 2-4m high and up to 90m across. A pipe is built into the dam, and over one to three seasons the dam fills with water and then sand, which filters the water through the pipe. The sand behind the dam sucks up the water like a sponge, acting like a water tank on a grand scale, storing, filtering and protecting the water from evaporation. People can then dig for water in the sand (just as you dig a hole on the beach) or collect it from the pipe in the dam wall. Without a dam, rainwater in this part of Kenya cascades downstream and washes away into the Indian Ocean.
Maddrell says sand dams, which have been around for 2,000 years, are a simple and cheap way to conserve water – each dam costs about £8,000 to build, providing fresh water for a community for up to 50 years for roughly £7 a head. In this part of Kenya, 65 per cent of people do not have an adequate supply of clean water. Since 2002 Maddrell’s charity has built 200 dams over 15,000 sq km, from the hills of Machakos, a district near Nairobi, to the flatter, more arid lands of Makueni and Kibwezi to the south.
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