Sand dams are a simple, low cost and low maintenance,
replicable rainwater harvesting technology. They provide a clean, local water
supply for domestic and farming use and are suited to semi-arid areas
The Operation
A sand dam is a reinforced concrete wall (or a similarly
robust and impermeable weir)
built 1–5 metres high across a seasonal sand river.
When it rains the dam captures soil laden water behind it –
the sand in the water sinks to the bottom, whilst the silt remains suspended in
the water.
Research on Kenyan dams shows that only 1 to 3% of rainwater
is retained behind any individual dam, the remainder continues its natural flow
towards the ocean. Eventually the dams fill with sand - sometimes after only
one rainfall or over 1 – 3 seasons. 25 to 40% of the volume of the sand held is
actually water.
A mature sand dam can store millions of litres of water –
refilling after each rainfall providing a year round supply of clean water to
over 1,000 people.
Application
The highest concentration of sand dams with the strongest
track record is found in Kenya although examples are found throughout world’s
semi-arid regions from Angola to Zimbabwe. Further examples are recorded in
Japan, India, Thailand, SW USA and Brazil.
Sand dams can be built in the upper and middle courses of
seasonal sandy river valleys (also known as wadis). Typically,
sand dams are built in the transition zone between hills and plains where the
gradient of the river bed is between 0.2 - 4%, but in extreme cases sand storage
dams have been constructed on slopes of 10-16% (Nilsson 1988). Dams must be
built on bed rock or highly compacted sub-soil. For the obvious reason of
economy on materials and labour, the stream must be reasonably narrow with well
defined and stable river banks and the bedrock or impermeable subsoil within a
few metres of the stream bed. Rocky banks and gorges are the most apt features.
Further advice on the siting, design and construction of sand dams is available
from the organisations and in the manuals listed below
Extracting stored water
There are two simple ways:
- Scooping a hole in the sand. The water will naturally emerge to the surface. Scope holes used for domestic water should be protected from contamination by livestock (by using acacia fencing or similar)
- A slotted pipe buried in the sand that either passes through the dam wall or is connected to a simple hand pump situated on the river bank
Benefits
- Low cost and maintenance: Sand dams are the lowest cost form of rainwater harvesting known – 3 to 100 times lower cost than other technologies. A sand dam is built to last over 50 years, has no operational costs and requires little maintenance.
- Community owned and managed: Most examples of sand dams are built by members of the communities they serve with support local and international development agencies. Community ownership and management is key to their successful operation.
- They save time and enable increased food production and tree planting: During drought periods in semi-arid regions, people might have to walk up to 12 hours a day to fetch water, which is often unsafe to drink. Freed from the chore of walking sometimes the whole day to collect the water, farmers are able to improve food production and generate surplus income.
- They provide a clean, secure and local year-round water supply in water scarce environments: The sand effectively acts as a large slow sand filter resulting high quality potable water. As the water is held under the sand, evaporation is minimal, people are protected from water-borne diseases and mosquitoes can’t breed – reducing the threat of malaria.
- Wider benefits for health, education and communities: In areas where sand dams have been built, communities have observed a dramatic fall in water related disease, an increase in school attendance and a significant increase in household income and food production.
- They transform the local ecology: The water held in the sand behind the dam spreads horizontally creating a permanent increase in the water table, allowing trees grow naturally and transforming the local ecology. The dams create a natural buffer that reduces the threat posed by flooding and drought and builds the resilience of communities to cope with the impacts of climate change in semi-arid regions.
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