Securing water supplies with sand dams

A large part of the world, over 40% of the earth’s surface, consists of drylands – hyper-arid, arid, semi-arid and dry subhumid areas with concentrated periods of seasonal rainfall. These drylands which cover large areas of Africa and Asia are home to around 2.3 billion people, mostly living in rural poverty trying to eke out a living in a tough climate. Millions do not have easy access to a safe water supply and are isolated from goods and services due to poorly maintained roads. Ironically, the same people tend half of the world’s livestock and produce over 40% of the world’s food.

The UK charity Excellent Development has been pioneering a low-tech solution to help combat poverty in Africa and Asia by building sand dams with local partners and community self-help groups. The charity has enabled the building of 884 sand dams for over 850,000 users since 2002 and is aiming to considerably increase the rate of construction in the coming years.

Simon Maddrell, Executive Director at Excellent Development, became aware of sand dams in 1984. He explains that these dams are based on structures that have been built since 400 BC and have existed in Kenya for at least 100 years. They were built sporadically during colonial times and then in 1980 a farmer and development worker, Joshua Mukusya, introduced the sand dam in South East Kenya. Mukusya’s work inspired Maddrell and led to the creation of the three main organisations in Kenya that build sand dams.

Excellent Development has supported work in nine different countries, with partners in India (Rajasthan) and Africa (including Zimbabwe and Mozambique). While much of their effort has previously been focused in Machakos and Makueni Counties in South East Kenya – now the charity is also supporting community conservancies in the Northern Rangelands of Kenya.

Extreme weather patterns mean that drylands often get rain so intense that it damages crops, causes flooding and erodes the topsoil. “We’re six inches from extinction; that’s the crust of soil around the earth which keeps all land species alive” says Maddrell. He maintains that the only solution to poverty in drylands is to conserve water and soil. Sand dams capture seasonal rain for year-round use and have the potential to work in a large percentage of drylands. Not only that, they save communities up to five hours a day fetching water, (up to 11 hours during droughts) time that they can invest in sustainable land management.

What is a sand dam?

A sand dam is a reinforced rubble masonry wall consisting of cement mortar, various sized rocks reinforced with steel rebar and barbed wire. It is built across a seasonal, sandy riverbed and captures and stores water within the sand behind it. Typically, sand dams are built in the transition zone between hills and plains where the gradient of the riverbed is between 0.2–4% – the upper-middle sections of a river catchment. The dams must be built on impermeable bedrock. Those built in South East Kenya most often range in height from 1.5 m to 4 m from the riverbed to the central spillway and span channels 10 m to 30 m wide. Dams of this size store in the region of 2–30 million litres of water and recharge after each rainfall – the larger ones store enough water to supply 1,200 people all year round.

Sand dams only work when built in seasonal rivers where there is sandy sediment, enabling water to be stored and abstracted. Eventually, the dam will fill up with the sediment that is already in the river. During the rainy season, the lighter silt is washed downstream and it is the heavy sand and rainwater that remains in the reservoir behind the dam wall. The sand in the water sinks and the remaining silt goes over the top and eventually the dam is totally full of sand holding water. The remaining sand protects the water from contamination and evaporation. Sand dams do not deprive communities downriver of water because between 97% and 99% of the water flowing in the river continues downstream.

Building sand dams

There are three technical prerequisites for locating sand dams. Firstly, the river needs to be highly seasonal – flowing for weeks rather than many months of the year. Secondly the river needs to have sufficiently sandy river sediment – see Sediment testing. Thirdly, there must be accessible bedrock – within four metres for manual digging and six metres using a standard JCB. Excellent Development has established two golden rules for sand dam design. The first is that dams must be built on bedrock (or, in rare cases, impermeable, compacted sub-soil) to a point at least 1.5 m wider than the flood width of the river. Secondly, the dam must allow the river to flow as it did before.

To ensure a smooth construction process there needs to be clarity over the ‘ownership’ and management of and access to the dam. This will be documented in legal agreements and requires recognition by both the local and traditional authorities. Construction only commences once all the local materials (sand, rocks and water) are available on site and the catchment immediately above the dam is protected from erosion. Sand dams are gravity dams which must be strong enough to hold the forces of the seasonal river flows and what they may carry. In the past, large baobab trees and even a train engine with its two carriages have been washed over Excellent sand dams – which remained intact. To prevent the dams from slipping they must also be anchored into the bedrock with steel rebars drilled 5 cm into the bedrock.

Skilled artisans familiar with stone-masonry supervise construction, checking that the foundations are cleaned and well prepared in order to form a water tight seal.

The dam is filled in so that it contains as much rock as possible, ensuring that the rocks do not touch each other or the formwork and that there are no planes of weakness. This is achieved by first filling the shuttering with 15–25 cm depth of mortar, then placing very large stones in the dam without touching, and then manually filling the spaces with medium then smaller sized stones. Every 50 cm depth, barbed wire is criss-crossed around the steel rebar and opposite stones in a helix pattern (to increase tensile strength of the dam). The process is then repeated until the given heights of the spillway and wings are achieved. Once the dam walls are complete, they are plastered inside and out and watered three times a day for three to four weeks to ensure that the cement cures correctly to maximise strength.

Abstraction methods

Sand dams have two main abstraction methods, depending on the context in which they are applied.

Many dryland communities traditionally collect water from sand rivers using simple holes scooped into the sand. Sand dams permanently raise the water table, making water easily accessible from scoop holes all year-round by enhancing existing sources of water.

Where the scoop hole is used for domestic purposes, acacia and thorns are often used to protect the hole from livestock. A separate livestock watering point is usually established below the dam, in order to mitigate contamination of the water.

An alternative way to collect water is to install an infiltration gallery immediately after the dam is built. Pipes are built into a sand dam wall and connected to an infiltration gallery which provides water ‘on-tap’. Sometimes these are connected to a tank behind the dam which allows water to be pumped for irrigation, or connected to a shallow well in the adjacent river bank as shown in the diagram.

Once a sand dam has matured, it reduces the average time taken to collect water to between half an hour and an hour and a half – water that is filtered clean by the sand. A clean water source closer to home is crucial to combat deep seated poverty.

A small sand dam costs about £10,000, a medium one is £15,000 and the largest ones between £20–30,000. Properly located and built sand dams last for well over 30 years and there is one still in use that is more than 100 years old. Sand dam failure ratios are less than 2% (half of which can be repaired), with less than 5% of sand dams needing maintenance according to Maddrell, which makes them such a sustainable solution.

Lessons learnt

Maddrell says that the secret to sand dams’ success is to understand seasonal river structures, as they require a different approach than traditional dam designs, with local knowledge being critical to successful design. Sand dams need to cope with constant overflows, so spillways need to be in the middle of the dam, not the side, and they need to cope with the flow and volume of flood water.

In addition, because the slope of the dam (where it is narrower at the top than the bottom) is upstream and not downstream as with traditional dams, this prevents large rocks flowing over the dam and damaging it. Finally, sand dams are not always built straight across a river, sometimes it is necessary for the dam to be angled to bring the river back into the original river channel.

In terms of using community knowledge, Maddrell insists that you cannot design a sand dam without knowing how a river has flowed over the last 25–50 years, “Because water flowing down a seasonal river is, metaphorically, the life blood of the people disappearing before their eyes, they consequently know better than anyone – water engineer or not – how their river flows.”

The charity runs expeditions each summer where volunteers spend a week building dams. See its website for more information at www.excellentdevelopment.com