Sand Dams

What would you do if you had no water in a place where it rained only once or twice a year? Your wells are dry and the local streams are now just sandy riverbeds. Don't lose hope! Across the world and for untold millennia, ingenious people have invented ways of capturing the scant rain that falls in deserts and semi-deserts, through check-dams, weirs, and other structures made from local materials such as branches and rocks.

When the rains come, some of it is retained in temporary reservoirs, to be used for a crop, or stored in containers, until it is gone and the anxious wait for the next rain resumes. In arid southeastern Kenya, local people have taken this a step further and figured out a way to store water in sand dams for the whole year.

Sand dam definition

Sand dams are a method of harvesting rainwater. Sand dams are simple, reinforced concrete structures that capture water and sand in rivers that only flow once or a few times a year (figure 1). They are found in arid, semi-desert regions, mostly in the tropics, without permanent streams. Kenya has thousands of them.

After the dams are built, they are designed to gradually fill up with sand washed downstream by occasional floods, while lighter sediment flows over the top of the dams. The sand then holds water during the rest of the year that local people can access for drinking, cooking, cleaning, irrigating crops, and other needs. They take five to ten years to mature, and after they are filled with sand, they last 50 years or more.

Sand dam diagram

Below is a diagram of a typical sand dam as you might encounter it in Africa.

Sand dam construction

Sand dams can only be constructed in stream valleys with sufficient sand and hard bedrock that water will not penetrate. Once an adequate place is identified, a wooden form is constructed across the dry stream bed, perpendicular to the flow of the stream. This form holds the poured, reinforced concrete that is the dam itself. Dams average two to 4 metres in height and 20 metres in length, but sizes vary greatly from place to place.

Once the dam is completed, villagers await the expected annual rain or rains that will carry streamwater to start filling in behind the dam. Silt, which is lighter than sand, flows over the top of the dam, along with around 98% of the water, and continues downstream, as the above diagram illustrates.

Fig. 2 - Tanzanian woman collects water from a sandy riverbed

During the first years after construction, an open pond or lake forms, with sand underneath that has settled to the bottom, but eventually, the entire area fills in with sand. Villagers dig holes and wells directly into the sand, which acts as an insulator, protecting the precious water from the rays of the tropical sun so that it can last until the next rainy season.

Pros and cons of sand dams

Sand dams can store millions of litres of water. In the modern era, spreading from Kenya, where they have long been plentiful, sand dams began to be heavily promoted as a sustainable development option in the 1990s. They are often cited as an excellent example of a bottom-up approach to water management.

Sand dams in Africa

Outside of its equatorial zone and the temperate highlands in the south, Africa is a water-stressed continent afflicted by desertification and climate change that causes periods of drought, scarcity, and even famine. But many seasonal rivers- over 1,000 in east Africa alone--naturally store water underground in areas where they deposit sand, and sand dams are a way of enhancing this process so that people can easily benefit.

Though found primarily in Kenya, examples of sand dams are also known in countries such as Ghana, Angola, Zimbabwe, Burkina Faso, Ethiopia, and Tanzania.² These and other countries are following the example of Kenya, where several development groups that promote sand dams are located.

Sand dams in Kenya

Every year, dozens of new sand dams are built in Kitui, Machakos, and Makueni, counties in the arid southeast of the country with the right geographic conditions (figure 3).

These dams have been the focus of intense study by professionals interested in learning how effective they are, and how to adapt their experiences to conditions elsewhere.

One important lesson that has been learned is that caution must be taken in assuming that sand dams are the best or only solution to water scarcity. Often, the most successful cases are promoted by development agencies, but do sand dams really deliver as promised?

Problems with Kenyan sand dams

A major claim is that once an area has a sand dam, its water problems are over. This is far from the truth! Concepts like sustainability and resiliency teach us that people who live in water-stressed, rural areas confronting desertification and climate change should not put all their eggs in one basket, so to speak.

Just as there are examples of sand dams built correctly and still functioning after many decades, the Kenyan landscape is also littered with failed sand dams. Cracked cement, leakage, siltation- all of these have contributed to dams not working at all, or ceasing to function after a few years. They were likely constructed in the wrong areas, using the wrong materials, and/or not built to the right specifications.

A second problem is the claim that sand dams can provide everyone with water for the entire year. This turns out to be a bit short of reality. Most households are not able to rely on the water from sand dams all the time.

Health issues related to sand dams

In terms of health problems, contamination occurs in areas where people are not properly trained in good hygiene, and the areas where people get water become contaminated with faecal bacteria and other dangerous pathogens. Just building a sand dam is not enough- Kenyans have learned that it is supremely important to have a community that works constantly to educate people, monitor water quality, and perform other important functions that keep the sand dam and its reservoir clean, strong, and functioning properly.