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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.

Sand Dams Tanzanian woman collecting water StudySmarterFig. 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.

Pros
Cons
  • They are inexpensive and use appropriate technology available to local villagers
  • They supply critical water needs in impoverished regions where other options are not feasible
  • Sand dams help villagers improve their hygiene and health by providing regular and year-long access to fresh water
  • They help villagers improve crop yields by providing a permanent source of irrigation water for gardens
  • Sand dams lessen the amount of time people have to travel to get water
  • Groundwater (the aquifer) is recharged, so water is available for replanting natural vegetation as well as for crops beyond the edges of the sand dam
  • Sand dams help combat desertification
  • They reduce conflict over access to water
  • Overall, they are a sustainable solution to unpredictable droughts and the ravages of climate change
  • Bacterial contamination occurs, particularly in 'scoop holes' where people gather water

  • Water is lost because it evaporates (evapotranspiration) and/or seeps into the ground
  • Catchment areas are silted up (siltation) because not all silt flows over the dam and downstream
  • Dysfunctional sand dams are risky when they are the only water solution people depend on
  • Sand dam models used in one area and applied in another area may not work because local geographic factors like climate, bedrock, sand availability, and so forth are not taken into account1

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.

The overall conclusion for Kenya, which can be applied wherever sand dams are found or planned, is that there are risks as well as benefits. 'Bottom up' means that in the absence of central government control and outside expertise, local villagers should be able to benefit from sand dams that they create and manage, and confront the uncertainties of climate change. They should be able to manage and overcome risks and, above all, avoid becoming overly dependent on a single sand dam.

Sand Dams - Key takeaways

  • Sand dams are concrete structures built across dry river courses that trap sand during occasional floods; the sand becomes saturated with water and serves as a valuable water source for local people
  • Sand dams are found principally in Kenya, but are in other countries as well
  • Advantages of sand dams include providing water, helping lessen the impacts of drought, desertification, and climate change, and recharging the groundwater
  • Problems with sand dams include loss of water, siltation, contamination, and people becoming too dependent on a single source of water

References

  1. Hannah, R., E. Jessica A., P. Alison. ‘Sand dams as a potential solution to rural water security in drylands: Existing research and future opportunities.’ Frontiersin.org. 2021.
  2. Nzwili, F. ‘Sand dams: Simple way to save water in Africa takes hold.’ Csmonitor.com. 2013.

Frequently Asked Questions about Sand Dams

Sand dams are rainwater harvesting systems that involve a reinforced concrete dam structure that traps sand brought downstream. The sand gradually fills up behind the dam and contains saturated water valuable for people who live in arid climates where rains occur infrequently.

Sand dams work by trapping sand behind and on top of impervious barriers of concrete and bedrock. Silt and most water in floods is not trapped. The sand becomes saturated with water which people can access via simple wells or holes long after surface water has dried up.

There are thousands of sand dams in Kenya, with dozens of new ones constructed every year.

Many sand dams do not work properly, resulting in siltation, loss of water, and contamination.

Sand dams provide inexpensive sources of fresh water for people who live in arid regions without permanent water sources. They reduce the amount of time people have to travel to get water and help restore groundwater resources.

Final Sand Dams Quiz

Question

What are three problems with sand dams?

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Answer

Siltation, contamination, and water loss

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Question

Why is a sand dam considered a 'bottom up' solution?

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Answer

Because it can be built and managed by villagers and people at the local level, without the need of government or outside experts

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Question

What two geographic requirements are needed for a sand dam?

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Answer

Bedrock that water can't seep through, and available sand in the drainage basin

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Question

Why is it not recommendable to have a sand dam as the sole source of water?

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Answer

Because over-reliance on a single water source is risky in an arid climate. If the sand dam fails or runs out of water, water may not be available until the next rain.

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Question

(True or False) Sand dams typically provide water to people for an entire year.

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Answer

False. Sand dams are rarely able to provide for a community's water needs for this long.

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Question

The typical amount of water that is stored by a sand dam is

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Answer

millions of litres

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Question

What material is used to construct sand dams?

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Answer

Reinforced concrete

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Question

What country has the most sand dams?

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Answer

Kenya. it has thousands.

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Question

Why might a Kenyan sand dam model not work in Ethiopia or Burkina Faso?

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Answer

Because local geographic conditions such as bedrock or climate may be different, requiring adaptations of the model to local conditions, or a different model altogether.

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Question

What three major global issues do water dams help people adapt to?

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Answer

Desertification, drought, and climate change

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