The natural water purifier that also stores water in drought stricken areas

Drought is the most serious hazard related crisis facing many poverty stricken areas in terms of severity and frequency of occurrences. When there is no water to sustain crops for food or to sell at market, families face hunger and desperation. The worst affected are women and children who walk day long in search of water. 

Due to limited and unreliable rainfall most rivers are sandy bed streams and only experience heavy water run off for short periods of time after rain. During such periods of high flows large quantities of sand are transported downstream while others get trapped on the upstream sides of rocks ledges along the stream. Such sand traps are natural purifiers that are capable of providing clean adequate water if well harnessed. Using appropriate technologies this can be exploited for water storage in the form of sand dams.

Sand dams

To construct the sand dam a deep trench is first dug across the valley or stream, reaching the bedrock or other stable layer like clay.

A concrete or masonry wall is then built on the underlying rock bars across the river channels so that it can trap and hold back the sand brought by the river during flooding.

At either end of the dam, wing walls may be added at an angle to the main dam to direct and confine the flows of channel as the sand stores water in its spores. Since the natural sorting and deposition of sediments in streams is a function of channel slope and the shape of channel cross section, channel geometry is quite important in sighting the prospective sand dam.

Sand filters

This technique can also be used to purify water coming out of haffirs and other water reservoirs.

Step one: stop the haffirs filling with silt
By digging a simple silt trap at the mouth of the haffir, it is possible to substantially reduce the amount of sediment, silt and other organic matter that goes into the haffirs and which, if untreated, can render them useless in less than six years.

Step two: build a slow sand filter
To make sure the water intended for human consumption is drinkable, we then build a simple slow sand filter at the outlet of the haffir. Part sand and part gravel, the slow sand filter works largely through the formation of its own layer of bacteria, protozoa and fungi that helps filter out the majority of harmful elements.

cross-section showing the relationship of the silt trap (right) to the main haffir (left) - click for larger image longitudinal section of the filter tank - click for larger image

Earth dams

Although dams vary in size, they typically measure 1-2.5kms. Water is collected behind a 15m by 3m earth wall spanning the wadi land. This leads to shallow flooding of areas in excess of 2,000 hectares (ha), depending on the length of the wall.

Key modifications implemented by Practical Action:

  • Each wall has sluice gates across the central channel to capture periodic flooding. A typical channel ranges from 10-100m. The gates are closed during flood periods when water covers the cropping area behind the wall. Sluices and spillways either side of the channel are 1.5m high to help prevent breakages. The sluice gates wash away the silt that deposits upstream.
  • Pitching the earth embankment to protect it from being washed away
  • Introducing a trench below the earth embankment to minimise eroding

Lubna Mohamed Adam from Abu Degaise in North Darfur

Lubna lives in the remote rural village of Abu Degaise in North Darfur. She has lived here for 40 years, making a home for her family in a one roomed, mud-walled hut. Despite the state capital, El Fashir, only being 20kms from Lubna’s village she has only visited it three times, her days are consumed by the struggle to survive. 

“My life is about my family: about farming enough food for them to grow strong and succeed”.

“Losing my husband to the fighting has made me more determined to give my children a future. As a mother it is my duty to provide for my family but sometimes I failed them, I could not grow enough food to stop their hunger.”

Times have been tough for Lubna, but they are changing. Working with Practical Action, she and other women and men from Abu Degaise began constructing an earth dam in 2005 (completed 2008), to ensure that the short but heavy rains which fall in Darfur can be used for good. Lubna and the other women in the community provided much of the labour to build the dam.

“When I look at the dam I know that my hands helped to make it. Each stone that I carried is strengthening the structure and giving our village the chance to farm our lands once more”.

“Building the dam was hard but it felt good: working together and watching it take shape stone by stone. When the rains fell we rejoiced, knowing that we would have enough water to survive”.

“Now there is water where there was none. My land is good enough to grow sorghum, watermelon and cucumber and I even have enough to sell some at market.”  

“I feel pride; I can feed my family through my own hard work and now my children smile.”

Mohammed Nor, Eastern Sudan

Most recently, Practical Action constructed a dam in Kassala (eastern Sudan). The Girgir dam has provided a secure source of food for 2,000 households.

It floods over 3,000 feddans which are used for farming activities as well as natural pasture. A demonstration farm was also established where crops such as watermelon, cucumber, okra and hibiscus were planted. The productivity was high so the farm is continuing into 2009.

Mohammed Nor is a member of the Hadandawa tribe who live in rural areas of Northern Kassala. Drought and conflict hit Mohammed hard: he could no longer farm and had lost the livelihood so proudly practiced by his father and grandfather.

Without the ability to farm, Mohammed struggled to feed his family. His wife and children became desperate; leaving their home, land and community seemed like their only option.

But last year the introduction of a simple earth dam changed Mohammed’s prospects. By working with the families of Girgir village, Practical Action constructed a dam which collects and diverts precious water onto local farm lands. Now the soil is fertile and productive.

However, families here simply cannot afford for their crops to fail. Our demonstration farm is giving confidence to poor rural communities, through enabling them to see first-hand that more, and more diverse, crops can be grown on the wadi soils.

Mohammed was trained in new farming methods at the demonstration farm and started growing crops such as sorghum, cucumber and okra on a plot alongside the dam.

Within just a few months, the results were visible: “Every day I watch my crops grow. Never did I think I would return to farming or be able to provide the food my family needs. How lucky I am”.

“I can see our lands changing before my eyes. The rains may not fall so frequently, but I am beginning to see the landscape of my childhood returning”.

In just the first season, Mohammed harvested 20 sacks of cucumbers, which generate a high price at market. He earned £70 from the sale of this crop and is likely to do the same from the sale of his okra and sorghum once they have been harvested.

“For too long we felt forgotten. I was determined that my family wouldn’t move but were struggling to find a way to survive. Now we are farming, growing crops to sell at market: we can rely on ourselves”.

Umm Bronga Dam, North Darfur

Umm Bronga, located 40Km west of El Fashir has a population of 45,000 people. Despite difficulties arising from the conflict, in 2004, Practical Action worked with the local community to build a dam.

In its first season, even before its completion and with very low rainfall, the dam irrigated 2,000 feddans of land including sorghum, okra, tomatoes and cucumber. Over 200 families were able to grow their own food and even provide seasonal employment for surrounding villagers.

2,617 families are now benefiting from irrigation of between 2-4 feddans of land.

The dam consists of 2.3km of earth embankments and a control structure in the middle composed of a 14m spill way and 4 sluice gates.

Community members and local government were involved in the planning and implementation of the dam. The village elders helped map out the flooding history and water course, enabling technicians to design the maximum possible storage. Communities provided local materials and unskilled labour. Communities also selected local people to be trained in managing the dam, including opening and closing of the gates, silt clearing and assessing potential damage.

In spite of continuing insecurity caused by the attacks from the Janjaweed, very few displaced people moved to the IDP camps at El Fashir. Most people preferred to stay where they were because they felt a sense of control over their future and their ability to provide for themselves.

Mohammed Adam, the secretary of the Dam Management Committee said, “This project provided the means for the school to stay open, families could pay the fees as a result of their harvests and children could continue to learn. It is hard to believe we achieved all this with such low rainfall. Once the rains are good the amount of wadi we can farm will increase. Our community will be strong as a result of this dam”.

With support from Practical Action, local farmers have been able to dramatically develop their livelihoods. For example, farmer, Abdalla Lutfi learned that watermelon could be grown successfully with access to irrigation water. His first watermelon harvest sold for close to £2,400: almost 6 times more than the tobacco that he had previously farmed.

"I am so happy to have the chance to change the way I farm. This knowledge has opened my eyes to a new life for my family. Now my children can be fed. For the first time, I can look to the future: I am going to double the amount of land I farm.”

£15 can provide tools for one worker to help build a dam;

£30 could cover the cost of reinforcing steel for one gate on a dam;

£150 could buy one ton of cement for building a dam.

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You can download further information on rainwater harvesting from Practical Answers, the technical information service of Practical Action, or you can submit a technical enquiry to the Practical Action staff via the online form.

Building Small-scale Water-harvesting Dams

North Darfur faces acute deficit in its water. The need for water has forced farmers to try and adapt their practices and conserve more water.


Sand Dams

Sand dams are a means of capturing water in arid and semi-arid regions where erratic rains are trapped in river sands.

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