In-channel modifications

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General information

1. Name technique:Sand dam Kenya

2. Pictures

Figure 1: Sand dam in ephemeral stream bed in Kitui district, Kenya (Source: Borst & Haas, 2006)

Figure 2: Schematic cross-section of sand dam (Source: Borst & Haas, 2006).

3. Brief description of the situation:
Irrigation potential in South and East African countries is much higher than the presently irrigated area. A variety of soil moisture and water conservation technologies could be adopted to reduce the cost of irrigation, extend it throughout and promote sustainable small-scale irrigation on a watershed basis. These technologies are essential especially in drought-prone areas. Even though drought is a purely natural calamity caused by the failure of (monsoon) rain, it can be minimized by careful planning and operation. During good rainy years, excess rainwater should be stored in the soil and also underground using suitable soil moisture conservation measures and water harvesting structures on a watershed basis. This stored water can subsequently be used for irrigation.

An example of a rural water conservation programme is the construction of sand dams in the Kitui district in Kenya (Figure 1). This programme is a co-operation between the community and the Sahelian Solution Foundation (SASOL). SASOL, founded in 1990, assists Kitui communities to address household and production water scarcity through the sand dam technology. Although the sand dam technology has been known for 3000 years since the time of the Babylonian Kingdom, it has not been applied at a large scale. This might be because it is a low-key technology and there is no grandeur to it. As a result its full capacity has not been realized and developed, even though it is one of the major systems to aid communities living in arid and semi arid lands with ideal condition for its application.

Technical information

4. Brief description of the technique:
The sand-storage dam is a small-scale concrete checkdam, constructed above ground in an ephemeral river bed (Figure 2). During periods of high flow sand and gravel accumulates against the dam. Runoff water can easily infiltrate these highly permeable soil deposits, creating an artificial aquifer upstream of the dam capable of recharging the groundwater and providing clean water if well harnessed. Sand accumulated against the dam can store up to 35 percent of its total volume as groundwater.

5. Attributes overview:

6. Construction:
The design and construction of sand dams is variable. Each site is different. Rivers vary in width and discharge, rock foundations vary in depth and susceptibility to leakage, riverbanks may be high or low. The location of a sand dam should be chosen such that: it is feasible on technical grounds, it has high storage capacity, it has minimum cost and it is convenient to the community using it. The selected location is excavated to reach a firm impermeable layer as founding layer for the dam. This layer may be base rock, clay or murram (coarse gravel with clay matrix) and is usually uneven.

The height of a sand dam is usually about 1.5 to 2 m in the centre, but some dams up to 4 m have been constructed. At either end the wall is raised to prevent the river cutting round during a flood. Where the valley sides are flat, wing walls may be added at an angle to the main dam for the same purpose. Normally the base width of a wall is 1.5 m and top width 0.75 m. The upstream side is vertical, the downstream side angled. In Kitui, Kenya, dams are made of masonry, because this is relatively cheap. Also masonry has a long lifetime and requires minimal maintenance.

7. Capacity:
The maximum amount of water that can be harvested per year from an average sand dam is around 5300 m3 (Alvarado, 2006) or 8100 m3 (Borst and Haas, 2006).

8. Experiences with Operation and Maintenance:
Sand dams are permanent structures with a long lifetime, provided they are well maintained. If a sand dam has been well constructed, there should be little or no maintenance. However, it is necessary to check after floods and repair any damage that is found. In the Kitui district in Kenya, sand dams built in the 1950s and 1960s continue to function to date.

9. Experiences with Monitoring and Evaluation:
In Kenya, SASOL Foundation has conducted a number of studies on their use and socio-economic benefits, supplemented with work carried out by students. A hydrological and socio-economic evaluation of existing dams implemented by SASOL, Acacia, IVM and the PWN with financial support from Aqua4all has started in 2006. Some conclusions of this evaluation are:

• The dams fill rapidly during the floods and an substantial amount of additional water is stored in the riverbanks
• A preliminary water balance shows that only a minor portion of the total run-off water (<5%) is intercepted, indicating minor impacts on the downstream users.
• The occurrence of a serious drought during the period of investigations confirms that the dams provide water during drought periods.
• The socio-economic study shows an significant improvement on income and economic growth

10. Experiences with related subjects (e.a. erosion prevention, quality of drinking water, ..):

• Ecosystem regeneration: Where sand dams have been constructed, substantial regeneration of natural vegetation and ecosystem function is evident.
• Water quality: The water is captured for use through a hand dug well or tube well that is put into the sand in the dry season. This water is clean and of good quality for consumption due to the filtering effect of the sand. Quality issues may develop from livestock drinking from scoop-holes, causing bacteriological pollution. Therefore it is recommended that abstraction wells are sited some distance upstream of the dam wall. Livestock may be allowed to drink from scoop-holes near the dam wall so that they do not pollute the surroundings of the well.

Financial information

11. Experiences with management:
The economic conditions in Kenya are such that participatory or bottom-up approaches are essential in the dam construction (and obtain maximum socio-economic benefits). Using locally available materials and community labour reduces costs and enhances efficiency, acceptance and dam life-span. Cost of an average dam in Kenya is about US$8,500. Some 40% of overall construction cost is provided by the community, resulting in community ownership and commitment to maintain the dam. The investment cost per consumer is about 5-10 US$.

12. Benefits:
In the Kitui region of Kenya, water is now available within short distances from homesteads and people have improved their livelihoods significantly by engaging in small-scale irrigation for growing food- and cash crops.

In the Kitui area, nearly 200,000 households have benefited through:

• cutting the average time spent on water collection
• increased crop production and better quantity and quality of drinking water-supply
• improving the hygiene and nutrition of people, livestock and poultry
• increased income

Table 1: Measured social and economic impact of sand dams in the Kitui region, Kenya

General conclusions

13. Generic factors of success and traps ('do's and don'ts'):

• The need for wing-walls and a spillway must be carefully assessed. In some cases it is considered preferable to construct the dam in layers. The idea is to keep a sufficiently high velocity in the reservoir so that light particles cannot settle and diminish the infiltation capacity of the accumulated material upstream.
• Community involvement is essential for the success of sand dams. It is important that communities reach consensus on the initiation and benefits of new activities.
• Dams can be built in cascade, providing water for the whole catchment area. Ecological damage on a single point water source is avoided and it is likely to get a higher rise in water table than in individual units. Recharge into soil storage spaces is hence much more effective.
• Site selection is important. Errors in site selection may result in:
  • insufficient storage potential
  • insufficient depth to reach relatively impermeable bedrock
  • location in soil types with very low infiltration capacity
  • severe groundwater salinization

16. Links to detailed information of this project / technique:

Sasol Foundation
P.O. Box 85
90200 Kitui, Kenya
Tel: +254.44.22873

Acacia Institute
De Boelelaan 1085
1081 HV Amsterdam, The Netherlands
Tel: +31.20.5987330
Fax: +31.20.6462457

17. References

• Borst L. and Haas S.A. (2006). Hydrology of Sand Storage Dams, A case study in the Kiindu catchment, Kitui District, Kenya. Master thesis, Vrije Universiteit, Amsterdam.
• Foster S. and Tuinhof A. (2005) Brazil, Kenya: Subsurface dams to augment groundwater storage in basement terrain for human subsistence. The world bank, Sustainable groundwater management. Mutiso S. 2002. The significance of subsurface water storage in Kenya. In Management of Aquifer Recharge and Subsurface Storage NNC-IAH publication #4. Utrecht, The Netherlands
• Thomas D.B. 1999. Where there is no water: A story of community water development and sand dams in Kitui District, Kenya. SASOL and Maji Na Ufanisi.

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