|Sourcebook of Alternative Technologies for Freshwater Augmentation in Africa (International Environmental Technology Centre - United Nations Environment Programme, 1998, 182 p.)|
|Part B - Technology profiles|
|2. Domestic water supply|
|2.1 Fresh water augmentation technologies|
There are three elements which comprise a spring catchment installation (Figure 27); namely, a) the “effective” catchment, consisting of a perforated pipe within a trench or dry walled channel (stone package), b) a supply pipe leading to an inspection chamber, and c) an inspection chamber, which consists of an entry basin for receiving the spring water and an operation chamber which helps to control water quantity and quality. Sometimes it can also serve as a sedimentation basin, and, in such cases, may be called silt trap.
Construction should be done during the peak of dry season in order to identify and use the most reliable springs. Nevertheless, spring protection structures have to be designed with overflow pipes so that they can function during peak flows during the rainy season. Usually, the excavation around the spring, necessary for construction of the catchment, is started from the point where the groundwater emerges. Once that excavation is completed, the construction of the other system components starts. There are two parts; preparation of a permeable construction into which the source waters enter, and a dam which prevents the water from bypassing the catchment or reservoir. The dam, or barrage, is constructed opposite to the point of entry of the water into the catchment. It has to direct the source waters into the supply pipe, which conveys the water to the inspection chamber. The barrage has to be built into the impermeable layer, as well as into both side walls, to prevent the water from bypassing the system. The foundation of the barrage is cast into the excavation directly against the ground in order to create a tight seal with the ground. The barrage is then constructed, of either concrete or stone masonry, on top of the foundation. The height of the dam should be positioned lower than the level of the top of the water bearing layer. Difficulties may arise when the source waters have to be bypassed during construction of the foundation. (The flow should never be obstructed!) Usually a temporary dam is constructed of clay behind the excavation, and water is diverted with a temporary pipe or syphoned by a tube.
The permeable construction consists usually of a drain in the dry-stone masonry or of perforated pipes. The cross-section of this catchment drain should be sufficient to ensure that the maximum yield of the spring can be drained off without obstructing the natural spring flow. The drain has to be sloped at 1% to 2%. In the case of a solid substrate, no flooring is normally provided, but, for sandy ground, a dry pavement is needed. The velocity of water should be limited by providing additional catchment drains, considering the maximum flows to be expected during rainy season. Around the drains, a sand and gravel filter should be built up with gravel. The purpose of this filter package is to support the water bearing layer and prevent the fine particles that often comprise this layer from being washed out into the protection structure, resulting in the subsequent collapse of the water-bearing layer. A watertight cover, in the form of a 5 to 10 cm concrete cap, should be placed on top of the drains and the gravel filter. This cover needs to extend 20 cm into the slopes on all sides of the structure. Surface water reaching this cover should be drained off to minimise the potential for groundwater contamination.
Extent of Use
This technology is extensively used for projects in Africa. In Malawi, huge, gravity-fed, piped water schemes have been built, tapping spring water. Likewise, in Lesotho, a number of villages are supplied this way.
Operation and Maintenance
The operation and maintenance of spring protection structures is simple. They require few skills to construct and manage, making them suitable for management by user communities. Where steep drops are encountered (such as in the Lesotho Highlands), good structural designs are required to cater to the increased pressures built up in the supply pipes.
Maintenance activities may include protection of the catchment area from potential contamination, periodic maintenance of the filter package, and cleaning the spring area of leaves and other terrestrial debris. Maintenance is carried out by controlling human and animal activities around the spring, repairing the perimeter fence, and repairing the surface water drainage system. It is also necessary to control of the growth of trees around the spring to prevent roots from causing piping to occur in the sand and gravel filter beds and/or breaching the impervious seals around the reservoir and dam. Periodic testing of the water for bacterial contamination is also recommended.
Level of Involvement
Local input of skills and materials from the beneficiary community is often needed to implement this technology. Technical support may be needed from government, NGOs, and other implementing agencies in the conduct of hydrogeological investigations, structure design, and construction. These activities require the inputs of technically-qualified staff, which depends on the size and nature of the scheme.
Spring protection is an inexpensive in comparison to the development of a conventional point source. The cost of the protection structure, itself, is largely a material cost (cement, pipes). However added costs may be incurred in the form of costs associated with the delivery mechanisms, which are dependant upon the length of piping, the number of storage reservoirs, and/or the number of pressure break tanks needed.
Effectiveness of the Technology
Springs have been used by local communities as a source of water supply for many years. Their relatively good quality water, and generally very low operation and maintenance costs, coupled with the ease of community management, make them quite effective for supplying rural communities with water for domestic purposes. Protecting these water sources from contamination is an natural way of ensuring the continuity of this supply.
This technology is suitable in locations where springs occur and no unresolved pollution problems prevail. They may be managed as point sources for communities or distributed to individual households by connection to a distribution system.
No environmental impacts have been reported.
The advantages of this technology are several-fold: groundwater is a relatively safe water source for use without treatment, springs are the most inexpensive source of groundwater, and spring protection structures can be constructed using local skills and materials. Further, this technology incurs few or no operating costs, and requires very little maintenance, if the water is obtained at its source.
Service level is dependent on groundwater yields, which seldom can be improved (unlike in conventional systems). Further, there is difficulty in ensuring the hygiene of the springs, especially during the rainy season when it is not always possible to protect the spring from surface water intrusion. The location of springs is not always near the point of consumption and, in many cases, access is difficult. Springs may also run dry during times of drought.
Spring water is associated with witchcraft amongst some East African communities. It is also the belief in some communities that women who have given birth to twins and/or whose husbands have died must not use springs before certain cleansing rituals are performed for fear that their “unclean” condition would cause the springs to dry up.
In Southern Africa, communities often associate the placement of cement on springs with the spring drying. Such communities would be reluctant to install concrete catchments around their springs.
Further Development of the Technology
The technology does not require any further technical development, but it may be necessary to carry out social research on the cultural beliefs of communities to determine their basis and the effect this has on spring protection.
Ministry of Land Reclamation Regional and Water Development, Post Office Box 30521, Nairobi, Kenya, tel (254 2) 716103.
Blair Research Laboratory, Post Office Box CY 573, Causeway, Harare, Zimbabwe, Tel. 792747
SKAT 1987. Manual for Spring Catchments.
Kenya-Finland Western Water Supply Programme 1990. Water Supply Development Plan 1990 -2005. Ministry of Land Reclamation Regional and Water Development, Nairobi.
WHO (World Health Organization) 1992. Fact Sheets on Environmental Sanitation for Cholera, WHO Publication No. WHO/CWS/92.17, Geneva.