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1.1.11 Sand Abstraction
Water is abstracted from sand-filled river beds during periods in which
there is no surface water flowing in the river. In such situations, the
sand is fairly saturated. The method involves the use of slotted cast iron
or PVC pipes drilled into the sand-filled river bed, connected to a
mainline or manifold, and a pumping system. The water pumped out and
distributed for use in agricultural or domestic applications. In
small-scale, domestic systems wells or casing sunk into the sandy river
bed are fitted with simple hand pumps, or a bucket and windlass assembly.
Extent of Use
Freshwater abstraction from sandy river beds is widely used in Zimbabwe
and Botswana for farming and domestic supply purposes. Commercial
enterprises, such as ARDA and the Hippo Valley Estates in Zimbabwe, have
successfully used sand abstraction for irrigation purposes. At
Chisumbanje, ARDA has installed some sand abstraction units for winter
irrigation of wheat and early cotton crops. Hippo Valley Estates
successfully used sand abstraction during the drought of 1991-1992 for
sugarcane irrigation in one of their Estate sections. Sand abstraction has
great potential for freshwater augmentation in agriculture in areas where
runoff is stored in surfacial aquifers such as "dry" river beds.
Operation and Maintenance
Adequately designed pumping units are essential for the successful
operation of this technology. Care must be taken to balance the yield of
the sand aquifer and the pumping rate, and also spacing of abstraction
points, to minimize excessive drawdowns.
Maintenance involves ensuring that well points do not collapse. This is
achieved by casing the well. Further, the screening at the well-point
intake has to be good to minimise the intake of sand grains which can
damage pump impellers and shorten the life of pumping plant.
Level of Involvement
For large-scale operations, the technology has been limited to
applications in the governmental and private sectors. For small-scale
operations, local communities may be involved.
For large-scale schemes, there is a relatively high capital cost in the
form of pumps and pumping accessories, and relatively moderate operating
costs, primarily related to the need for fuel for the pumping equipment.
Effectiveness of the Technology
The technology is quite effective as has been proven by its application
in Zimbabwe. As an example, the sand abstraction units at Chisumbanje
(ARDA) are capable of delivering over 340 l/s, which is adequate for
irrigation of up to 300 ha of summer cotton and winter wheat. The volume
of water available is a function of the depth of sand in the river bed. A
general guideline, based on the ARDA experience, is that 20% of the volume
of sand is water.
This technology is suitable for sandy river beds that are usually
This technology may permit maintenance of ground cover vegetation during
drought periods. However, over abstraction may reduce downstream flows.
Use of this technology has the advantages of:
- Lower financial requirements compared to surface water developments
- Providing a reliable source of water even during the dry season
- Requiring management and operational skills not much different from
those required by borehole systems
- Being applicable over a range of operations from small scale- to
The technology has the disadvantages of having:
- Relatively high capital costs related to the purchase of hardware
- The potential for considerably shortened equipment lifespans,
primarily arising from the effect of sand on the pumps.
Traditionally, sand abstraction has always been practised in the whole
region, albeit on a small scale, for human supply and livestock watering.
Therefore, it has a high degree of acceptability.
Further Development of the Technology
A study for relating to the various productivity parameters (e.g.,
drawdown and well spacing, yield, casing and sand depths, sustainable
pumping rates, etc.) is required before this technology is widely used on
a large-scale. In Zimbabwe, research to answer some of these questions is
The hardware requirements necessary to implement this technology are not
a problem in the region.
Agricultural and Rural Development Authority, Box CY
1420, Causeway, Harare, Zimbabwe.
Department of Geology, University of Zimbabwe, Box MP
167, Mt Pleasant, Zimbabwe.
Departments of Water Resources Development (Botswana,
South Africa, Namibia and Swaziland).