|
Photographic Feature |
British Geological Survey, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire, OX10 8BB, UK (Current address: Water Resource Associates, Robin-Herbert@compuserve.com)
The term ‘sand river’ is used in Botswana for a particular type of ephemeral river channel similar to the ‘wadi’ of North Africa and elsewhere. Figure 1 shows a typically linear stretch of the sand river near Chadibe in Botswana. Up to 100 m across, sedimentological studies show sand rivers to be filled with a complex mixture of clays, silts and sands. The most recent deposits overlie the main water-bearing deposits and were laid down since the last major flood which has been dated as occurring about 100 years ago. At Borolong the river was 50 m wide, the most recent deposits were 0.5 m thick, the full depth was 3.5 m and the high yielding sands were 10 m from the crystalline embankment.
There is perennial groundwater flow in the sand rivers even though the riverbed is dry for most of the year. It has been estimated that a total of over 900 km of sand rivers in Botswana could supply as much as one third of the country's domestic water needs from the water stored in them.
Collector wells
One of the most efficient ways of extracting water from thin shallow aquifers is via collector wells. These are large diameter shafts from the base of which an array of horizontal holes extends into the aquifer. Figure 2 illustrates how a collector well works. Collector wells have been constructed by the BGS in several parts of the world in weathered rock and flood-plain deposits. Sand rivers present special drilling
...
This 250-word extract was created in the absence of an abstract.
