Dabane Trust
Water Workshops
___________________________ Reg No. MA 2011/92
________________________
Siqala ngamanzi
Water from Sand Rivers
Sand filled river channels, often referred to as sand rivers are an extensive phenomena
in dryland, arid and semi-arid areas. Depending on rainfall, rainfall intensity,
topography and vegetation cover there is often extensive soil erosion in dryland areas.
Material eroded from the land surface is washed toward riverbeds and considerable
quantities of eroded material are transported through river channels. Depending on
the gradient of the river channel and the nature of the riverbed huge volumes of
sediment can be retained throughout the riverbed. Where this occurs the sand filled
riverbeds become shallow aquifers retaining 20 – 25% water where they are saturated.
Where material is not retained either by gradient or natural obstacles to sediment
transport, constructed barriers such as sand dams and subsurface dams will impound
sediment that will store water.
There is no single or ‘best’ method of harvesting water from river sand. As with any
potential water supply system, particularly when considering a new area, it is
necessary to carry out an assessment for the most appropriate system. A technical
feasibility study of the locality and possible sites is required together with an insight
into the area, topography, rainfall etc. Significant quantities of water are available
from sand aquifers with differing systems of abstraction required for different
situations. My experience is primarily with naturally formed sand aquifers where in
effect sand rivers form an extensive natural dam, dry on the surface but with a vast
reserve of water beneath.
Sand-abstraction
The system of abstracting water from the underlying water bearing sand of surface dry
river beds is well established. In Zimbabwe where the system is erroneously but
commonly referred to as “sand-abstraction”, abstraction systems are based on basic
borehole well-screen technology and installations have been undertaken since the
1950’s.
Although a natural occurrence sand rivers have expanded exponentially in the last
century due to more intensive use of marginal land. Sand rivers are a result of high
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rates of soil erosion that remains in the river channel with copious amounts retained in
large slow flowing rivers that cross extensive plains below the water-shed. Conditions
for sand-abstraction are ideal where rivers cut through igneous soils. In these areas
soils tend to contain a large proportion of coarse grained sands which are carried into
and remain in the river systems and allow for large volumes of water to be retained.
Wellpoint sand-abstraction is inappropriate in small fast flowing rivers such as the
headwaters of larger rivers that contain small amounts of unstable sand. However
useable quantities of water can be abstracted from smaller rivers through the
construction of sand dams. This is an impoundment, generally a weir, constructed to
allow sediment to accrue over several years that will in turn retain water. To maximise
water storage capacity the dam is designed so that coarse grained sand is retained and
finer silt is washed through the weir each year.
Sand-abstraction is dependent on water freely percolating through sand to the point of
abstraction. Silt and clay create an impenetrable barrier within an abstraction system
and thus in such conditions wellpoint sand-abstraction is not an appropriate option.
The ideal situation is a slow flowing, wide river with deep coarse sand. The ideal site is
above a natural rock barrier or in a depression in the riverbed which is continually
recharged by water percolating from an expanse of sand above.
There are several methods of abstraction that have been developed over the years.
Each is dependent on equipment which can be installed into the water bearing river
sand and at all times remain in free moving water.
Wellpoints - Well Screens, Sand Spears
Recently developed systems utilising plastic or styrene pipes with inward tapering
slots, generally 0,5; 1,0 or 1,5 mm wide, used according to the coarseness of river
sand, prevent the entry of sand with a grain size of a greater diameter. Smaller sand is
initially drawn through the slots but coarser sand quickly collects around the
immediate screen and by degrees blocks the entry of finer sand; thus a graded natural
filter is developed. One or more well screen is connected to a suction pump on the
river bank. Installation is easiest when the river sand is saturated to full depth, when
well screens can be driven or ‘screwed’ into the lower levels of river sand. An effective
installation method is ‘jetting’ through the creation of an artificial ‘quicksand’. A jet of
water from a motorised centrifugal pump allows a wellpoint to sink through the sand
into the depths of the riverbed. This system is however relatively more costly and
technically complex.
Manifold & Wellpoints
This method is utilised on larger, commercial installations. The system depends on a
low velocity of water at the point of abstraction, sufficient to draw off water without
drawing sand into the system. The system is best installed towards the end of the dry
season when the river water level is at its lowest. River sand is removed to water
bearing level, a ‘manifold’, a large diameter pipe, is laid on the water-bearing sand and
slotted pipes driven at an angle into the sand and connected to the manifold. The
manifold is also connected to a suction pump on the river bank. The number of
wellpoints, size of manifold and piping to the river bank are calculated according to the
velocity of water in certain parts of the system, and on the quantity of water required.
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This is probably the most common system employed in Zimbabwe today, though it is
doubtful if every installation is correctly calculated.
Infiltration Galleries
Most of the original installations consist of a pipe system with several slotted pipes at
the end. Sand has to be kept out of the pipes so that water may flow in and gravitate
to a sump or false well on the river bank from where it can be pumped out. Prsent-day
synthetic materials have recently improved the potential of this system, before their
introduction it was difficult to keep sand out of the system. However it is still difficult
to ensure that the pipe system is sufficiently deep in the sand to maintain it in water
year round. Further it necessitates extensive digging into the river bank. It is relatively
inexpensive and does not require complex equipment or expertise. A windlass or
commonly used hand pump can be utilised to draw water.
Caisson
Water is abstracted through the lower sides or slotted base of a large (1,0 to 1,5m
diameter) flat cylindrical or slightly conical ‘caisson’ sealed on the top to prevent
ingress of river sand. The caisson either contains a wellpoint or is connected by pipes
to a suction pump on the river bank. The ‘caisson’ is dug into river sand as deep as
possible and is lowered as necessary, as the water level drops during the season in
order to keep it in water. As with any of the systems, the installation is complete once
the abstraction point reaches a level where it will remain in water year round. This
system is more awkward with the continual re-digging and lowering but, once at its
lowest point, because of the large surface area for abstraction it can be used where
there is fine sand or where silt tends to accumulate.
Recharge
1.
Once a system has been correctly surveyed and designed it is relatively easy to
monitor and maintain. Unlike the recharge of ground water, which occurs slowly
during and initially after the rainy season, recharge of a ‘sand-abstraction’ system is
immediate from a flowing river. It must however be noted that the river will continue
to drain through the sand, the sand merely impedes the drainage. Thus the longer or
more frequently the river flows to recharge the river basin, the longer the river will
take to drain, thus keeping the system in water.
2.
There is no water table or aquifer at the point of abstraction. As well as water
percolating into the abstraction area, water is also draining away from the site. In less
than optimum sites, water can drain away, leaving the system high and dry before the
next recharge.
Water Quality
1.
Where there are extensive sands the entire river basin acts as an enormous
sand filter bed. Where abstraction is not excessive and provided there are not large
herds of cattle polluting the sand, the water may be considered potable and require no
treatment.
2.
Excessively used river sites, however, can become clogged and polluted from
clay or livestock droppings.
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Village Level Operation & Maintenance (VLOM)
1.
“Sand-abstraction” systems range from massive multiple wellpoint domestic
water supply or irrigation schemes, with diesel or electric powered pumps on large
rivers to small scale single wellpoint hand operated pump and well screen units on
small rivers.
2.
In consideration of the large number of possible sites and relatively low river
banks on typical rivers, small scale units which can be sustained by rural communities
are a useful option.
3.
In some rivers there is the possibility that the river in flood will wash the
abstraction point of systems away. A further complication in slow flowing rivers is that
silt layers may move through the river sand to clog installations. Observation of the
river and consultation with the local community will provide useful, practical
experience to prepare for this.
USES
1.
Human Consumption - Drinking, Washing, Cooking: A properly managed “sandabstraction” system provides a suitable source of water for domestic use. The
limitation being that water is not easily moved a great distance away from the river
bank without mechanical means.
2.
Livestock Watering: Even in less than optimum conditions “sand-abstraction”
systems provide a very suitable source of water for livestock. Whenever possible
during the rains, farmers will allow their animals to drink directly from the river.
However once water levels in the river have dropped below the surface, water must be
drawn by container. A simple pump discharging into a water trough on the riverbank
makes for an appropriate livestock watering system
3.
Small Irrigation Schemes: Particularly where gardens can be established on low
river banks (above the flood line), not too distant from the river edge, sand-abstraction
systems provide an excellent, low cost, option to convey water to nutrition gardens.
One small handpump is sufficient to adequately water more than 200m² of garden - an
average size brushwood fenced garden.
Stephen W Hussey
Dabane Trust
P.O. Box 3331
BULAWAYO
Zimbabwe
Reg No. MA 2011/9`
Dabane Trust - Water Workshops
Telephone
Fax
E mail
web site
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+263 (0)9 244775/240198
+ 263 (0)9 241647
s.w.hussey@dabane.org
www.dabane.org
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