Newsletter and Technical Publications
<Sourcebook of Alternative Technologies for
in Small Island Developing States>
PART B - ALTERNATIVE TECHNOLOGIES
3. TECHNOLOGIES APPLICABLE TO SMALL HIGH, VOLCANIC ISLANDS
3.1 Freshwater Augmentation Technologies
3.1.2 Runoff Collection Using In-Stream Structures
In-stream weirs or buried collector pipe systems laid in, or adjacent
to, the stream bed are installed to provide sufficient head for water to
enter either a gravity-fed pipe or a pumping system (Figure 29). Sometimes
an intermediate chamber, fitted with screens, is provided to filter out
coarse material such as branches and leaves. Occasionally, settling tanks
are also provided to screen out finer material near the intake (UNESCO,
1991). Buried collector pipe systems extract water from the stream, and,
during low- or zero-flow periods, from the surrounding groundwater. This
type of in-stream system is less susceptible to seasonal fluctuations in
streamflows than weir-type intakes. In addition, the filtering effect of
sand and gravel backfill can have a beneficial effect on water quality
(UNESCO, 1991). Intake structures should be sited and designed to maximise
water quantity; maximise water quality; optimise water pressure in the
demand centre(s); be sited in geologically-sound areas; minimise
construction costs; minimise environmental effects; and, provide good
access for maintenance. Where possible, gravity, rather than pumped,
systems should be used to minimise pumping costs. In high-elevation
streams, it may be desirable to reduce the pressure in the transmission
line by the use of break-pressure tanks. Excessive pressure will require
expensive, high-pressure rated pipe, and can lead to wastage of water at
leaking joints and fittings. A simple rectangular tank fitted with a ball
valve or some other flow-control device can be installed in the line to
reduce the pressure to atmospheric at that point. Hydrological studies
should be carried out in selecting suitable intake sites, to ensure the
required amount of water is available and that the source does not dry up.
Streamflow data collection is recommended as a necessary part of such
Figure 29. Stream intake and settlementnin small mountain
streams in Seychelles (UNESCO, 1991).
This technology is suitable for use in perennial streams with solid
stream beds. The in-stream weir option is generally preferable in this
situation since it avoids difficult and costly rock excavations. Weir
intakes, in addition to their previously noted design features, should be
designed to divert sediment and organic matter; allow for ease of
maintenance; and, be able to withstand the damaging effects of periodic
high flows and debris movements (UNESCO, 1991).
In contrast, in alluvial stream beds, buried collector pipes are
generally a more suitable technology, especially where the streamflow is
unreliable, since they can obtain groundwater even when the stream ceases
flowing (as long as the groundwater level does not drop below the level of
the collector pipes). This technology is also more suited to streams with
high levels of suspended solids and sediment, as the alluvium around the
collector pipes acts as a sand filter. However, this technique may not be
suitable if there is a high percentage of fine particulate material, as
the natural stream bed filter may become choked, causing a restriction or
cessation of flow to the collector pipe (UNESCO, 1991).
Extent of Use
This technology is being utilised on most of the small islands that have
surface streams. Examples of the use of weirs are known on islands in
Seychelles; high islands in French Polynesia; Western Samoa; Rarotonga in
the Cook Islands; most 'high' islands in the Caribbean Sea; and, Fiji.
Examples of the buried collector pipe system are found in some streams in
French Polynesia, including those near Papeete on the island of Tahiti;
and, streams on Rarotonga, Cook Islands.
Operation and Maintenance
In the case of gravity-fed systems, operation and maintenance
requirements are limited. Maintenance will consist mainly of keeping the
intake structure clear of debris and checking the pipeline for leaks. For
stream-bed collector systems, replacement of the gravel and sand filter
above the pipes may be necessary if erosion occurs as a result of flood
flows. With pumped-intake structures, pump operation and mechanical
maintenance are additional requirements.
Level of Involvement
Hydrological studies are normally required in order to appropriately
site surface water intakes. Such studies should be undertaken by qualified
personnel. Engineers should also design, and supervise the construction
of, water supply systems. Once constructed, gravity-fed systems could be
operated and maintained at the community level. However, pumped systems
require trained operators.
Costs are very variable and site-specific. In 1995, the cost of stream
intakes in Fiji was $1 500 to $3 500 depending on the size of the intake.
Labour is usually supplied without cost by the village. Transportation
costs vary from $500 for transportation on the main island to as much as
$3 000 for transportation to remote islands.
Effectiveness of the Technology
With proper site selection, design, installation, operation, and
maintenance, surface water supply systems are very effective.
Surface water sources are less expensive to abstract since they can be
gravity-fed rather than pumped as in the case of groundwater abstraction.
Surface systems also have more limited, but necessary, operation and
maintenance requirements that do not require highly-skilled operators.
Surface water can be an unreliable source of water that often needs
treatment due to the moderate to high risk of contamination. Many surface
supplies fail after a few years due to incorrect siting and lack of
maintenance and spare parts.
Ownership of the stream and the water may belong to another village and
could create conflict in some circumstances. If disputes are to be
avoided, early negotiations and agreement with the owners of the stream is
Further Development of the Technology
This is a well-developed technology. It is not anticipated that further
development will be required.
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