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
3.1.3 Runoff Collection Using Surface Structures
Large water storages (also called reservoirs or impoundments) can be
formed in-stream by constructing dams, or off-stream by lining natural or
artificial depressions with impermeable liners (UNESCO, 1991). The stored
water may be used during periods when surface water flows are insufficient
to meet water demands. An example of an impoundment is shown in Figure 30.
One important aspect to be considered when using this technology is the
need to prevent in-stream storages from rapidly silting up, thus reducing
the effective storage capacity. Siltation problems can be reduced by
constructing siltation ponds upstream of the main storage to trap silt
that would otherwise enter the storage reservoir, and by ensuring that
land use practices within the drainage basins feeding these storages are
such that they minimise sediment loss from the land surface.
Figure 30. Schematic of an impounded pond.
Extent of Use
Large storages are not common on small islands due to a combination of
unsuitable topography, unsuitable geological conditions, lack of perennial
streams, and economic factors that do not favour the large investment in
infrastructure required. However, they are found on some of the larger
islands. For example, there are 22 dams on Xiamen Island, China, with a
combined capacity of 13.5 x 106 m3 and a yield of about 77 000
m3/day; an earth-filled dam on Guam with a storage capacity of
about 9.5 x 106 m3 and a yield of about 50 000 m3/day;
a butyl-rubber lined storage with a capacity of 5.3 x 106 m3
on the island of Molokai, Hawaii (the liner was necessary to prevent
leakage through highly permeable rocks); a dam on Penang, Malaysia, with a
storage capacity of 2.5 x 106 m3 and a yield of about 23 000 m3/day;
an off-stream storage, built using an artificial liner at Akoao, Rarotonga
in the Cook Islands; and, 218 small impounding ponds on West Nusa
Tenggara, Indonesia, that have been built since 1980. On East Nusa
Tenggara, 129 impoundments have been constructed, with a total storage
capacity of 22 x 106 m3 and serving approximatley 6 400
families. On Upolu Island, Western Samoa, a dam has been constructed for
hydropower and water supply purposes.
Operation and Maintenance
Proper operation and maintenance of dams is essential to ensure the
structural safety of the dams and the safety of people downstream. Safety
and surveillance inspections should be an integral part of operational
procedures. The overflow spillway has to be maintained to cope with
storm-related overflows. The dam structure must be checked regularly for
signs of seepage. Some impoundments may require desilting to increase
effective storage capacity. As part of the general maintenance of the
dams, it is also essential that steps be taken to protect the catchment
area, as the quality of surface water is often affected by heavy sediment
loads and turbidity levels.
Level of Involvement
The design and construction of dams (large or small) and off-stream
storages is an highly-specialised task requiring specialist expertise.
Hydrogeological and geotechnical investigations are required, as well as
specialist contracting and construction skills, especially in the case of
very large dams. Operation and maintenance can be performed by properly
trained local people.
Costs are variable and depend on the type and size of the structure, and
the source of construction materials. An example is the estuary reservoir
on Nusa Dua, Bali, Indonesia, which has a supply rate of approximately 300
l/sec. The costs of the Nusa Dua Reservoir consisted of general costs,
equipment costs, construction costs, administration costs, and engineering
costs, and totalled $800 000 in 1995.
Effectiveness of the Technology
Where the naturally-occurring surface water resources are inadequate to
continuously provide sufficient water, the storage of surface water in
dams or impoundments is an effective water resource development option,
provided that suitable topographical and geological conditions prevail.
Suitability This technology is only appropriate for larger islands, where
the topography and geological conditions, and economic factors, are
Impoundments can provide for the storage of moderate to large volumes of
water that can be abstracted to meet dry season shortages. This technology
has limited but necessary operation and maintenance requirements.
Dam can create potential environmental problems, and there is a possible
risk of catastrophic failure. Construction of dams often incurs high
investigational and construction costs. The stored water quality is
variable and may require treatment.
The flooding of land behind dams may be a sensitive issue in SIDS, where
land area is at a premium.
Further Development of the Technology
This technology is very specialised, but relatively well-developed for
continental use. The development of procedures for dam construction in
SIDS would be useful. Such procedures should cover investigation, design,
construction, operation, and maintenance aspects of the technology, with
particularly emphasis on safety and surveillance aspects.
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