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Newsletter and Technical Publications

<Sourcebook of Alternative Technologies for Freshwater Augumentation
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

Technical Description

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

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

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 suitable.

Advantages

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.

Disadvantages

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.

Cultural Acceptability

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.

Information Sources

AWRC [Australian Water Resources Council] 1989a. Guidelines for the Design and Operation of Surface Water Information Networks. Water Management Series No. 18, Australian Water Resources Council, Canberra.

AWRC [Australian Water Resources Council] 1989b. Guidelines for Low Cost Water Supplies for Small Communities. Water Management Series No. 17, Australian Water Resources Council, Canberra.

Chan, P.K. and K.W. Chan 1989. Water Resources Development in Hong Kong. In: Proceedings of the Seminar on Water Management in Small Island States. Commonwealth Engineers Council, London. pp. 25-37.

Hall, A.J. 1983. Surface Water Information Network Design for Tropical Islands. In: Proceedings of the Meeting on Water Resources Development in the South Pacific, United Nations Water Resources Series No. 57, 83-95.

Hofkes, E.H. 1981. Small Community Water Supplies. International Reference Centre for Community Water Supply and Sanitation Technical Report No. T18, IRC, The Hague.

Kerr, C. 1988. Community Water Development. Intermediate Technology Publications, London.

Law, F.M. 1984. Surface Water Resources on Small Islands - A Consultant's Viewpoint. In: Proceedings of the Regional Workshop on Water Resources of Small Islands, Commonwealth Science Council Technical Publications Series No. 154, Part 2, 411-421.

Lee, Y.C. 1989. Development of Water Supply in Penang Island, Malaysia. In: Proceedings of the Seminar on Water Management in Small Island States. Commonwealth Engineers Council, London. pp. 38-43.

McMahon, T.A. and R.G. Mein 1986. River and Reservoir Yield. Water Resources Publications, Littleton, Colorado.

Mink, J.F. 1976. Groundwater Resources of Guam: Occurrence and Development. Water Resources Research Center Technical Report No. 1, University of Guam, Guam.

Peterson, F.L. 1972. Water Development on Tropical Volcanic Islands - Type Example: Hawaii. Groundwater, 10(5):18-23.

USBR [United States Bureau of Reclamation] 1974. Design of Small Dams. Government Printing Office, Washington.

Waikato Valley Authority 1987. General Guidelines for the Design of Small Homogeneous Earthfill Dams. Waikato Regional Council, Hamilton, New Zealand.

Wood, J.L. and J. Richardson 1975. Design of Small Water Storage and Erosion Control Dams. Colorado State University, Fort Collins, Colorado.

Zhang Zongwang and Liang Zhixin 1988. Study of Water Supply to Xiamen Island. In: Proceedings of the Southeast Asia and the Pacific Regional Workshop on Hydrology and Water Balance of Small Islands, UNESCO-ROSTSEA, Nanjing, China. pp.134-140.

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