Sourcebook of Alternative Technologies for Freshwater Augumentation in Small Island Developing States


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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

Technical Description

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 studies.

Figure 29. Stream intake and settlementnin small mountain streams in Seychelles (UNESCO, 1991).

Suitability

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

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.

Advantages

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.

Disadvantages

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.

Cultural Acceptability

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 essential.

Further Development of the Technology

This is a well-developed technology. It is not anticipated that further development will be required.

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. 74 pp.

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.

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. (Ed.) 1981. Small Community Water Supplies. International Reference Centre for Community Water Supply and Sanitation Technical Report No. T18, The Hague.

Jordan, T.D. 1984. Handbook of Gravity-flow Water Systems. Intermediate Technology Publications, London. pp. 12-123.

Kerr, C. 1988. Community Water Development. Intermediate Technology, London. 192 pp.

McMahon, T.A. and A. Diaz Arenas 1982. Methods of Computation of Low Streamflow. UNESCO, Paris.

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

Reynolds, P. 1982. Aspects of an Appropriate Rural Water Supply Technology. In: Proceedings of the Regional Seminar on Water Technology, Towards Rural Development. Faculty of Agricultural Engineering, University of Agriculture, Malaysia. pp. 92-103.

Singh, V.P. (Ed.) 1982. Applied Modeling in Catchment Hydrology. Water Resources Publications, Littleton, Colorado.

Spangler, C.D. 1980. Low-cost Water Distribution - A Field Manual. Appropriate Technology for Water Supply and Sanitation, The World Bank, Washington.

Tao X. and P. Chen 1988. The Designed Runoff for Mountainous Basins of Small Size, Fiji. In: Proceedings of the Southeast Asia and the Pacific Regional Workshop on Hydrology and Water Balance of Small Islands. UNESCO-ROSTSEA, Nanjing, China. pp. 180-188.

Warner, D.B. 1984. Rural Water-supply and Sanitation Planning: the Use of Socioeconomic Preconditions in Project Identification. Journal of Hydrology, 68:443-459.

WHO [World Health Organization] 1976. Typical Designs for Engineering Components in Rural Water Supplies. World Health Organisation South-East Asia Series No. 2, New Delhi, India.

Wilson, R. 1986. Standardisation of Materials, Construction Techniques and Operational Methods in the Water and Sanitation Sector of a Small Island Population. Water Supply, 4(2):103-116.

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