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Stormwater harvesting and management/Surface water/Floating intake

< Stormwater harvesting and management | Surface water

Floating intakes for drinking-water systems allow water to be abstracted from near the surface of a river or lake, thus avoiding the heavier silt loads that are transported closer to the bottom during floods. The inlet pipe of a suction pump is connected just under the water level to a floating pontoon that is moored to the bank or bottom of the lake or river. The pump itself can be located either on the bank or on the pontoon. The advantages of placing the pump on the pontoon are that the suction pipe can be quite short and the suction head will be constant (less risk of cavitation). If the river currents frequently carry logs or large debris, a floating inlet needs extra protection or it will be damaged. To construct the pontoon, a steel or wooden frame can be attached to floats made from empty oil drums, plastic containers, or sealed steel tubes at least 30 cm in diameter.

When choosing a river intake location, select one with few rocks and boulders, so that the intake system does not get damaged. These types of locations are further downstream, as opposed to higher elevations.

Suitable conditions

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Rivers or lakes.

Pond and lake intakes should be located so that the best quality water enters the system. Their location must prevent the drawing of water contaminated by contact with humans and animals or by organic material and suspended matter. Proper location of the intake will help ensure that suitable water is available to the community.

Generally developing intakes for ponds and lakes is expensive. The further away the users are from the source, the more expensive the system. Where possible, the source and intake should be located near the community. Especially in times of drought, a system far from the community that is piped to the community, could stop short of providing consistent water. In this case, make sure the dry and wet season volumes of water are known beforehand, so that water sources may be diversified in times of low to no water.

Whenever practicable a river intake should be sited where there is adequate flow; at a level that allows gravity supply to minimise pumping costs; upstream of densely populated and farming areas to reduce silt inflow; upstream of cattle watering places, washing places and sewer outlets (to eliminate pollution of the water); upstream of bridges (to reduce velocity/turbulence).

Intake designs aim to avoid clogging and scouring and to ensure the stability of the structure even under flood conditions. Where the river transports no boulders or rolling stones, an unprotected intake may be adequate.

Resilience to changes in the environment

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Drought effects on cement

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Effects of drought: Badly made concrete or cracked linings (e.g. in tanks, dams, waterways, wells, and other structures).

Underlying causes of effects: Less water used for curing; Impure water used for mixing.

To increase resiliency of WASH system: Ensure adequate mixing, ratios, purity of ingredients; Minimize water content in mixture; Ensure adequate curing.


More information on managing drought: Resilient WASH systems in drought-prone areas.

Construction, operations and maintenance

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A floating-intake system is usually operated by a caretaker. The pump and inlet pipe must be checked before and during pump operation, and any obstructing debris removed and damage repaired. This is particularly important during the rainy season. Every day, the mooring cables should be checked and adjusted if necessary, and the flexible pipe connections checked for leaks. Any damage to the mooring or the pontoon structure must be repaired immediately, which may require the assistance of several people. Depending on the materials used, the pontoon should be painted regularly, at least once a year for steel parts.

Potential problems

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— floating objects collide with the floating pontoon;
— the pipe connectors between the pontoon and the bank wear out;
— the lake or river water may be of poor quality.

Screens

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In water supply engineering, screens are used for various purposes:

  • Removal of floating and suspended matter of large size which otherwise might clog pipelines, damage pumps and other mechanical equipment, or interfere with the satisfactory operation of the treatment processes. Fixed screens are used for this purpose and they are cleaned on site by hand or mechanically.
  • Clarification of the water by removal of suspended matter even of small size, to lighten the load on the subsequent treatment processes. In particular screens are used to prevent filters from becoming clogged too rapidly. Screening is done by passing the water through closely spaced bars, gratings or perforated plates. It does not change the chemical or bacteriological quality of the water. It serves to retain coarse material and suspended matter larger than the screen openings. Even when screened-out material forms a filtering mat of deposits, the screening still is purely of a mechanical nature. Bar screens usually consist of steel strips or bars spaced at 0.5-5 cm. If the amount of material ex-pected to be screened out is small, the bars are set quite steeply, at an angle of 60-75° to the horizontal, and cleaning is done by hand using rakes. If larger amounts will be retained, cleaning by hand should still be feasible; to facilitate the cleaning work, the bars should be placed at an angle of 30-45° to the horizontal.

The water should flow towards the bar screen at a quite low velocity, 0.1-0.2 m/s. Once the water has passed the screen, the flow velocity should be at least 0.3-0.5 m/s in order to prevent the settling out of suspended matter. In the openings between the bars the velocity of flow should be limited to a maximum of 0.7 m/s; otherwise soft, deformable matter will be forced through the screen openings. A clean screen will allow the water to pass with a head loss of only a few centimetres. However, the head loss rises sharply when the clogging of the screen builds up. Regular cleaning should keep the head loss limited to 0.1-0.2 m head of water. Allowing for delayed cleaning and mechanical failures, it is good practice to design a bar screen for a head loss of 0.5-1.0 m.

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Acknowledgements

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