Water Portal / Rainwater Harvesting / Surface water / River-bottom intake

River-bottom or Tyrolean intakes (different than a Tyrolean weir) for drinking-water systems are usually used in small rivers and streams where the sediment content and bed load transport are low. The water is abstracted through a screen over a canal (usually made of concrete and built into the river bed). The bars of the screen are laid in the direction of the current and sloping downwards, so that coarse material cannot enter. From the canal, water enters a sand trap and then may pass a valve and flow by gravity, or be pumped into the rest of the system.

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.

Suitable conditions

• Rivers with little sediment and bed load.
• 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).

Resilience to changes in the environment

Drought effects on concrete

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

River-bottom intake mechanics.
Click image to zoom.

For small community water supplies only small quantities of water are needed and often very simple intake structures can be used. With a per capita water use of 30 litres/day and the peak intake 4 times the average water demand, 1000 people would require an intake capacity of only 1.4 l/s. A 150 mm diameter intake pipe would be sufficient to keep the entrance velocity 0.1 m/s. If an entrance velocity of 0.5 m/s were allowed, a pipe as small as 60 mm would be adequate.

The bottom of the intake structure should be at least 1m above the riverbed to prevent any boulders or rolling stones from entering. The intake structure must always include one or more baffles or screens to keep out debris and floating matter such as tree trunks and branches. It is advisable to use “passive” screening that does not create turbulent flow conditions. To reduce the drawing in of silt and suspended matter, the velocity of flow through the intake should be low, preferably less than 0.1 m/s. To make use of the natural current to help cleaning of screens, the following tips are suggested:

• The screen axis must be parallel to the current flow
• Dead-end approach channels should be avoided as they collect debris
• Adequate water cover must be provided all around the screen

A river intake always requires a sufficient depth of water in the river. A submerged weir may have to be constructed downstream of the intake to ensure that the necessary depth of water will be available even in dry periods. This type of weir is only a small structure and cannot be expected to provide any storage or flow balancing. Ideally the weir should be founded on rock, to provide the best conditions with regard to bearing capacity, seepage and safety against sliding. Soils with a clay/silt content that reduce permeability and increases cohesion (but with low plasticity) are also acceptable as foundation materials. A key at the upstream toe of the weir will improve stability and an upstream apron will extend the seepage path and thus reduce seepage.

Screens

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 expected 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 quite a 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 barscreen for a head loss of 0.5-1.0 m.

Maintenance

A river-bottom intake is usually operated by a caretaker. The inlet must be checked regularly and obstructing debris removed and any damage repaired. The sand trap must be cleaned regularly. Preventive maintenance consists of painting the screens and other metal parts, such as sluices or valves. Depending on silt and bed load transport, the sand trap and screen will have to be cleaned regularly, and the screen or valve may need repairing. Any erosion undermining the structure must be repaired immediately. Every year, the concrete structure should be checked for cracks and repaired if needed. The water users may be required to help with annual cleaning and major repairs.

Potential problems

• Clogging by silt or debris
• Undermining by river currents
• The river or lake water may be polluted
• During the dry season, there may not be enough water in the river or stream to supply all users.

Manuals, videos and links

• Choosing Where to Place Intakes. Water for the World.

• Surface water intake and small dams. Chapter 11. Revised by Nhamo Masanganise.

Acknowledgements

• Masanganise, Nhamo. Surface water intake and small dams.
• Brikke, François, and Bredero, Maarten. Linking technology choice with operation and maintenance in the context of community water supply and sanitation: A reference document for planners and project staff. World Health Organization and IRC Water and Sanitation Centre. Geneva, Switzerland 2003.