General river intake and site selection

General river intake

A river intake is a tower or well built in (or at the edge of) the river with openings at different levels. Water enters the openings through screens, passes into the well and then flows by gravity (or is pumped) through the intake pipe to the treatment plant.

   Foot bridge
  =========|===========
           |   Intake tower
 HFL ~~~~~ |  [ ]  gate/valve
           | __|__ upper port
 NWL ~~~~~ |/  |  \  <-- screens
           |   |__ middle port
 LWL ~~~~~ |   |
           |___|__ lower port
  River bed ///|\\\
              \|/ intake pipe to
                   pump well/treatment

Working

• Ports at two or three levels let the operator draw the clearest water at any river stage: the upper ports when the river is high (the surface water carries less silt), and the lower ports when the river is low.
• Coarse and fine screens at the ports stop floating matter, leaves, fish and debris.
• Gates or valves on each port regulate flow and isolate a port for repair.
• The water travels from the intake well through the intake pipe to the pump well or the plant.
• The entrance velocity is kept low (0.15 to 0.3 m/s) so that little silt and debris are drawn in. A foot bridge gives access for operation and cleaning.

Points to consider in site selection

1. Water quantity: the source must give the design flow even in the driest season; the minimum flow must exceed the demand.
2. Water quality: the site should be upstream of towns, sewage and industrial discharges, and away from tributaries that bring silt or pollution.
3. Stable bank and bed: choose a stable, non-eroding reach, preferably on the concave (outer) side of a bend where the water is deep and the current scours silt away.
4. Depth of water: enough depth at all stages so the intake port stays submerged at low water.
5. Flood level: the structure must stand above the highest flood level (HFL) or be safe against it, and the site should be free from flood and drift damage.
6. Foundation: sound strata for the foundation of the intake tower and pipes.
7. Navigation: the site must not obstruct navigation and should not be in a navigation channel.
8. Distance and elevation: close to the treatment plant and consumers (shorter pipeline, lower cost); for gravity schemes, enough head.
9. Accessibility: access for construction, inspection and maintenance in all seasons, and nearness to power for pumping.
10. Future expansion and ownership of land/legal rights at the site.
11. Ice and silt: the site should not be affected by ice formation or heavy sedimentation.