Reservoir intake and site selection factors

Factors for selecting an intake site

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.

Characteristics of a wet reservoir intake

A reservoir intake draws water from an impounding reservoir (a natural lake or an artificial lake formed by a dam). A wet intake tower is a vertical tower, usually of masonry or concrete, set in the reservoir near the dam, with ports at different depths. The water level inside the tower is the same as in the reservoir.

  Foot bridge ==============+
                            | operating
  HFL ~~~~~~~~~~~~~~~~~~    | stem
  NWL ~~~~~~~[ ]~~~~~~~~~~~ |
       top port + screen    |
  MDDL ~~~~~[ ]~~~~~~~~~~~~ |
       middle port + screen |
       ~~~~~[ ]~~~~~~~~~~~~ |
       bottom port          |
  bed //////|  intake well  |////
            +--------pipe/conduit--> outlet to plant

Characteristics and working

• The tower has ports (usually 3) at different levels. Water is taken from the level of best quality: top water in summer when the deeper water is cold and poor (low oxygen, high iron and manganese), and deeper water in winter when the surface is cold or frozen.
• Water entering the port passes through coarse and fine screens, then the sluice gate or valve, into the well, and from the well by the intake pipe or conduit to the plant.
• Gates are operated from the top by stems and handwheels on the service platform; access is through a foot bridge.
• Low entrance velocity avoids drawing silt and floating matter. The bottom port is placed above the sludge/silt level.