Intake site factors; spring intake

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.

Construction of a spring intake

A spring intake (spring box or spring capture) is a small chamber built at the spring eye to collect all the flow from the spring, protect it from pollution, and send it to the pipeline by gravity. It is the most common intake for rural hilly areas of Nepal because springs give clean water at a high level.

Parts of a spring intake

• Collection chamber (spring box) of stone masonry or concrete, with a pre-cast cover and manhole.
• Gravel filter / perforated pipes at the back wall to collect water uniformly from the spring eye.
• Outlet pipe with a gate valve (and a strainer) to the pipeline.
• Overflow pipe to take excess water; washout (drain) pipe with a valve for cleaning; vent pipe with a screen.
• Protection works: clay/impervious cap over the spring, a diversion drain (catch drain) uphill to divert surface water, a fence, and a vegetation or plantation cover.

Plan

   Diversion drain (uphill)
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  |    fence                    |
  |   +---------------------+   |
  |   | gravel filter ::::  |   |
  |   |  (spring eye)       |-->| overflow
  |   | collection chamber  |   |
  |   |  [manhole]   outlet |==>| pipe to tank
  |   +----------+----------+   |
  |         washout pipe        |
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Section / elevation

        Vent      Manhole cover
         |   ____|_____
  ground |  |          |  clay cap (0.5 m)
  =======+--+          +=============
         |  |  air     |
 Spring->|::| water lvl|--> overflow
 gravel  |::|~~~~~~~~~~|
 filter  |::|          |==> outlet+valve
         |__|__________|
              washout pipe