RoR headworks sketch; intake importance and location

Typical arrangement of the headworks

PLAN
    river flow ==============================>
        left bank wall
   +-------------------------------------------+
   |  intake  | undersluice | weir (spillway)    |
   | (side)   |  (gates)    | ||||||||||||||||| |
   +----+-----+-------------+--------------------+
        |  gravel trap
        v                         fish ladder
   approach canal --> settling basin --> headrace

SECTION through weir, undersluice and intake
  pond level ~~~~~~~~~~~~~~~~~~~~~~~~~~
        _____      weir crest  ______
  intake|rack |gate          /      \  stilling basin
  sill  |_____|-> canal     /        \____ apron
 ///////////  undersluice /////////  river bed
   (sill 1.0-1.5 m above bed)   flushing gate

Importance of the intake

• It is the link between the river and the waterway: the whole plant depends on it for the design discharge.
• It controls the quality of water entering the plant: sediment, gravel and trash that pass the intake reach the settling basin, tunnel and turbines and cause abrasion and blockage.
• It decides the head loss and so the energy generated; a poor intake gives vortices, air entry and heavy losses.
• It provides flow control and safety: gates close during floods and for maintenance.
• A wrongly placed intake can silt up, so its location and design decide the life and reliability of the headworks.

Deciding the location of an intake

1. On the outer bank of a bend (about one-third to one-half way along the bend, downstream of the bend apex), where the helical flow carries bed load away from the opening; on a straight reach put it on the stable bank.
2. In a stable reach with a good foundation, free of landslides, bank erosion and large boulder deposits.
3. At a point that gives sufficient head and an easy alignment of the canal to the settling basin.
4. Upstream of the weir and next to the undersluice so that the flushing current keeps the intake clear.
5. Away from the confluence of tributaries, with good access and space for the gravel trap and settling basin.
6. Where the river is narrow, to reduce the length and cost of the weir, and the flood level is limited.