RoR headworks arrangement and functional requirements

General arrangement of a typical RoR 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

Main components:

• Diversion weir / barrage across the river to raise the water level for the intake and pass floods over its crest.
• Undersluice (scouring sluice) at the intake side, with gates, to keep a flushing channel in front of the intake and to pass bed load.
• Intake (side intake with trash rack and gate) on the outer bank, with its sill above the undersluice level.
• Gravel trap and approach canal leading to the settling basin.
• Fish ladder, river training works (guide bunds, flood walls), stilling basin and apron downstream of the weir.

Functional requirements of the headworks

1. Safe passage of floods: the structure must be stable against the design flood (usually 1 in 100 year) and pass it with a limited afflux.
2. Reliable diversion: divert the design discharge at all river stages, including low flow, with the intake always submerged.
3. Minimum sediment entry: bed load must be kept out by a high sill, undersluice and gravel trap, and suspended load limited by the settling basin.
4. Trash and floating debris control: trash rack, trash boom and log deflector.
5. Stability and safety: weir safe against sliding, overturning and uplift; scour protected by cut-offs, aprons and a stilling basin.
6. Environmental flow and fish passage as per law; minimum release downstream.
7. Easy operation and maintenance with small head loss in the intake.

In rivers carrying a high sediment load

• Place the intake at the outer bend, with the sill at least 1.0-1.5 m above the undersluice crest.
• Make the undersluice large enough to give a flushing velocity that carries the boulders and gravel.
• Provide a gravel trap and a settling basin with efficient flushing; allow frequent flushing of the pond in floods.
• Use a flood-operation rule: close the intake during the high sediment-laden flood.