Hydropower intake sketch; minimizing head loss

Sketch of a hydropower intake (non-pressurized side intake of a RoR plant)

PLAN
 river ==========================>
   weir |||||||| undersluice
   bank +----------------------+
        | rack    gate  bell   |--> gravel trap --> canal
        +----------------------+

SECTION
 pond ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  boom o  | trash rack | gate | hoist deck
          |###\        |      |
          |####\_______|______|=====> conduit
   sill (1.0-1.5 m above bed)   transition
 ///////////////////////////////////////// bed

Major components: trash boom, trash rack (inclined), intake sill, regulating gate with stop-log slot, hoist deck, bell-mouthed transition, flushing undersluice and the conduit to the gravel trap or settling basin.

Minimising head loss in the intake

Head loss reduces the net head and so the energy produced. It is reduced by:

• Low velocity at the trash rack (0.6-1.0 m/s) by providing a large rack area; the loss varies with $V^2$.
• Streamlined bars (rounded or tapered upstream edges, $\beta$ of 0.76-1.8 instead of 2.42), larger spacing, and regular raking to avoid clogging.
• Bell-mouthed, rounded entrance instead of a sharp-edged entry (loss coefficient 0.05-0.1 instead of 0.5).
• Gradual transitions (contraction or expansion not steeper than about 1:5) and smooth curved wing walls.
• Few gate slots and piers, and a fully open gate in operation.
• Smooth lining, short conduit and large bend radius; avoiding sharp bends at the entry.