Intake and its functions

An intake is the structure at the head of a water conductor system that diverts the required design discharge from a river or reservoir into the canal, tunnel or penstock leading to the powerhouse, while keeping out debris, ice and as much sediment as possible.

Functions of an intake

1. Diversion of design flow: draws the required discharge (design discharge plus flushing flow) into the water conductor at all river stages, including the lean (dry) season.
2. Control of flow: gates at the intake regulate the flow and can shut it off completely for maintenance of the waterway or during floods.
3. Exclusion of floating debris: trash racks stop logs, branches, leaves and ice from entering and damaging the turbines.
4. Exclusion of bed load: a raised sill, undersluice and gravel trap keep coarse sediment (gravel, boulders) out of the waterway.
5. Minimising head loss: a smooth bell-mouth entrance with low approach velocity (about 0.6–1.0 m/s through the racks) reduces entry losses.
6. Prevention of vortex and air entry: adequate submergence prevents vortices that would draw air into a pressure conduit.
7. Flood protection: limits the flow entering the waterway during floods so that the canal or tunnel is not overloaded; excess is spilled back to the river.

 River --> [Trash rack] -> [Gate] -> [Gravel trap]
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                                        v
                         Headrace canal / tunnel