Spillways, gates and types of spillways

Spillways

A spillway is a structure built with a dam to release surplus flood water from the reservoir safely to the downstream river when the reservoir is full, protecting the dam from overtopping and failure.

Gates

Spillway gates are movable steel barriers placed on the spillway crest between piers. They store water above the crest during normal operation (increasing storage and head) and are opened during floods to control the outflow. Common types: vertical lift (fixed-wheel, Stoney), radial (Tainter), drum, sector, rolling gates, flash boards and stop logs.

Types of spillways used in hydropower projects

1. Free overfall (straight drop) spillway: water drops freely over a thin crest; used for low arch or thin dams on hard rock, with a plunge pool or apron below.
2. Ogee (overflow) spillway: crest shaped to the lower nappe of a sharp-crested weir; high discharge coefficient ($C \approx 2.2$); most common on concrete gravity dams and barrages.
3. Chute (trough) spillway: short crest followed by a steep lined open channel to the river; used with earth and rockfill dams; placed on an abutment or saddle.
4. Side channel spillway: crest along the side of a channel; water spills laterally and turns 90° into a chute or tunnel; useful in narrow valleys where a long crest is needed.
5. Shaft (morning glory) spillway: water enters a funnel-shaped circular crest, falls down a vertical shaft and exits through a horizontal tunnel (often the diversion tunnel). Suitable where there is no room for other types.
6. Siphon spillway: inverted U-shaped closed conduit; once primed it gives large discharge for small rise in water level, keeping the reservoir level nearly constant.
7. Tunnel (conduit) spillway: closed tunnel through the abutment or a conduit through the dam carries the flood.
8. Labyrinth spillway: zigzag crest in plan gives a long crest in a narrow width.
9. Stepped spillway: steps on the downstream face dissipate energy along the chute.

  Ogee         Chute            Shaft
 ~~__        ~~|--\           ~~\   /~~
   / \_          \  \            | |
  |    \__        \__\___        | |____
  |_______\~~      stilling      |______ tunnel