Purpose and types of spillways

A spillway is a structure built with a dam to safely pass surplus flood water from the reservoir to the downstream river when the reservoir is full.

Purpose of a spillway

• To protect the dam from overtopping (especially earth and rockfill dams) during floods.
• To keep the reservoir level below the maximum water level (MWL).
• To release water safely without eroding the dam toe or downstream bed.

Types of spillways

1. Free overfall (straight drop) spillway: water falls freely from a thin crest; used for low weirs and thin arch dams on firm rock.

 ~~~~~~~|\
        | \  nappe
        |  \
 _______|___\~~~~~

2. Ogee (overflow) spillway: crest shaped to fit the lower nappe of a sharp-crested weir; most common for concrete gravity dams.

 ~~~~~~~_
       /  \__
      |      \
      |       \___
 _____|___________\___~~

3. Chute (trough) spillway: water flows over a short crest into a steep open channel (chute) to the river; used with earth dams.

 reservoir  crest
 ~~~~~~~~~~|----\
                 \  chute
                  \______ stilling basin

4. Side channel spillway: crest is parallel to the channel; water spills sideways into a trough and then into a chute or tunnel; used in narrow valleys.

  ~~~~~~~ reservoir ~~~~~~
  ======== crest =========
  |   side channel  --->  | -> chute

5. Shaft (morning glory) spillway: water enters a circular funnel crest, drops through a vertical shaft and leaves through a horizontal tunnel.

   ~~~\    /~~~
       |  |  shaft
       |  |______
       |_________ -> tunnel

6. Siphon spillway: closed duct shaped like an inverted U; flow primes automatically when water rises above crest, giving a large discharge for small head rise.

     ____
   /  __  \
  ~|~|  |  \
   |_|  |   \__ outlet

7. Tunnel (conduit) spillway: flow is carried through a closed tunnel or conduit around or under the dam.
8. Labyrinth and stepped spillways are also used (longer crest in plan; steps to dissipate energy).