The spillways may be broadly classified as:

  1. Emergency spillways, and
  2. Main spillways.


  • Main spillway is the spillway which is called upon to work under normal conditions of flood.
  • Emergency spillway is provid to deal with abnormal conditions of flood.
  • Under normal flood conditions, the emergency spillways are not required to work.
  • They may be called upon to work under following abnormal conditions.
  • Rise is water level in the reservoir is continuing above the maximum permitt level even when main spillway and other outlet works are working at their full capacity.
  • This happens when flood greater than design flood has occurred.
  • When, due to certain reasons, enforced shut down of the outlet works have to be applied and flood water is entering the reservoir with full vigour.
  • If spillway gates have got struck down and cannot be opened immediately.
  • In all the above three conditions the level of reservoir may rise above the maximum contemplat level and thus may cause failure of the dam and extra submergence on the U/S side.
  • In all such conditions, the emergency spillways are opened.
  • Emergency spillways are generally formed by lowering the crest of a dike section below that of the main embankment.
  • Natural saddle, if available, is an ideal site for emergency spillway.
  • Emergency spillways are construct low height earth embankments, but before earth embankments are made the surface of the saddle
  • Lowered dike section is made pucca, so that when emergency spillway is opened it is not unnecessary eroded and widened.

Fig. 14.1. Overflow or Ogee spillway.


  • The crest or top of the emergency spillway is kept at or little above the designed maximum reservoir water surface.
  • When rising water level in the reservoir reaches the maximum high flood level and is still rising, the emergency spillway, which is an ordinary earth fill low dam, gets either automatically washed away or is deliberately cut.
  • This causes very large amount of flood water to escape, and further rise in reservoir level stops.
  • When flood waters are discharged D/S, and the reservoir level has dropp below the maximum flood level, the flow through the emergency spillway stops.
  • After this main spillways are capable to handle the situation.
  • When flow through emergency spillway has stopped, it is again filled with earth embankment and emergency spillway again restored.
  • Since emergency spillway section either breaches itself or is deliberately cut, it is sometimes also know breaching section.


  • We have already stated that these spillways work under the design flood conditions.
  • Emergency spillways only help the main spillways to save the situation created by abnormal conditions.
  • There are several types of spillways which may be used as main spillways.

They are as follows:

1. Overflow or ogee spillway.

2. Free overfall or straight drop spillway.

3. Side channel spillway.

4. Chute spillway, which is also called trough or open channel spillway.

5. Tunnel spillway.

6. Siphon spillway.

7. Shaft, or drop inlet or morning glory spillway.

  • The spillways may be gate or ungated. Gated spillways are referred controll spillways while others uncontroll spillways.
  • Shape, discharge features, and discription of each type of spillway is given one by one.