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What is Spillway?

by Swati


  • Spillway is the passage provided for disposal of surplus flood water on the downstream side without affecting stability of water.
  • This is necessary as water will go on rising even above high flood level.
  • Also it will start flowing from top of the dam which as a result may affect the stability of dam.
  • Therefore,it is essential to provide spillway to dispose surplus water.


The various essential requirement of spillways are:

  • Firstly, it must have adequate discharge capacity.
  • Secondly, One should provide some devices with it for energy dissipation.
  • Thirdly, its discharge should be such that it should not cause flooding on downstream side.
  • Fourthly, it must be safe.
  • Fifthly, it’s surface must be erosion resistant.
  • Lastly, it should be located such that it should not erode downstream toe of dam.

Factors Affecting its capacity:

The various factors which affect its capacity are as follows:

  1. Inflow
  2. Capacity of outlet.
  3. Possible Damage
  4. Available shortage Capacity.
  5. Gates of Spillway.
  6. Site condition.
  7. Type of dam and its purpose.
  8. Height of the crest of spillway.
  9. Geological condition.

Component Parts of Spillway:

The component parts are as follows:

  1. Body,Weir or Spillway
  2. Approach Channel
  3. Energy Dissipators
  4. Tail Channel
  5. Guide wall
  6. Discharge Channel

1.Body,Weir or Spillway:

  • It may be Ogee shaped, U shaped, semicircular or circular.
  • These are sometimes provided with gates to control flow of water.

2.Approach channel:

  • It removes excess water from reservoir and further direct it to downstream side.
  • Moreover, it helps in controlling discharge.

3.Energy dissipators:

  • These are provided at downstream end of the discharge channel.
  • It avoids damage to the dam as a result the high velocity of water may scour the downstream side.
  • Generally, Ski jump bucket, roller bucket are provided to dissipate the energy.

4.Tail Channel:

  • It conveys spillway discharge to the downstream side.

5.Guide wall:

  • It guides the water to flow directly into the reservoir downstream.

6.Discharge Channel:

  • The main function is to convey the water safely from reservoir downward to the river.

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