What is spillway? Different types of spillway and their uses

What is spillway? Different types of spillway and their uses

A spillway is basically a structure which helps to pass the excess water coming from upstream of the dam. Spillway can be divided a/c to their usage and requirement.



A spillway is a type of structure which helps to divert excess water from the upstream side to the downstream side safely.

In other words, the spillway is a passageway which controls the flow of excess water.
It acts as a safety valve in a reservoir or dam. Due to the function of the spillway, water doesn't rise above the maximum reservoir level.

Due to flood or other activity, the water level on the upstream side rises high enough to make uncontrolled water flow towards the downstream side. 

This phenomenon can damage the toe part of the dam and even flood may occur in the downstream side of the dam.

Hence we may conclude that the necessities of a spillway are-

  1. Safe diversion of water from u/s side to d/s side.
  2. provide structure stability of the dam.
  3. minimizes the chance of damage of toe of the dam due to surplus water.
  4. control floods or excess water.

Now you might have a clear idea about the spillway.
Let's move forward.

The function of the spillway is pretty simple but the selection process is not that simple.

There are various types of spillway available in hydraulic structure.

Here you will get to know about some of the most important types of the spillway.



Let us briefly discuss the types of spillway mentioned above.


chute spillway

Chute spillway is an open channel-like structure constructed on a steep slope to transfer excess water with a smooth decline (see the image given above).

Out of all types of the spillway, chute spillway is most suitable for earthen and rockfill dams. 

a smooth and controlled flow of Surplus water from the reservoir can be done by constructing chute spillway.

Feature and component of chute spillway-

  • Open channel-like structure.
  • constructed on a steep slope.
  • low-cost.
  • the base of the channel made of RC slabs of 25-50 mm thick.
  • expansion joints required at intervals of about 10m
  • Underdrains also provided.
A chute is usually narrowed and widened near the end to reduce discharge velocity.


image Source: steemkr.com

It is also known as 'S' shaped or 'overflow' spillway. This type of work generally constructed when the width of the river is sufficient to provide the required crest length.

The flow of excess water guided over the crest.

Basically, it is a different form of drop spillway and suitable for the rigid dam.

downstream profile of spillway is made to coincide with the shape of the lower *nappe of free falling excess water and form a sharp-crested weir.

With ogee spillway, one major con that may occur is that-

When the actual head of water becomes more than the designed head lower nappe of free water-jet does not follow the ogee profile and gets separated from the spillway surface.


siphon spillway

This is named as siphon spillway because this type of spillway consists of siphon pipe.

This siphon pipe can be installed within the body of the dam(known as Tilted siphon) or it can be adopted separately and constructed over an ordinary overflow section of gravity dam(Known as hooded siphon).

One end of the siphon pipe is kept on the upstream side and is in contact with the reservoir. Other end discharges water on the downstream.

The working principle of siphon spillway is based upon pressure due to the difference between the water level in U/S side and D/S side.

When the water rises above the reservoir level, the siphonic action gets started and priming process begins.


Side-channel spillways are most suitable for narrow valleys where enough space is not available to accommodate chute spillway.

Here, the flow of water after spilling over the crest is turned by 90°.

Which means side-channel spillway divert the flow of water by 90°.

To increase the discharge capacity, it is necessary to provide a sufficient slope in the side channel.

The main differences between chute and side-spillway are that- For chute direction of flow of water is not changed.

But in case of side-channel water flow turned by 90°.

Sometimes hybrid spillway can be constructed at the side in which angle of water diversion lies between 0° to 90°.


shaft spillway

In case of shaft-spillway water from reservoir comes to the vertical shaft. Then water travels from a vertical shaft to the horizontal tunnel which finally goes to the river downstream.


From the name itself, you might understand that the flow of water either controlled or un-controlled by gates.

In the case of the controlled spillway, gates are present at the top of the spillway crest and control the flow of water.

On the other hand, gates are not installed in the uncontrolled spillway. Water flows only when the water level in upstream rises above the crest of the spillway.


What is spillway? Different types of spillway and their uses What is spillway? Different types of spillway and their uses Reviewed by Sk rafiqul islam on December 28, 2018 Rating: 5

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