spillway and type of spillway

                               SPILLWAY

A spillway is a structure used to provide the controlled release of flows from a dam or levee into a downstream area, typically the riverbed of the dammed river itself. Spillways ensure that the water does not overflow and damage or destroy the dam.

 How Do Spillways Work?

We normally build a dam to hold water back and store it for use in water supply, irrigation, hydropower, or flood control. But sometimes we have to let some water go. Whether we need it downstream or the impounded water behind the dam is simply too full to store any more, nearly every dam needs a spillway to safely discharge water.

In fact, some of the worst human-caused disasters in history have been failures of dams. For this reason, they’re often required to withstand the biggest storm that we could possibly conceive, called the Probable Maximum Flood. It’s too expensive to build a dam so tall that it can store the entirety of this flood. On the other hand, we can’t just let the flood overtop the dam, because flowing water can damage and destroy the structure. So in most cases, dams are designed with at least one spillway, a structure that can safely discharge floodwaters without causing injury or deterioration to the dam.

                                            Types of Spillways

1. Straight Drop Spillway
2. Ogee Spillway
   
3. Shaft Spillway
4. Chute Spillway
5. Side Channel Spillway
6. Siphon Spillway
7. Labyrinth Spillway

 

1.      Straight Drop Spillway

A Straight drop spillway consists of low height weir wall having its downstream face roughly or perfectly vertical. When the water level in the reservoir rises above the normal pool level, the surplus water falls freely from the crest of the weir and hence it is known as Straight drop spillway .

 

2. Ogee Spillway

Ogee spillway, as the name says, represents the shape of the downstream face of the weir. It is an improved form of a straight drop spillway. In this case, the downstream face of the weir is constructed corresponding to the shape of lower nappe of freely falling water jet which is in ogee shape.

The ogee shape of the downstream face is designed on the basis of the principle of a projectile. In general, the shape of lower nappe of the water jet is not constant for all water heads hence, the shape obtained for the maximum head is taken into account while designing ogee spillway.

3. Shaft Spillway

A Shaft spillway is a type of spillway which consists of a vertical shaft followed by a horizontal conduit. The surplus water enters into the vertical shaft and then to the horizontal conduit and finally reaches the downstream of the channel.

4. Side Channel Spillway

Side channel spillway is similar to chute spillway but the only difference is the crest of side channel spillway is located on one of its sides whereas crest of chute spillway is located between the side walls. In other words, the water spilling from the crest is turned to 90 degrees and flows parallel to the crest of side channel spillway unlike in chute spillway.

5. Siphon Spillway

A siphon spillway is a type of spillway in which surplus water is disposed to downstream through an inverted U shaped conduit. It is generally arranged inside the body or over the crest of the dam.

In both types of siphon spillways, air vents are provided at the bent portion of the upper passageway to prevent the entrance of water when the water level is below the normal poll level. Whenever the level rises above normal pool level, water enters into the conduit and is discharged to the downstream of the channel by siphonic action.