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
- Straight Drop
Spillway
- Ogee Spillway
- Shaft Spillway
- Chute Spillway
- Side Channel
Spillway
- Siphon Spillway
- 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.
