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Types of spillways
Presented by: Bakht Zamin
SUBJECT: DAM ENGINEERING
A spillway is an integral part of a Dam (large wall built to hold back
water usually across rivers). A spillway is usually used to remove
excessive water from a reservoir to prevent overflow and to release
pressure on a dam from increasing quantities of water. It is generally
composed of four major components:
Entrance and outlet channels
Different types of spill ways
ways are generally classified as controlled and
uncontrolled spill ways.
controlled spillway has mechanical structures or gates
to regulate the rate of flow of water from the reservoir.
This design allows nearly the full height of the dam to be
used for water storage year-round, and flood waters can
be released as required by opening one or more gates.
An uncontrolled spillway, in contrast, does not have gates;
when the water rises above the lip or crest of the spillway
it begins to be released from the reservoir. The rate of
discharge is controlled only by the depth of water within
the reservoir. All of the storage volume in the reservoir
above the spillway crest can be used only for the
temporary storage of flood water, and cannot be used as
water supply storage because it is normally empty.
of controlled and uncontrolled spillways are further
classified on the basis of the their shapes, structures and
main working procedure.
drop spillway (free over fall).
(open channel) spillway.
Straight drop spill way
Flow drops freely from the crest of dam.
It may be provided with gates.
Flows may be freely discharging or may be supported by narrow
This type of spillways may be economical for low heads as compared
to other types.
Floor blocks and an end sill are provided.
Widely used on Gravity dams, Arch dams, and Buttress dams.
Not recommended for very high dams.
Side channel spillway
A side channel spillway is one whose control weir is placed alongside
and approximately parallel to the upper portion of the spillway
Flow may be directed into an open channel, or a closed conduit or in
an inclined tunnel.
It may be used as principal spillway or emergency spillway.
Lining of the side channel spillway with concrete is required.
If a sufficient crest length is not available for an over flow spillways in
narrow valleys, excess water is removed from the reservoir through a
side channel spillway.
The ogee spillway has a control weir that is ogee shaped (sshaped)
It is also an overflow type spillway.
Reduce the impact of water at downstream.
To drop off the water at downstream from the foundation of dam.
Reduce scouring etc.
Used for small concrete dams
Provide recreational impact at downstream side.
The concept behind labyrinth spillway is to provide added crest for a given
spillway width, so that less head is required to pass a given discharge.
The additional crest length is obtained by a series of trapezoidal or
Labyrinth design is particularly beneficial when spillway width is fixed, u/s
water surface elevation is restricted and large discharge must be passed.
An excellent alternative to traditional method of adding another spillway.
Labyrinth spillways are more economical than gated structures.
Flow pattern are very complicated.
Ratio of Length of spillway width to the spillway height is an important
parameter responsible for flow pattern.
A spillway whose discharge is conveyed from the reservoir to the d/s
through an open channel or through a saddle (chute).
Used for earth fill dam.
Factors influencing the selection of chute spillways are simplicity of
their design, and construction, their adoptability to any foundation
condition and overall economy.
Profile of chute spillway is greatly influenced by site topography.
These types of spillways may be provided with gated orifice, a side
channel crest or other control device etc.
Abrupt slope changes should be avoided.
Conduit and tunnel spillway
It is a spillway in the form of a closed channel used to convey the
discharge around or under the dam.
The closed channel may be in the form of vertical or inclined shaft, a
horizontal tunnel through earth or rock filled dam.
For free flow in the tunnel, the ratio of the flow area to the total
tunnel area is often limited to about 75%.
Designed for partially full flow throughout the length of tunnel.
Air slots are provided for prevention of cavitations.
Suitable in rockslide and snow slide areas.
Expensive type of spillways.
Baffled chute spillway
The spillway where water is to be lowered from one level to another
level without a stilling basin.
Similar to chute type spillway with exception of baffled structure.
The baffle pier obstruct the flow partially, dissipating energy so that the
flow velocity entering the d/s is lowered.
Slope is normally 2:1.
Economical , smooth operation, and no effect on d/s condition are the
advantages of such types of spillways.