Spillway and its types

Subject: Design Of Hydraulic Structures
(ID-701)

1. Hyder Ali Khaskheli (2K19-ID-27)
2. Ali Anwer Shah (2K19-ID-20)
3. Aftab Ahmed Rahu (2K19-ID-63)
4. Asif Raza Noonari (2K19-ID-23)
5. Aqeel Ahmed Solangi (2K19-ID-21)
6. Ayaz Faqeer (2K19-ID-24)

 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.
In the United Kingdom, they may be known
as overflow channels. Spillways ensure that the water
does not overflow and damage or destroy the dam.
 Other uses of the term "spillway" include bypasses of
dams or outlets of channels used during high water.
Water normally flows over a spillway only during flood
periods

It is normally composed of three major components:
1. The approach facility admits flow to the spillway
2. The discharging conduit evacuates the flow from the
approach facility to an outlet structure.
3. The outlet structure (tail water channel )dissipates the
excessive energy of the flow from the discharging
conduits and convey tranquil flow to the downstream

 The spillway has the function of discharging all the water not
utilized for generation. The maximum discharge capacity of
the spillway is 62.2 thousand m3/s; 40 times greater than the
mean discharge of the Iguaçu Falls.
 Spillways release water so that the water does not overtop
and damage or even destroy the dam.
Function of Spillway

Location of Spillway
 Generally, the spillways are provided at the following places
Spillways may be provided within the body of the dam.
Spillways may sometimes be provided at one side or both
sides of the dam.
 Sometimes by-pass spillway is provided which is completely
separate from the dam

 Spillways are classified into different types on the basis
of the arrangement of the control structure
1. Straight Drop Spillway or free over fall spillway
2. Ogee Spillway
3. Shaft Spillway
4. Chute Spillway
5. Side Channel Spillway
6. Siphon Spillway
7. Labyrinth 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 or free
over fall spillway. This type of Spillway is suitable
for Arch dam.

 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 nape of freely falling water
jet which is in ogee shape. It is the most commonly used spillway. It is widely used with
gravity dams, arch dams & buttress dams
2. Ogee 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. A Shaft spillway is used at the sites where the
conditions are not favorable for an overflow or a chute spill-
way.
3.Shaft Spillway

 Chute spillway is a type of spillway in which surplus water
from upstream is disposed to the downstream through a
steeply sloped open channel. It is generally constructed at one
end of the dam or separately away from the dam in a natural
saddle in a bank of the river.
4. Chute 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. Side Channel 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. It is commonly used in practice.
6. Siphon Spillway

 A labyrinth spillway is a type of spillway in which the weir
wall is constructed in a zigzag manner in order to increase
the effective length of the weir crest with respect to the
channel width. This increase in effective length raises the
discharge capacity of the weir and hence higher water flow at
small heads can be conveyed to the downstream easily.
7. Labyrinth Spillway

 During excess rainfall, through ogee spillway, more amount
of water can be discharged to downstream to prevent the
dam from over-topping
 Very stable. The likelihood of serious structural damage is
less than for other types of structures.
 The rectangular weir is less likely to be clogged by debris
than the openings or other structures of comparative
discharge capacities.
 They are relatively easy to construct. The concrete block type
can be built with farm labor, while the reinforced concrete or
steel sheet piling type usually requires the services of a
contractor.
ADVANTAGES

 It is more costly than some other types of structures where
the required discharge capacity is less than 100 c.f.s. and the
total head or drop is greater than 10 feet.
 It is not a favorable structure where temporary spillway
storage is needed to obtain a large reduction in discharge.
 A stable grade below the structure is essential.
DISADVANTAGES

 As water passes over a spillway and down the chute, potential
energy converts into increasing kinetic energy. Failure to
dissipate the water's energy can lead to scouring and erosion
at the dam's toe (base). This can cause spillway damage and
undermine the dam's stability. To put this energy in
perspective, the spillways at Tarbela Dam could, at full
capacity, produce 40,000 MW about ten times the capacity of
its power plant.
ENERGY DISSIPATION

 The energy can be dissipated by addressing one or more parts of a spillway's design.
 Steps
 On the spillway surface itself by a series of steps along the spillway
 Flip bucket
 At the base of a spillway, a flip bucket can create a hydraulic jump and deflect water
upwards.
 Ski jump
 A ski jump can also direct water horizontally and eventually down into a plunge pool or
two ski jumps can direct their water discharges to collide with one another.
 Stilling basin
 A stilling basin at the terminus of a spillway serves to further dissipate energy and
prevent erosion. They are usually filled with a relatively shallow depth of water and
sometimes lined with concrete.
ENERGY DISSIPATION

1. Spillway gates may operate suddenly without warning,
under remote control. Trespassers within the spillway run
the risk of drowning. Spillways are usually fenced and
equipped with locked gates to prevent casual trespassing
within the structure. Warning signs, sirens, and other
measures may be in place to warn users of the downstream
area of sudden release of water. Operating protocols may
require "cracking" a gate to release a small amount of water
to warn persons downstream.
2. The sudden closure of a spillway gate can result in the
stranding of fish, and this is also usually avoided.
SAFETY

Spillway and its types

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