Description about spillways, its types as well as Diversion Headwork's in detail.

1. Unit 4
Spillways and Diversion Headwork's
By
Prof. kartiks. chourasia

2. INTRODUCTION
• A passageway through which surplus water escapes
from a reservoir, lake, or the like.
• Discharges the flood water to downstream of river
without overtopping the dam.
• A structure which will not let the water rise above the
maximum reservoir level.
• A spillway is essentially a safety valve for a dam.
Location of Spillway
• Within the body of the dam
• At one end of dam.
• Entirely away from it, independently in a saddle.

4. Requirements of Spillway
Provide structural stability to dam under all conditions.
Able to pass the designed flood without raising the
water level above H.F.L. Safe disposal of water without
toe erosion. Should have an efficient operation and
should be economical.
Types of Spillways
• Straight drop spillway
• Overflow spillway
• Chute spillway
• Side channel spillway
• Shaft spillway
• Siphon spillway

5. Straight drop Spillway
• In this type of spillway, the water freely drops
down from the crest.
• It is a low weir and simple vertical fall type
structures.
• The water falls freely from the crest under the
action of gravity.
• To prevent scouring at the downstream, an
auxiliary dam of Artificial pool is to be constructed
at the place of fall of water.

7. Overflow spillway
• It is generally known as Ogee spillway.
• It represents the S-shape curve so, it is called ogee
spillway.
• It is an improved form of straight drop spillway.
• It is mainly used in gravity dams.
• It has got the advantage over other spillways for its
high discharging efficiency.

9. Chute spillway
• It is often called as trough or open channel spillway.
• For earthen and rockfill dams, spillway is to be
constructed separately away from the main valley.
• Chute Spillway is the simplest type of a spillway
which can be easily provided independently and at
low costs.
• It is lighter and adaptable to any type of
foundations.

11. Side channel spillway
• The flow in this spillway is turned 90 ̊ after passing
the crest such that the flow is parallel to the weir
crest.
• Best suitable for non rigid dams like earthen dams.
• It is preferred where space is not available for
providing sufficient crest width for chute spillway.
• The discharge carrier may be an open channel type
or a conduit type.

13. Shaft spillway
• The water from the reservoir enters into a vertical
shaft which conveys this water into a horizontal
tunnel which finally discharges the water into the
river downstream.
• This type of spillway is preferred where the space is
not available for providing the above type of
spillways If the inlet leg is provided in shape of a
funnel, it is called Morning Glory Spillway.
• It has maximum discharge even at low heads.

15. Siphon Spillway
• It works on the principle of siphonic action.
• It consists of a siphon pipe whose inlet leg is kept just below the normal pool
level and an air vent kept at normal pool level is connected to the crown of
siphon.
• When the water raises the pool level, siphonic action starts automatically and
the water discharges to downstream side.
• When the water level falls below the pool level, air is entered through air vent
and the discharging of water stops.
There are two types of siphon spillways:
1. Saddle siphon spillway
2. Volute siphon spillway
1. Saddle siphon spillway: Saddle siphon spillway can be constructed in
many designs but basic concept for all of them is the same. One type
of saddle siphons is shown in Figure below. The diagram consists of a
U-shaped siphon pipe whose D/S end remains submerged in tail
water. An air inlet is provided near the top of the bend.

16. • Crest of the inside of U-tube is maintained at the normal reservoir
level and air inlet slightly above this level. At normal reservoir level
no water flows through the siphon. During floods as the water
level in the reservoir rises above the normal reservoir level, water
will start flowing through the conduit of the spillway. Siphonic
action is established after the air is the bend over the crest has
been exhausted. This action is known as Priming.
• When rising water level blocks the air inlet, the flow through the
conduit immediately starts under siphonic head.
A number of such siphonic units may be installed depending upon
the diameter of the conduit of the siphon and the amount of flood
water to be handled. When water level in the reservoir drops
below the air inlet, siphonic action is broken. Closing of the air
inlet by rising water is known as priming and opening of the air
inlet by dropping level of water is known as depriming of the
siphon spillway.

17. • Discharging capacity of a saddle siphon spillway is obtained
by following formula –
• Q = CA*
Where A = Area of cross-section at crown, H = Operating
head. It is taken as vertical height from reservoir level to the
centre of the outlet if outlet is discharging in atmosphere and
up to tail water level if outlet is submerged, C = Coefficient of
discharge whose average value is taken as 0.65.
• Siphon spillways can be further classified as high head,
medium head and low head siphons.
2gH

18. Priming Devices
The following are the common methods employed for early priming in
spillways:
1. Provision of cup type basin: This is shown in the figure (1) below the lip of
the lower limb is kept immersed in the water stored in the cup. This will
prevent the entry of air.
2. Provision of tilted outlet: This is another type of saddle siphon in which
the vertical shaft has a tilted outlet which maintains a water seal figure
(2). The air inlet is maintained just at the reservoir level.
3. Provision of auxiliary or baby siphon: a small auxiliary or baby siphon is
formed just below the crest. When the water level reaches the crest, the
baby siphon will run full. The sheet of water issuing from it is arranged to
shoot across the lower limb of the siphon so as to seal it and prevent air
entering from below. The priming thus takes place
4. Provision of tudol or step Figure (a): A horizontal tudol or step, provided
near the lip, deflects the jet to strike the lower limb thus sealing the air
entry.
5. Provision of step just after crest: A step is provided just after the crest to
deflect the water which strikes to the opposite side, thus providing a
water seal

19. Figure 1 Figure 2

20. 6. Provision of hinged adjustable plate. Fig. (b) shows a system
suggested by Heyn in which a flexible steel tongue directs the
over-flow across the siphon until priming begins when weight of
water presses down the steel tongue against the inner wall so that
an unobstructed flow is ensured.
7. Provision of priming weir: A priming weir will be connected with
the main reservoir is made. When the overflow commences, the
falling nappe completely seals the air in the crown of the siphon
and priming ensured.

21. Conclusion
• Straight drop spillway is suitable for arch dams and for small drops.
• Ogee spillway is the most commonly used spillway for its high
discharging efficiency.
• Chute spillway can be provided in any type of foundation.
• Side channel spillways are hydraulically less efficient.
• Shaft spillway becomes undesirable where a discharge more than
the design capacity is to be passed.
• siphon spillway is an automatically operated type spillway.

22. Ogee Spillway
(Design Principles)

24. Energy Dissipation
Hydraulic Jump
• When flood discharge passes over the spillway
crest, it has high potential energy which gets
converted into kinetic energy as it glides along it.
• This high energy has to be dissipated otherwise it
would cause erosion at the downstream toe.

26. JHC vs TWC
Jump Height Curve (JHC):
• It is a curve representing post jump depth (d2) with
discharge (q)
Tail Water Curve (TWC):
• It is a curve representing tail water depth (D) with
discharge (q)

27. Refer text book
(B.C.PUNMIA) for more
explanation REGARDING
JHC vs TWC

28. Five conditions that govern the
relationship between JHC and TWC

29. JHC lies lower than TWC at all Discharges
(PROTECTION MEASURES)

30. JHC lies above TWC at all Discharges
(PROTECTION MEASURES)

32. JHC lies lower than TWC at Small Discharges & JHC lies
above TWC at large discharge & Vice-Versa
(PROTECTION MEASURES)

33. SPILLWAY GATE
Spillway gate, also called stop gates, are adjustable
gates used to control water flow in reservoir, river,
stream systems.
→They also acts as barrier for the storage of
additional water.
→By these the height of the dam will be more such
that land acquisition will be more.
→ Gates can be provided to all types of spillways

34. TYPES OF SPILLWAY GATES
1. DRIPPING SHUTTERS OR PERMENENT FLASH
BOARDS
2. STOP LOGS AND NEEDLES
3. RADIAL GATE OR TAINTER GATES
4. DRUM GATES
5. VERTICAL LIFT GATES OR RECTANGLE GATES

35. DRIPPING SHUTTERS OR PERMENENT
FLASH BOARDS
• Consists of wooden panels of usually 1.0-1.25m HIGH
• Hinged at bottom and are supported against water
pressure by structs. The shutters falls flat on the crest
when the downstream supporting structs are tripped.
Hence they are not suitable for curved crests.
• These shutters can be raised or lowered from an over
head cable way or a bridge.
• Various automatic gates which drop themselves have
been designed these days.

36. STOP LOGS
• They consists of wooden beams or planks placed
one upon the other and spanning in the grooves
b/w the spillway piers .
• →they can be romed either by hand or with
hoisting mechanism. It takes lot of time for
removing them, if they become jammed in slots.
• →Leakage bw logs is a big problem hence they are
used in minor works.

37. NEEDLES
• Needles are wooden logs kept side by side with
their lower ends resting in a keyway on the spillway
and upper ends supported by a bridge.
• It is difficult to handle these at the time of flow
hence these are not used on any major works.
• They are sometimes used for emergency bulk
heads, where they need not be replaced until the
flow has stopped.

39. RADIAL GATE OR TAINTER GATES
• Radial gates are rotary gates consisting of cylindrical
sections.
• They may rotate vertically or horizontally. Tainter
gates are a vertical design that rotates up to allow
water to pass underneath.
• Low friction trunnion bearings, along with a face
shape that balances hydrostatic forces, allow this
design to close under its own weight as a safety
feature.

41. Drum Gates
• Drum gates are hollow gate sections that float on
water. They are pinned to rotate up or down. Water
is allowed into or out of the flotation chamber to
adjust the dam's crest height.
• These are desirable to longer spans in order of 40 or
so and medium heights of 10 or so.

45. Diversion Headwork’s
The works which are
constructed at the head of
the canal in order to divert
the river water toward the
canal, so as to ensure a
regulated continuous supply
mostly silt free water with
certain minimum head into
the canal, are known as
diversion headwork’s.

46. Objectives of Diversion Head Works
The Following are the objective of Diversion Head works
• To Raise the water level at the head of canal.
• To form a storage by construction of dykes on both side
of banks of the river so that water is available
throughout the year.
• To control the entry of silt into the canal and to control
the deposition of silt at the head of canal.
• To control the fluctuation of water level in the river
during different seasons.

48. Selection of Site for Diversion Head Works
• The following points should be considered to select
a site for this diversion headwork's.
• The river should be straight and narrow at the site
• The elevation of site should be higher than the area
to be irrigated for gravity flow.
• River banks at site should be well defined and
stable.
• Valuable land upstream of the barrier like weir or
barrage should not be submerged.

49. • Material of construction should be locally
available.
• Roads or railway communication to the site is
essential to carry the material of construction.
• Site should be close to the cropland to minimize
loss of water due to seepage and evaporation of
canal.
• The site should provide a good foundation for
construction of weir or barrage.

50. Components of Diversion Headwork's
• The components of diversion headwork's are:
• Weir or barrage
• Canal head regulator
• Divide Wall
• Fish Ladder
• Scouring Sluices Under sluices
• Silt excluder
• Silt ejector.
• Marginal embankment or dikes
• Guide bank
• Silt pocket or trap.

53. Weir or Barrage
• Weir is a solid obstruction placed across the river.
Its main function is to raise the water level so that
water can be diverted by canal to crop field due to
difference of head.
• Barrage is practically a low weir with an adjustable
gate over this low weir. Heading up of water is
affected by gate.

54. If the major part or the entire ponding of water is achieved by a
raised crest and a smaller part or nil part of it is achieved by the
shutters, then this barrier is known as a weir.

55. Weir

56. Barrage: If most of the ponding is done by gates and a smaller or
nil part of it is done by the raised crest, then the barrier is known as
barrage or a river regulator

57. Barrage

58. Comparison Between Weir & Barrage

59. Weir Barrage
High set crest Low set crest
Ponding is done against the raised crest or
partly against crest and partly by shutters
Ponding is done by means of gates
Shutters in part length Gated over entire length
Shutters are of smaller height, 2 m Gates are of greater height
No control of river in low floods Perfect control on river flow
Excessive afflux in high floods High floods can be passed with minimum
afflux
Shorter construction period Longer construction period
No means for silt disposal Silt removal is done through under sluices

60. Types of Weir

61. Types of Weir
• Weir may be of different types based on material of
construction, design features and types of soil
foundation as:
• Vertical Drop Weir
• A crest gate may be provided to store more water
during flood period. At the upstream and downstream
ends of impervious floor cut off piles are provided.
Launching apron are provided both at upstream and
downstream ends of floor to safeguard against
scouring action. A graded filter is provided
immediately at the downstream end of impervious
floor to relieve the uplift pressure. This type of weir is
suitable for any type of foundation.

64. Sloping Weir of Concrete
• This type is suitable for soft sandy foundation. It is
used where difference in weir crest and
downstream riverbed is not more than 3 m.
Hydraulic jump is formed when water passes over
the sloping glacis. Weir of this type is of recent
origin.

65. • A parabolic weir is almost similar to spillway section
of dam. The weir body wall for this weir is designed
as low dam. A cistern is provided at downstream.
Parabolic Weir

66. Dry Stone Slopping Weir
• It is dry stone or rock fill weir. It consists of body wall and
upstream and downstream dry stones are laid in the form
of glacis with some intervening core wall.

67. Barrage
• When the water level on the upstream side of the weir is
required to be raised to different levels at different time,
then the barrage is constructed. Practically a barrage is an
arrangement of adjustable gates or shutters at different
tiers over the weir. The water level can be adjusted by the
opening of gates.

68. Divide Wall
• The Divide Wall is a long wall constructed at right angle to the
weir or barrage, it may be constructed with stone masonry or
cement concrete. On the upstream side, the wall is extended
just to cover the canal regulator and on the down stream
side, it is extended up to the launching apron. The functions
of the divide wall are as follows,
• (a) To form a still water pocket in front of the canal head so
that the suspended silt can be settled down which then later
can be cleared through the scouring sluices from time to
time.
• (b) It controls the eddy current or cross current in front of the
canal head.
• (c) It provides a straight approach in front of the canal head.
• (d) It resists the overturning effect on the weir or barrage
caused by the pressure of the impounding water.

70. Scouring Sluices or Under Sluices
• The Scouring sluices are the openings provided at the
base of the weir or barrage. These openings are
provided with adjustable gates. Normally, the gates are
kept closed. The suspended silt goes on the depositing
in front of the canal head regulator. When the silt
deposition becomes appreciable the gates are opened
and the deposited silt is loosened with an agitator
mounting on a boat. The muddy water flows towards
the downstream side through the scouring sluices. The
gates are closed. But, at the period of flood, the gates
are kept opened.

72. Fish Ladder
• The Fish Ladder is provided just by the side of the divide
wall for the movement of fishes. Rivers are important
source of fishes. There are various types of fish in the river.
The nature of fish varies from type to type. But in general,
the tendency of fish is to move from upstream to
downstream in winters and from downstream to upstream
in monsoons. This movement is essential for their survival.
Due to construction of weir or barrage, this movement gets
obstructed, and is detrimental to the fishes. For the
movement of the fishes along the course of the river, the
fish ladder is essential. In the fish ladder, the baffle walls
are constructed in the zigzag manner so that the velocities
of flow within the ladder does not exceed 3 m/s. The width,
length, and height of the fish ladder depends on the nature
of the river and the type of the weir or barrage.

75. Canal Head Regulator
• A structure which is constructed at the head of the
canal regulator to regulate the flow of water is
known as canal head regulator. It consists of a
number of piers which divide the total width of the
canal into a number of spans which are known as
bays. The pier consists of a number of tiers on
which the adjustable gates are placed. The gates are
operated from the top by suitable mechanical
device. A platform is produced on the top of the
piers for the facility of operating the gates. Again
some piers are constructed on the downstream side
of the canal head to support the roadway.

77. Silt Excluder
• When still pocket is formed in front of the canal head
by constructing the divide wall, then it is found that the
lower layer of water contains heavy silt and the upper
layer contains very fine silt. The fine silt is very fertile
and it may be allowed to enter the canal. But the heavy
silt causes sedimentation in the pocket.. To eliminate
the suspended heavy silt, the silt excluder is provided.
It consists of a series of tunnels starting from the side
of the head regulator up to the divide wall. The tunnel
nearest to the head regulator is longest, and the successive
tunnels decrease in length, the tunnel nearest to the divide
wall is shortest. The tunnels are covered by R.C.C. Slab.

78. The top level of the slab is kept below the sill level
of the head regulator. So, the completely clear
water is allowed to flow in the canal through the
head regulator. The suspended heavy silt carried by
the water enters the silt excluder tunnels and
passes out through the scouring sluices. Silt
excluders are those works which are constructed on
the bed of the river, upstream of the head regulator.
The clearer water enters the head regulator and
silted water enters the silt excluder. In this type of
works, the silt is, therefore, removed from the
water before in enters the canal.

79. Refer Textbook for Clear Diagram

80. Silt Ejectors
• Silt ejectors, also called silt
extractors, are those
devices which extract the
silt from the canal water
after the silted water has
• Traveled a certain distance
in the off-take canal.
These works are,
therefore, constructed on
the bed of the canal, and
little distance downstream
from the head regulator. Refer Textbook for Clear Diagram

81. Marginal Embankments or dykes
The marginal embankments or dykes are earthen
embankments which are constructed parallel to then
river bank on one or both the banks according to the
condition. The top width is generally 3 to 4 m and side
slope is generally 1 ½ : 1 to 2: 1. The height of the
embankment depends on the highest flood level. A
suitable margin is provided between the toe of the
embankment and the bank of the river. To resist the
effect of erosion on the embankment, wooden piles are
driven along the river banks throughout the length of
dyke. The length of the dyke is protected by boulders
pitching with cement grouting and the downstream
side is protected by turfing.

82. The Marginal Bunds are constructed for the following
purposes.
• (a) It prevents the flood water or storage water
from entering the surrounding area.
• (b) It retains the flood water or storage water within
a specified section.
• (c) It Protects the towns and village from
devastation during the heavy flood.
• (d) It protects valuable agricultural lands.

83. Guide Bank
• When a barrage is constructed across a river which
flows through the alluvial soil, the guide banks must be
constructed on both the approaches to protect the
structure from erosion. It is an earthen embankment
with curved head on both the ends.
• The Guide Bank serves the following purposes.
• It protects the barrage from the effect of scouring and
erosion.
• It controls the tendency of changing the course of the
river.
• It controls the velocity of the flow near the structure.

85. Causes of Failure of weir or barrage
on permeable foundation
The combined effect of surface flow and surface flow may cause the
failure of the weir or barrage.
(1) Failure due to subsurface flow
(a) By piping or undermining: The water from the upstream side
continuously percolates through the bottom of the foundation and
emerges at the downstream end of the weir or barrage floor. The
force of percolating water removes the soil particles by scouring at
the point of emergence. As the process of removal of soil particles
goes on continuously, a depression is formed which extends
backwards towards the upstream through the bottom of the
foundation. A hollow pipe like formation thus develops under the
foundation due to which the weir or barrage may fail by subsiding.
This phenomenon is known as failure by piping or undermining.

88. (b) By uplift Pressure: The percolating water exerts an upward
pressure on the foundation of the weir or barrage. If this uplift is
not counterbalanced by the self weight of the structure, it may fail
by rapture.
2. Failure by Surface Flow:
(a) By Hydraulic Jump: When the water flows with a very high
velocity over the crust of the weir or over the gates of the barrage,
then hydraulic jump develops. This hydraulic jump causes a suction
pressure or negative pressure on the downstream side which acts
in the direction of uplift pressure. If the thickness of the
impervious floor is not sufficient, then the structure fails by
rapture.
(b) By Scouring During floods: The gates of the barrage are kept
open and the water flows with high velocity. The water may also
flow with very high velocity over the crest of the weir. Both the
cases can result in scouring effect on the downstream and on the
upstream side of the structure. Due to scouring effect on the
downstream and on the upstream side of the structure, its stability
gets endangered by shearing.

89. By uplift Pressure

90. By Hydraulic Jump

92. Causes of Failure of Weir, and Remedies
• If a weir is constructed on permeable soil, weir may fail
by piping, uplift force, suction caused by standing wave
and scouring on both upstream and downstream of the
weir.
• When hydraulic gradient or exit gradient exceeds the
critical value of soil, surface soil at down end starts
boiling first and is washed away by percolating water.
This process of removal or washing out of soil
continuous and eventually a channel in the form of
pipe is formed by seepage water. This is called piping
which may cause the failure of foundation. Similarly
uplift force of percolating water is acting on the floor
from bottom and if the weight of floor is not enough to
resist this uplift force, floor may fail by cracking or
bursting.

93. The main remedies against failure are:
• Path of percolation or creep length of seepage
water should be increased by providing sheet piles
at upstream, downstream or at intermediate point
to reduce the hydraulic gradient.
• Floor thickness should be increased to increase its
self weight to balance the uplift force.

94. Precautions Against Failure
The following precautions can be taken to prevent failure.
(a) The length of the impervious layer should be carefully
designed so that the path of the percolating water is
increased consequently reducing the exit gradient.
(b) Sheet piles should be provided on the upstream side and
the downstream side of the impervious floor to increase to
the length of percolating water so that the uplift pressure is
considerable reduced.
(c) The thickness of the impervious floor should be such that
the weight of the floor is a sufficient to counterbalance the
uplift pressure.
(d) Energy dissipater blocks like friction blocks, impact blocks,
should be provided.

95. (e) Inverted filter should be provided with concrete
blocks on the top so that the percolating water does
not wash out the soil particles. Deep foundation like
well foundation should be provided for the barrage
pier.

96. Location of Headwork’s

97. Location of Diversion Headwork’s
The location of the diversion headwork’s depends
on the stages of flow in the river. Most of the large
rivers in our country have the
following four stages.
1. Rocky stage or Hilly stage (Mountainous stage)
2. Boulder stage
3. Trough or alluvial stage
4. Delta stage

98. Rocky stage or Hilly stage (Mountainous stage)
• In this stage river is in the hills. The bed slope and velocities are high. It is generally
not suitable for the location of a diversion headwork’s.
• Advantages:
• A sound rocky foundation is usually available at the site. Thus cost of construction of
weir is less.
• High heads are available for hydroelectric work.
• Due to high velocities of flow, there are no chances of the supply channel getting
silted.
• Disadvantages:
• The discharge in the river is low because of small catchment.
• The land in the hilly area is not suitable for agriculture.
• The ground has a steep slope and therefore a number of canal falls are required.
• The weir constructed in the mountainous stage would be located away from the
commanded area of the canal and therefore the idle length of the canal would be
large.
• The water in this region does not contain silt and thus devoid of fertilizing materials.
• The river is usually very flashy and there is sudden rise and fall of the water level.
• The number of cross drainage works on the canals is usually very large.

99. Boulder Stage
From the rocky stage the river passes on to the boulder stage. It is also called sub-
mountainous stage. In this stage the bed and banks of the river are composed of boulders
and gravels. The width of river is small and the river has well-defined boundaries. The bed
slope and velocity are less than those in the rocky stage. There is large subsoil flow in this
region.
• Advantages:
• The length of weir is generally shorter in boulder stage.
• Because the banks are high, the cost of river training works is low.
• Construction materials such as stone, aggregates, sand, gravel are locally available.
• Since the ground slope is quite steep, the falls on the canals can be utilized fo hydropower
generation.
• The silt charge is less and the associated problems of excessive silt are small.
• Disadvantages:
• There is a large loss of stored water due to sub soil flow at the site of diversion headwork’s.
• Idle length of canal is more.
• More cross drainage works are required.
• In the head reaches of canal, seepage losses are high.
• The demand of irrigation water is low because the land is not fertile.

100. Trough or Alluvial stage
• From the boulder stage the river passes on to the alluvial plain created by
itself. In this stage the cross section of river is made up of alluvial sand and
silt. The bed slope and velocity are small.
• Advantages:
• Sub soil flow is comparatively less.
• The land is fertile demand of irrigation water is high.
• The site is near the commanded area and hence the length of the canal is
small.
• The seepage losses are less in the canal.
• Less number of cross drainage works required.
• The water contains silt which has manurial value.
• Disadvantages:
• The river section is quite wide and hence the length of the weir structure is
large.
• Construction material is usually not available locally.
• The cost of headwork’s is usually more due to poor foundation
• Because of meandering tendency more river training works are required.
• There is problem of silt in the canal.

101. Delta stage
• From the trough stage the river passes on to the
delta stage as it approaches the ocean.
• The delta stage is not suitable for location of a
diversion headwork’s because the river section is
excessively wide, and the river has a shifting
tendency.
• There is no suitable compact commanded area.
• Since the water table is also high, there is not much
need of irrigation.
• Therefore the choice is usually between the
locations of diversion headwork’s in the boulder
stage and in the trough stage.