Department of Civil Engineering

University of Engineering and Technology Peshawar

CE-402: Irrigation Engineering

Lecture 8
Diversion Headworks

8th Semester (4th Year)

Civil Engineering
Spring 2021

Lecturer: Alamgir Khalil

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Headwork
➢ Any hydraulic structure which supplies water to
the off taking canal is called a headwork.
➢ Headworks may be divided into two classes:
1) Storage headwork
2) Diversion headwork

1) Storage headwork
STORAGE HEADWORK
➢ It comprises the construction of a dam across
the river. It stores water during the period of
excess supplies in the river and releases it when
demand overtakes available supplies.

2) Diversion headwork
➢ It serves to divert the required supply of water
into the canal from the river. Diversion
headworks are constructed on perennial rivers. DIVERSION HEADWORK
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Diversion Headworks
Definition:
➢ The works, which are constructed at the head of the canal, in order to divert
the river water towards the canal, so as to ensure a regulated continuous
supply of silt-free water with a certain minimum head into the canal, are
known as diversion headworks.

Amandara Headworks Khanki Headworks

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Diversion Headworks (cont.)

➢ A diversion headwork serves the following purposes:

✓ It raises the water level in the river so that the command area can be
increased.
✓ It regulates the intake of water into the canal.
✓ It controls the silt entry into the canal.
✓ It reduces fluctuations and stores water in case of short supplies.

Islam Headworks Balloki Headworks Sulemanki Headworks

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Diversion Headworks (cont.)

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Weir
➢ A weir is a solid obstruction put across the river to raise its water level and
divert the water into the canal.

➢ 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.

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Weir (cont.)
➢ If a weir also stores water for tiding over small periods of short supplies (with
less volume), it is called a storage weir.

➢ The main difference between a storage weir and a dam is only in height and
the duration for which the supply is stored. A dam stores the supply for a
comparatively longer duration.

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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 a barrage or a river regulator.
➢ A barrage is a diversion structure across a river fitted with a series of gates
over its entire length for creating the required ponding.

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Barrage (cont.)

Kotri Barrage
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Barrage (cont.)

➢ If most of the ponding or the entire ponding is done by a permanent raised

crest, as a weir, then the afflux caused during high floods is quite high. On the
other hand, if most of the ponding is done by gates, as in a barrage, then
gates can be opened during high floods and the afflux (i.e. rise in HFL near
the site) will be nil or minimum.

➢ A barrage type construction can be

easily supplemented with a roadway
across the river at a small additional
cost. Hence, barrages are almost
invariably constructed these days on
all important rivers.

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Afflux

➢ Afflux is the rise in the high flood level of the river, upstream of the weir (or the
bridge in case of non-erodible soils), or barrage, as a result of its construction.
➢ This rise in water level is maximum just near the site of constriction and reduces
as we go away from it, upstream. The afflux, extends for a long distance on the
upstream side, as shown.

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Afflux (cont.)

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Afflux (cont.)

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Comparison between Weir and Barrage

Barrage

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Comparison between Weir and Barrage (cont.)

Weir Barrage
High Set Crest Low set Crest
Ponding is done against the raised crest or partly Ponding is done by means of gates
against the raised crest and partly by shutters
Shutters in part length Gated over entire length
Shutters are of smaller height Gates are of greater height
Shutters are dropped to pass flood Gates are raised to pass flood
Not perfect control on river flow Perfect control on river flow
Operation of shutters is slow, involve labor and time Gates are convenient to operate
Raised crest causes silting upstream Less silting upstream due to low set crest
No means for silt disposal Silt removal is done through under sluices
Shorter construction Period Longer construction period
Relatively cheaper structure Costly structure
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Components of Diversion Headworks

A typical layout of
canal headworks is
shown. It consists of:

1) Weir portion
2) Under-sluices
3) Divide wall
4) River Training works
5) Fish Ladder
6) Canal Head Regulator
7) Weir’s ancillary works,
such as shutters,
gates, etc.
8) Silt Regulation Works

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Components of Diversion Headworks (cont.)

Weir
➢ The weir is a solid obstruction put across the river to raise its water level and
divert the water into the canal.
➢ Length of weir is divided into number of bays by means of divide piers.
➢ Weir is aligned at right-angle to the direction of
main river current. The right-angled alignment
is better and is common when riverbed is silty
or sandy. This ensures lesser length of the weir,
better discharge capacity and lesser cost.
➢ Sometimes oblique alignment is necessary
when riverbed consists of gravel or shingle,
which could otherwise enter the head
regulator of main canal and get deposited into
head reach of main canal.
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Components of Diversion Headworks (cont.)

Classification of Weirs
A) Classification according to design of Floor
1) Gravity Weir
2) Non-Gravity Weir

1) Gravity Weir

➢ A gravity weir is the one in

which the uplift pressure
due seepage of water
below the floor is resisted
entirely by the weight of
the weir (i.e., its body and
floor).

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Components of Diversion Headworks (cont.)

2) Non-Gravity Weir

➢ In the non-gravity weir, the floor

thickness is kept relatively less, and
the uplift pressure is largely resisted
by the bending action of the
reinforced concrete floor.

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Components of Diversion Headworks (cont.)

Classification of Weirs
B) Classification according to material of construction
1) Masonry weirs with vertical drop
2) Rock-fill weirs with sloping aprons
3) Concrete weirs with sloping glacis

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Components of Diversion Headworks (cont.)

B) Classification according to material of construction
1) Masonry weirs with vertical drop

✓ Suitable for hard clay and consolidated gravel foundations.

✓ At the downstream end of impervious floor, a graded inverted filter is provided
to relieve the uplift pressure.

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Components of Diversion Headworks (cont.)

B) Classification according to material of construction

2) Rock-fill weirs with sloping aprons

✓ It is also called “dry stone slope weir”.

✓ Simplest type of construction and suitable for fine sandy foundations.
✓ Such a weir requires huge quantities of stones and suitable where stone is
easily available.

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Components of Diversion Headworks (cont.)

B) Classification according to material of construction
3) Concrete weirs with sloping glacis

✓ Suitable for soft sandy foundations.

✓ Suitable where difference in weir crest and downstream riverbed is limited to
3 m (10 ft).
✓ When water passes over such a weir, hydraulic Jump is formed on the sloping
glacis.

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Components of Diversion Headworks (cont.)

Parabolic Weir

✓ It is similar to the spillway section of a dam. The body wall of such a weir is
designed as a low dam.
✓ A cistern is provided at the downstream side to dissipate the energy.
✓ The upstream and downstream protection works are similar to that of a vertical
drop or sloping glacis weir.

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Components of Diversion Headworks (cont.)

Under Sluices or Scouring sluices
✓ It is a deep channel in front of the head regulator and dispose off heavy silt and a
part of flood discharge on to the D/S of the barrage.
✓ It creates less turbulent pockets of water near canal head regulator.
✓ A divide wall separates the main weir portion from the under sluices portion of the
weir.
✓ The crest of the under sluices portion of the weir is kept at a lower level than the
crest of the normal portion of the weir.
✓ The under-sluiced length of weir or barrage is divided into a number of ways by
piers with separate gates.
Functions of scouring sluices
✓ To draw water during dry weather flow.
✓ To reduce flow of silt into canal head regulator therefore also called scouring sluices.
✓ To scour the silt deposits in the pockets periodically. Accumulated silt can be
released by opening gates.
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Components of Diversion Headworks (cont.)

Under Sluices or Scouring sluices

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Components of Diversion Headworks (cont.)

Under Sluices or Scouring sluices

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Components of Diversion Headworks (cont.)

Fish Ladder

➢ Fish move from upstream to down

stream in search of relatively warm
water in the beginning of winter and
return to upstream for clear water
before the onset of monsoons.
➢ The fish ladder is provided just by
the side of the divide wall for the
free movement of fishes.
➢ In the fish ladder, the optimum
velocity is 6-8 ft/sec (2-2.5m/s).
➢ Low velocity is obtained by baffle
walls.
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Components of Diversion Headworks (cont.)

Fish Ladder

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Components of Diversion Headworks (cont.)

Fish Ladder

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Components of Diversion Headworks (cont.)

Fish Ladder

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Components of Diversion Headworks (cont.)

Divide Wall

➢ The ‘divide wall’ is a masonry or a

concrete wall constructed at right angle
to the axis of the weir and separates
the weir proper from the under sluices.
➢ If there are two canals off taking from
each flank, then there will be two
divide walls and two under sluices.
➢ The top width of divide wall is about
1.5 m to 2.5 m.
➢ Its length on the upstream side should be sufficient to keep the heavy turbulence at
the nose of the wall well away from the upstream protection of the sluices.
➢ Its length on the downstream side should extend to cover the hydraulic jump and the
resulting turbulence.
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Components of Diversion Headworks (cont.)

Divide Wall
➢ The main functions served by divide wall are:
✓ The floor level of the under sluices is generally kept
lower than the floor level of the main weir. Hence, a
divide wall is essential to separate the two floors.
This prevents the turbulence action.
✓ It helps in providing a comparatively less turbulent
pocket near the canal head regulator, resulting in
deposition of silt in this pocket and, thus, to help in
the entry of silt-free water into the canal.
✓ Divide wall may keep the cross-currents, if formed, away from the weir. Cross-
currents lead to vortices and deep scours. For this purpose, additional divide walls
are sometimes provided.
✓ To serve as a sidewall of the fish ladder.
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Components of Diversion Headworks (cont.)

River Training Works

➢ River training works are required near the weir site in order to ensure a smooth and
an axial flow of water, and thus, to prevent the river from outflanking the works due
to a change in its course. The river training works required on a canal headwork are:

(a) Guide banks

(b) Marginal bunds
(c) Spurs or groynes

(a) Guide banks

➢ The guide banks force the river into a
restricted channel, and thus, ensuring
a smooth and an almost axial flow near
the weir site.
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Components of Diversion Headworks (cont.)

River Training Works

(a) Guide banks

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Components of Diversion Headworks (cont.)

River Training Works

(b) Marginal bunds

✓ Marginal bunds are flood embankments

in continuation of guide banks designed
to contain the floods within the flood
plain of the river.
✓ Both height and length vary according to back
water effect caused by the weir/barrage.
✓ Partially help in controlling and guiding the river flow between the guide banks.
For this reason, the length of guide banks do depend on the layout of the
marginal bunds.
✓ Marginal bunds are nothing but earthen embankments, protected by groynes
wherever needed.
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Components of Diversion Headworks (cont.)

River Training Works

(c) Spurs or groynes

✓ Groynes are the embankment type

structures, constructed transverse to the
river flow, extending from the bank into
the river. These groynes are also known
as spurs, dikes, and transverse dikes.

Purpose of a Groyne

✓ Contracts a river channel to improve its depth.

✓ Protects the riverbank.
✓ Silts up the area in the vicinity by creating a slack flow.
✓ Trains the flow along a certain course.
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Components of Diversion Headworks (cont.)

River Training Works

(c) Spurs or groynes

Types of groynes

1) Classification according to materials of

construction
✓ Permeable groyne
✓ Solid impermeable groyne
2) Classification according to its height
below high water
✓ Submerged groyne
✓ Non-submerged groyne.
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Components of Diversion Headworks (cont.)

River Training Works

(c) Spurs or groynes

Types of groynes

3) Classification according to the

function it serves

✓ Attracting groyne
✓ Deflecting groyne
✓ Repelling groyne
✓ Sedimenting groyne

4) Special type of groynes include:

Denehy’s T-headed groyne, Hockey
type groyne, and Burma type groyne.
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Spurs or groynes

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Spurs or groynes (cont.)

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Spurs or groynes (cont.)

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Components of Diversion Headworks (cont.)

Canal Head Regulator

➢ A canal head regulator is

provided at the head of the
off-taking canal and serves
the following functions:

✓ It regulates the supply of water

entering the canal.

✓ It controls the entry of silt in

the canal.

✓ It prevents the river floods

from entering the canal.

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Components of Diversion Headworks (cont.)

Canal Head Regulator

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Components of Diversion Headworks (cont.)

Silt Regulation Works

➢ The entry of silt into a canal, which takes off from a headworks, can be reduced
by constructed certain special works, called silt control works.
➢ Two types of Silt regulation works:
(a) Silt Excluders
(b) Silt Ejectors

(a) Silt Excluders

➢ 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 it enters the canal.

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Components of Diversion Headworks (cont.)

(a) Silt Excluders

➢ The fundamental principle behind silt control is that most of the silt tries to settle
down in water, thus, confining itself mostly in the bottom layers of water.
➢ The silt excluder is a device constructed in the riverbed just upstream of the
regulator to exclude silt from the water (source) entering the canal.
➢ It is so designed that the top and bottom layers of flow are separated with the least
possible disturbance, the top sediment-free water being led towards the canal
while the bottom sediment-laden water is discharged downstream of the diversion
structure through under sluices.
➢ The device basically, consists of a number of tunnels in the floor of the deep pocket
of the river, isolated by a dividing wall.
➢ The sill level of the regulator is kept the same as that of the top level of the R.C.C.
roof slab of the tunnels.
➢ The capacity of the tunnel(s) is usually kept at about 20% of the canal discharge,
and they are designed to maintain a minimum velocity of 2–3 m/s (to avoid
deposition in tunnels). 46
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Components of Diversion Headworks (cont.)

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Components of Diversion Headworks (cont.)

A view of the Completed Silt Excluder at Taunsa Barrage 48

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Components of Diversion Headworks (cont.)

Silt Regulation Works

(b) 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.
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Silt Ejector

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Silt Ejector (cont.)

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Site Selection For Diversion Headworks

➢ The site for headwork should be selected after giving proper consideration to the
following points:

1) Economical Considerations

✓ The construction materials should be

available in large quantities in close
proximity of the site.
✓ At the site the river section should be
preferably narrow, straight with high
non- submersible banks. It reduces the cost of river training work.
✓ The cost of connecting the site by rail and roadway should be less.
✓ To reduce the cost of construction of the canal system the site should be close to
the culturable area.
✓ Number of cross drainage works required to be constructed should be minimum.
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Site Selection For Diversion Headworks (cont.)

2) Functional Considerations
✓ Moderate height of the obstruction should give good command.
✓ So far as possible the canal take-off should be at right angles to the river.
✓ The river reach should have central flow without tendency of scouring or silting.
✓ The canal should give maximum command.

3) Structural Considerations

✓ Good foundation should be

available at moderate depth.

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Site Selection For Diversion Headworks (cont.)

➢ The following considerations should be kept in mind while selecting the site for a
barrage:

❖ The site must have a good command over the area to be irrigated and also must
not be at too far distance to avoid long feeder channels.
❖ The width of the river at the site should be preferably minimum with a well-defined
and stable river approach.
❖ A good land approach to the site will reduce expenses of the transportation and the
ultimate cost of the project.
❖ There must be easy diversion of the river after construction.
❖ Existence of central approach of the river to the barrage after diversion, this is
essential for proper silt control.
❖ If it is intended to convert the existing inundation canals into the perennial canals,
site selection is limited by the position of the head-regulator and the alignment of
the existing inundation canals.
❖ A rock foundation is the best but in the alluvial planes the bed is invariably sandy.
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Site Selection For Diversion Headworks (cont.)

➢ The common practice in
Pakistan has been to build
the barrage on dry land in
a bye river and after
completion to divert the
river through it. This gave
an oblique approach and
created many problems.

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Site Selection For Diversion Headworks (cont.)

➢ The following guidelines have now been proposed by the Irrigation Research
Institute, Lahore. Their recommendations are based on extensive hydraulic
model experiments for each individual case.

✓ Where the angle b/w the headwork axis and the river axis exceeds 10
degree, the problem arises of concentration of flow on one side and island
formation within the guide banks on the other side occurs due to heavy
silting as in case of Islam, Sidhnai and Balloki barrages in Pakistan.

✓ If the river axis is to the right of the headwork axis, the concentration of flow
is generally on the left side with the consequent tendency to form an island
on the right and vice versa.

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Site Selection For Diversion Headworks (cont.)

✓ When a barrage is located below the confluence of two rivers, it should be

located sufficiently far below the confluence and consideration must be
given as to which of the two rivers dominate the confluence.

✓ The barrage should be located as far as possible in the center of the flood
plain. Asymmetry of location increases the likelihood of shoals forming and
calls for expensive training works.

✓ The most suitable site for a barrage when constructed on dry land, is below
the outer side of the convex bend which is followed by the straight reach of
the river.

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Amandara Headworks

Swat River

Amandara Headorks

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Amandara Headworks

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Amandara Headworks (cont.)

HISTORY

➢ To meet the requirements of irrigation in Malakand valley and district Mardan and
Swabi, an irrigation scheme was planned by the British Government in 1903.
➢ The plan consists of headwork at Amandara near Batkhela on River upper Swat,
unlined irrigation channel 7.2 Km long up to Malakand Pass and excavation of 17.5
ft dia, 3.4 km long tunnel underneath of Malakand Pass.
➢ A network of irrigation canals including Abazi and Machai branches and their
distributaries were also part of the project. The project started in 1907 and
completed in 1914.

http://www.wapda.gov.pk/index.php/kurram-dam-menu/item/408-history 60
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Amandara Headworks (cont.)

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