Components of a Barrage and Theories of Seepage

This document provides detailed information about the components of a barrage, including the main barrage portion, sheet piles, inverted filter, flexible apron, under sluices, divide wall, fish ladder, canal head regulator, silt regulation works, and river training works. It also covers the theories of seepage in irrigation engineering and water management.

Department of Civil Engineering
University of Engineering and Technology Peshawar
CE-402: Irrigation Engineering and
Water Management
Lecturer: Alamgir Khalil
8th Semester (4th Year)
Civil Engineering
Spring 2022
Lecture 9
Components of a Barrage,
Theories of Seepage
1

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Barrage
2
➢ 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.)
3
➢ A barrage is a type of low-head diversion dam which consists of a number of large
gates that can be opened or closed to control the amount of water passing
through. This allows the structure to regulate and stabilize river water elevation
upstream for use in irrigation and other systems.
➢ The main difference between a weir and a barrage is of gates, that is the flow in
barrage is regulated by gates and that in weirs, by its crest height. Barrages are
costlier than weirs.
Kotri Barrage

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Barrage (cont.)
4
✓ It diverts the required quantity of water from the river to the channels.
✓ It raises the level of supply so that water can reach the area proposed for
irrigation by gravity flow.
✓ It allows proper silt control.
✓ It provides permanent headworks for the canals in order to protect them
during floods by providing for complete closure.
✓ It provides better regulation than a weir.
➢ A barrage serves the following purposes:
Islam Headworks Balloki Headworks Sulemanki Headworks

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Components of a Barrage
5
The following are the components of a barrage:
1) Main barrage portion
2) Sheet piles
3) Inverted filter
4) Flexible apron
5) Under sluices
6) Divide wall
7) Fish ladder
8) Canal Head Regulator
9) Silt Regulation Works
10) River Training works

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Components of a Barrage (cont.)
6

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Components of a Barrage (cont.)
7
Main Barrage Portion
➢ This is the main body of the barrage
normally of R.C.C. slabs which support the
steel gates. In cross-section it consists of;
✓ U/S concrete floor to lengthen the path
of seepage and to protect the middle
portion where the piers, gates and bridge
are located.
✓ A crest at the required height above the floor on which the gates rest in their closed
position.
✓ U/S glacis having the necessary slope to join the u/s floor level to the highest point,
the crest.
✓ D/S glacis of suitable shape and slope. This joins the crest to the d/s floor level
(which may be at riverbed level or below). The hydraulic jump forms on the glacis.

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Components of a Barrage (cont.)
8
Main Barrage Portion
✓ D/S floor is built of concrete and is constructed so as to contain the hydraulic jump.
Thus, it takes care of turbulence which would otherwise cause erosion. It is also
provided with friction blocks of suitable shape and at distances determined by
hydraulic model experiments in order to increase friction and destroy residual K.E.

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Components of a Barrage (cont.)
9
➢ An inverted filter is provided between the d/s sheet piles and the flexible
protection.
➢ It typically consists of 6” sand, 9’’ coarse sand and 9” gravel. The filter material may
vary with the size of the particles forming riverbed. It is protected by placing
concrete blocks of sufficient weight and size, over it.
Inverted Filter
➢ Its primary function is to
check the escape of fine soil
particles in the seepage
water. In the case of scour, it
provides adequate cover for
the d/s sheet piles against
the steepening of exit
gradient.

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Components of a Barrage (cont.)
10
➢ From the functional point view in
a barrage, these are classified
into three types i.e.
✓ u/s sheet piles
✓ intermediate sheet piles
✓ d/s sheet piles
Sheet Piles
➢ There are generally three or four sheet piles. These are made of mild steel, each
portion being with sufficient width and in thickness of the required length and
having grooves to link with the other sheet piles.

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Components of a Barrage (cont.)
11
➢ u/s sheet piles are located at the u/s end of the u/s concrete floor. These piles are
driven into the soil beyond the maximum possible scour that may occur. Their
functions are:
✓ To protect the barrage structure from scour.
✓ To reduce uplift pressure in the barrage floor.
Upstream Sheet Piles
Intermediate Sheet Piles
➢ They are situated at the end of u/s and d/s glacis and
serve as second line of defense. In case of the u/s or
d/s sheet piles collapse due to advancing scour or
undermining, then these sheet piles give protection
to the main structure of the barrage. The
intermediate sheet piles also help to lengthen the
seepage path and reduce uplift pressure.

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Components of a Barrage (cont.)
12
Downstream Piles
➢ These are placed at the end of the d/s concrete floor and their main function is to
check the exit gradient. Their depth should be greater than the maximum possible
scour.
Shutters or Gates
➢ The gates or shutters are made of steel
plates welded on the fabricated steel
framework.
➢ Their function is:
✓ To maintain pond level.
✓ To raise the water level during low
supplies.

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13
Launching Apron
Upstream Launching Apron
➢ It is constructed with boulders or
stones arranged in layers without
any joint. It protects the
impervious floor and sheet piles
from the scour holes which may
develop.
Components of a Barrage (cont.)
Downstream Launching Apron
➢ It is constructed with loosely packed boulders or stones. It covers completely the
zone of exit gradient. This apron protects the barrage/weir from the effect of piping
or undermining which may occur due to seepage of water.

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Components of a Barrage (cont.)
14
Launching Apron
Placement of D/S Block Apron Placement of D/S Stone Apron
PCC Block and Stone Aprons at Taunsa Barrage

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Components of a Barrage (cont.)
15
Launching Apron
Lacy’s regime scour depth
𝑅 = 1.35𝑞2
𝑓
1/3
xR
measured from HFL
𝑓 = 1.76 𝑑𝑚𝑚
D = xR – Y
x = 1.25 to 2 Y = HFL – Floor level
= flow depth of unscoured depth
(Y)
𝑞 = discharge per
unit width (cumec/m)
(Maximum possible scour depth below floor level

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Components of a Barrage (cont.)
16
1) Under sluices
2) Divide wall
3) Fish ladder
4) Canal Head Regulator
5) Silt Regulation Works
6) River Training works
Details of the above
components are
same as covered in
the previous lecture.

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Weir vs Barrage
17

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Causes of Failure of Weirs/Barrages on Permeable Foundation
18
➢ The combined effect of subsurface flow and surface flow may cause the failure of
the weir or barrage.
1) Failure due to subsurface flow
a) By piping or undermining
b) By uplift pressure
2) Failure due to surface flow
a) By hydraulic jump
b) By scouring

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Causes of Failure of Weirs/Barrages on Permeable Foundation (cont.)
19
➢ At permeable soils water seeps at the base of the weir/barrage. When the flow lines
at the downstream end emerge at d/s end of the impervious floor, the hydraulic
gradient may exceed a certain critical value of the soil. At this stage, soil particles
starts removal. Further concentration of flow results more soil removal. This process
of soil removal further progresses towards the upstream and results in the
formation of a channel or a pipe under the floor of weir/barrage causing its failure.
a) Piping or undermining
1) Failure due to subsurface flow
✓ Providing sufficient length of impervious
floor so that path of percolation is
increased, and the exit gradient is
decreased.
✓ Providing pile at downstream ends.
Control

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Causes of Failure of Weirs/Barrages on Permeable Foundation (cont.)
20
1) Failure due to subsurface flow
b) Uplift Pressure
➢ If the weight of floor is not sufficient to resist the uplift pressure, the floor may
burst, and the effective length of impervious floor is thereby reduced. In this way
the final failure is due to reduction of effective length and consequent increase in
exit gradient. Example of such failure is Khanki weir on Chenab.
✓ Providing impervious floor of sufficient
length and of appropriate thickness.
✓ Providing pile at the u/s end so that the
uplift pressure to the d/s is reduced.
Control

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Causes of Failure of Weirs/Barrages on Permeable Foundation (cont.)
21
➢ The standing waves or hydraulic jump formed at the d/s of the weir/barrage causes
suction that acts in the direction of uplift pressure. If the floor thickness is not
sufficient; it may fail by rupture. Examples of such failures are Marala weir on the
Chenab and Rasul weir.
a) Hydraulic jump
2) Failure due to surface flow
✓ Providing additional thickness of floor to
counterbalance the extra pressure due
to the standing waves.
✓ Constructing the floor thickness in one
concrete mass instead of in masonry
layers.
Control

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Causes of Failure of Weirs/Barrages on Permeable Foundation (cont.)
22
➢ When the natural waterway of a river is contracted, the water may scour the bed
both at upstream and downstream of the structure. The scour holes may progress
toward the structure, causing its failure. Example of such failure was Islam weir.
b) Scouring
2) Failure due to surface flow
✓ Using the piles at upstream and downstream ends of the impervious floor, much
below the calculated scour level.
✓ Providing suitable length and thickness of launching aprons at u/s and d/s sides, so
that stones of the apron may settle in the scour holes.
Control

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Failure of Weirs/Barrages on Permeable Foundation
23
➢ The following precautions can be taken to prevent failure:
1) 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.
2) Sheet piles should be provided on the upstream side and the downstream side of
the impervious floor to increase the length of percolating water so that the uplift
pressure is considerably reduced.
3) The thickness of the impervious floor should be such that the weight of the floor is
sufficient to counterbalance the uplift pressure.
4) Energy dissipater blocks like should be provided.
5) Inverted filter should be provided with concrete blocks on the top so that the
percolating water does not wash out the soil particles.
6) Deep foundation like well foundation should be provided for the barrage piers.
Precautions Against Failure

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Theories of Seepage
24
➢ Hydraulic Structures such as dams, weirs, barrages, head regulators, cross-
drainage works etc. may be founded on an impervious solid rock foundation or
on a pervious foundation.
➢ Whenever, such a structure is founded on a pervious foundation, it is subjected
to seepage of water beneath the structure, in addition to all other forces to
which it will be subjected when founded on an impervious foundation.
➢ Hydraulic structures founded on alluvial soil foundations do allow seepage
beneath them.
➢ The water seeping below the body of the hydraulic structure, endangers the
stability of the structure and may cause its failure, either by:
✓ Piping or
✓ Direct uplift
➢ The above concepts of the failure of hydraulic structures due to subsurface flow
were introduced by Bligh, based on experiments and the research work
conducted after the failure of Khanki weir in 1895, which was designed on
experience and intuition without any rational theory.

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Theories of Seepage (cont.)
25
Bligh’s Creep Theory for Seepage Flow
➢ According to Bligh’s Theory, the percolating water follows the outline of the base
of the foundation of the hydraulic structure. In other words, water creeps along
the bottom contour of the structure.
➢ The length of the path thus traversed by water is called the length of the creep.
Further, it is assumed in this theory, that the loss of head is proportional to the
length of the creep.
➢ If 𝐻is the total head loss between the upstream and the downstream, and 𝐿is
the length of creep, then the loss of head per unit of creep length (i.e. 𝐻/𝐿) is
called the hydraulic gradient. Further, Bligh makes no distinction between
horizontal and vertical creep.

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Bligh’s Creep Theory for Seepage Flow
26

A B
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Bligh’s Creep Theory for Seepage Flow (cont.)
27
➢ Consider a section as shown in Fig. Let 𝐻be the difference of water levels between
upstream and downstream ends. Water will seep along the bottom contour as
shown by arrows. It starts percolating at A and emerges at B. The total length of
creep is given by
𝐿 = 𝑑1 + 𝑑1 + 𝐿1 + 𝑑2 + 𝑑2
= 𝐿1 + 𝐿2 + 2(𝑑1 + 𝑑2 + 𝑑3)
𝐿 = 𝑏 + 2(𝑑1 + 𝑑2 + 𝑑3)
+𝐿2 + 𝑑3 + 𝑑3

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Bligh’s Creep Theory for Seepage Flow (cont.)
28
Head loss per unit length or hydraulic gradient = 𝐻
𝑏 + 2(𝑑1 + 𝑑2 + 𝑑3) = 𝐻
𝐿
Head losses equal to 𝐻
𝐿 × 2𝑑1
𝐻
𝐿 × 2𝑑2
𝐻
𝐿 × 2𝑑3 ; will occur respectively, in the
planes of three vertical cut offs. The hydraulic gradient line (H.G. Line) can then be
drawn as shown in figure above.

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Bligh’s Creep Theory for Seepage Flow (cont.)
29
➢ According to Bligh, the safety against piping can be ensured by providing sufficient
creep length, given by 𝐿 = 𝐶𝐻, where 𝐶 is the Bligh’s Coefficient for the soil.
Different values of 𝐶for different types of soils are tabulated in table below:
S.No. Type of Soil Value of
C
Safe Hydraulic gradient
should be less than
1 Light sand and mud 18 1/18
2 Fine micaceous sand 15 1/15
3 Coarse grained sand 12 1/12
4 Sand mixed with boulder and gravel,
and for loam soil 5 to 9 1/5 to 1/9
Note: The hydraulic gradient i.e. 𝐻/𝐿is then equal to 1/C. Hence, it may be stated that the
hydraulic gradient must be kept under a safe limit in order to ensure safety against piping.
1) Safety against piping or undermining

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Bligh’s Creep Theory for Seepage Flow (cont.)
30
➢ The ordinates of the H.G line above the bottom of the floor represent the
residual uplift water head at each point. Say for example, if at any point, the
ordinate of H.G line above the bottom of the floor is 1 m, then 1 m head of water
will act as uplift at that point. If ℎ′ meters is this ordinate, then water pressure
equal to ℎ′ meters will act at this point and has to be counterbalanced by the
weight of the floor of thickness say 𝑡.
2) Safety against uplift pressure
Downward pressure = 𝛾𝑤 × 𝐺 𝑡
Uplift pressure = 𝛾𝑤 × ℎ′
[Where 𝛾𝑤 is the unit weight of water]
[Where 𝐺is the specific gravity of the floor material]

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Bligh’s Creep Theory for Seepage Flow (cont.)
31
2) Safety against uplift pressure
For equilibrium,
Subtracting 𝑡 on both sides, we get
𝛾𝑤 × ℎ′ = 𝛾 𝑤 × 𝐺 𝑡
ℎ′ = 𝐺 𝑡
Where, ℎ′ − 𝑡 = ℎ, is the ordinate of the H.G line above the top of the floor
𝐺 − 1is the submerged specific gravity of the floor material
ℎ′ − 𝑡 = 𝐺 𝑡 − 𝑡
ℎ′ − 𝑡 = (𝐺 − 1)𝑡
𝑡 = ℎ′ − 𝑡
𝐺 − 1 = ℎ
𝐺 − 1

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Bligh’s Creep Theory for Seepage Flow (cont.)
32
2) Safety against uplift pressure
➢ Hence, the thickness of the floor can be easily determined by using the equation.
This is generally increased by 33%, so as to allow a suitable factor of safety.
✓ It may be mentioned that the floor thickness has to be designed according to the
above equation only for the downstream floor and for the worst conditions i.e. when
maximum ordinates of the H.G. line occur. The water standing on the upstream floor,
more than counterbalances the uplift caused by the same water, and hence, only a
nominal floor thickness is required on the upstream side, so as to resist wear, impact
of flowing water, etc.
𝑡 = 4
3
ℎ′ − 𝑡
𝐺 − 1 = 4
3
ℎ
𝐺 − 1

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Bligh’s Creep Theory for Seepage Flow (cont.)
33
Bligh’s Theory Limitations
✓ Bligh made no distinction between horizontal and vertical creep.
✓ Did not explain the idea of exit gradient - safety against undermining cannot
simply be obtained by considering a flat average gradient but by keeping this
gradient well below critical.
✓ No distinction between outer and inner faces of sheet piles or the intermediate
sheet piles, whereas from investigation it is clear, that the outer faces of the end
sheet piles are much more effective than inner ones.
✓ Losses of head does not take place in the same proportions as the creep length.
✓ The uplift pressure distribution is not linear but follow a sine curve.
✓ Bligh’s does not specify the absolute necessity of providing a sheet pile at d/s
whereas it is essential to have a deep vertical cut off at d/s end to prevent
undermining.

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Bligh’s Theory – Example (Punmia)
34
Figure shows the sections of a hydraulic structure founded on sand. Calculate the
average hydraulic gradient. Also, find the uplift pressure at points 6, 12, and 18 m from
the u/s end of the floor and find the thickness of the floor at these points.

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Theories of Seepage (cont.)
35
Lane’s Weighted Creep Theory
➢ Bligh, in his theory, had calculated the length of the creep, by simply adding the
horizontal creep length and the vertical creep length, thereby making no distinction
between the two creeps.
➢ However, Lane, on the basis of his analysis carried out on about 200 dams all over
the world, stipulated that the horizontal creep is less effective in reducing uplift (or
in causing loss of head) than the vertical creep.
➢ He, therefore, suggested a weightage factor of 1/3 for the horizontal creep, as
against 1.0 for the vertical creep.
𝐿𝑙 = 1
3𝐿 + 𝑉
Where, 𝐿= sum of all horizontal contacts and all sloping contacts having slope less than 45o
𝑉= sum of all vertical contacts and all sloping contacts steeper than 45o

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Lane’s Weighted Creep Theory
36
Thus, in Fig., the total
Lane’s creep length (𝐿𝑙)
is given by
𝐿𝑙 = (𝑑1 + 𝑑1) +1
3𝐿1
𝐿𝑙 = 1
3 𝐿1 + 𝐿2 + 2(𝑑1 + 𝑑2 + 𝑑3)
+(𝑑2 + 𝑑2) +1
3𝐿2
+(𝑑3 + 𝑑3)
𝐿𝑙 = 1
3𝑏 + 2(𝑑1 + 𝑑2 + 𝑑3)

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Lane’s Weighted Creep Theory (cont.)
37
➢ To ensure safety against piping, according to this theory, the creep length 𝐿𝑙 must
not be less than 𝐶𝑙𝐻, where 𝐻is the head causing flow, and 𝐶𝑙 is Lane’s creep
coefficient given in table.
✓ Lane's theory was an improvement over Bligh's theory, but however, was purely
empirical without any rational basis, and hence, is generally not adopted in any
designs. Bligh's theory, though is still used (even after the invention of modern
Khosla's theory), but Lane's theory is practically nowhere used, and is having only a
theoretical importance.

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Lane’s Weighted Creep Theory (cont.)
38
Values of Lane’s Safe Hydraulic Gradient for different types of Soils
S.No Type of Soil Value of Lane’s
Coefficient 𝑪𝒍
Safe Lane’s Hydraulic
gradient should be less than
1 Very fine sand or silt 8.5 1/8.5
2 Fine sand 7 1/7
3 Coarse sand 5 1/5
4 Gravel and sand 3.5 to 3 1/3.5 to 1/3
5 Boulders, gravels and
sand
2.5 to 3 1/2.5 to 1/3
6 Clayey soils 3 to 1.6 1/3 to 1/1.6