CE302 DESIGN OF HYDRAULIC STRUCTURES

MODULE 1

Headwork

Any hydraulic structure which supplies water to the off taking canal is called a
headwork.

Headwork may be divided into two classes;

1. Storage headwork
2. Diversion headwork

Storage headwork

 It comprises the construction of a dam across a river.

 It stores water during the period of excess supplies in the river and releases it
when demand overtakes available supplies.

Diversion headwork

 It serves to divert the required supply into the canal from the river.

Purpose of Diversion Headwork;

 Raises the water level in the river so that commanded area can be increased.
 Regulates the intake of water on to the canal.
 Control the silt entry into the canal.
 Reduces the fluctuations in the level of supply in the river.
 Stores water for tiding over small periods of short supplies.

Layout and functions of components of a Diversion headwork

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1. Weir and barrage

2. Under sluice
3. Divide wall
4. Fish ladder
5. Canal head regulator
6. Silt regulation works
7. River training works (Marginal bunds and Guide banks)

1. Weir and Barrage

a) Weir
 Normally the water level of rivers is such that it cannot be diverted into
the irrigation canal since the bed level of canal may be higher than the
existing water level of the river.
 Weir is constructed across the river to raise the water level to divert
the water from the river into the canal.
 Surplus water passes over the crest of weir.
 Adjustable shutters are provided on the crest to raise the water level to
some required height.

b) Barrage
 When the water level on the upstream side is to be raised to different
levels at different time, barrage is constructed.
 Barrage is an arrangement of adjustable gates or shutters.
 The function of a barrage is similar to that of the weir, but the heading
up of water is mainly affected by the gates.

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Weir Barrage
 It is a solid construction put  No solid construction is put across
across the river to raise the the river. Heading up of water is
water level. affected by gates.
 Less costly.  More costly.
 No attention is required at the  At the time of flood, gates are to
time of flood. be raised or lowered.
 May become ineffective due to  No silting problem.
silt deposits.  Better control over the water
 No control over the water level. level.
2. Under sluice
 Also known as scouring sluice.
 These are the openings with adjustable gates provided at the base of the
weir or barrage.
 Maintains a deep channel in front of the head regulator and dispose-off
heavy silt.

Functions of under sluice

 Preserves a clear and defined river channel approaching the regulator.

 They control the silt entry into the canal.
 They scour the silt deposited in the river bed above the approach channel
 Pass low floods without dropping main shutters.
3. Divide wall
 A long wall constructed at right angles in the weir or barrage, it may be
constructed with stone masonry or cement concrete.
 On the u/s side, the wall is extended just to cover the canal head
regulator and on the d/s side, it is extended up to the launching apron.

Functions of divide wall:

 Controls the eddy current or cross currents in front of the canal head.
 Provides a straight approach in front of the canal head.
 Resists the overturning effect on the weir or barrage caused by the
pressure of the impounding water.
4. Fish ladder
 Provided just by the side of divide wall for the free movement of fishes.
 Tendency of fish is to move from u/s to d/s in winter and d/s to u/s in
monsoons.
 Movement is essential for their survival.
 Due to construction of weir/barrage, this movement is obstructed.
5. Canal head regulator
 Structure which is constructed at the head of the canal to regulate flow
of water is known as canal head regulator.

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 Consists of a no. of piers which divide the total width of the canal into a
number of spans which are known as bays.
 Piers consists of the adjustable gates.

Main functions:

 Regulate water supply into canal.

 Controls entry of silt into canal.
 Completely exclude high flood entering into canal.
6. Silt regulation works
 Entry of silt into a canal, which takes off from a head work can be
reduced by constructing certain special works called silt control works.
 Mainly two types;
i. Silt excluders (in river)
ii. Silt ejectors (in canal)

Silt excluders

 Silt excluders are those works which are constructed on the bed of the
river, upstream of the head regulator.
 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.
 The top water is led towards the canal while the bottom water containing
high silt content is wasted.

Silt excluder

(please refer text books for more details about the components of weir)

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Headwork

Storage Headwork Diversion Headwork

(stores water during excess (used for diverting the supply from
supply and releases whenever river into the canal)
demand exceeds the supply)

Temporary spurs or Bunds

Permanent weirs and Barrages
(these are temporary constructions
(these are constructed for
and are constructed every year after
important works)
floods)

Weirs Barrages
Solid obstruction constructed across a Construction in which heading up is
river for raising water level and achieved by gates without any solid
diverting water into the canal. obstruction.
Stores water forr short supplies. Crest level is kept low

Non-gravity weirs
Gravity weris
(floor thickness small and uplift
(uplift pressure is resisted pressure is resisted mainly by bending
by the weight of the floor. action of the reinforced concrete
floor)

Vertical drop
Sloping wier Parabolic wier
weir

Concrete/ masonry sloping

weir

Dry stone/ rock-fill sloping

weir

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Types of weirs

Depending upon the criterion of the design of floors, weirs are classified as;

1. Gravity weirs
A gravity weir is the one in which the uplift pressure due to the seepage of
water below the floor is resisted entirely by the weight of the floor.
2. Non-gravity weirs
Floor thickness is kept relatively less, and the uplift pressure is largely resisted
by the bending action of the reinforced concrete floor.

Depending upon the material and certain design features, gravity weirs (or simply
weirs) can further be sub-divided into;

1. Vertical drop weir

2. Sloping weir
a. Masonry or concrete slope weir
b. Dry stone slope weir
3. Parabolic weir

1. Vertical drop weir

 Consists of a vertical drop wall or crest wall with or without crest gates.
 Cutoff piles are provided at upstream and downstream ends.
 Launching aprons are provided at the end of u/s and d/s floor for safety
against scouring action.
 Vertical drop weirs are suitable for any type of foundation.

2. Concrete or Masonry sloping weir

 Suitable for soft sandy foundations.
 Generally used where the difference in weir crest and downstream river
bed is limited to 3m.
 When water passes over such weir, hydraulic jump is formed on the
sloping glacis.

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3. Dry stone slope weir (rock-fill weir)

 Consists of a body wall (or weir wall).
 Upstream and downstream rock-fills laid in the form of glacis with few
intervening walls.
 Eg:- Okhla weir on Yamuna river

4. Parabolic weir
 Similar to the spillway section of a dam.
 Body wall for such a weir is designed as a low dam.
 A cistern is provided at the downstream to dissipate

Failure of Hydraulic Structures Founded on Pervious Foundations

Hydraulic structures may either be founded on an impervious solid rock foundation or

on a pervious foundation. On pervious foundations, it is subjected to seepage of water
beneath the structure in addition to other forces. The seeping water endangers the
stability of the structure and may cause failure by;

1. Piping (or Undermining)

2. Direct uplift

(1) Failure by piping or undermining:

 When the seepage water retains sufficient residual force at the emerging
downstream end of the work, it may lift up the soil particles.

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 This leads to increased porosity of the soil by progressive removal of soil from
beneath the foundation.
 The structure may ultimately subside into the hollow so formed, resulting in the
failure of the structure.
(2) Failure by direct uplift:
 The water seeping below the structure, exerts an uplift pressure on the floor of
the structure.
 If this pressure is not counterbalanced by the weight of the concrete/masonry
floor, the structure will fail by a rupture of a part of the floor.

Bligh’s theory

 The percolating water creeps along the contact surface of the base profile of
the structure with the subsoil. In other words, water creeps along the bottom
contour of the structure.
 The length of the path thus traversed by the percolating water is called the
length of creep or creep length.
 If HL the total head loss between u/s and d/s, and L is the length of creep, then
the loss of head per unit creep length is called the hydraulic gradient.
 As the water creeps from u/s to d/s end, the head loss occurs. This head loss is
assumed to be proportional to the creep length. (𝐻𝐿 𝛼 𝐿)
 Bligh’s theory does not differentiate between vertical and horizontal creep.

Consider the section shown in the figure.

Let, HL be the difference of water levels between upstream and downstream ends.
(no water is there on the d/s). 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 + 𝑑2 + 𝐿1 + 𝑑2 + 𝑑2 + 𝐿2 + 𝑑3 + 𝑑3
𝐿 = (𝐿1 + 𝐿2 ) + 2(𝑑1 + 𝑑2 + 𝑑3 )
𝐿 = 𝑏 + 2(𝑑1 + 𝑑2 + 𝑑3 )

Head loss per unit length, or hydraulic gradient

𝐻𝐿 𝐻𝐿
=[ ]=
𝑏 + 2(𝑑1 + 𝑑2 + 𝑑3 ) 𝐿

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𝐻𝐿 𝐻𝐿 𝐻𝐿
Head losses equal to ( × 2𝑑1 ) , ( × 2𝑑2 ) and ( × 2𝑑3 ); will occur respectively, in
𝐿 𝐿 𝐿
the planes of three vertical cutoffs. The hydraulic gradient line (H.G. line) can be
drawn as shown in the figure.

i. Safety against Piping or Undermining

According to Bligh, the safety against piping can be ensured by providing
sufficient creep length.
𝐿 = 𝐶. 𝐻𝐿
Where, C = Bligh’s creep coefficient of the soil.

Values of Bligh’s safe hydraulic gradient for different types of soil

Sl. Type of soil Value of Safe
No. C Hydraulic
gradient
1 Fine micaceous sand 15 1/15
2 Coarse grained sand 12 1/12
3 Sand mixed with boulder and gravel, and for 5 to 9 1/5 to 1/9
loam soil
4 Light sand and mud 8 1/8

ii. Safety against uplift pressure

 The ordinates of H.G. line above bottom of the floor represent the
residual uplift water head at each point.
 If h’ meters is the 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 t.

Uplift pressure caused by the seeping water = w h’

where w is the unit wt. of water

Downward pressure due to weight of the concrete floor of thickness t

= ( w . G c) t

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Where Gc is the specific gravity of floor material (concrete).

For equilibrium,

w h’ = ( w . Gc) t

h’ = Gc . t

subtracting t on both sides,

h’ - t = (Gc . t) – t

ℎ′ − 𝑡 ℎ
𝑡=( )= ( )
𝐺𝑐 − 1 𝐺𝑐 − 1

Where,

(h’-t = h) is the ordinate of the H.G. line above the top of the floor.

(Gc-1) = the submerged specific gravity of the floor material.

Allowing 33% increase in length as a factor of safety,

ℎ
𝑡 = 1.33 ( )
𝐺𝑐 − 1

(* for concrete, Gc value is usually taken as 2.4)

Limitations of Bligh’s theory

o It does not differentiate b/w the vertical creep and the horizontal creep and
gives the same weightage to both, actually, the vertical creep is more effective
than the horizontal creep.
o The head loss variation is assumed to be linear, while actual head loss is non-
linear.
o No distinction is made b/w head loss on the outer faces and that on the inner
faces of the sheet piles. Actually, outer faces are more effective than the inner
faces.
o It does not emphasise the importance of the d/s pile without which piping
failure occurs. It considers the d/s pile only as a component of the total creep
length and not as a controlling factor for the exit gradient and piping.
o Does not give any theoretical or practical method for the determination of the
safe gradient.
o Bligh does not consider the effect of the length of the intermediate pile.

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Design of vertical drop weir on Bligh’s theory

Even though this theory has now been replaced by modern Khosla’s theory, yet it is still
used at certain places and especially for minor works, owing to its simplicity.

 Design of Pucca-floor and Aprons

Total length of pucca floor,

𝐿 = 𝐶. 𝐻𝐿
Thickness of floor,
ℎ
𝑡 = 1.33 ( )
𝐺𝑐 − 1

Bligh has further given certain empirical formulas,

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 Head over the weir during maximum flood discharge (H)

𝑞 = 1.7𝐻3/2
𝑄
Where 𝑞 = 𝐿
Q = maximum flood discharge
L = length of weir

 Length of cutoff
The cutoffs may be provided as per the provisions of Khosla’s theory;

Depth of u/s sheet pile below u/s HFL = 1.5 𝑅

where R = Lacey’s regime scour depth.

1
𝑞2 3
𝑅 = 1.35 ( )
𝑓
where f is the silit factor taken as 1.
𝑄
𝑞=
𝐿
where Q is the discharge and L is the length of weir.
Since afflux is not known, the depth of d/s cut off cannot be determined. So it is
taken same as u/s pile.

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The balance floor length [i.e., (total length of floor) – (downstream floor(L2) + twice
cutoff length u/s and d/s (2d1 + 2d2) + length under the weir (B)] is calculated and
provided on the upstream side (i.e., L1).

Example:
A weir with a vertical drop has the following particulars:
Nature of bed: Coarse sand with Bligh’s C = 12
Flood discharge = 300 cumecs
Length of weir = 40 m
Height of weir above low water = 2.0 m
Height of falling shutter = 0.6 m
Top width of weir = 2.0 m
Bottom width of weir = 3.5 m
Design the length and thickness of aprons and draw the cross section of the weir.

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Use a similar cutoff of 3m depth below the weir floor.

The length of u/s talus may be kept as equal to ½ the length of d/s talus, i.e, 10 m.

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Khosla’s theory
Some of the structures designed on Bligh’s theory showed some serious troubles later.
Khosla studied the problems and arrived at these conclusions
1. The outer faces of the end sheet piles were much more effective than the inner
ones and the horizontal length of the floor.
2. The intermediate piles if smaller in length than the outer ones were ineffective.
3. Undermining of the floor started from the tail end.
4. It is absolutely essential to have a reasonably deep vertical cut-off at the d/s
end to prevent undermining.

Khosla’s theory can be summarised as below;

1) The seeping water does not creep along the bottom contour of pucca floor, but it
moves along a set of stream lines.
This steady seepage in vertical plane for a homogeneous soil can be expressed by
Laplacian eqn.
𝑑2 ∅ 𝑑2 ∅
+ =0
𝑑𝑥 2 𝑑𝑧 2
where ∅ = Flow potential = Kh, where K = coefficient of permeability and h =
residual head.

The above eqn. represents two sets of curves intersecting each other orthogonally.
One set of lines is called streamlines and the other set is called equipotential lines.
The resultant flow diagram is called a Flow net.

Stream lines:
It is the paths along which the water flows through the sub soil. The first stream
line follows the bottom contour of the structure as stated by Bligh. The remaining
streamlines follow smooth curves, transiting slowly from the outline of the
foundation to a semi-ellipse.
Equipotential lines:
Every stream line possesses a head while entering the soil and when it emerges at
the d/s end, its head is zero. At every intermediate point on its path, there is
certain residual head (h) still to be dissipated in the remaining length to be

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traversed to the d/s end. There will be points on the stream lines having the same
value of residual head. If such points are joined together, the curve obtained is
called an equipotential line.

2) The seepage water exerts a force at each point in the direction of flow and
tangential to the streamlines. This force has an upward component from the point
where the streamlines turns upward. For stability of soil particles, the upward
component should be counterbalanced by the submerged weight of soil grain.

This force has maximum disturbing effect at the d/s exit end, since the direction
of force is vertically upward. The disturbing force at any point is proportional to
the gradient of pressure of water at that point (dp/dl).
This gradient of pressure at the exit end is called the exit gradient. In order that
the soil particles to be stable, the exit gradient should be safe.

Critical exit gradient & Safe exit gradient:

The exit gradient is said to be critical, when the upward force on the grain is just
equal to the submerged weight of the grain at the exit.
When a factor of safety of 4 or 5 is used, the exit gradient can then be taken as
the safe exit gradient. i.e., an exit gradient equal to 1 /4 or 1 /5 of the critical exit
gradient is ensured to keep the structure safe against piping.

3) Undermining of the floor starts from the downstream end of the d/s pucca floor,
and if not checked, it travels upstream towards the weir wall. The undermining
starts only when the exit gradient is unsafe for the subsoil on which the weir is
founded.
It is therefore, absolutely necessary to have a reasonably deep vertical cutoff at
the downstream end of the d/s pucca floor to prevent undermining.
The depth of d/s vertical cutoff is governed by two considerations;
i) Maximum depth of scour
ii) Safe exit gradient.

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Khosla’s method of independent variables

 This method is used for the determination of pressure and exit gradient for
seepage below a hydraulic structure.
 For designing hydraulic structures on pervious foundations, Khosla has developed a
simple quick and accurate approach called the method of independent variables.
 In this method of independent variables, a complex profile like that of a weir is
broken into a number of simple profiles, each of which can be solved mathematically.
The simple profiles are;
1. A straight horizontal floor of negligible thickness with a sheet pile on the u/s
end or d/s end. [fig (a) and fig (b)]
2. A straight horizontal floor of negligible thickness with a sheet pile at some
intermediate point. [fig (d)]
3. A straight horizontal floor depressed below the bed but without any vertical
cutoff. [fig (c)]

fig (a) fig (b)

fig (c) fig (d)

 The Khosla’s curves can be used for the determination of percentage pressures at
the various key points.
The key points are;
o The junctions of the floor and the pile lines on either side.
o Bottom part of the pile line.
o Bottom corners in case of a depressed floor.
 The values obtained for the simple profiles should be corrected for obtaining the
values corresponding to the complex profile.

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The corrections are;

 Correction for the mutual interference of piles.
 Correction for the thickness of floor.
 Correction for the slope of the weir.

1. Correction for the mutual interference of piles

The correction C to be applied as percentage of head due to this effect is given
by;
𝐷 𝑑+𝐷
𝐶=√ ′ [ ]
𝑏 𝑏
Where;
b = total floor length
b’ = distance b/w the two pile lines
d = depth of influenced pile (or the pile being considered)
D = depth of influencing pile

This correction is positive for the points in the rear and negative for the points
forward in the direction of flow.
This equation does not apply to the effect of an outer pile on an intermediate
pile, if the intermediate pile is equal to or smaller than the outer pile and is at a
distance less than twice the length of the outer pile.

2. Correction for the thickness of the floor

In the simple profiles, the floor is assumed to have negligible thickness. The
percentage pressures calculated by Khosla’s theory pertain to the top levels of
the floor. While the actual junction points E and C are at the bottom of the
floor. Hence, the pressure at the actual points is calculated by assuming a
straight line pressure variation.

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The corrected pressures at E1 is less than the calculated pressure at E1’. Hence,
the correction applied for the point E1 shall be negative.
The pressure calculated at C1’ is less than the corrected pressure at C1, and
hence the correction to be applied at point C1 is positive.

3. Correction for the slope of the floor

The correction to be applied is taken as positive for downward slope and
negative for upward slope.

The correction factor given in the above table is to be multiplied by the

horizontal length of the slope and divided by the distance b/w the two pile lines
b/w which the sloping floor is located
The correction is applicable only to the key points of the pile line fixed at the
start or end of the slope.

Exit gradient

 The gradient of pressure of water at the exit end is called the exit gradient.
 The exit gradient is said to be critical exit gradient, when the upward force on
the soil grain is just equal to the submerged weight of the grain at the exit.
 When a factor of safety is used, the exit gradient can then be taken as safe
exit gradient.

For a standard form consisting of a floor length b with a vertical cutoff of depth d, the
exit gradient is given by
𝐻 1
𝐺𝐸 =
𝑑 𝜋√

Where,

1 + √1 + 𝛼 2
=
2
𝑏
𝛼=
𝑑
The exit gradient should lie within the safe limits given in the table below.

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Exit gradient is calculated from Plate 11.2 shown below.

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(refer text book for solutions worked out with equations instead of charts)

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Previous University Questions

1. Draw a neat sketch of layout of a Diversion headwork and explain the functions

of components.

2. What are the limitations of Bligh’s theory of design of impermeable foundation?

3. What are the assumptions of Khosla’s theory for design of impermeable

foundation?

4. Using Khosla’s theory, determine the pressure at C1 with interference

correction (Use Khosla’s curves).

5. What is the difference between weir and barrage?

6. Distinguish between Bligh’s theory and Khosla’s theory.

7. Two end sheet piles of length 6m and 8m are provided below an impervious floor

of 25m length. Total head created on the floor is 5m. Calculate the average

hydraulic gradient. Also find the uplift pressures at points 6, 12 and 18m from

the u/s end of the floor and find the thickness of the floor at these points

using Bligh’s creep theory. Take specific gravity of concrete as 2.25.

8. Explain the terms piping and uplift. How can this be controlled?

9. Explain the failure of hydraulic structures by sub surface flow.

10. Explain the different types of weir with neat sketches.

11. Write down the procedure for the design of a vertical drop weir.

12. Two sheet piles of unequal length are provided at the two ends of an impervious

floor of 15m length and 1m thick. Total head created on the floor is 3m. Using

Khosla’s method of independent variables, calculate the uplift pressure at the

key points, if the upstream pile is 3m deep and downstream pile is 5m deep.

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