Chapter 2 Canal Falls at Mnnit Allahabad .pptx

Civil Engineering Department
Motilal Nehru National Institute of Technology
Allahabad-211004
Motilal Nehru National Institute of Technology
Allahabad-211004
Water Resources Engineering-II (CE-18102)

Syllabus
WATER RESOURCESENGINEERING – II CE-18102 3-1-0 CREDIT:4
UNIT 1: Principles of Design of Canal Masonry Works: Types of Masonry work, Principle of design,
BLIGH’S Theory, Khosla’s theory for determination of pressure and exit gradient, Hydraulic jump. 7(L)
UNIT 2: Canal Regulation Works and Cross Drainage Works: Falls, Classification of falls, Design of
falls, Distributary head regulator and cross-regulator, Escape, Bed bars. Necessity and types, Aqua duct,
Syphon Aqua duct, Super passage, Canal syphon, Level crossing, Design of cross drainage work. 7(L)
UNIT 3: Canal Head Works: Canal Head Works, Functions, Location, Layout of Head work, Weir,
Canal head regulator, Design of Weirs on permeable foundation, silt control at headwork. 7(L)
UNIT 4: River Training Planning of Dams & Reservoirs: Objective, scope & classification of river &
river training, stages, Methods of River Training, bank protection. Selection of Dam sites, Investigation,
Estimation of storage capacity, Principle of Reservoir planning. Flood Routing, Reservoir loss, Reservoir
sedimentation. 7(L)
UNIT 5: Types of Dams and Their Characteristics: Gravity Dams, Forces acting, method of analysis,
Modes of failure and factors of safety, Elementary Profile of a gravity dam, Stability analysis, galleries.
Earth dam, Foundation, Materials, Criteria for safe design, typical sections, compaction of Rock fill dam.
spillway, spillway capacity, Types of spillway, Energy dissipation below spillway, Gates. 10(L)
UNIT 6: Water Power: Hydro-Electric Power: Assessment of potential, Classification of power plants,
Types of turbine, Powerhouse. 4(L)

References
1. Singh, Dr. Bharat - Fundamentals of Irrigation Engineering, Nem Chand and Bros.
2. Varshney, Dr. R.S., Gupta & Gupta - Theory and Design of Irrigation Structures Vol. I &
II., Nem Chand and Bros.
3. Punmia, Dr. B.C. and Pandey B.B. Lal, Irrigation and Water Power Engineering, Laxmi
Publications (Pvt.) Ltd.
4. Modi, P.N. – Irrigation Water Resources and Water Power Engineering, Standard Book
House
5. Bedient and Huber- Hydrology and Floodplain Analysis, Prentice Hall.
6. Asawa, G.L. – Irrigation and Water Resources Engineering, New Age International.
7. Walker, W.R. and Skogerboe, G.V. 1986. Surface irrigation theory and practice. Prentice‐
Hall, Inc.
8. Journal of Hydraulic Engineering, American Society of Civil Engineers (ASCE).
9. Journal of Irrigation and Drainage Engineering, American Society of Civil Engineers
(ASCE).

Introduction
Fig. 1. Canal Fall
A fall is a structure constructed across a channel to permit lowering down
of its water level and dissipate the surplus energy possessed by the falling
water which may otherwise scour the bed and banks of channel.
Canal Fall:
Designed Slope
Available Ground slope
Vertical drop

Necessity of a Canal Fall
The necessity of a canal fall arises because the available ground
slope usually exceeds the designed bed slope of a canal.
Disadvantages of a canal in embankment
 Higher construction and maintenance cost
 Higher seepage and percolation losses
 Adjacent area being flooded due to any possible breach in the
embankment.
 Difficulties in irrigation operation

Location of Fall
 Main canal:- Economy in the “cost of excavation and filling” versus cost
of Fall.
 Branch Canals and Distributary Channels:- The Falls are located with
consideration to command area.
Procedure – To fix FSL required at the head of the off taking channels
and outlets and mark them on the L-section of the canal.
The FSL of the canal can then be marked as to cover all the
commanded points, thereby deciding suitable locations for falls in
canal FSL, and hence in canal beds.
 The location of Falls may also be influenced by the possibility of
combining it with a bridge, regulator, or some other masonry work, since
such combinations often result in economy and better regulation.

Types of Falls
Different falls based on shapes, length and height of crest have been used since
the 19th century considering the large irrigation projects such as the Ganga, the
Kavery, and the Eastern and Western Yamuna canals.
 Ogee Fall
 Rapids
 Stepped Fall
 Trapezoidal Notch Falls
 Well Type falls or cylinder falls or syphon well drops
 Simple vertical drop type and Sarda type falls
 Straight glacis fall
 Montague type fall
 Inglis fall or Baffle fall

1. Ogee Fall
 The ogee fall was developed by Sir Cautley on Ganga Canal.
 The water was gradually led down by providing convex and concave curves.
 The shortcomings in the design are as follows:
 Heavy drawdown on the u/s side resulting in lower depth, higher velocities
and consequent bed erosion
 Due to smooth transition, kinetic energy of flow was not at all dissipated,
causing erosion of d/s beds and banks.
Fig.2. Ogee Fall

2. Rapid Fall
 In Western Yamuna canal, long rapids at slopes 1:10 to 1:20 (gently
sloping glacis) with boulder facings, were Provided.
 This worked quite satisfactorily, due to long reach and assured the
formation of the hydraulic jump.
 But were very expensive and so it was not considered for a long period.
Fig.3. Rapid Fall

Fig.4. Stepped Fall
3. Stepped Fall
 After Stepped fall, it was recognized that better dissipation of energy
could be achieved through vertical impact of falling jet of water on the
floor.
 As such, vertical falls with cistern were evolved.
 However, earlier types of vertical falls were not well developed and gave
trouble.
 These were superseded by trapezoidal notch, for sometime.
 But it lead the development of vertical drop type fall and glacis type fall.

4. Trapezoidal Notch Fall
 Developed by Reid (1894)
 Consists of a number of trapezoidal notches constructed in a high crested wall across
the channel with a smooth entrance and a flat circular lip projecting d/s from each
notch to spread out the falling jet.
 Notches could be designed to maintain the normal water depth in the u/s channel at
any two discharges, as the variation at intermediate value is small.
 These falls remained quite popular, till simpler, economical and better modern falls
were developed

4. Well Type Falls or cylinder falls or syphon well
 Downstream well is necessary for falls greater than 1.8 m and for discharges greater
than 0.29 cumec.
 Very useful for affecting larger drops for smaller discharges.
 Commonly used as tail escapes for small canals, or where high levelled smaller
drains do outfall into a low level bigger drain.

5. Simple vertical drop type and Sarda type falls
 It was installed to replace the notch fall on the Sarda canal system in U.P because of
its economy and simplicity.
 In that area, a thin layer of sandy clay overlaid a stratum of pure sand.
 The clear nappe leaving the crest is made to impinge into a cistern below.
 The cistern provides a water cushion and helps to dissipate the surplus energy of the
falling jet.

6. Straight Glacis Fall
 This is modern type of fall.
 Hydraulic jump is made to occur on the glacis, causing sufficient energy
dissipation.
 Suitable up to 60 cumecs discharge and 1.5 m drop.

7. Montague type of fall
 Energy dissipation on a straight glacis remains incomplete due to vertical
component of velocity remaining incomplete.
 An improvement in energy dissipation may be brought about in this type of fall by
replacing the straight glacis by a parabolic glacis, commonly known as Montague
profile.
 The curve glacis is difficult to construct.
 Generally not adopted in India.

8. Inglis fall or Baffle fall
 A straight glacis fall added with a baffle platform and a baffle wall, was developed
by Inglis and is called “Inglis fall” or “baffle fall”.
 Quite suitable for all discharges and for drops of more than 1.5 m

Design of Sarda type Fall
The design provisions of Sarda type fall consists of the following
components:
1. Crest wall
2. Cistern
3. Impervious floor
4. Downstream protection
5. Upstream approach

1. Design of Crest wall
(i) Length of crest wall
normally length of crest wall = bed width of channel
In case of future development of irrigation and increase in discharge capacity
Length of crest wall = bed width of channel + water depth

Rectangular Crest- Sarda Type Fall

Trapezoidal Crest- Sarda Type Fall

(ii) Shape of crest wall
For discharge < 14 cumec → crest wall rectangular in section with both u/s
and d/s faces vertical.
For discharge > 14 cumec → crest wall TRAPEZOIDAL in section with u/s
face having a slope of 1 in 3 and d/s face 1 in 8.

(ii) Shape of crest wall
Trapezoidal crest : Top width of crest,
Rectangular crest: Top width of crest,
Base width (meters) determined by batter: (H+d)/G
G is the specific gravity of the material of crest and may be taken as
2 for masonry crests.
d
H
B 
 55
.
0
d
B 55
.
0


(iii) Discharge Capacity
Trapezoidal crest :
Rectangular crest:
Where, L is the length of crest wall.
(iv) Crest level for free overfall condition:
Crest Level= Upstream FSL - H
6
/
1
2
/
3
)
/
(
2
45
.
0 B
H
LH
g
Q 
6
/
1
2
/
3
)
/
(
2
415
.
0 B
H
LH
g
Q 

(iv) Bed protection u/s of crest wall:
A few meters of brick pitching should be provided and laid on the upstream
bed sloping down towards the crest at 1 in 10 (for 2 to 4 m length) and a few
drain holes should be provided in the crest at this level to drain out the
upstream bed during the closure of the channel

2. Design of Cistern:
 The cistern element is that portion of the work in which the surplus
energy of water leaving the crest is dissipated and the subsequent turmoil
stilled before the water passes on to the lower level channels.
 It includes the sloping glacis, if any, the devices for securing the complete
formation of the hydraulic jump.
 U.P Irrigation Research Institute formula:
L
C H
E
L .
5

3
/
2
)
.
(
4
1
L
H
E
x 
(Neglecting the small velocity of approach head, E may be replaced by H)

3. Design of Impervious Floor:
Bligh’s theory – for small & medium falls
Khosla’s theory – for large falls
Maximum seepage head :
when there is water on the u/s side up to the top of crest wall and there
is no flow on the d/s side. Thus the Maximum seepage head = d
Out of the total length of the impervious floor a minimum length ld is to
be provided on the d/s of toe of the crest wall, which is
ld = 2(D1+1.2) + HL

 Draw HGL and determine thickness of floor
 Minimum thickness on u/s side = 0.3 m
 Minimum thickness on d/s side = 0.3-0.4 for small falls = 0.4-0.6 for large
fall
3. Design of Impervious Floor:
Cutoffs /curtain walls
(a) u/s cutoff Minimum depth = D1/3
D1 = u/s Full supply depth
(b) d/s cut off Minimum depth = D2/2
D2 = d/s Full supply depth

4. Downstream protection works:
i. Bed protection
ii. Downstream wings
iii. Side protection
iv. Energy dissipator

(i) Bed protection
 The bed of the channel needs to be protected for some length on the d/s
of the impervious floor.
 consists of dry brick pitching (i.e. brick laid dry without mortar), about
200 mm thick (one brick on edge laid over one flat brick) resting on 100
mm ballast.
 Table gives length of pitching and no. of curtain walls (depending upon
Head over crest), required to hold the pitching.
 It is provided horizontal up to end of masonry wings then sloping
downwards at 1 in 10.

(ii) Downstream wings (Wing walls)
 After the crest wall the wings are stepped down to the required level of
the downstream wings.
 The d/s wings are kept vertical for a length varying from 5 to 8 times
square root of (E. HL) from the crest.
 The wings are then flared or warped, that is their water face is
gradually inclined from vertical to a slope of 1.5:1 or 1:1.
 In the latter case, warping is continued in the side pitching till a slope of
1.5:1 is attained.

(iii) Side protection
After the warped wings, pitching protection is provided on the sides.
 The side pitching consists of either one brick on edge or 1.5 brick on
edge (i.e. one brick on edge placed over one flat brick) laid in cement
mortar.
 In the latter case, warping is continued in the side pitching till a slope
of 1.5:1 is attained.
 The side pitching is curtailed at an angle of 45o from the end of the bed
pitching in plan.
 A toe wall is provided between the bed pitching and the side pitching to
provide a firm support for the side pitching
 .The toe wall is usually 1 ½ brick (i.e. about 0.4 m) thick and of depth
equal to D2/2.

(iv) Energy Dissipators
 Energy dissipators are not provided for small discharges. However, for
large discharges, additional energy dissipators are provided.
 The energy dissipators consist of two rows of friction blocks in the
cistern and two rows of cube blocks on the impervious floor at its d/s
end.
 Both the friction blocks and cube blocks are staggered.

Size and position of friction blocks
a) Length of block = 2dc
b) Width of block = dc
c) Height of block = dc
d) Distance of first row of blocks from d/s toe of the
crest wall = 1.5dc
e) Spacing between two rows of block = dc
f) Spacing between blocks in same row = 2dc Where dc is
the critical depth.

Size and position of cube blocks
Length of block = 0.1 D2
Width of block = 0.1 D2
Height of block = 0.1 D2
Spacing between the two rows of blocks = 0.1 D2
Spacing between blocks in the same row = 0.1 D2 Where, D2 is
d/s full supply depth
One of the two rows of cube blocks is provided just at the d/s
end of the impervious floor and the other one is provided on
the u/s side at above noted spacing

 Design of u/s approach
 The u/s approach consist of wing walls (wings)
 For discharge <14 cumecs, the wing walls may be splayed
straight at an angle of 45 degree from the upstream edge of the
crest wall.
 For greater discharges, the wing walls are made segmental
(curve) from the u/s edge of crest wall, with radius equal to 5
to 6 times H, subtending an angle of 600 at the center and then
carried along straight lines tangential to segment.

Design a Sarda type Fall for the following data:
upstream / downstream
Full supply discharge, (cumec) = 45 / 45
Full Supply level (m) = 118.30 / 116.80
Full supply depth (m) = 1.8 / 1.8
Bed width (m) = 28 / 28
Bed level (m) = 116.50 / 115
Drop = 1.5 m
Design the floor on the basis of Bligh’s creep theory, taking coefficient of
creep =18
Design problem of Sarda type fall

Step 1, Calculation of H and d
Since Q > 14 cumec, trapezoidal crest wall will be provided.
L = Length of crest wall = bed width of channel = 28 m
H + d = D1 + drop in bed level, or
= u/s FSL – d/s CBL
= 118.30 – 115
= 3.30
6
/
1
2
/
3
)
/
(
2
45
.
0 B
H
LH
g
Q 

Therefore, we know that
Shape of crest wall
Trapezoidal crest : Top width of crest,
Hence, putting, H+ d= 3.30, we get B= 1.0 m
d
H
B 
 55
.
0
Now putting the value of B in the discharge capacity equation for
trapezoidal crest, we get H= 0.88 m

Step 1,
Calculation of H & d
Since, H = 0.88 m H + d = 3.30 m
Therefore, d = 3.30 – 0.88 d = 2.42 m
Height of crest wall above u/s bed (h)
h = D1 – H = 1.8 – 0.88 = 0.92 m

Step 2,
Design of Crest wall
u/s TEL = u/s FSL + velocity head
= 118.30 + 0.036 = 118.336 m
Crest level = u/s FSL – H
=118.30 – 0.88
= 117.42 (above d/s FSL 116.80 m, free flow)
E = u/s TEL – crest level
= 118.336 – 117.42 = 0.916 m
036
.
0
81
.
9
2
84
.
0
2
head
velocity
m/s
84
.
0
)
8
.
1
28
(
8
.
1
45
2
2







g
v
v
a
a

Step 3,
Design of Cistern
E = 0.916 m
HL = u/s FSL – d/s FSL
= 118.30 – 116.80
= 1.5 m
L
C H
E
L .
5

3
/
2
)
.
(
4
1
L
H
E
x 
RL of bed of cistern = RL of d/s bed – x = 115 – 0.31 = 114.69 m
m
31
.
0
)
5
.
1
916
.
0
(
4
1
m
86
.
5
5
.
1
916
.
0
5
3
/
2







Step 4,
Design of Impervious floor
Seepage head, Hs = d = 2.42 m
Bligh’s coefficient, C = 8
Therefore, required length of creep
= 8 × 2.42 = 19.36 m
u/s cutoff d1 = 1.0 m (Minm D1/3 = 1.8/3 = 0.6 m)
d/s cutoff d2 = 1.5 m (Minm D2/2 = 1.8/2 = 0.9 m)
The vertical length of creep = 2 (1.0 + 1.5) = 5 m
Hence, length of horizontal impervious floor = 19.36 – 5
= 14.36 m say 15 m

Step 4, Design of Impervious floor
Provide 15 m length of impervious floor.
Minimum length of impervious floor to be provided on the
d/s of the crest wall ld
ld = 2 (D1 +1.2) + HL
= 2 (1.80 + 1.2) + 1.5
= 7.5 m
Provide ld = 8 m. The balance of the length 15 – 8 = 7 m is
provided under and u/s of the crest wall.

Step 4,
Calculation of uplift pressure and thickness of floor
Total creep length = 15 + 2 (1.0 + 1.5)
(i) The uplift pressure under the u/s floor will be counter balanced by
the weight of water and hence no thickness is required.
However, provide a minimum thickness of 0.4
Provide ld = 8 m.
The balance of the length 15 – 8 = 7 m is provided under and u/s of
the crest wall.
(ii) For other points the minimum vertical ordinate between
Bligh’s HGL and the floor level gives the uplift pressure

Step 4,
Calculation of uplift pressure and thickness of floor
(ii) For other points the minimum vertical ordinate between Bligh’s
HGL and the floor level gives the uplift pressure.
The maximum unbalanced head under the d/s toe of the crest wall
m
32
.
1
1
24
.
2
64
.
1
1
-
G
head
unbalanced
required
Thickness
m
64
.
1
31
.
0
)
45
.
0
1
(
42
.
2
20
1
2
7
1
42
.
2















 


 x

Step 4, Design of Impervious floor
Provide 1.4 m thick cement concrete floor overlaid by 0.2 m
brick pitching.
Provide a minimum thickness of 0.6 m overlaid by 0.2m thick
brick pitching at the d/s end of the floor.
The thickness of the floor at intermediate points may be varied
as per requirements of uplift pressures.

Step 5, Design of d/s wings
Provide d/s wings vertical for a length of
Then,
the wings would be warped to 1:1 slope at a splay of 1 in 3.
Height of top of d/s wings above bed
= water depth + freeboard
= 1.8 + 0.5 = 2.3 m
Therefore, Horizontal projection of this on 1:1 slope = 2.3 m, with a
splay of 1 in 3, length of warped wings measured along the centre
line of the channel = 2.3 × 3 = 6.9 say 7 m
m
7
say
03
.
7
5
.
1
916
.
0
6
.
6 


L
H
E

Step 5,
Design of d/s bed protection - (a) Bed pitching
Provide about 200 mm thick dry brick pitching consisting of
one brick on edge laid over one flat brick resting on 100 mm
ballast.
From Table, for H = 0.88 m,
Length of bed pitching = 9.0 + 2 HL= 9.0 + 2×1.5 = 12 m
This should be provided horizontal up to the end of masonry
wings and then sloping downwards at 1 in 10.
Since, the warped wings commence from 1 m u/s of the d/s end
of the impervious floor,
the length of the horizontal pitching = 7-1 = 6m
the length of sloping pitching is therefore = 12- 6 = 6 m

Step 5, Design of d/s bed protection -
(b) Curtain wall
Thickness of curtain wall = 1 ½ brick (0.4 m)
From Table, for H = 0.88 m,
Depth of curtain wall = 0.75 m, say 1.0 m
Provide 0.4 m thick and 1 m deep curtain wall at the d/s end of
bed pitching.
(c) Side pitching
Provide about 200 mm thick side pitching consisting of one brick
on edge laid over one flat brick in cement mortar. The side
pitching should be warped from a slope of 1:1 to 1.5:1 and it
should be curtailed at an angle of 45o from the end of bed
pitching in plan.

(d) Toe wall
Thickness of toe wall = 1 ½ brick (0.4 m)
Depth of toe wall = d/s water depth / 2 = 0.9 m say 1m
Provide 0.4 m thick and 1 m deep toe wall between the bed
pitching and side pitching.
(e) Energy Dissipator: As discussed in the previous slide.

Step 5,
Design of u/s approach
Radius of segmental (or curved) portion of the u/s wings
= 5 to 6 times H
= 5 to 6 times 0.88
= 4.4 to 5.28 m
Thus provide u/s wings having segmental (or curved) portion of
radius 5 m and subtending an angle of 60o at the center from u/s
edge of the crest wall.
The wings should then be carried along the straight lines
tangential to the segment and embedded in the earthen banks of
the channel by a minimum of 1 m from the line of FSL.

DISTRIBUTARY HEAD REGULATORS
The distributary head regulator is the head work constructed at the upstream end
(i.e., the head) of a channel where it takes off from the main canal or a branch canal
or a major dis-tributary.
The distributary head regulator should be distinguished from the canal head
regulator which is provided at the canal headworks where a canal takes its supplies
from a river source. The distributary head regulator serves to:
(1) Divert and regulate the supplies into the distributary from the parent channel
(2) Control silt entering the distributary from the parent channel
(3) Metering or Measure the discharge entering the distributary.

1. Regulation, or control of the work:
In order to control the discharge, there should be abutments on either side of the
regulator crest with grooves in which either planks can be placed or gate can be
operated to control the discharge.
2. Silt control:
 The silt entry into the channel generally influence by the design of the
distributary/channel.
 The mechanics of sediment transport states that the silt deposition is more at the bed
and less at the surface. But it is observed that, eddies and turbulence is more liable for
the suspension of sediment particles.
 The difference of velocity is now responsible for the deposition of sediment particles
near the banks. Furthermore, the bottom water having lesser velocity than surface
velocity is more easily diverted and has a greater tendency to move into the offtake.
 Therefore, offtakes draw excessive amount of silts and disturbs the sediment
continuity and flow in any channel.
Functions of the Head works

The principles of silt control are:
(a) Concentrate silt charge at the bottom by providing smooth bed in the parent
channel and reducing the force of vertical eddies (which keep the silt in
suspension).
(b) Separate surface water from the bottom/bed water and divert the top water
into the offtake channel without disturbing the bottom and bed water.
(c) Side velocity should be augmented, so that cross waves can be decreased
from the center towards the sides.
The regular siltation in some of the channels is due to the defective head and
can be eliminated by designing a head in accordance with the above-mentioned
principles.

Kumar et al. (2021)- Journal of Irrigation and Drainage Engineering (ASCE)

Metering or measurement of discharge:
 The measurement of discharge is needed to understand the capacity
of the canal/headworks.
 To design any hydraulic structure, it is quite essential to know the
discharge capacity of that structure.
 A gauge indicating the water depth above the bed, is fixed 200
meters or so downstream of the head regulator, so that the
disturbance on account of the head is stilled out.
 By actual measurement of discharge at different gauge readings a
gauge discharge curve is prepared.
Gauge
Discharge

Discharge Measurement Methods:
1. Discharge Measurement in open channel
 Surface floats,
 Double Floats and velocity rods,
 Current meter,
 Pitot tube
2. Discharge measurement by the chemical method: The chemical method is a direct
method for the estimation of the discharge without observing the mean velocity and
cross sectional area.
Introduce a solution a solution of known strength of some chemical, usually salt,
uniformly over a cross-section at a known rate and determining the average
concentration of the chemical by taking out samples some distance downstream where
the solution to have mixed properly.

A simple square design of a head regulator:
 A suitable layout of wings is provided to connect the parent channel to the
offtaking channel.
 The crest level can be determined by the drowned weir formula.
 Grooves are provided in the wings for insertion or removal of logs.
 A bridge carries the bank or the road of the parent channel across the offtake.
 The impervious floor may be designed on the basis of the Bligh's theory for the
worst conditions which will occur when the parent channel is running full and
the offtake is closed.
)
(
2
2
/
3
L
d
L
d H
h
H
g
L
C
Q 


Venturihead
 It was designed during the Sarda canal system’ construction basically as a
measure of economy.
 Its main advantage was a flume throat of the regulator with suitable wing
connections to restore the full bed width of the offtake downstream.
 The theoretical maximum discharge of an open channel venturi-flume in which
critical flow is obtained, and no velocity of approach is given by Q=1.7 wd3/2;
where d is the depth of the water upstream of the throat and w = width of
throat.
 Venturi heads may be constructed for any angle of offtake from 60 degrees to
90 degrees for any bed width up to 6.0 m.
 The width of the throat should not be less than one third the bed width of the
off taking channel nor less than the width determined for the formula,
w= Q/1.2 d3/2, d being measured from F.S.L in parent channel to cill.
 The throat should be set back by (1.4w +0.60) m from the F.S.L line in the
parent channel.
 The side slope of the parent channel should be taken as 1/2:1.
The throat should be setback by (1.4w+0.60) m from the F.S.L line in the parent
channel. The side slope of the parent channel should be taken as ½:1.

Devices to control silt entry into the off taking channel
 It was observed that a raised cill is not enough by itself to control silt entry into the
offtake.
 If there is turbulence at the head, the top water may carry as much, or even more,
silt at the bottom water.
 Furthermore, the sediment particles would heap against the crest at an easy grade
making it still more convenient for the silt particles in the bottom layers to climb
over into the offtake. (Fig. 3)
King’s Vanes: The vanes are thin vertical walls curved at a radius of 8 to 10 m
provided in the parent channel to deflect the bottom silt laden water to an angle of
about 30 degree from the direction of flow.
The walls could be 7 to 8 cm thick made up of R.CC or steel plate.
The height is 0.75 to 0.25 times the depth of water and their spacing is 1.5 times the
height.
The vane should extend from 0.6 to 1.5 meter beyond a line drawn at an angle of
inclination of 2:1 to the axis of the parent channel from the downstream end of the
intake.
These vanes prove to be the quite effective unless the design of the head is so defective
as to cause violent turbulence and draw the silt out of the vanes.

Gibbs’s Groyne Wall:
 Gibb’s groyne wall is an extension of the downstream wing into the parent channel
in a smooth curve.
 The groyne wall is usually so provided that it divides the discharge of the parent
channel in the proportion of the discharge requirement of the offtake and that of the
channel downstream.
 It is extended upstream into the parent channel to cover ¾ to full of the throat
width of the offtake.
 If the groyne wall divides the discharge proportionately between the offtake and
the downstream parent channel, the silt is also divided proportionately.
Cantilever Skimming Platform :
 This type of device could be provided in ordinary or flumed head regulator.
 It consists of a slab cantilevered into the parent channel for some distance a little
below the cill level.
 The cantilevered slab cuts apart the top water and the bottom water and prevents
their intermixing.
 It also makes it very difficult for the silt particles at the bottom to make a ramp to
climb over into the offtake.

Fig. 3 (Left) heaped sediment (Right) King’s Vanes
Fig.4 (Left) Gibb’s Groyne wall (Right) Cantilever skimming platform

Sharma’s Silt Selective Head:
 This kind of head has been designed by Shri K.R. Sharma of Punjab Irrigation to deal
with regulation, silt control as well as metering.
 The structure can be divided into three parts: the approach chamber, the regulator and
the flume.
 The approach chamber selects silt.
 The supply is regulated upstream of the weir flume which meters the supply.

C-OTHER WORKS
Cross Regulators
 A regulator which regulates the water level in a channel and controls the
quantity of water passing below it. A ‘head regulator provided at the head of
each channel controls the supplies entering the channel.
 A cross regulator may be required in the main channel downstream of an off
taking channel or escape. It is utilized to head up the water level upstream of
itself when necessary to enable the offtaking channel to take its required supply
even though the main channel is running with low supplies.

Functions of Cross Regulator
 To raise the water level in the parent channel to feed the off-taking
channel.
 To shut the supply in the parent channel on its d/s for repairs.
 Usually a bridge is combined with cross regulator for communication.
 Helps in absorbing fluctuations in the canal system and in preventing
the possibilities of breaches in the tail reaches.
 Useful for the effective regulation of entire canal system.

Design Steps
 Vent way of The Regulator
 Fixing The Vent way by Drowning Method
 Downstream Of Regulator
 Roadway
 Pier
 Shutters
 Abutments
 Wing Walls
 Solid Apron For The Regulator
 Revetments

Design steps
1. Discharge capacity:
The discharge formula applicable in this case will be Q=CLH3/2, where H is the
upstream water depth above cill + velocity of approach head.
The value of C may however be less than the theoretical 1.71, if the jump is
drowned out.
Note: It is economical to flume the channel at the site of a regulator from the
sides thereby reducing the width of the floor required. The cill may also be raised
a little above the bed, reducing the height of the gates. A sloping glacis and
cistern should be provided downstream of cill (If cill is provided)
Drowning Ratio F.P.S Metric system

The following empirical guidelines for the design of piers, segmental arches, and
abutments have been provided by the Bligh for the design of ordinary bridges
supported on segmental arches and will be quite useful in setting preliminary
dimensions.
Width of piers: If S is the span in meters, this should be approximately equal to
And should vary from 1/3 rd to 1/5th S, the smaller ratio being used for larger spans
and larger ratio for smaller spans.
Thickness of arch: If R is the radius of intrados of the segmental arch, the thickness
at crown should be kept
Section of Abutment: The following rule known as Trautwine’s rule gives the
section of abutment-
Width at springing in meters = 0.60+1/5 radius of arch+ rise of arch/10
S
55
.
0
R
25
.
0

 Canal Escape
 A canal escape is an essential structure designed to manage the controlled
release of surplus water from the canal as needed.
 Basically, acting as a safety valve for the canal system, it plays a vital role in
safeguarding the canal from potential harm caused by an excess water supply.
 This surplus may arise from errors in water release at the head regulator or
unexpected factors such as heavy rainfall leading to a sudden, irregular demand
for water.
 The canal escape becomes instrumental in stopping potential damage to the
canal banks, mitigating the risk of failure due to overtopping or hazardous leaks
caused by an excess water supply.

What is Canal Escape?
 Canal escape refers to a designed system or channel within a canal network
that allows excess water to be safely diverted or released to prevent flooding
and maintain the stability of the canal.
 This escape route is crucial in managing water levels during periods of high
flow, heavy rainfall, or other conditions that could lead to an overflow. By
providing a controlled path for surplus water, canal escape systems help
prevent damage to the canal infrastructure and surrounding areas, ensuring
effective water management in irrigation and drainage systems.

A canal escape is an engineered structure positioned along an irrigation canal to
facilitate the controlled discharge of water. Based on its specific function, there are
three distinct types of escapes:
Canal Scouring Escape
A canal scouring escape is a specialized structure strategically placed along an
irrigation canal to manage and regulate the controlled release of water for scouring
purposes. Its primary function is to facilitate the removal of sediment and debris that
may accumulate within the canal, ensuring optimal water flow and preventing
potential blockages. This type of escape plays a critical role in maintaining the
efficiency and longevity of the canal system by preventing the buildup of materials
that could hinder water distribution.

Surplus Escape
A surplus escape, an integral component of canal infrastructure, is designed to
address situations where there is an excess supply of water within the
irrigation canal. This escape serves as a controlled outlet, allowing the
discharge of surplus water to prevent overtopping of canal banks or potential
damage. By providing a regulated release mechanism, the surplus escape acts
as a safety valve for the canal system, ensuring its resilience in the face of
unexpected variations in water supply.
Tail Escape
Tail escapes are essential structures located at the downstream end of an
irrigation canal, serving as the final point of controlled water release.
Positioned to prevent overflows and potential damage to the canal banks, tail
escapes efficiently manage the water flow as it exits the canal. This type of
escape ensures that the downstream section of the canal remains protected
from the risk of bank overflow, contributing to the overall stability and
functionality of the irrigation system.

Advantages of Canal Escape
The advantages of Canal Escape are:
•Regulates water levels to prevent canal overflows.
•Safeguards against potential damage caused by excess water.
•Facilitates controlled discharge for canal maintenance.
•Acts as a safety valve, protecting the canal system.
•Preserves downstream sections from bank overflow risks.

Bed Bars
 Bed bars are constructed in order to serve as a permanent mark of reference to indicate
the correct alignment and theoretical bed level of a channel.
 Bed bars should be constructed at intervals of 200 m in small channels and 800 m or 1 km
in large channels.
 Bed bars for small channels basically consist of a masonry wall partly extending into the
bed and flush with it and partly flush with the bank.
 The foundation should be deep and substantial as shown in Figure below.
 For large branches and canals, a bed bar consists of a masonry or concrete block with its
upper face flush with theoretical bed levels and its depth and cross section substantial
enough to withstand action of water. A typical size is 1.0×1.2×1.5 m deep. The centre line
of the canal is indicated by the centre of the block.
 The bed bars enable a continuous watch on the behavior of the canal, particularly as to
whether it is silting or scouring in a given reach.

Cross Drainage Works
 A cross drainage work is a structure carrying the discharge of a natural stream
across a canal intercepting the stream.
 When a canal is to be taken to the watershed, it crosses a number of natural streams
in the distance between the reservoir to the watershed.
A cross drainage work is generally a very costly item, and should be avoided as far as
possible by (i) diverting one stream into another, or (ii) changing the alignment of the
canal so that it crosses below the junction of two streams.

In an irrigation project, when the network of main canals, branch canals,
distributaries, etc. are provided, then these canals may have to cross the natural
drainages like rivers, streams, nallahs, etc. at different points within the commanded
area of the project. The crossing of canals with such obstacle cannot be avoided.
Therefore, suitable structures must be constructed at the crossing point for the easy
flow of water of the canal and drainage in the respective directions. These structures
are called cross – drainage works. Thus, a cross – drainage work is a structure
carrying the discharge from a natural stream across a canal intercepting the stream.

Necessity of Cross Drainage Works:
 The cross drainage work is required to dispose of the drainage water so that the
canal supply remains uninterrupted.
 The canal at the cross – drainage work is generally taken either over or below the
drainage. However, it can also be at the same level as the drainage. As we know
that, canals are usually aligned on the watershed so that there are no drainage
crossings. However, it is not possible to avoid the drainages in the initial reach of
a main canal because it takes off from a diversion head works (or storage works)
located on a river which is a valley.
 The canal, therefore, requires a certain distance before it can mount the watershed
(or ridge). In this initial reach, the canal is usually a contour canal and it intercepts
a number of natural drainages flowing from the watershed to the river.
 After the canal has mounted the watershed, no cross-drainage work will normally
be required, because all the drainage originate from the watershed and flow away
from it.
 However, in some cases, it may be necessary for the canal to leave the watershed
and flow away from it. In that case, the canal intercepts the drainages which carry
the water of the pocket between the canal and the watershed and hence the cross-
drainage works are required.

Types of Cross-Drainages Works:
Depending upon the relative positions of the canal and the drainage, the cross-
drainage works may be classified into 3 categories as:
(1) By passing the irrigation canal over the drainage. This is achieved through (i) an
aqueduct, or (ii) a syphon aqueduct.
(2) By passing the drainage over the canal. This is achieved through (i) a super
passage or (ii) a syphon.
(3) By passing the drainage through the canal so that the drainage and irrigation
water are intermixed. This is affected by (i) a level crossing, or (ii) an inlet and
outlet.

1. Irrigation Canal over the drainage (a) Aqueduct: An aqueduct is a structure in
which the canal flows over the drainage and the flow of the drainage below is open
channel flow. An aqueduct is similar to an ordinary road bridge (or railway bridge)
across drainage, but in this case, the canal is taken over the drainage instead of a
road (or a railway). A canal trough is to be constructed in which the canal water
flows from upstream to downstream. This canal trough is to be rested on a number
of piers. An aqueduct is provided when the canal bed level is higher than the H.F.L.
of the drainage.
Fig. 1: Aqueduct

In this C.D work, the canal is carried over the natural drain. The advantage of such
arrangement is that the canal, running perennially, is above the ground and is open
to inspection.
Also the damage done by the floods is rare.
This is basically constructed when the natural drain is very big in comparison to the
section of the canal.
In aqueduct, the HFL of the natural drain is much below the trough of the canal, so
that the drainage water flows freely under the gravity.

(b) Syphon aqueduct: In syphon aqueduct, the HFL of the natural drain is much
higher above the canal bed, and the water runs under syphonic action through the
aqueduct barrels.
In a syphon aqueduct also the canal is taken over the drainage, but the flow in the
drainage is pipe flow (i.e. the drainage water flows under syphonic action and
there is no atmospheric pressure in the drainage). A syphon aqueduct is
constructed when the H.F.L. of the drainage is higher than the canal bed level.
When sufficient level difference is not available between the canal bed and the
H.F.L. of the drainage to pass the drainage water, the bed of the drainage may be
depressed below its normal bed level. Syphon aqueducts are preferred than
Aqueducts, though costlier.
Fig.2. Syphon Aqueduct

2. Irrigation Canal below the drainage
(a) Super passage: In a super passage, the canal is taken below the drainage and
the flow in the channel is open channel flow. A super passage is thus reverse of an
aqueduct. A super passage is required when canal F.S.L is below the drainage bed
level. In this case, the drainage water is taken in a trough supported over the piers
constructed on the canal bed. The water in the canal flows under gravity and
possess the atmospheric pressure.
Fig.3. Super passage

(b) Canal syphon: A canal syphon (or Simply a syphon) is a structure in which
the canal is taken below the drainage and the canal water flows under symphonic
action and there is no presence of atmospheric pressure in the canal. It is thus the
reverse of a syphon aqueduct. A canal syphon is constructed when the F.S.L. of
the canal is above the drainage bed level. Because some loss of head invariably
occurs when the canal flows through the barrel of the canal syphon, the command
of the canal is reduced. Moreover, there may be silting problem in the barrel. As
far as possible, a canal Syphon should be avoided.
Fig.4. Canal Syphon

3. Canal at the same level as drainage
(a) Level crossing: A level crossing is provided when the canal and the drainage are
practically at the same level. In a level crossing, the drainage water is admitted into the
canal at one bank and is taken out at the opposite bank. A level crossing usually
consists of a crest wall provided across the drainage on the upstream of the junction
with its crest level at the F.S.L. of the canal. The drainage water passes over the crest
and enters the canal whenever the water level in the drainage rises above the F.S.L. of
the canal. There is a drainage regulator on the drainage at the d/s or the junction and a
cross-regulator on the canal at the d/s of the junction for regulating the outflows.

(b) Inlet and outlet: An inlet-outlet structure is provided when the drainage and the
canal are almost at the same level, and the discharge in the drainage is small. The
drainage water is admitted into the canal at a suitable site where the drainage bed is at
the F.S.L. of the Canal. The excess water is discharged out the canal through an outlet
provided on the canal at some distance downstream of junction. There are many
disadvantages in use of inlet and outlet structure, because the drainage may pollute
canal water and also the bank erosion may take place causing the deterioration of the
canal structure so that maintenance costs are high. Hence, this type of structure is rarely
constructed.
Fig.6. Inlet and Outlet

Design Considerations for Cross Drainage Works:
1. Determination of Maximum Flood Discharge: The high flood discharge for
smaller drains may be worked out by using empirical formulas; and for large
drains, other reliable methods Such as Hydrograph analysis, Rational formula,
etc. may be used.
2. Fixing the Waterway Requirements for Aqueducts and Syphon-Aqueducts:
An approximate value of required waterway for the drain may be obtained by
using the Lacey's equation, given by
where P is the wetted perimeter in metres and Q is the Total discharge in cumecs.
Q
P 75
.
4

For wide drains, the wetted perimeter may be approximately taken equal to the
width of the drain and hence, equal to waterway required.

3. Afflux and Head Loss through Syphon Barrels: The velocity through syphon
barrels is limited to a scouring value of about 2 to 3 m/sec. A higher velocity may
cause quick abrasion of the barrel surfaces by rolling grit, etc. and shall definitely
result in higher amount of afflux on the upstream side of the syphon or syphon-
aqueduct, and thus, requiring higher and longer marginal banks.
The head loss (h) through syphon barrels and the velocity (V) through them are
generally related by Unwin's formula as
HL = [1 + F1 + F2. L/R] V2/2g, where
L= Length of the barrel. R= Hydraulic mean radius of the barrel. V= Velocity of
flow through the barrel. Velocity of approach and is often neglected. Coefficient
of head loss at entry = 0.505 for unshaped mouth = 0.08 for bell mouth is a
coefficient such that the loss of head through the barrel due to surface friction

Chapter 2 Canal Falls at Mnnit Allahabad .pptx

Chapter 2 Canal Falls at Mnnit Allahabad .pptx

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