measurement of discharge in channels & Design of lined canal

1. MEASUREMENT OF DISCHARGE IN
CHANNELS
&
DESIGN OF LINNED CANALS
Submitted by:
C014205, C014215,
C014224, C014238.

2. Measurement of Discharge in
Channels
Volume of water passing through a section of
a canal, channel or river in a unit time is
known as Discharge. It is worked out in
cum/sec. or cumec.

3. Discharge is determined usually for the
following objects:
• For forecasting the normal and highest
supplied from the irrigation projects
• For designing various hydraulic structures,
bridges, cross drainage works, regulators,
head works, canals etc.
• For distributing the irrigation water properly
among the farmers.

4. Selection of site for observations
• The site should be selected on the straight
reach of the river.
• At the site river should not be too wide or
shallow.
• In hill streams the site should accessible in all
seasons.
• The section line should at right angle to the
direction of flow.
• The river should be too tight at the site,
otherwise during floods velocities would be
excessively high and dangerous.

5. For taking discharge observations following
points should kept in mind:
• The site should be away from hydraulic
structures.
• The site should have straight reach not less
than 10 times the width of canal.
• At site channel should have silting or scour.
• Channel should have stable uniform section
at the site.

6. Methods of discharge measurements
Following are the common methods used for
determining the discharge of river, canal or
channel:
• Area velocity method
• Weir method
• Chemical method
• Venturi flume method

7. Area velocity method
• In this method mean velocity of river or canal and
cross-section area are determined. The discharge
is worked out by multiplying the mean velocity
with the cross-sectional area
• In practice as velocity is not uniform throughout
the section due to various resistive forces. The
cross-section of river or channel is divided into
different parts and the area of each is part is
determined. The mean velocity of each part is
worked out.

8. Cross-section of a river

9. Suppose the channel is divided into 8 parts
as shown in figure on previous slide.
Let A1, A2 …. A8, be the area of each
compartment and V1, V2, V3….. V8 be the mean
velocity of each compartment respectively.
Total discharge of river shall be determined
by the following:
Q = [A1V1+A2V2+A3V3………+A8V8]
In other words Q = ∑A.V

10. Measurement of Area
For the measurement of Area of each
compartment, width and depth at various
sections are required. The width and depth of
various compartments are determined as in
following slide

11. Width Measurement
For determining the width of open channel having
width upto 150m, a wire rope is stretched across
the channel and segments are marked on the wire
rope by means of pendants. Precautions are taken
in marking positions of pendants taking into
account the sag correction.
But when the width of the channel is more
than 150m, the segments are marked by means of
pivot point method which is based on the principle
of similar triangles properties.

15. If the width of river is more than 600m, than 2 pivot
points are fixed one on each bank as shown in figure and
the width is calculated accordingly.

16. Measurement of Depth
• By sounding rod: In this method a graduated
wooden rod 5-8 cm in diameter, enamelled steel
pipes of flat gauge is used. The graduation in
these sounding rods are done in m and cm. The
bottom of sounded rod is fitted with 10-15cm dia
disc, to prevents sinking in bed. The depth is
directly by lowering the sounding rod in the
channel. It is used for determining depth of
shallow channels. The depths measurements
should be taken on downstream side.

17. • By lead line: For deeper channel of channel
having high velocity, it is not possible to
determine depth by sounding rod, so help of
weighted lead line which is essentially consist
of copper cores covered with hemp and
weight fixed at one end is used. These lines do
not shrink, stretch, remain free from knots.
The lead weighed attached to it weighs 5-30kg
depending on velocity of flow. For starting
depth measurement, first a reference point is
marked on section line below which depth of
water is measured.

18. • By Echo sounder: This method is based on
principle of electricity. Echo sounder transmits
sound waves from surface of bed. When these
waves reflect back from the bed they are
arrested by the transmitter. The time of
transmission and time of reception is
determined by the echo sounder
arrangement. The depth of channel is worked
out from these records. This method is usually
adopted by ship for determining the depth of
channel. This method gives accurate results.

19. • By Haigh’s depth meter: This method is
based on boyle’s law. It consist of coiled tube
in cylinder container, figure on next slide
shows the Haigh’s depth meter. In this cylinder
the water is entrapped under pressure. The
compressing volume of air in cylinder, the
volume of water is proportional to the Depth
upto which depth meter is immersed. The
depth of river is worked out from the table
along with Haig’s meter.

20. Haig’s Depth Meter Curve for determining depth

21. • By Kelvin tube: It consist of glass tube
closed at one end open at another end. It is
placed in a iron cylinder with holes. Heavy
weight is fixed at the bottom of the tube due
to which container rest at the bed, when the
meter is lowered with wire rope. Depending
upon the depth of water which enter in the
tube, depth of channel is worked out. Using
formula:
D = 10.33 L1/L2

22. Measurement of velocity

23. Velocity distribution curve

24. The velocity of flow can be determined by
any of the following methods:
• By surface floats
• By velocity rods
• By double or sub surface floats
• By current meter

25. By Surface floats
• Surface floats are usually wooden disc 7.5 to
15 cm in dia weight and size of these float is
designed in such a way that it is least effected
by disturbing forces. Three wire ropes are
stretch 15m apart across the channel over
polls. The width of channel is divided into
equal parts by hanging pendants. Floats are
released in middle of each section and their
timing are recorded.

26. By surface floats

27. The surface velocity of the current shall be
calculated by the formula:
Velocity = Distance travelled by the float in m
time taken in sec
The velocity determined by the surface float is
not mean velocity for obtaining mean velocity it
is multiplied by coefficient whose value is 0.8.

28. By velocity rods
• Velocity rods are wooden or still tubes of 2.5 to 5
cm in dia the length of velocity rod is kept 0.94 D,
where D is = depth of channel. Weights are
provided at the bottom to maintain it vertical
position with 2.5 cm above the water. For
visibility flag is provided on top. The velocity rods
are released in the same way as the floats, and
time taken in travelling is noted and velocities are
calculated. This method gives direct mean
velocity of channel.

29. Velocity rod

30. By double or sub surface floats
• In this method two floats are employed. One
is known as surface float and other is sub
surface float. The weight of surface float is less
than sub surface float
• Both the floats are connected with each other
by means of cord. They are so adjusted that
sub surface float remains at 0.2 D distance
from bed of channel. These floats also give the
mean velocity of channel.

31. Double or sub surface floats

32. By current meter
Current meter is an instrument used for
determining the velocity of channel and the
rivers it mainly consist of wheel, contact breaker,
tail and the weights. A chart, known as rating
chart, is supplied with the current meter, which
give relation between the velocity of water and
number of revolutions of the wheel per minute.

33. Component part of a current meter

34. Method of using current meter

35. Usually for using the current meter two
methods are followed:
• One point method
• Two point method
In first method the current meter is
positioned at 0.6 D from the top and it gives the
mean velocity. And in second method it is
placed at two place i.e. at 0.8 D and 0.2 D the
mean of these two velocities is the mean
velocity of the channel.

36. The velocity of flow can be easily
calculated by the following rating formula of the
given current meter:
V = (a + b.N)
where, V = velocity of flow in m/sec
N = Number of revolutions made by
wheel per sec a and b are the
constants, whose value are
given by the manufacturer or
calculated by the experiments.

38. Weir Method
When the discharge of an irregular or regular
channel is moderate, sharp crested weir may be
used for measuring it. Practically it has been
observed that weirs with suppressed end
contractions give fairly good results. In this
method ceppoletti weir having trapezoidal
section with 1 : 4 side slopes is used for
determining the discharge, figure shows the
section of a ceppoletti weir.

39. Cippoletti weir

40. In this the increase in the sectional area
is equal to the area of contractions. Therefore,
ceppoletti weir acts as a rectangular weir
having suppressed end contractions.
The discharge is worked out from the
Fancis’s formula:
Q=1.85L*H3/2
where Q =discharge in m3/sec
L = length of the notch in m
H = Depth of water over notch still
in m

41. Venturi flume method
Venturi flume is a hydraulic structure
constructed in the channel for measurement of
discharge observation. Figure shows the venturi
flume. It mainly consist short channel reach for
restricting water way. This channel known
throat. The in size for normal section to the
throat and back to the normal section is done
gradually.
To be continued on next page

42. From pre-slide:
Due to decrease in the water way at the throat
the velocity and the discharge per unit
increases, due to which the reduction in depth
of the flow occurs at the throat. The discharge
of the channel is obtained by taking
observations of the reading on upstream and
inside the throat.

43. Venturi flume

44. The principle is the same as of venturi
meter, and the discharge of the channel is
determined by formula (refer to figure).
𝑄 =
𝑎1 𝑎2 2𝑔(ℎ1 − ℎ2)
𝑎1
2 −𝑎2
2
Where Q =discharge of channel
a1 =waterway of the channel
a2 =waterway at throat section
h1 =depth of water in upstream side
h2 =depth of water in venturi flume.

45. Design of Lined Canals
•The lined canals are not designed making use
of Lacey or Kennedy Theory because the
section is rigid
•Generally Manning’s equation is used in
design. To carry a certain discharge number of
channel sections may be designed with different
bed widths and side slopes.

46. •But it is clear that each section is not equally good
for the purpose.
•The section to be adopted should be economical
and at the same time it should be functionally
efficient
•It has been found that the most suitable cross-
section of a lined canal is a circular section with
sloping sides. That is, the bed is not flat but it is an
arc of a circle. This arc is tangential to the sloping
sides

48. Side Slopes
•The side slope is selected in such a way that it
nearly equals the angle of repose of the soil in
the subgrade.
•Care is taken to ensure that no earth pressure
is exerted on the back of the lining. From the
knowledge of hydraulics it is clear that the
section is economical when cross- sectional area
is maximum for minimum wetted perimeter.

49. •This condition is achieved when the centre of an arc lies
at FSL of the canal. This section is also efficient in the
sense that as the velocity of flow is higher silt carrying is
also higher than a wide and shallow section. Thus the
problem of silting is completely eliminated and
functioning is efficient
•It may be mentioned here that such section with
circular bed may be designed up to a discharge of 85
m3/sec. When the discharge is more than 85 m3/sec the
section best suited is one with a flat bed and sloping
sides with rounded corners

50. •This section is certainly better than trapezoidal
section, because it is more stable and economical to
construct.
The canal sections of this type with two standard
side slopes.
When r = 3.6 m or less, side slopes may be taken 1: 1
When r = 3.6 m side slopes may be taken 1.25: 1

51. Velocity of Flow:
Mean velocity of flow may be calculated using
Manning’s formula.
V = 1/N .R2/3. S1/2
where N is coefficient of rugosity and may be
taken as 0.018
S is the slope of bed and expressed in fall of
bed in m in 10,000 m length
For the given value of N formula may be
reduced to
V = 0.556 R2/3. S1/2

52. where V is velocity of flow in m/sec;
R is hydraulic mean radius in m; and
S is bed slope expressed in metres/metre length,
since (10,000)1/2 is merged in the constant 0.556

53. Velocity of Flow:
Mean velocity of flow may be calculated from
Manning’s formula for value of N = 0.018.
Then V = 0.556 R2/3. S1/2
It may be remembered that lined canal could be
given steep slope to achieve recommended
velocity of 2 m/sec and value of ‘f’ about 1.2.
Critical velocity ratio is not applicable to lined
canals. But to avoid possibility of sitting CVR
should be aimed at more than unity

54. Coefficient of Rugosity (N):
In general practice for lined canal average value
of N may be taken as 0.018. For different types
of linings the value of W varies. The values
given for straight channels in Indian Standard
4745 are given in Table 10.3. When the
alignment is not straight loss of head increases
and a small increase in the value of W may be
made to allow for additional loss of energy.

55. Freeboard:
Freeboard is measured from the (FSL) full
supply level to the top of lining. For lined canals
having less than 10 cumec discharge 0.6 m free
board is recommended. For bigger lined canals
freeboard not less than 0.75 m is generally
provided

56. Bank Widths:
The Indian Standard recommended following
values for bank widths for main and branch
canals:
Main canal in cutting and filling = 8.0 m
Branch canal in cutting = 6.5 m
Branch canal in filling, left bank = 6.5 m
Right bank = 5.0 m

59. The canal sections shown have trapezoidal shape. For
circular bottomed canal arrangement will be similar. It
may be noted that angle 0 shown in the section varies
with the side slopes adopted. 9 for side slope 1: 1 has 45°
value and for 1.25: 1 is 38° 40′ as shown in Fig. 10.3.
Maximum height of the spoil bank is limited to 6 m than
10.5 m filling each section shall be designed and tested
for stability. when the excavation is done by machinery. In
case work is accomplished manually maximum height is to
be restricted to 4 m only. Also when a canal section in
filling involves more

61. Problem:
Design an irrigation lined canal to carry a discharge of 34
m3 / sec. The mean diameter of the average soil particles
is 0.464 mm. Assume side slopes 1.25 : 1 and width zero.

63. • Suitable provision of dowel, roadway, catch water drain,
under drainage has to be made for every lined canal. Figure
10.4 shows three typical cross-sections of the lined canal in
which provision of various components is illustrated.

64. THANK YOU