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Title: CHAPTER SIX:


1
CHAPTER SIX
  • DESIGN OF CHANNELS AND IRRIGATION STRUCTURES

2
6.1 DESIGN OF CHANNELS FOR STEADY UNIFORM FLOW
  • Channels are very important in Engineering
    projects especially in Irrigation and, Drainage.
  • Channels used for irrigation are normally called
    canals
  • Channels used for drainage are normally called
    drains.

3
6.1.1 ESTIMATION OF CANAL DESIGN FLOWS (Q)
  • For Irrigation Canals, Design Flows are
    estimated Using the Peak Gross Irrigation
    Requirement
  • For Example, in a Location with the Peak Gross
    Irrigation Requirement of 7.69 mm/day.
  • Peak flow (Q) 7.69/1000 m x 10000 x
    1/3600 x 1/24 x 1000
  • 0.89 L/s/ha
  • For a canal serving an area of 1000 ha, canal
    design flow is then 890 L/s or 0.89 m 3 /s.
  • Typically, for humid areas, magnitude of
    discharges are in the range of 0.5 to 1.0
    L/s/ha.

4
6.1.2        Dimensions of Channels and
Definitions
5
Definitions
  • a) Freeboard Vertical distance between the
    highest water level anticipated in the design and
    the top of the retaining banks. It is a safety
    factor to prevent the overtopping of structures.
  •  
  • b) Side Slope (Z) The ratio of the
    horizontal to vertical distance of the sides of
    the channel. Z e/d e/D

6
Table 6.1 Maximum Canal Side Slopes (Z)
Sand, Soft Clay 3 1 (Horizontal Vertical)
Sandy Clay, Silt Loam, Sandy Loam 21
Fine Clay, Clay Loam 1.51
Heavy Clay 11
Stiff Clay with Concrete Lining 0.5 to 11
Lined Canals 1.51
7
  • 6.1.3 Estimation of Velocity in Channels
  • The most prominent Equation used in the design is
    the Manning formula described in 6.1.3. Values
    of Manning's n can be found in standard texts
    (See Hudson's Field Engineering).
  • 6.1.4 Design of Channels
  • Design of open channels can be sub-divided into
    2
  • a) For Non-Erodible Channels (lined)
  • b) Erodible Channels carrying clean water

8
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9
Design of Non-Erodible Channels
When a channel conveying clear water is to be
lined, or the earth used for its construction is
non-erodible in the normal range of canal
velocities, Manning's equation is used. We are
not interested about maximum velocity in design.
Manning's equation is  
Q and S are basic requirements of canal
determined from crop water needs. The slope of
the channels follows the natural channel.
Manning's n can also be got from Tables or
estimated using the Strickler equation n
0.038 d1/6 , d is the particle size
diameter (m)
10
Design of Non-Erodible Channels Contd.
  • LHS of equation (1) can be calculated in terms of
    A R2/3 termed section factor. For a
    trapezoidal section
  • A b d Z d2 P b 2 d (1
    Z)1/2
  • The value of Z is decided (see Table 6.1) and the
    value of b is chosen based on the material for
    the construction of the channel.
  • The only unknown d is obtained by trial and
    error to contain the design flow. Check flow
    velocity and add freeboard.
  •  

11
Example 6.1
  • Design a Non-Erodible Channel to convey 10
    m3/s flow, the slope is 0.00015 and the mean
    particle diameter of the soil is 5 mm. The side
    slope is 2 1.
  • Solution Q 1/n AR 2/3 S 1/2 .. (1)
  • With particle diameter, d being 5 mm, Using
    Strickler Equation, n 0.038 d 1/6
  • 0.038 x 0.005 1/6
    0.016

12
Solution of Example Contd.
Z 2. Choose a value of 1.5 m for 'b For
a trapezoidal channel, A b d Z d2
1.5 d 2 d2 P b 2 d (Z2 1)1/2
1.5 2 d 51/2 1.5 4.5 d Try
different values of d to contain the design flow
of 10 m3/s  
13
Soln of Example 6.1 Contd.
d(m) A(m2 ) P(m) R(m) R2/3
Q(m3/s) Comment 2.0 11.0
10.5 1.05 1.03 8.74 Small
flow 2.5 16.25 12.75 1.27 1.18
14.71 Too big 2.2 12.98
11.40 1.14 1.09 10.90 slightly
big 2.1 11.97 10.95 1.09 1.06 9.78
slightly small 2.13 12.27 11.09 1.11 1.07
10.11 O.K. The design parameters
are then d 2.13 m and b 1.5 m
Check Velocity Velocity Q/A 10/12.27
0.81 m/s Note For earth channels, it is
advisable that Velocity should be above 0.8 m/s
to inhibit weed growth but this may be
impracticable for small channels. Assuming
freeboard of 0.2 d ie. 0.43 m, Final design
parameters are D 2.5 m
and b 1.51 m
14
Final Design Diagram
T 11.5 m
D 2.5 m
Z 21
d 2.13 m
b 1.5 m
T b 2 Z d 1.5 2 x2 x 2.5 11.5 m
15
Design of Erodible Channels Carrying Clean Water
  • The problem here is to find the velocity at which
    scour is initiated and to keep safely below it.
    Different procedures and thresholds are involved
    including maximum permissible velocity and
    tractive force criteria.
  • Maximum Permissible Velocities The maximum
    permissible velocities for different earth
    materials can be found in text books e.g.
    Hudson's Field Engineering, Table 8.2.

16
Procedure For Design
  • i) Determine the maximum permissible velocity
    from tables.
  • ii) With the permissible velocity equal to
    Q/A, determine A.
  • iii) With permissible velocity 1/n S1/2 R2/3
  • Slope, s and n are normally given.
  • iv) R A/P , so determine P as A/R
  • v) Then A b d Z d and
  • P b 2 d (Z2 1)1/2 ,
  • Solve and obtain values of b and d

17
Example 6.2
  • From previous example, design the channel using
    the maximum permissible velocity method.
  • Solution Given Q 10 m3 /s , Slope
    0.00015 , n 0.016
  • , Z 2 1
  • i) From permissible velocity table, velocity
    0.75 m/s
  • A Q/V 10/0.75 13.33 m
  • iv) P A/R 13.33/0.97 13.74 m
  • v) A b d Z d2 b d 2 d2
  • P b 2 d (Z2 1)1/2 b
    2 d 51/2 b 4.5 d
  • ie. b d 2 d2 13.33 m 2
    ........(1)
  • b 4.5 d 13.74 m
    ........ (2)

From previo
18
Solution of Equation 6.2 Contd.
From (2), b 13.74 - 4.5 d
.......(3) Substitute (3) into (1), (13.74 -
4.5 d)d 2 d2 13.33
13.74 d - 4.5 d2 2 d
13.33
13.74 d - 2.5 d2 13.33 ie. 2.5 d2 -
13.74 d 13.33 0 Recall the quadratic
equation formula
d 1.26 m is more
practicable From (3), b 13.74 - (4.5 x
1.26) 8.07 m Adding 20 freeboard, Final
Dimensions are depth 1.5 m and width 8.07
m

19
6.1.5 Classification of Canals Based on Capacity
  • Canals can be classified as
  • (a)   Main Canal It is the principal channel of
    a canal system taking off from the headworks or a
    reservoir or tail of a feeder.
  • It is a large capacity channel and usually there
    is no direct irrigation from it.
  • Small capacity ditch distributaries running
    parallel to the canal are taken off from the main
    canal to irrigate adjoining areas.
  • Main canals deliver supply to branch canal and
    main distributaries.

20
Canals Contd.
  • (b)   Branch or Secondary Canal
  • Branch canals take their supply from the main
    canal and convey to the distributaries.
  • Very little direct irrigation is done from the
    branch canals.
  • Sub-branch is a canal, which takes off from the
    branch canal but has capacity higher than a
    distributary.

21
Canals Contd.
  • (c)   Major Distributary
  • It is a distributing channel, which may take off
    from a main canal, branch canal or sub-branch and
    has discharge capacity less than that of a branch
    canal.
  • It supplies water to another distributary.
  • Distributaries and minors take off from it.
  • Irrigation is done through outlets fixed
    along it.

22
Canals Contd.
  • (d)   Distributary
  • It is a channel receiving supply from branch
    canal or major distributary and has discharge
    less than that of major distributary.
  • Minors take off from it, besides irrigation is
    done from it through outlets.

23
IRRIGATION STRUCTURES
  • Structures are widely used in Irrigation, water
    conservation, flood alleviation, river works
    where water level and discharge regulation are
    required.
  • These are hydraulic structures that are used to
    regulate, measure, and/or transport water in open
    channels.
  • These structures are called control structures
    when there is a fixed relationship between the
    water surface elevation upstream or downstream of
    the structure and the flow rate through the
    structure.
  • Hydraulic structures can be grouped into three
    categories

24
IRRIGATION STRUCTURES
25
Hydraulic Structures Contd.
  • (i)  Flow measuring structures, such as weirs
  • (ii) Regulation structures such as gates and
  • (iii) Discharge structures such as culverts.

26
Weirs
  • Weirs Weirs are elevated structures in open
    channels that are used to measure flow and/or
    control outflow elevations from basins and
    channels.
  • There are two types of weirs in common use
  • Sharp-crested weirs and the broad-crested weirs.
  • The sharp-crested weirs are commonly used in
    irrigation practice

27
Sharp-Crested Weirs
  • Sharp-crested or thin plate, weirs consist of a
    plastic or metal plate that is set vertically
    across the width of the channel.
  • The main types of sharp-crested weirs are
    rectangular, V-notches and the Cipolletti or the
    Trapezoidal weirs.
  • The amount of discharge flowing through the
    opening is non-linearly related to the width of
    the opening and the depth of the water level in
    the approach section above the height of the weir
    crest.

28
Sharp Crested Weirs Contd.
  • Weirs can be classified as being contracted or
    suppressed depending on whether or not the nappe
    is constrained by the edges of the channel.
  • If the nappe is open to the atmosphere at the
    edges, it is said to be contracted because the
    flow contracts as it passes through the flow
    section and the width of the nappe is slightly
    less than the width of the weir crest (see
    figure).
  • If the sides of the channel are also the sides of
    the weir opening, the streamlines of flow are
    parallel to the walls of the channel and there is
    no contraction of flow.

29
Figure 6.2 Rectangular Weirs
(b) Unsuppressed Weir (Contracted)
(a) Suppressed Weir
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31
Sharp Crested Weirs Contd.
  • In this case, the weir is said to be suppressed.
    Some type of air vent must be installed in a
    suppressed weir so air at atmospheric pressure is
    free to circulate beneath the nappe. (See Figure
    6.2 for suppressed and unsuppressed weirs).

32
Sharp Crested Weirs Contd.
The discharge, Q (m3/s) over a rectangular
suppressed weir can be derived as

Where Cd is the discharge coefficient, b is the
width of the weir crest, m (see Figure 6.2 above)
and H is the head of water (m) above weir crest.
According to Rouse (1946) and Blevins( 1984),

..(2) Where Hw is the height of the
crest of the weir above the bottom of the
channel.
33
Weirs Contd
This equation is valid when H/Hw lt5, and is
approximated up to H/Hw 10. If H/Hw lt 0.4,
Cd can be approximated as 0. 62 and equation (1)
reduces to Q 1.83 b H1.5
. (3)
This equation is normally used to compute flow
over a rectangular suppressed weir over the usual
operating range. It is recommended that the
upstream head, H be measured between 4H and 5H
upstream of the weir. For the unsuppressed
(contracted) weir, the air beneath the nappe is
in contact with the atmosphere and venting is not
necessary. The effect of side contractions is to
reduce the effective width of the nappe by 0.1 H
and that flow rate over the weir, Q is estimated
as Q 1.83 (b 0.2 H) H1.5
(4) This equation is acceptable as long as b is
longer than 3 H
34
Cipoletti Weir
  • A type of contracted weir which is related to the
    rectangular sharp-crested weir is the Cipoletti
    weir (see Figure 6.3 below) which has a
    trapezoidal cross-section with side slopes 14
    (HV). The advantage of a Cipolletti weir is
    that corrections for end contractions are not
    necessary.

35
Cipolletti Weir Contd.
The discharge formula can be written as   Q
1.859 b H1.5 .. (5)   Where b is the
bottom width of the Cipolletti weir. The minimum
head on standard rectangular and Cipolletti weirs
is 6 mm and at heads less than 6 mm, the nappe
does not spring free of the crest.  
   
Figure 6.3 A Trapezoidal of Cipolletti Weir
36
Example 6.3
  • A weir is be installed to measure flows in the
    range of 0.5 to 1.0 m3/s. If the maximum depth
    of water that can be accommodated at the weir is
    1 m and the width of the channel is 4 m,
    determine the height of a suppressed weir that
    should be used to measure the flow rate.

37
Solution to Example 6.3
The flow over the weir is shown in the Figure 6.4
below. The height of water is Hw and the flow
rate is Q. The height of water over the crest of
the weir, H is given by H 1
Hw Assuming that H/Hw , 0.4, then Q is related
to H by equation (3), where   Q 1.83 b H 1.5


Figure 6.4 Weir Flow
38
Solution to Example 6.3Concluded
Taking b 0.4 m, Q 1m3/s (the maximum flow
rate will give the maximum head, H), then  
The height of the weir, Hw
is therefore given by Hw 1 0.265 0.735
m   And H/Hw
0.265/0.735 0.36   The initial assumption
that H/Hw lt 0.4 is therefore validated, and the
height of the weir should be 0.735 m.  
39
V-Notch Weir
A V-notch weir is a sharp-crested weir that has a
V-shaped opening instead of a rectangular-shaped
opening. These weirs, also called triangular
weirs, are typically used instead of rectangular
weirs under low-flow conditions ( mainly lt 0.28
m3/s), where rectangular weirs tend to be less
accurate. It can be derived that the flow rate,
Q over the weir is given by    

40
V-Notch Weirs Contd.
41
Parshall Flume
  • Although weirs are the simplest structures for
    measuring the discharge in open channels, the
    high head losses caused by weirs and the tendency
    for suspended particles to accumulate behind
    weirs may be important limitations.
  • The Parshall flume provides an alternative to the
    weir for measuring flow rates in open channels
    where high head losses and sediment accumulation
    are of concern.
  • Such cases include flow measurement in irrigation
    channels.
  • The Parshall flume (see Figures 6.7 and 6.8
    below) consists of a converging section that
    causes critical flow conditions, followed by a
    steep throat section that provides for a
    transition to supercritical flow.

42
Parshall Flume
43
Parshall Flumes
44
Parshall Flume Contd.
  • The unique relationship between the depth of flow
    and the flow rate under critical flow conditions
    is the basic principle on which the Parshall
    flume operates.
  • The transition from supercritical flow to
    subcritical flow at the exit of the flume usually
    occurs via a hydraulic jump, but under high tail
    water conditions the jump is sometimes submerged.

45
Parshall Flume Contd
  • Within the flume structure, water depths are
    measured at two locations, one in the converging
    section, Ha and the other at the throat section,
    Hb. The flow depth in the throat section is
    measured relative to the bottom of the converging
    section as illustrated in the figure below.
  • If the hydraulic jump at the exit of the Parshall
    flume is not submerged, then the discharge
    through the flume is related to the measured flow
    depth in the converging section, Ha by the
    empirical discharge relations given in Table 6.2,
    where Q is the discharge in ft3/s, W is the width
    of the throat in ft, and Ha is measured in ft.

46
Parshall Flume Contd
  • Submergence of the hydraulic jump is determined
    by the ratio of the flow depth in the throat, Hb,
    to the flow depth in the converging section, Ha,
    and critical values for the Hb/Ha are given in
    Table 6.3.
  • Whenever, the ratio exceeds the critical values
    in the table, the hydraulic jump is submerged and
    the discharge is reduced from the values given by
    the equations in Table 6.2.
  • Corrections to the theoretical flow rates as a
    function of Ha and the percentage of submergence,
    Hb/Ha are given in the Figures 6.8 and 6.9 below
    for throat widths of 1 ft and 10 ft.

47
Parshall Flumes Contd.
48
Parshall Flumes Contd.
  • Flow corrections for the 1 ft flume are applied
    to larger flumes by multiplying the correction
    for the 1 ft flume by a factor corresponding to
    the flume size given in Table 6.4.
  • Similarly, flow corrections for flume sizes
    greater than 10 ft. are applied to larger flumes
    by multiplying the correction for the 10 ft flume
    by a factor corresponding to the flume size
    given in Table 6.5.
  • Parshall flumes do not reliably measure flow
    rates when the submergence ratio, Hb/Ha exceeds
    0.95.

49
Parshall Flume Correction
50
Tables For Parshall Flume Correction
51
Example 6.4
  • Example 6.4 Flow is being measured by a
    Parshall flume that has a throat width of 2 ft.
    Determine the flow rate through the flume when
    the water depth in the converging section is 2.00
    ft and the depth in the throat section is 1.70ft.

52
Solution to Example 6.4
From the given data W 2 ft, Ha 2 ft,
and Hb 1.7 ft. According to Table 6.2, Q
is given by In this
case Hb/Ha 1.7/2 0.85 Therefore,
according to Table 6.3, the flow is submerged.
Figure 6.8 gives the flow rate correction for a
1 ft flume as 2ft3/s, and Table 6.4 gives the
correction factor for a 2 ft flume as 1.8. The
flow rate correction, dQ for a 2 ft flume is
therefore given by DQ 2 x 1.8
3.6 ft3/s And the flow rate through the
Parshall flume is Q dQ, where Q dQ 23.4
3.6 29.8 ft3/s
53
Gates
  • Gates are used to regulate the flow in open
    channels.
  • They are designed for either over-flow or
    underflow operation, with overflow operation
    appropriate for channels in which there is a
    significant amount of floating debris.
  • The common types of gates are vertical and radial
    (Tainter) gates, which are illustrated below.
  • Vertical gates are supported by vertical guides
    with roller wheels, and large hydrostatic forces
    usually induce significant frictional resistance
    to raise and lower the gate

54
Diagrams of Gates
55
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56
Gates Contd.
Flow, Q through a gate could be
established to be Cc
Cc coefficient of contraction, y2/yg
0.61 for most vertical gates. For For Tainter
gates, Cc is generally greater than 0.61 and is
commonly expressed as a function of the angle
(degrees) shown in the diagram above.
57
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58
Gates Concluded
It can be expressed as This equation
applies as long as the angle is least than 900.
All the equations apply where there is free flow
through the gates. See texts for situations
where the flows through the gates are submerged.
59
Drop Structures
  • Drop structures, typically constructed out of
    concrete, can accommodate a sudden change in
    elevation of the channel bottom while maintaining
    control of the flow.
  • Drop structures are used in channels, which must
    be laid along relatively steep gradients to allow
    for dissipation of energy without causing scour
    in the channel itself.
  • In such applications, the drop structure allows
    the main channel to be laid on subcritical slope
    while the excess potential energy of the flow due
    to the steep topography is absorbed in the drop
    structure. See Figure 6.12 of a drop structure
    below

60
Diagram of Drop Structure
61
Example 6.5
  • An irrigation channel with a design discharge of
    2.265 m3/s is to be laid along a terrain having
    an average slope of 0.005 m/m. To maintain
    subcritical flow in the channel section, the
    bottom of the channel must be limited to 0.001
    m/m. The extra fall is to be absorbed by drop
    structures such as the one shown above in the
    diagram having a width of 3.048 m. Compute the
    number of structures required in a 16.09 km
    length of line if the drop height (dZ) is equal
    to 1.829 m.

62
Solution of Example 6.5
  • Solution The total drop to be absorbed by
    structures, ZT (St - So) L
  • Where St is the terrain slope, L is total
    distance, and So is the slope of the channel.
  • ZT ( 0.005 m/m - 0.001 m/m ) 16.09 km
    64.36
  • The number of drop structures required,
  • N ZT/dZ 64.36/1.829 36 Structures.
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