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DESIGN PROCEDURE
FOR
IRRIGATION CHANNELS
Cross-section of an Irrigation Canal
A typical and most desired section of a canal, i.e. partly in cutting and
partly in filling, is shown in the following figure.
NSL = Natural Surface Level
CBL = Canal Bed Level
FSL = Full Supply Level
When NSL is above the top of the bank, the canal section will have to be
cut, and it shall be called “canal in cutting”.
When NSL is lower than the CBL, the canal section will have to be built
in filling, and it is called “canal in filling”.
Side Slopes
Side slopes (H:V) are fixed according to stability requirements,
depending upon the type of the soil (angle of repose of soil).
A comparatively steeper slope can be provided in cutting because soil is
naturally consolidated, and hence, more stable.
Generally adopted slopes:
In cutting: 1:1 to 1½: 1
In filling: 1½:1 to 2:1
For channels with silt laden water, the actual capacity of the channel is
worked out with ½ :1 side slopes.
Berms
Berm is the horizontal distance left at ground level between the toe of
the bank and the top edge of cutting.
If s1:1 is the slope in cutting and s2:1 in filling, then initial berm width
= (s2-s1) d1
Since NSL fluctuates considerably, while canal bed level (CBL) varies
very slightly, d1 shall vary; and, therefore, the berm width shall vary.
After the water flows in the channel for some time, the silt gets
deposited on the sides giving them a slope of ½:1.
The position of the berm, therefore, changes from ground level to FSL.
as shown in Fig. and its width becomes equal to (s2- ½ ).y.
If s2 = 1 ½ then the final berm width = y, i.e. equal to the depth of the
canal. (where, y = Full Supply Depth)
The berms when fully formed, serves the following purposes:
(i) The silt deposited on the sides is very fine and impervious. It,
therefore, serves as a good lining for reducing losses, leakage and
consequent breaches, etc.
(ii) They help the channel to attain regime conditions, as they help in
providing a wider waterway, if required. Even fluctuations of
discharge do not produce much fluctuations in depths because of
wider waterway.
(iii) They give additional strength to the banks and provide protection
against erosion and breaches.
(iv) The possibility of breaches gets reduced because the saturation line
comes more in the body of the embankment.
(v) They protect the banks from erosion due to wave action.
(vi) They provide a scope for future widening of the canal.
(vii) Berms can be used as borrow pits for excavating soil to be used for
filling.
Freeboard
The margin between FSL and bank level is known as freeboard.
The amount of freeboard depends upon the size of the channel. The
generally provided values of freeboard are given in following table.
Values of Freeboard in Canals
Discharge (cumec) Extent of Free board (m)
1 to 5 0.5
5 to 10 0.6
10 to 30 0.75
30 to 150 0.90
Banks
The primary purpose of banks is to retain water. They can be used as
means of communication and as inspection paths.
They should be wide enough, so that a minimum cover of 0.5 m is
available above the saturation line, as shown in below Fig.
High banks will have to be designed as earth dams.
Service Roads/Inspection Path
Service roads are provided on canals for inspection purposes, and may
simultaneously serve as the means of communication in remote areas.
They are provided 0.4 m to 1.0 m above FSL, depending upon the size of
the channel.
Dowlas
As a measure of safety in driving, dowlas 0.3 m high and 0.3 to 0.6 m
wide at top, with side slopes of 1:1 to 2:1, are provided along the banks,
as shown in below figure.
They also help in preventing slope erosion due to rains, etc.
Back Berm or Counter Berms.
Even after providing sufficient section for bank embankment, the
saturation gradient line may cut the downstream end of the bank.
In such a case, the saturation line can be kept covered at least by 0.5 m
with the help of counter berms, as shown in following figure.
The straight saturation gradient line may be drawn with the following
slopes.Assumed Values of Saturation Gradients in Different
Soils
Type of soil Slope (H:V)
Clay 1 in 4
Clayey loam 1 in 6
Loam 1 in 8
Loamy sand 1 in 10
sand 1 in 15
Spoil Banks.
When the earthwork in excavation exceeds earthwork in filling, even
after providing maximum width of bank embankments, the extra earth
has to be disposed off economically.
To dispose off this earth by mechanical transport, etc. may become very
costly, and an economical mode of its disposal may be found in the form
of collecting this soil on the edge of the bank embankment itself.
The soil is, therefore, deposited in such a case, in the form of heaps on
both banks or only on one bank, as shown in following figure.
These heaps of soil are discontinued at suitable intervals and longitudinal
drains running by their sides are excavated for the disposal of rain water.
Cross drains through the spoil banks may also be excavated, if needed.
Borrow Pits
When earthwork in filling exceeds the earthwork in excavation, the earth
has to be brought from somewhere.
The pits, which are dug for bringing earth, are known as borrow pits.
If such pits are excavated outside the channel, they are known as external
borrow pits, and if they are excavated somewhere within the channel,
they are known as internal borrow pits.
It is a very costly affair to bring soil from distances. Even in the nearby
areas, these pits may cause mosquito nuisance due to collection of rain
water in these pits, and hence, external borrow pits are not preferred.
When needed, internal
borrow pits are excavated
on the bed of the canal, as
shown in figure.
The borrow pits should start from a point at a distance more than 5 m
from the toe for small channels, and 10 m for large channels.
The width of these pits (b) should be less than half the width of the canal
(B), and should be dug in the centre.
The depth of these pits should be equal to or less than 1 m.
Longitudinally, these pits should not run continuous but a minimum
space of 0.5 L should be left between two consecutive pits. (where L is
the length of one pit) as shown in following figure.
Balancing Depth for Excavating Canals
Maximum economy can be achieved in canal construction. if the
earthwork in excavation equals the earthwork in filling. Such a thing is
possible when a canal is constructed partly in filling and partly in cutting,
which mostly happens in practical life.
If this balance between cutting and filling can occur. then the need for
spoil banks or borrow pits is entirely eliminated. and moreover.
earthwork has to be paid only once in a single item.
For a given cross-section of a channel. there can be only one depth. for
which such a balance between cutting and filling will occur. This depth
is known as the Balancing depth.
This depth can be worked out easily by equating the areas of cutting and
filling.
Example
Calculate the balancing depth for a channel section having a bed width equal to 18 m
and side slopes of 1:1 in cutting and 2:1 in filling. The bank embankments are kept 3.0
m higher than the ground level (berm level) and crest width of banks is kept as 2.0 m.
Solution
The channel section is shown in Fig. Let d1 be the balancing depth, i.e. the depth for
which excavation and filling becomes equal.
Area of cutting = (18+d1).d1 m2
Equating cutting and filling, we get (18+d1).d1 = 48
Solving for d1, we get
Balancing depth = 2.35 m
2m 4832
1422 filling of Area
Example
Design and Prepare the longitudinal section, schedule of area statistics
and channel dimensions of an irrigation channel from the following data:
Longitudinal leveling of channel alignment gives the following results:
Assume a datum level of 150.00 m, Full Supply Level of parent channel
as 157.70 m, and Bed Level of parent channel as 156.00 m.
km Ground level km Ground level
0.0 155.20 2.6 155.35
0.2 157.26 2.8 155.17
0.4 157.29 3.0 155.12
0.6 156.21 3.2 154.95
0.8 156.56 3.4 154.70
1.0 156.52 3.6 154.39
1.2 156.51 3.8 154.28
1.4 156.28 4.0 154.19
1.6 156.11 4.2 154.10
1.8 156.14 4.4 153.74
2.0 155.83 4.6 153.58
2.2 155.68 4.8 153.38
2.4 155.58 5.0 153.18
Commanded areas are given as follows:
Total loss below km 5 is 0.5 cumec. Losses are to be calculated at the rate of 2 cumecs
per million square metres of the wetted perimeter.
Take side slope of the channel section as 1V:0.5H, m = 1, and Kutter’s N = 0.0225.
Below km GCA (ha) CCA (ha)
0 24000 16000
1 22000 14000
2 18000 12000
3 16000 11000
4 14000 10000
5 12000 8000
Intensity of irrigation Rabi (wheat) 30% Kharif (Rice) 12.5%
Kor-period (B) 4 weeks 2.5 weeks
Kor-depth (∆) 13.4 cm 19 cm
Solution:
Design Procedure:
1.The longitudinal section (L-section) of the ground is plotted first.
2.The slope of the channel is calculated with Lacey’s formula or taken from Lacey’s
diagrams and is tabulated in Schedule of Area Statistics.
3.A trial slope line for full supply level is marked. The position and depth of falls are
also tentatively marked.
4.The actual channel design is thereafter carried out by filling in the table of Schedule
of Area Statistics.
5.The outlet discharge factor for controlling crops is determined as follows:
(a)
where, B = kor period = 4 x 7 days; and ∆ = kor depth = 0.134 m
(b)
where, B = kor period = 2.5 x 7 days; and ∆ = kor depth = 0.19 m
B64.8 Rabifor factor dischargeOutlet
cumecha / 18000.134
748.64 factor dischargeoutlet Hence
B64.8 Khariffor factor dischargeOutlet
cumecha / 0760.19
72.58.64 factor dischargeoutlet Hence
The outlet discharge factor for the controlling crops is the one which gives maximum
discharge.
Let us assume the CCA as 16000 ha. (CCA below 0 km, as given in the data)
Hence outlet discharge factor to be adopted = 1800 ha/cumecs.
ha 48000.30 16000 30% as I.I. with crop Rabiunder Area
cumecD
A 67.2
1800
4800 crop Rabifor needed Discharge
ha 20000.12516000 12.5% as I.I. with crop Kharifunder Area
cumecD
A 63.2
760
2000 crop Rabifor needed Discharge
Design of Channel at various sections:
The design is started from the tail where the losses are known.
At km 5.0:
Culturable Commanded Area (CCA) = 8000 ha
Intensity of irrigation in Rabi = 30%
Area to be irrigated for Rabi crops = 8000 x 0.3 = 2400 ha
Outlet discharge factor = 1800 ha/cumec
Total losses after 5.0 km = 0.5 cumec (Given)
Total discharge at km 5.0 = 1.34 + 0.5 = 1.84 cumec
Adopt design discharge Q, 10% more than the total discharge.
Therefore,
Design discharge = 1.84 x 1.1 = 2.02 cumec
V/Vo = m = 1.0 (Given), and
N = 0.0225
Assume slope S = 0.25 m per km = 1/4000
cumecD
A 34.1
1800
2400 crop Rabifor needed Discharge
Garret's diagram:
It gives the graphical method of designing the channel dimensions based on Kennedy's Theory.
The diagram has discharge plotted on the abscissa.
The ordinates on the left indicate the slope and on the right water depth in the channel (D) &
critical velocity (Vo).
The discharge lines are curved and the bed width lines are shown dashed.
Procedure to use Garret's diagram in channel design:
1.Find out the discharge for which the channel is to be designed through Schedule of Area
Statistics.
2. Find out the slope of the channel through longitudinal section.
3. Follow the discharge line and find out its intersection with the horizontal line from the slope.
Interpolation may be done if needed.
Mark the intersection point P.
4. Draw the vertical line through the intersection point. This will cut several bed width curves.
Each point of intersection gives a depth (D) & Vo on right hand side of ordinates.
5. Choose a pair of bed width, depth and Vo corresponding to point of intersection obtained
above.
6. Calculate the area of channel section A corresponding to channel bed width and depth.
7. Calculate the velocity corresponding to the area A.
8. Calculate the ratio V/Vo. This should be equal to unity or as given for the channel design.
9. Repeat the procedure for other values of bed width and depth till the value of V/Vo is the same
value as given for channel design.
From Garret's diagram: At km 5.0
Discharge Q
(Cumecs)
Slope S
Bed width B
(m)
Water Depth D
(m)
Area A
Velocity V = Q/A (m/s)
Critical Velocity Vo
(m/s)
Critical Velocity
Ratio
5.50 0.73 4.28 0.472 0.450 1.05
2.02 1 in 4000 4.90 0.79 4.18 0.483 0.473 1.02
4.55 0.84 4.17 0.484 0.492 0.98
At km 4.0:
Culturable Commanded Area (CCA) = 10,000 ha
Intensity of Rabi irrigation = 30%
Area to be irrigated for Rabi crops = 10,000 x 0.3 = 3000 ha
Outlet discharge factor = 1800 ha/cumec
Losses below km 5.0 = 0.5 cumec (Given)
Losses in channel between km 5.0 and km 4.0
Note: For calculation of losses adopt section at km 5.0 as section at km 4.0 is not yet
calculated.
Total losses at km 4.0 = losses below km 5.0 + losses between km 5.0 and km 4.0
= 0.5 + 0.013 = 0.513 cumec.
Therefore total discharge required at km 4.0 = 1.67 + 0.513 = 2.183 cumec
Adopt design discharge Q 10% more than the total discharge.
Therefore design discharge = 2.183 x 1.1 = 2.40 cumec, Similar approach is used in
calculating discharge at km 3.0, 2.0, 1.0 and 0.0.
cumecD
A 67.1
1800
3000 crop Rabifor needed Discharge
mD 445.684.0555.45B= 5.0 kmat section channel ofperimeter Wetted
cumec 013.02000,000,1
10006.445 4.0 km to5.0 km fromreach in loss Thus,
Below
kmGCA CCA
Area to be irrigatedOutlet
discharge
factor
Outlet
discharge
Loss
in
reach
Total
losses
Total
discharge
Channel dimensions
Velocity V/VoRabi
30%
Sugar
Cane
Kharif
12.5%
Bed
slope
Bed
width
Water
depth
Design
Discharge
Height
of bank
Width of
bank
ha ha ha ha ha ha ha/cumec cumeccume
ccumec cumec
cm/
kmm m cumec m m m/sec
0 24000 16000 4800 Nil 2000 1800 2.667 0.015 0.569 3.236 22.5 5.500 1.040 3.56 2.0 0.570 1.015
1 22000 14000 4200 Nil 1750 1800 2.333 0.014 0.554 2.888 22.5 5.200 1.007 3.17 2.0 0.555 1.000
2 18000 12000 3600 Nil 1500 1800 2.000 0.014 0.540 2.540 22.5 4.850 0.975 2.80 2.0 0.538 1.000
3 16000 11000 3300 Nil 1375 1800 1.833 0.014 0.527 2.360 24.0 4.700 0.945 2.60 2.0 0.532 1.000
4 14000 10000 3000 Nil 1250 1800 1.667 0.013 0.513 2.180 24.0 4.550 0.915 2.40 2.0 0.521 1.000
5 12000 8000 2400 Nil 1000 1800 1.333 0.500 1.833 25.0 4.550 0.840 2.02 2.0 0.484 0.992
Schedule of Area Statistics
Schedule of area statistics and Channel Dimensions
The design of channel cross section from km to km is done in tabular form & is called
schedule of area statistics & channel dimensions. The column wise calculation is done
as follows:
Col 1: The actual design of the channel is done from km to km. If any channel is off
taking in between the kms the channel dimensions are also found downstream
of the cross regulator.
Col 2: It indicates the Gross Commanded Area (GCA) below that particular km.
Col 3: It indicates the Culturable Commanded Area (CCA) below that particular km.
Col 4 – 6: They indicate the % of area under the Rabi, Perennial, & Kharif crops.
Col 7: It indicates the outlet discharge factor for the controlling crop.
Col 8: It indicates the outlet discharge required which is found by multiplying the
area to be irrigated with the outlet discharge factor.
Col 9: It indicates the losses in the reach from km to km. Generally these
transmission losses are assumed as 2.5 cumecs per million square metres of
wetted perimeter for unlined channels and 0.6 cumecs per million square
metres of wetted perimeter for lined channels.
Schedule of area statistics and Channel Dimensions (continued)
Col 10: It indicates the total losses in the channel below any particular km.
Col 11: It indicates the total discharge for which the channel has to be designed which
includes outlet discharges and losses.
Col 12 – 17: These columns pertain to channel dimensions and are to be filled up after
designing the channel in each reach.
Col 18: It indicates the velocity in the channel
Col 19: It indicates the critical velocity ratio
