DESIGN PROCEDURE FOR IRRIGATION CHANNELS

1. 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.

The berms when fully formed, serve 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 CanalsDischarge (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 RoadsService 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

SoilsType of soil Slope (H:V)Clay 1 in 4Clayey loam 1 in 6Loam 1 in 8

Loamy sand 1 in 10sand 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 48321422 filling of Area

ExampleDesign 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 level0.0 155.20 2.6 155.350.2 157.26 2.8 155.170.4 157.29 3.0 155.120.6 156.21 3.2 154.950.8 156.56 3.4 154.701.0 156.52 3.6 154.391.2 156.51 3.8 154.281.4 156.28 4.0 154.191.6 156.11 4.2 154.101.8 156.14 4.4 153.742.0 155.83 4.6 153.582.2 155.68 4.8 153.382.4 155.58 5.0 153.18

Commanded areas are given as follows:

Total loss is between km 0.5 to 5.0. 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 160001 22000 140002 18000 120003 16000 110004 14000 100005 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.

Hence outlet discharge factor to be adopted = 1800 ha/cumecs.

ha 48000.30 16000 30% as I.I. with crop Rabiunder Area

cumecDA 67.2

18004800 crop Rabifor needed Discharge

ha 20000.12516000 12.5% as I.I. with crop Kharifunder Area

cumecDA 63.2

7602000 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 haIntensity of Rabi irrigation = 30%Area to be irrigated for Rabi crops = 8000 x 0.3 = 2400 haOutlet 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 cumecAdopt 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 = 0.0225Assume slope S = 0.25 m per km = 1/4000

cumecDA 34.1

18002400 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 & 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 & 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 5.0Discharge

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 1.052.02 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 haIntensity of Rabi irrigation = 30%Area to be irrigated for Rabi crops = 10,000 x 0.3 = 3000 haOutlet discharge factor = 1800 ha/cumec

Losses below km 5.0 = 0.5 cumec (Given)Losses in channel between km 5.0 and km 4.0Note: 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 cumecAdopt 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.

cumecDA 67.1

18003000 crop Rabifor needed Discharge

m 445.684.0555.45DB= 5.0 kmat section channel ofperimeter Wetted

cumec 013.02000,000,110006.445 4.0 km to5.0 km fromreach in loss Thus,

Below km GCA CCA

Area to be irrigated Outlet discharge

factor

Outlet discharge

Loss in

reach

Total losses

Total discharge

Channel dimensions

Velocity V/VoRabi 30%

Sugar Cane

Rice 12.5%

Bed slope

Bed width

Water depth

Design Discharge

Height of bank

Width of bank

ha ha ha ha ha ha ha/Cms Cms Cms Cms Cms cm/km m m Cms 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 1.0151 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 1.0002 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 1.0003 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 1.0004 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 1.0005 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