In this article we will discuss about how to design canals for irrigation.
Longitudinal Section of a Canal:
Actually the whole of the area where irrigation is proposed is surveyed, and contour plans prepared. The other features of the area are also marked over the plans. The contour plan on which other features of the area are also marked in known as shajra sheet. Alignment of the main canal is fixed on the main ridge of the area proposed to be irrigated, so that irrigation is possible on both the sides of the canal.
Branch and distributory channels are aligned along the main ridges of the area allotted for their command. In this way, the whole of the area to be irrigated is divided into several parts and each part is commanded by a branch, distributory or minor depending upon the extent of the area.
The area under the command of each distributory or minor, is further subdivided into small areas surrounded by small drainages and each area is known as clink. The chak is that area which is generally surrounded by minor drainages. One outlet for each chak is provided from the distributory. The outlet is located in such a way that water may flow to all the areas of a chak under gravity. Size of the outlet depends upon the command areas available in a chak for irrigation.
After having fixed the canal alignment, detailed leveling is done along the alignment, and longitudinal section is plotted on a drawing sheet. For preparing maps of the area, Horizontal scale of 1 cm 160 m and vertical scale of 1 cm = 1/2 m is used. If the area is very much undulated, the vertical scale may be different.
After drawing the longitudinal section, along the alignment, bed level of the proposed canal is marked. While marking bed level it should be ensured, that it will involve either too much cutting nor filling and secondly the full supply level of the canal will remain above the ground level so that irrigation is possible on both the sides, along the alignment.
Bed level and F.S.L. of the off-taking channels should be decided in relation to the Bed level and F.S.L. of the parent channel. When water is diverted to the off- taking channel, some head loss is bound to occur. To overcome this head loss, F.S.L. off-taking channel should be about 30 cm below the F.S.L. of the parent channel.
All the irrigation channels are given some longitudinal slope as these are gravity channels and water can flow only if some longitudinal slope is given to them. The slope of the canal is also decided in relation to the general slope of the area, in which canal is to run. If slope of the channel is almost same as general slope of the area, no fall will have to be constructed.
But such ideal conditions are seldom available. Most of the canals are designed by Lacey’s theory and this theory states that for a particular discharge and silt factor (f), there is a fixed slope for the canal. If this slope is changed to suit the general slope of the area, the canal will not remain regime canal. This fixed slope given by Lacey may not be same as general slope of the area. Generally designed slope of the channel is much less than the general slope of the area, and, as such a number of falls have to be constructed.
If somewhere, general slope of the ground is smaller than designed slope, the slope of the channel is changed to general slope of ground and section of the canal is accordingly modified.
After deciding the longitudinal slope and also the bed level at the head regulator, the bed levels of the channel at all the points are known. F.S.L. is marked parallel to the bed level. F.S.L. should always remain above the ground level so that water may flow to fields under gravity.
After having marked Bed levels on the longitudinal section of the proposed channel, mark the lengths where cuttings or filling are to be done. Extent of cutting or filling at a particular point is determined from the difference of ground level and Bed level. In order to economize the work, cutting should be just equal to filling at a particular point.
Following points should be taken care of, while drawing longitudinal section of canal:
1. Cutting and filling at all the points should be equal. In other words cutting and filling should balance each other.
2. F.S.L. of off-taking channel should be below the F.S.L. of parent channel. The difference in F.S.L. of off-taking channel and parent canal should be minimum 30 cm for distributory, 70 cm for branch and 1 m for main canal.
3. F.S.L. of the channel should remain above the ground level for most of the length. At isolated high spots, it may remain below the ground level.
4. F.S.L. should be above ground level only by 15 to 30 cm. This is considered sufficient because canals being aligned on water shed, will develop sufficient cross-slope and water will be flowing to fields under sufficient head.
5. Canals should not be too much in filling. The canals are always subjected to breaches at such reaches.
6. Bed slope as obtained by Lacey’s theory if equals general slope of the ground it will be an ideal situation. If design slope of the canal is less than general slope of ground, canal falls will have to be provided, at suitable intervals. The fall or drop structure should be such that F.S.L. of the canal D/S of fall remains below G.L. for about 1/2 km distance and then emerges out of G.L. The high ground on both the sides, D/S of fall is irrigated by taking outlets from U/S of the fall.
7. If designed slope of the channel is greater than general slope of the ground, the channel would go deep in cutting after running for a short distance running. In such a case general ground slope should be adopted and section of the channel should be accordingly amended.
Balancing Depth of Canal:
The canal section is considered to be most economical when cutting at a particular section equals the filling. For such a section payment has to be made only for one operation. More so the formation of borrow pits, or spoil banks is completely eliminated. The balancing depth is worked out as follows. See Fig. 19.1.
After solving this equation reduces to
The values of d and B are found out from this equation.
Design of a Canal:
In the design of a canal one has to find out bed width (B) depth (D), longitudinal slope (S), and velocity of flow (V). If discharge (Q) and silt factor (f) are given, the method of finding out all these elements either by Kennedy’s theory or by Lacey’s theory. This aspect limits only finding the sectional dimensions of the canal. But how to compute the discharge at a particular reach is an important aspect of canal design.
Discharge in the off-taking canal does not remain constant throughout the length. Outlets fixed on the canal at regular intervals draw discharge from the canal and supply it to the fields for irrigation. Evaporation and percolation losses also go on increasing with length of the canal.
Hence because of discharge withdrawn by outlets, and also continuous evaporation and seepage losses, the remaining discharge in the canal goes on decreasing as canal flows towards the tail. As discharge at various points on the canal is not constant, the section of the canal will also be changing.
While fixing the discharge for any canal to be withdrawn from a head regulator, one must know the following data:
(i) Gross command area of which the proposed canal is going to be incharge.
(ii) Percentage by which G.C.A. is to be multiplied, to determine the cultural command area.
(iii) Various crops that will grow after the commissioning of the proposed canal.
(iv) Duties of water for various crops.
(v) Intensities of irrigation during Rabi and Kharif crops.
(vi) Losses due to seepage and evaporation.
The discharge required at a particular point on the canal depends upon the area to be irrigated lying D/S of that point and also upon the seepage and evaporation losses occurring in the canal itself, lying D/S of that point. Since area under irrigation, and losses due to evaporation and seepage, go on increasing as we proceed towards the U/S side, we have to design the canal sections at different points.
Design of the canal is always started from the tail end of the canal and proceeded step by step towards U/S side till head regulator of the canal is reached.
An example as to how discharge is determined at various points on the canal is given here:
Let a 5 km long minor distributary is to be designed. Let discharge required at the tail, on irrigated area basis, is 2.02 cumec. Let canal length of 5 km be divided into 5 parts each 1 km long. All the 5 parts we denote by lengths measured from head regulator in kilometre. Thus first kilometre length canal is denoted by 0 — 1 kilometre and second kilometre length by 1 — 2 kilometre. Similarly other lengths are 2 — 3, 3—4 and 4 — 5 kilometres.
Discharge at the end of each kilometre is worked out as follows:
At 5 kilometre point – Discharge for which canal is to be designed = 2.02 cumec.
At 4 km point – Let from 4 km to 5 km point there be five outlets, each of 0.06 cumec. Also evaporation and seepage losses are say 0.08 cumec.
The discharge required at 4 km point should be as follows so that 2.02 cumec water remains available at 5 km point.
Required discharge at 4 km point
= 2.02 + 5 (0.06) = 0.08 = 2.40 cumec
At 3 km point – Let there by five outlets between 3 km and 4 km points, each of 0.03 cumec discharge. Let losses in the same length are 0.05 cumecs.
Total discharge required at 3 km point –
= 2.40 + 5 (0.03) = 0.05
= 2.60 cumecs
At 2 km point – Let there be 0.02 cumec sized six outlets between 2 km and 3 km points and losses are 0.08 cumec.
Total discharge required at 2 km point
= 2. 60 + (0. 02) 6 + 0.08
= 2.80 cumecs
At 1 km point – Let there be 5 outlets each of 0.06 cumec, between 1 km and 2 km points and losses are 0.07 cumec.
Total discharge required at 1 km point
= 2.80 + 5 (0.06) + 0.07 = 3.17 cumecs
At zero km point i.e. at head regulator – Let there be 6 outlets, each of 0.05 cumec discharge between zero km and 1 km points and losses be 0.03 cumec.
Total discharge required at Head regulator
3.17 + (0.05) 6 + 0.03 = 3.50 cumecs
If say a minor of 1 cumec is taking off from this distributory between 2 km and 3 km points. The discharge in the distributory D/S of 3 km point will continue to be same as calculated above but discharges at 2 km, 1 km, and head regulator will increase by I cumec. In this case discharge at Head regulator, 1 km point, and 2 km points, will be 4.50, 4.17 and 3.80 cumecs respectively.
All the information regarding discharge, canal section, slope, area under irrigation, losses at each point etc. is filled in a standard table known as schedule of area statistics and channel dimensions.
Now discharge in the canal at various points is known. Fixing appropriate value of silt factor (f), channel dimensions at various points can be easily found out, using garret diagrams or Lacey’s charts.
Bed Width and Depth Relationship:
Kennedy’s theory does not give any importance to B/D ratio. Very large number of sections with varying B/D ratio and all satisfying the C.V.R. are possible by this theory. But all these sections cannot be equally satisfactory. This drawback of Kennedy’s theory was made good to some extent by Mr. Woods, who gave B/D ratio table for various discharges. In Uttar Pradesh (U.P.) bed width, and depth are related by following equation –
(i) For discharge of the channel up to 15 cumec
(ii) For discharge of 15 cumecs and above depths for various discharges should be as follows:
Channel Cross Section:
The channel sections for an irrigation canal may be of following four types:
1. Canal in cutting.
2. Canal in filling.
3. Canal in heavy filling.
4. Canal partly in cutting and filling.
1. Canal in Cutting:
This canal does not require any bank as F.S.L. lies below the G.L. If F.S.L. is just at the G.L., small banks may have to be provided. In this section F.S.L. of canal lies just at G.L. or slightly below it. See Fig. 19.2 (a).
2. Canal in Filling:
In such a section the bed level of the canal lies at the G.L. Section whose bed level is slightly above the G.L. also comes under this category. See Fig. 19.3 (a).
3. Canal in Heavy Filling:
In this section, the bed level of the canal lies substantially above the G.L. Such a section should as far as possible be avoided. Such sections are easily liable to breach and cause lot of damage to canal itself and surrounding areas. See Fig. 19.3 (b).
4. Canal Partly in Cutting and Filling:
In such a section the ground level lies in between the F.S.L and bed level of the canal. See Fig. 19.2 (a).
Service road is usually provided on the left bank of the canal. One bank of the canal always has service road and the other bank is made banked section. Service road and canal berms are separated by a small bund called Dowla.
Side Slope:
The side slope of the canal depends upon the type of the soil. The canals in alluvial soils, are designed assuming 1/2:1 side slope, irrespective of the actual initial side slope. It is assumed that after due course of run, the canal section would ultimately acquire 1/2:1 slope. This happens because silt gets deposited on the berms. Had section been designed with 1/2:1 slope initially, the section would be reduced in due course of time due to silting and the section remaining would be inadequate.
As per recommendation of Central Water and Power Commission (C.W.P.C.) the side slopes for various soils should be give Table 19.2.
Berms:
It is a narrow strip of land, left on either side of a channel at G.L., between upper edge of the cut and the inside toe of the bank. The width of the berm depends upon the size of the channel. If canal is in partial cutting, the original width of the berm will be small but it becomes wider after silting over the berms and side slopes. The canals while constructing area excavated with 1:1 slope, but after a run for few months the section automatically acquires a side slope of 1/2: 1.
Berm performs following functions:
1. The width of the canal can be easily increased if required.
2. Slipping soils and boulders are held up at berms and do not allow them to be dropped into the channel.
3. It acts as a storage space for materials if some repair or construction work is to be clone in the canal.
4. Barrow pits may be made on banks for taking soil. These berms get silted up very soon. Such a need for barrow pits arises during canal breaches.
5. They strengthen the channel banks.
6. Rise in water level is marginal with substantial increase in discharge above full capacity of the canal. If by mistake excess discharge enters the canal, they do not allow water to rise much and thus possible breach of the canal is averted.
7. Because of silting of inside edge and top, the terms become impervious, and as such, loss of water by seepage is reduced.
8. Waves developed in the canal do not come in direct contact of the banks and hence possibilities of bank erosion are reduced.
9. They also provide easy path for inspection.
10. They increase the width of the bank, and thus, seepage line is not likely to be exposed.
For channels in full cutting, a berm of width equal to depth of water is provided at 50 cm above the F.S.L. In channels, partly in cutting, the berm provided is such that after silting its width at F.S.L. will not exceed twice the depth of water.
In channels, fully in embankment, a berm width varying from twice the depth to thrice the depth is provided at F.S.L.
Minimum berm width can also be found out from the Table 19.3.
Freeboard:
The vertical distance between F.S.L. and top of the lowest bank of the channel is known as free board. It is provided to prevent waves or fluctuations in water surface from overtopping the banks. Free board depends upon the canal size, wind action, soil characteristics and location. According to USER, free board may be worked out from following formula, under ordinary conditions.
F = √CD
Where F = freeboard
C = a constant whose value varies from 0.46 to 0.76
D = the depth of water in metres
Lacey gave following formula for the free board –
F = 0.20 + 0.15 Q1/3
According to CWPC the free board should be as follows:
Canal Banks:
The purpose of banks is to prevent spread of water beyond the specified limit. The width of the banks should be enough so that a minimum cover of 0.5 in soil is available everywhere above the saturation line. Hydraulic gradient in ordinary soils is kept 1 in 4 and for light soils 1 in 6. If banks become very high counter- berms may be provided on the outer slopes of the banks. Banks should be properly compacted while making.
CWPC has given following bank widths depending upon the discharge –
Service Road and Dowla:
A service road is provided usually on the left bank of the channel. In case of very small channel i.e. minor, it may or may not be provided. The top width of the bank carrying service road should not be less than 5 m. According to CWPC minimum road width should be 61 m. On large canals service roads may be provided on both the banks.
Service road level should be 50 cm to 1 in above the F.S.L. depending upon the size of the canal.
A Dowla is provided along the service road, separating service road from berms. This is provided as a measure of safety. Dowla is an earthen bond 50 cm high and 50 cm wise at the top. Side slope of Dowla is 1.5:1. It also prevents erosion of the slope due to rain.
Land Width:
The width of land, required to accommodate the canal cross-section and its connected elements, is known as land width for the canal. The distance between outer toes of canal banks plus a few metres on both the sides for the construction of side rain water drains or for growing tree rows, is known as the permanent land width of the canal. This land has to be acquired before canal construction is started.
During construction of the canal, some additional land is required for borrow pits and for stacking the materials. Such additional width of land is known as temporary land width. This land is acquired temporarily and returned to the owners after its use. Compensation is paid to the owners for their temporarily acquired lands.
Following are the recommendations of C. W.P.C. in regard width of the land:
(i) Width of land to be acquired clear of banks when canal is in less than balancing depth of cutting.
(a) For major canals – Width due to full height of bank + 5 m.
(b) For minors and distributaries – Width due to full height of banks above ground + 1.5 m.
In this case – (i) Extra land is required for borrow pits.
(ii) Width of land to be acquired clear of banks when canal cutting is more than balancing depth.
(a) For major canals – As per actual drawing + 5 m.
(b) For minors and distributaries – As per actual requirements +1.5 m.
Counter Berm:
It is also known as back berm. It is provided on the outer slope of the banks. It is required only in case of high banks and very permeable soils. Its main purpose is not to allow the seepage line expose on the outer slope of the bank.
Spoil Banks:
It is a method of disposal of surplus excavated soil from very deep reaches of the canal. When quantity of surplus excavated soils is not much it is used either to widen the banks or to raise the height of banks. If amount is large it is disposed of, by constructing spoil banks parallel to canal banks, but slightly away from the banks. The area enclosed between canal banks and spoil banks is properly drained.
Borrow Pits:
When amount of soil obtained from cutting is not enough to complete the banks of the canal extra earth is required. This extra earth is obtained from the borrow pits. Borrow pits may be constructed out of canal section or within the bed of the canal. Outside borrow pits are not preferred as they may become mosquito breeding centres during rains.
The outside borrow pits should not be deeper than 30 cm so that they may be easily reclaimed by the owners when land having borrow pits is returned to them. Outside borrow pits should be located at least 5 m away from the toe of the bank, in case of small canals, and 10 m in case of large canals.
Inside borrow pits are preferred to outside ones. The reason being that inside borrow pits get silted up during course of time, automatically. Inside borrow pits should not cover area more than half the bed width and suitable unexcavated bed should be left after each section so that water remains held up in them during running of the canal for silting purposes. Depth of inside borrow pits should not exceed 1 m.
Standards of Canal Cross-Section:
Usual dimensions of canal cross-section elements have been given here.
The standards as suggested by CWPC are given as follows:
Lacey’s Method:
According to Lacey, a canal is said to have attained regime condition when a balance between silting and scouring and dynamic equilibrium in the forces generating and maintaining the canal cross-section and gradient are obtained. If a canal runs indefinitely with constant discharge and sediment charge rates, it will attain a definite stable section having a definite slope.
If a canal is designed with a section too small for a given discharge and its slope is kept steeper than required, scour will occur till final regime is obtained. On the other hand, if the section is too large for the discharge and the slope is flatter than required, silting
will occur till true regime is obtained. In practice true regime conditions do not develop because of variations in discharge and sediment rates.
Lacey postulated that the required slope and channel dimensions are dependent on the characteristics of the boundary material which he quantified in terms of the silt factor (f) defined as
Where V = The mean velocity of flow in m/sec.
R = The hydraulic mean depth of an existing stable canal, and
D50 = The average particle size of the boundary material in mm.
The following three relationships may be used for determining required slope and canal dimensions:
Where S = longitudinal slope of the canal
Q = discharge in cumecs
P = wetted perimeter of the section in metres
R = hydraulic mean depth in metres.
Knowing the desirable values of P, R the curves given in Fig 19.7 given on next page may be used for determining the corresponding canal bed width (B) and depth (D) for a canal having internal side slopes of 0.5 :1 (it is assumed that the canal attains a slope of 0.5 :1 after running in regime.
Regime Type Fitted Equations for Design of Unlined Canals in Alluvial Soils:
The regime type fitted equations evolved on the basis of data collected from various states in India are given in Table 19.4. In these equations, average boundary conditions is taken care of by fitting different equations to data obtained from different states and assuming similar average boundary conditions in a state.
Tractive Force Approach for Design of Unlined Canals:
The unit tractive force exerted on bed of a running canal can be calculated from the formulae.
Z = Yω RS
When Z = unit tractive force in kg/ m3
Yω = unit weight of water in kg/m3 usually 1000 kg/m3.
R = hydraulic mean radius in metre and
S = canal slope.
The permissible tractive force may be defined as the maximum tractive force that will not cause serious erosion of the material forming the canal bed on a level surface. The permissible tractive force is a function of average particle size (D50) of canal bed in case of canals in sandy soils and void ratio in case of canal in clayey soils and sediment concentration.
The values of permissible tractive force for straight canal have been given by some authors on the basis of laboratory experiments, but the same can better be determined by analysis of observed data on existing canals. Once this is done, this would provide a rational approach to the design of section of regime channels. The value of permissible tractive for sinuous canal may be reduced by 10% for slightly sinuous once by 25% for moderately sinuous ones and by 40% for very sinuous ones.
In this approach, first the sediment concentration X of the canal flow and D50 size of the bed material in case of non-cohesive soils and void ratio of bed material in case of cohesive soils is determined and from these corresponding permissible tractive force shall be obtained by use of observed data of existing canals.
A suitable bed slope is then selected either with reference to average ground slope along the canal alignment or on the basis of experience and the value of R shall be obtained from equation Z = ω RS. Knowing the value of R and assuming a suitable value of N for the canal, the average desirable velocity of flow in the canal may be determined by using the Manning’s formula –
Thus the area of cross-section may be determined and knowing R and A, the desirable canal bed width (B) or depth (D) may be calculated.
Values of Rugosity Coefficient (N) for unlined canals as per IS: 7112 —1973 is as follows:
