DESIGN OF IRRIGATION CANALS

Irrigation Canals:-

The canals used for irrigation purpose are called irrigation canals. Canals may be defined as artificial channels constructed on the ground to carry water from one place to the other. The canals may be classified as an alluvial or non alluvial canal which is based on the nature of source of supply. It may be termed as inundation or a permanent canal. Depending on how the water is fed from one system to another.

Classification of canal:-

1.    Alluvial Canal:-

A canal flowing through alluvium soil (silt, sand and gravel) is called an alluvial canal. A canal flowing through such sediments transports some of this material along with the flowing water.  These canals take supplies from rivers which always carry sediments rolling on the bed or held in suspension , which is passed on to the offtaking canals . If the velocity in a canal is very high, the suspension particles are not deposited, but if the velocity is very low, the sediment held in suspension will get deposited.

2.    Non Alluvial Canal:-

Non alluvial canals are those that have been lined with some suitable material to provide a rigid bed banks so as to avoid the problems with alluvial sides (boundaries) of a canal.

3.    Inundations canal :-

Those canals which depend for their supply on the periodical rise in the water level of the river from which they are taken off. The supplies of these canals are not always of the desired level .these canals are fill with water in Rainey season or in monsoon.

4.    Permanent canal:-

A canal is said to be permanent when its source of supply is sufficiently well assured so as to warrant the construction of a regular grade channel supplied for regulation and distribution. These canals are provided with permanent masonry head works, regulator and distribution works and are constructed with engg. skills. The lining of the irrigation canal is also a protective work as it helps in minimizing the chances of water- logging.

5.    Productive Canals:-

The canals, which indicate at the time of design and planning, that the total income will exceed the annual maintenance charges, are called productive canals.

6.     Feeder or Link canal:-

Link canals supply water from a reservoir to another place wherefrom a given irrigation canal system is fed. These canals are used for diverting surplus water from one source to another.

Design of irrigation canals –

            Chezy’s formula:-

        Chegy ‘s formula for the velocity of uniform flow in an open channel is given by the relation:-

V=C √ ( RS )

 Where,

 V =  velocity of flow in m/s

S = bed slope of the channel

C = Chezy’s coefficient

C= 23+(1/N)+(0.00155/S)  /   1+[ 23+(0.00155/S] (N/ √ R)

OR

C = 87 / 1+(K / √ R)

N= rugosity coefficient depending upon the nature of the bed and sides of the channel.

R = hydraulic mean radius

R = area of cross section of flow / wetted perimeter

Manning’s formula:-

The mean velocity for a uniform flow as given by Mannings is given by

V= (1/n) R^2/3 S1^/2

The value of n depends upon the bed and side material of the channel.

Kennedy’s theory:-

Sediment in flowing canal is kept in suspension solely by the vertical components of the constant eddies (opposing forces by bed of canal) .In order to obtain an expression for the silt supporting power of a stream, it may be safely assumed that the quantity of silt supported power is proportional to the width of the bed, all other conditions remaining the same. Hence the amount of silt supported in a stream may be expressed by A.B V₀^n-1. 

Where,

A= some constant

B= bed width

V_{0 =} velocity in stable state.

n= some index. It is depends on the type of silt.

Design of irrigation canal using Kennedy’s theory:-

When an irradiation canal is to be designed by the Kennedy’s theory it is essential to know ,F.S.D (Full supply discharge ,Q ) , coefficient of rugosity (N) ,C.V.R (m) critical velocity ratio & longitudinal slope  of  channel (s) . By making use of the following three equations a canal section can be designed by trials.

1.      V =0.546 MD^0.64

2.     Q = A.V

3.     V=C √ ( RS )

R = Hydraulic mean Radius

S = bed slope

Procedure:-

1.     Assume a reasonable full supply depth D.

2.     Using equation 1 find out the value of V.

3.     With this value of V, using equation 2 & find out the value of A.

4.     Assuming side slopes & from the knowledge of A & D, find out bed width B.

5.     Calculate R, which is ratio of area & wetted perimeter.

6.     Using equation 3 , find out the value of actual velocity V.

                       When the assumed value of D is correct, the value of V in step no 6 will be same as V calculated in step no 2. If not another suitable value of velocity come out to be the same.

Lacey‘s theory:-

  According to lacey the dimensions, i.e width, depths and slope of a regime channel to carry a given discharge & given silt charge are fixed by nature.

Regime channel:-

A channel will be regime if it flows in incoherent unlimited alluvium of the same character as that transported & the silt grade charge are all constant, a channel is said to be in regime when

1.     The discharge of the channel is constant.

2.     The silt grade & silt charge are constant.

3.     The channel is flowing in unlimited in coherent.

4.     The channel has freedom to form its own section alluvium of the same character as that transported.

     Incoherent alluvium:-

Soil composed of loose granular grade material which can be scoured with the same case with which it is deposited.

These conditions are very rarely achieved & very difficult to maintain in practice.

Hence according to Lacey’s conception regime conditions may be subdivided as initial and final.

                                    Fig 1

Initial Regime:-

If a channel is excavated initially with some defective slope & somewhat narrow section ,it is free immediately to throw down the incoherent silt on the bed there by increasing its bed slope & generating the increased velocity to achieve a non silting regime.

Final Regime:-

A channel which has formed its shape & slope in its own silt finally is said to be in final regime.

Design of irrigation canal making use of the Lacey theory:-

When Q & f = m² are known, design can be done in the following ways.

1.     Find out the value of V by using equation V = 0.4382 (Q.f^2)1/6.

2.     Calculate the value of R using equation R =4.825 Q^1/2.

3.     Calculate wetted perimeter (the area which covered with water) Pw using equation Pw = 4.825 Q^1/2.

4.     Calculate cross sectional area A , using equation Q = A.V

5.     Assuming side slopes , calculate the full supply depth from A , Pw & R using equations      a)   A = BD+ D² /2       OR      b)    Pw = B + √ 5 D

Where B = width of canal

6.     Calculate longitudinal slope of the canal using equation  = f^5/3  / 3316 Q^1/6

Comparison between Kennedy’s and Lacey’s theories:-

1.     The basic concept regarding silt transportation is the same in both.

2.     Kennedy assumes that the eddies are produced on the bed of canal only but Lacey proposed that eddies are produced along complete wetted perimeter.

3.     Lacey states that as the shape of an irrigation canal is fixed to particular geometrical figure, it cannot achieve final regime conditions and hence may be said to achieve initial regime only. Kennedy assumes that when there is neither silting nor scouring the channel is in its regime condition.

4.     Kennedy selects Kutter’s formula for designing irrigation canal. But in Kutter’s formula value of N is fixed.

5.     Kennedy use term C.V.R (m) but he did not give any basis for calculating m.  He simply states that it depends on the silt change & silt grade flowing in canal. But lacey has introduced silt factor f. He related f to mean diameter of the bed material & given basis to calculate f. The formula is f = 1.76 √ mr

6.     Kennedy gives no idea for calculating longitudinal regime slope . but lacey gives a regime slope formula.

7.     Design based on kennedy’s theory can only be achieved by making trials . lacey gave important wetted regime perimeter equation

Pw = 4.825           Q^1/2

Canal lining :-

 The impervious layer which protects the bed and sides of the canal from seepage is called canal lining.

Advantages of lining :-

1.     To save water for irrigation.

2.     To maintain the stability of section which reduces the change of shifting of outlets.

3.     To minimize the costs of maintenances.

4.     To carry water at higher velocities.

5.     To prevent canal’s bank erosion.

6.     To increase in canal capacity.

7.     Removal of silt & plants from the sides of beds.

8.     Minimizing of flood dangers.

Disadvantage:-

The cost are increased.

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