CANALS, RIVER TRAINING

Canals
A canal is an artificial channel, generally trapezoidal in shape, constructed on the ground to carry water to the fields either from a river or tank or reservoir.

Classification of Canals: The irrigation canals can be classified in different ways based on the following considerations. 

1. Classification based on the nature of source of supply:

a) Permanent canals 

- A permanent canal is one which draws water from a permanent source of supply. 

- The canal in such cases is made as a regular graded canal (fixed slope). 

- It is provided with permanent regulation and distribution works. 

- A permanent canal may also be perennial canal or non-perennial canal depending on whether the source supplying water is a perennial one or a non-perennial. 

b) Inundation canals 

- An inundation canal is one which draws water from a river when the water level in the river is high or the river is in floods. 

- These canals are not provided with any regulatory works, but an open cut is made in the banks of the canal to divert water. 

2. Classification based on the function of the canal:

a) Feeder canals : A feeder canal is constructed for the purpose of supplying water to two or more canals only but not directly irrigating the fields.

b) Carrier canals : A carrier canal carries water for irrigating the fields and also feeds other canals for their needs.

c) Navigation canals : A canal serving the purpose of in-land navigation is called a navigation canal. 

d) Power canals : A power canal supplies water to a hydro electric power generation plant for generation of electrical power.

3. Classification based on the discharge and its relative importance in a given network of canals: 

a) Main canal

A main canal is the principal canal in a network of irrigation canals. It directly takes off from a river, reservoir or a feeder canal. It has large capacity and supplies water to branch canals and even to major distributaries.

b) Branch canal 

Branch canals take off from a main canal on either side at regular intervals. They carry a discharge of about 5 cumec and are not usually used to directly irrigate the fields.

c) Major distributory

A major distributory takes off a branch canal or a main canal. It has a discharge capacity of 0.25 to 5 cumec. They are used for direct irrigation and also to feed minor distributaries.

d) Minor distributory

Minor distributaries are canals taking off from the branch canals and major distributaries. They carry a discharge less than 0.25 cumec. These canals supply water to field channels.

e) Water course or Field channel 

A water course or field channel takes off from either a major or minor distributory or a branch canal also. These are constructed and maintained by the cultivators/farmers.  

4. Classification based on Canal alignment:

a) Ridge canal or watershed canal

A Ridge canal or watershed canal is one which runs along the ridge or watershed line. It can irrigate the fields on both sides. In case of ridge canals the necessity of cross drainage works does not arise as the canal is not intercepted by natural streams or drains. 

b) Contour canal

A contour canal is one which is aligned nearly parallel to the contours of the country/area. These canals can irrigate the lands on only one side. A contour canal may be intercepted by natural streams/drains and hence cross drainage works may be essential.  

c) Side slope canal

A Side slope canal is one which is aligned at right angles to the contour of the country/area. It is a canal running between a ridge and a valley. This canal is not intercepted by streams and hence no cross drainage works may be essential.

5. Classification based on the financial output: 

a) Productive canals 

b) Protective canals 

A productive canal is one which is fully developed and earns enough revenue for its running and maintenance and also recovers the cost of its initial investment.

Protective canals are those constructed at times of famine to provide relief and employment to the people of the area. The revenue from such a canal may not be sufficient for its maintenance. The investment may also not be recovered within the stipulated time.

6. Classification based on the soil through which they are constructed: 

a) Alluvial canals 

b) Non-alluvial canals. 

Canals constructed in alluvial soils are known as alluvial canals. Alluvial soils are found in the IndoGangetic plains of North India. The alluvial soils can be easily scoured and deposited by water. 

Canals constructed through hard soils or disintegrated rocks are called non-alluvial canals. Such soils are usually found in Central and South India.

7. Classification based on Lining being provided or not: 

a) Unlined canals 

b) Lined canals 

An unlined canal is one which the bed and banks of the canal are made up of natural soil through which it is constructed. A protective lining of impervious material is not provided. The velocity of flow is kept low such that bed and banks are not scoured. 

A lined canal is one which is provide with a lining of impervious material on its banks and beds, to prevent the seepage of water and also scouring of banks and bed. Higher velocity for water can be permitted in lined canals and hence cross sectional area can be reduced. 

IMPORTANT RIVER TRAINING WORKS 

Guide bunds or banks

- Guide bunds are provided for  guiding the river flow past the diversion structure without causing damage to it and its approaches. 

- They are constructed on either or both on the upstream and downstream of the structure and on one or both the flanks as required.  

Afflux bund 

Afflux bunds are provided on upstream and downstream to afford flood protection to low lying areas as a result of floods due to afflux created by the construction of bridge/structure and to check outflanking the structure. 

Approach embankment

Where the width of the river is very wide in an alluvial plain, the diversion structure is constructed with a restricted waterway for economy as well as better flow conditions.

Groynes or Spurs

- Groynes or spurs are constructed transverse to the river flow extending from the bank into the river. 

- This form of river training works perform one or more functions such as training the river along the desired course to reduce the concentration of flow at the point of attack, creating a slack flow for silting up the area in the vicinity and protecting the bank by keeping the flow away from it. 

 Cut-offs

The cut-off is artificially induced with a pilot channel to divert the river from a curved flow which may be endangering valuable land or property or to straighten its approach to a work or for any other purpose.

 

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