6.1 Canal

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

Classification of Canal

a) Classification based on nature of source of supply

i. Permanent Canal – when it is fed by a permanent source of supply. It is also sometimes known as perennial canal.

ii. Inundation Canals – They draw their supplies when there is high stage in the river.

b) Classification based on financial input

i. Productive canal – those which yield net revenue to the nation after full development of irrigation in the area.

ii. Protective canal – is a sort of relief work constructed for protection from famine.

c) Classification based on function of canal –

i. Irrigation canal – carries water to agricultural fields

ii. Carrier canal – carries water to another canal

iii. Feeder canal – Feeds two or more canals

iv. Navigation channel -

v. Power canal – carries water for power generation

d) Classification based on discharge and importance in network

i. Main canal – carries heavy supplies directly from river or reservoir. Generally no used for direct irrigation but feed branches and major distributaries.

ii. Branch canals – They act as feeder for major and minor distributaries. Sometimes direct outlets are provided. Carry discharge of over 5 cumec.

iii. Major distributaries – take off from branch canal or main canal. They supply water for irrigation to the fields through outlets.

iv. Minor distributaries – take off from branch canals or distributaries. They supply water to the water courses which ultimately feeds to irrigation fields.

e) Classification based on canal alignment

i. As Watershed Canals or Ridge canals : The dividing line between the catchment area of two drains (streams, water bodies) is called the watershed. In between two major streams there is main watershed similarly in between tributaries and main streams there are subsidiary watersheds, dividing the drainage between two streams on either side.It is advantageous in plain areas to align all channels on watersheds of the areas. Such a canal aligned along any natural watershed is called watershed canal. In such canal water flows by gravity on either side of canal. Cross drainages works are avoided as natural drainage will never cross a watershed. Sometimes watersheds are abandoned in order to bypass localities settled on watershed.

ii. Contour Canal: Watershed is not possible in hilly areas. As river flows troughs valley and ridge line may be 100m or above it. The canals in such cases are aligned parallel to contour in those areas. A contour channel irrigates only on one side because areas on the other side are higher. These types of canals would require cross drainage works.

iii. Side Slope Canal: A side slope channel is that which is aligned at right angles to the contours. Such a cannel is parallel to natural drainage flow and hence does not intercepts cross drainage and hence no cross drainage works are required.

f) Classification based on type of soil

i. Alluvial Canals : Alluvial soil is a soil which is formed by deposition of silt of river by long period of time. The area is even and having flat surface slope. Hard foundations are generally not available. Rivers have tendency to shift their courses. The canals when excavated through such soils are called alluvial canals. They required special precaution and design methods. Alluvial soil is very fertile as it can absorb fair percentage of rainfall and retain it in substratum making it highly productive e.g. North India.

ii. Non Alluvial Soils: Mountainous regions may go on disintegrating over a period of time in formation of rocky plain area called non alluvial area. Rivers passing through these areas have no tendency to change their courses they do not have pose much problems for designing irrigation structures on them. Canals passing through such areas are called non alluvial canals. Generally soils are non permeable. e.g. Maharashtra

Important points to be considered while fixing canal alignment

· Canal should be taken by direct route to command. This will reduce length of canal and transit losses

· In order to ensure better command canal should be aligned on ridge.

· Canal should be aligned such that it has minimum cd works.

· Curves should as far as possible avoided.

· Canal should not pass through alkaline, waterlogged or costly land.

· Canal alignment should avoid deep cuttings and high embankments.

Components of Canal cross section

· Side slopes - side slopes should be such that they are stable. Steeper slopes are provided in cutting. in cutting 1:1 or and to 2:1 in filling are the slopes generally adopted. Actual capacity is worked out with

side slopes but flatter slope is provided. Flatter is the slope it is more stable but there is more risk of silting.

· Berms – berms are horizontal offset at ground level provided between toe of bank and top edge of cutting. When fully developed berms serve following purposes

o Silt deposited on berms serves as good lining for reducing leakages and breaches.

o They give additional strength to banks and provide protection against erosion.

o As saturation line comes more in body of embankment possibility of breach is reduced.

o It protects bank from wave action.

o They provide scope for further widening.

o Berms can be used for borrow pits fro excavating soil.

· Free board – Margin between FSL and bank level is known as freeboard. It is 0.5m to 0.9m depending upon discharge.

· Banks – Primary purpose is to retain water but can be used as means of communication or inspection paths. High banks should be designed as earth dams.

· Service roads – It is necessary to give access to all parts of canal for inspection. They are provided 0.4m to 1.0m above FSL.

· Dowla – As a measure of safety in driving, dowlas 0.3m high and o.6m wide are provided along the bank.

· Spoil bank – When canal section is in cutting, excavated earth has to be disposed off economically. To dispose it by mechanical transport becomes costly. Therefore it is disposed in form of heaps on one or both the banks. Longitudinal drains running by their sides are excavated for disposal of rain water. Cross drains through spoil banks may also be excavated at 100m interval if needed, at these places spoil banks are discontinued.

· Borrow pits – When the canal section is in filling or embankment, and amount of earthwork exceeds earthwork in excavation, the required earth is brought from the pits known as borrow pits. The borrow pit should start from a point at a distance more than 5m for toe of small channels and 10m for large channel.

· Balancing Depth- A canal section will be economical when the earthwork involved at a particular section has an equal amount of cut and fill. Then it has to be paid once only and necessity of borrow pit or spoil bank is entirely avoided. For a given cross section there is always only one depth for which cutting and filling will be equal. This depth is known as balancing depth.

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