Canals are waterways channels or artificial waterways for the conveyance or to service of water transport vehicles. They may also help with irrigation. It can be thought of as an artificial vision of a river.
Classification of Canal
A) According to function
1) irrigation canal
It carries water directly to the agricultural fields.
2) Navigation canal
The canal is used for navigation besides doing irrigation.
3) Power canal
The canal which is utilized for power generation.
4) Feeder canal
It is constructed with the concept of feeding two or more canals.
5) Carrier canal
A carrier canal besides doing irrigation carries water for another canal.
B) Based on financial output
1) Productive canal
The canal which yields net revenue to the nation after the full development of the irrigation in the area is known as a productive canal.
2) Protective canal
The canal which is constructed with the idea of protecting a particular area from famine is termed a protective canal.
C) Based on the soil through which constructed
1) Alluvial canal
The soil, which is made by transportation and deposition of silt through the agency of water, over a course of time, is known as alluvial soil.
The canals when excavated from this type of soil are known as alluvial canals. Canal irrigation is usually preferred in such areas, as compared to storage irrigation.
2) Non-Alluvial canal
It is an uneven topography, and hard foundations are usually available. The rivers, passing through such areas, have no tendency to shift their courses, & they don’t pose many problems for designing irrigation structures on them. The canals, which is passing through such areas are called non-alluvial canals.
D) Based on canal alignment
1) Watershed or ridge canal
The Watershed or ridge line is the highest line between two drainage areas.
2) Contour canal
A channel aligned parallel to the counter except for the necessary longitudinal slope is called a counter canal. This type of canal is generally constructed in hilly areas.
3) Side slope canal
It is a channel roughly at a right angle to counters. Such a channel is roughly parallel to the natural drainage & hence doesn’t intercept cross drainage & hence no cross drainage structure is required.
E) Based on the lining
1) Unlined canal
The canal whose surface is left unlined with any types of impervious materials is called a canal.
2) Lined canal
The canal whose surface is lined/hard with impervious material like brick, stone, PCC, etc. is called a lined canal. The lining of the canal decreases the seepage loss to about nil.
F) On the basis of the nature of the source
1) Permanent canal
When a provision of a regular graded channel & masonry works for regulation & distribution is justified by a well-assured source of supply is said to be a permanent canal.
The canal which gets water throughout the year is called a permanent canal. A permanent canal taken off from the source which is dry for part of the year is called a seasonal permanent canal.
2) Inundation canal
They draw their supplies from the river when there is a high stage in the river. They aren’t provided with headworks for the diversion of river water to the canal.
G) Based on discharge and relative importance;
- Main Canal
- Branch canal (Discharge > 5 cumec)
- Major distributary (Discharge ¼ to 5 cumecs)
- Minor distributary (Discharge < ¼ cumec)
- Field canal
Losses in canal
i) Loss due to evaporation
ii) Loss due to seepage
iii) Loss due to transpiration It can be minimized by the lining of the canal.
Cross Section of Canal
A most desirable section of the canal is one which is partly in cutting and partly in filling.
a) Berm:- It is the narrow width of land left at the G.L. between the inner toe of the bank & top edge of the cutting.
b) Banks:- The primary purpose of the bank is to store water. It may also be used as a service road. The minimum cover should be 0.5m
c) Back berm or counter berm:- It is a horizontal benching provided on the outer slope to maintain sufficient cover saturation line even if bank embankment width is sufficient.
d) Borrow pit:- When the E/W in filling exceeds the E/W in excavation, then the pit which is dug for bringing earth is known as the borrow pit. It should be external as well as internal. The width of the internal pit < ½ of the width of the canal.
It should be discontinuous. Minimum gaps between two consecutive pits should be L/2 (where L-Length of the pit). The depth of the borrow pit should not be greater than 1 m. It is normally provided along the center line of the canal.
e) Bed bars:- Bed bars are those masonry/concrete toe walls that are constructed at suitable intervals, so as to serve as permanent marks to indicate the correct alignment and theoretical bed level of the canal.
f) Critical velocity:- It is defined as the velocity where neither scouring nor silting occurs is known as critical velocity. The ratio of critical velocity & mean velocity is known as the critical velocity ratio(m).
g) Counterbalance:- A counterbalance is a vertical benching provided on the inner edge of the bank
h) Dowla:- It is a structure/construction provided on the bank of the canal for the safety of automobiles during inspection of the canal.
It is provided on the side of the roadway towards the canal, Its width varies from 0.3 to 0.6m and height 0.3 m above the service road. It is also known as a dowel.
i) Freeboard:- It is the difference between FSL and bank level which depends upon the capacity of the canal. It depends upon the size of the canal, water surface fluctuations, location of the canal, etc.
The main purpose of the freeboard is to keep the saturation gradient line below the top of the bank.
j) Spoil bank:- When the E/w in excavation exceeds E/w in filling, then excess soil is disposed of normally parallel to the canal on the edge of the embankment which is known as spoil bank. It may be continued or discontinued.
k) Service road:- The road provided either one side or both sides of the canal bank for inspection as well as maintenance works, is known as a service road. It is also known as an inspection road.
Advantages of irrigation canal
The main advantage of irrigation canal is;
- It helps in the development of un-irrigated land.
- Development of unirrigated wasteland.
- Dangerous droughts may be avoided to expedite economic development.
- The water requirement of crops during fluctuation in rainfall intensity will be met by having a
proper irrigation system.
- The canals constructed are permanent that need regular maintenance.
- Canal irrigation doesn’t let the groundwater level go down
- Canals also serve the aim of hydroelectricity, drink supply, fishery development, and navigation.
- Increasing food production.
Disadvantages of Canal Irrigation
The disadvantages of canal irrigation are:
a) Any imbalance in the water distribution process results in scarcity of water in some areas and water logging in other areas. And hence, this makes the soil unproductive.
b) Canal construction demands economic investment and time.
c) Breeding of bacteria and mosquitoes causing outbreaks of diseases like malaria.