In this article we will discuss about the layout of a canal system.
When a canal takes off from headwork’s it carries maximum discharge. As it traverses the land it goes on satisfying the water needs. Thus the discharge of the canal goes on reducing as it proceeds further in the irrigable area. Naturally when the canal carries small quantity of water same section need not be maintained.
Also, when small patches of land in different directions are to be irrigated canals with smaller section are required. Depending upon the size of the canal adopted and the function performed by that canal various components of a canal system can be recognised.
The components of a canal system are shown in Fig. 6.6 and are described below:
Main Canal (M):
This canal takes off from the river and derives water through a head regulator. For a certain length in the beginning the canal may be in full cutting. Obviously direct flow irrigation from such a channel is not possible as its water surface is below the general ground surface. Main function of the canal is to take the total water required for irrigation from the head and to deliver it in the canal system below.
Branch Canals (B):
To distribute the water over the entire commanded area main canal is branched. The bifurcated canals are called branch canals or simply branches. From the branches also direct irrigation is very rarely done. Main function of the branches is to make the water available in different parts of the tract for further distribution.
Distributaries:
They can be subdivided into:
(a) Major distributaries, and
(b) Minor distributaries.
Major Distributaries (D):
They are also named Rajbaha. Major distributaries are smaller in section than the branches. They are taken off mainly from the branches to distribute the water to various parts for direct irrigation purposes. Sometimes major distributaries may take off directly from main canal. A distributary supplies water to field channels through outlets.
Minor Distributaries (m):
They are smaller in section than the major distributaries. Main purpose of these distributaries is to reduce the length of the field channels. They are taken off from the major distributaries and sometimes from the branch canals. When the length of the field channels is more than 3 km minor distributaries are introduced to reduce the length. They are also called minors.
Table 6.1 shows the ranges of discharges of each type of canal:
Water Courses:
They are also called guls or field channels. They take irrigation water to the fields. The water courses derive their supply from the distributaries through outlets. The amount of water discharged through one outlet is in proportion to the area to be irrigated below that point. The length of a water course should not be more than 3 km to ensure minimum loss of water.