CHAPTER FOUR CANAL IRRIGATION SYSTEM AND ...

CHAPTER FOUR

CANAL IRRIGATION SYSTEM AND CONSTRUCTION OF CANALS

4.1 Alignment of Canals

Irrigation canals can be alignment in any of the three way:-

4-1-1 Watershed Canal

4-1-2 Contour Canal

4-1-3 Side Slope Canal

4-1-1 Watershed Canal

Watershed canal

4-1-2 Contour Canal

Contour canal

4-1-3 Side Slope Canal

Side slope canal

4-2 Canal Irrigation network

The direct irrigation scheme using a weir or a barrage, as well as the storage

irrigation scheme using a dam or a reservoir, require a network of irrigation

channels. The entire network as irrigation channels is called canal system. The

canal system consists of:-

1- Main canal

2- Branch canals

3- Distributaries

4- Minors

1- Water courses

4-2-1 Main canal or head canal

The canal head works are generally situated in a valley, and the canal should

mount the watershed in the shortest possible distance. The canal, in this reach

must be aligned very carefully and has to be generally excavated in deep cutting

below N.S.L.( natural surface level ).

4-2-2 Branch canals

When a main canal leaves the high ground and bifurcate into branches,

covering the whole tract required to be irrigated, the canals in such portions

are called branch canals. Very little irrigation is done from the branch canals.

Discharge in a branch canal is generally is more than 30m3/sec.

4-2-3 Distributaries

Smaller channels while it takeoff from the branch canals and distribute their

supply through outlets into minors or water-courses are called distributaries.

They are aligned either as watershed channels or as side slope channels.

Discharge in a distributaries is generally less than 30m3/sec.

4-2-4 Minors

When the distance between the distributaries outlet and field is very high, the

small channels called minors. They are takeoff from the distributaries, so as to

supply water to the cultivators at the point nearer to their fields. Discharge in

a generally less than 2.5 m3/sec.

4-2-5 Water-courses

They are small channels excavated by cultivators, so as to take water from

the government-owned outlet point, provided in the distributaries or the minor.

Layout of an irrigation canal network

4-3 Design Capacities of irrigation Channels

Before discussing the techniques employed to determine the design discharge

for a channel, the following terms should be expressed.

4-3-1 Gross Commanded Area (G.C.A.)

It is the total bounded within the irrigation boundary of a project, which can

be economically irrigated without considering the limitation of the quantity of

available water. It include the cultivated and non-cultivated areas.

4-3-2 Cultivable Commanded Area (C.C.A.)

It is that part of G.C.A. in which cultivation is possible.

C.C.A. = G.C.A. – uncultivable area of G.C.A. --------- (4-1)

4-3-3 Intensity of Irrigation (II)

It is the ratio of the actually irrigation area during a crop season to the

cultivable commanded area (C.C.A.)

Intensity of irrigation Ii=𝑨𝒄𝒕𝒖𝒂𝒍𝒍𝒚 𝒊𝒓𝒓𝒊𝒈𝒂𝒕𝒆𝒅 𝒂𝒓𝒆𝒂

𝑪.𝑪.𝑨. ------------ (4-2)

4-3-4 Area to be irrigated

The area proposed to be irrigated at any one time is called the area to be

irrigated, and it obtain as:-

Area to be irrigated = C.C.A. × II -------------- (4-3)

Capacity factor = Mean supply discharge / Full capacity discharge------ (4-5)