The following points highlight the three main types of water conveyance system used in irrigation. The types are: 1. Open Canals 2. Earthen Canals 3. Lined Canals.
Type # 1. Open Canals:
An open canal, channel or ditch, is an open waterway whose purpose is to carry water from one place to another. Channels and canals refer to main waterways supplying water to one or more farms. Field ditches have smaller dimensions and convey water from the farm entrance to the irrigated fields.
According to the shape of their cross-section (Fig 10.6), canals are called rectangular (a), triangular (b), trapezoidal (c), circular (d), parabolic (e) and irregular or natural (f).
Most commonly used canal cross-section in irrigation and drainage is the trapezoidal cross-section.
Typical cross-section of a trapezoidal canal is shown in Fig 10.7:
Capacity of field channel depends on the extent of area to be irrigated, frequency of irrigation and the quality of water to be conveyed for each irrigation.
where, A = Area to be irrigated (ha)
d = Depth of water conveyance in the channel (cm)
F = Irrigation period (days)
H = Hours of irrigation in a day.
Conveyance losses over the length of the channel must be added to arrive at capacity of the channel.
Type # 2. Earthen Canals:
Earthen canals (Fig 10.8) are simply dug in the ground and the bank is made up from the removed earth.
Disadvantages of earthen canals are the risk of the side slopes collapsing and the water loss due to seepage. They also require continuous maintenance in order to control weed growth and to repair damage done by livestock and rodents.
The steeper the canal, the more will be the velocity and more the discharge for the same cross section. Normally, a channel should slope about 0.1 per cent. Silting may take place if the slope is less than 0.05 per cent. For determining the bed slopes, the velocities should be checked and maximum grade designed so that the velocities do not exceed the limits given in the following Table 10.2.
Field observations and experience indicated that it is adequate if the field channel has 30 cm bead width, 30 cm full supply depth and provide a free board of 15 cm. A slide slope of 1:1 in light soils and 1.5:1 in heavy soils can be allowed. Entire length of field channel should have the same carrying capacity equivalent to the flow at the top of the outlet.
Type # 3. Lined Canals:
Lining of channels is necessary to:
1. Reduce seepage losses during water conveyance
2. Prevent weed growth, which obstruct free flow in the channel
3. Reduce erosion in the channel.
Earthen canals can be lined with impermeable materials to prevent excessive seepage and growth of weeds. Lining canals is also an effective way to control canal bottom and bank erosion. Materials mostly used for canal lining are concrete (in precast slabs or cast in place), brick or rock masonry and asphaltic concrete (a mixture of sand, gravel and asphalt). Construction cost is much higher than for earthen canals. Maintenance is reduced for lined canals, but skilled labour is required.