International Commission on Irrigation & Drainage Commission Interationale des Irrigation et du Drainage



Canal Irrigation and Drainage

The conveyance and distribution systems consist of canals transporting the water through the whole irrigation system. Canal structures are required for the control and measurement of the water flow.

 

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.

 

The most commonly used canal cross-section in irrigation and drainage is the trapezoidal cross-section. Earthen canals are simply dug in the ground and the bank is made up from the removed earth. The 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.

 

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. The materials mostly used for canal lining are concrete slabs, brick or rock masonry and asphaltic concrete.The construction cost is much higher than for earthen canals. Maintenance is reduced for lined canals, but skilled labour is required.

 

Water flowing in steep canals can reach very high velocities. Soil particles along the bottom and banks of an earthen canal are then lifted, carried away by the water flow, and deposited downstream where they may block the canal and silt up structures. The canal is said to be under erosion; the banks might eventually collapse.

 

A drainage system is necessary to remove excess water from the irrigated land. This excess water may be waste water from irrigation or surface runoff from rainfall. It may also include leakage or seepage water from the distribution system.Excess surface water is removed through shallow open drains. Excess groundwater is removed through deep open drains or underground pipes.

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