A proper design of canal irrigation system also consist provision of a suitable drainage system for removal of excess water. Of course, this may not be required all over the command area of the canal, but may be necessary in areas of high water table and in river deltas. The drainage system may also help to drain out storm water as well, and thus to prevent its percolation and to ensure easy disposal. There are two types of drainages that may be provided, which are either of surface-type or of sub-surface types. These are briefly described in the following paragraphs.
- Surface Drainage
These constitute open ditches, field drains, proper land grading and related structures. The open drains which are broad and shallow are called shallow surface drains and carry the runoff to the outlet drains. The outlet drains are termed as the deep surface drains. Land grading, or properly sloping the land towards the field drains, is an important method for effecting surface drainage. The Bureau of Indian Standards code IS: 8835-1975 may be referred to for further details.
- Sub-Surface Drainage
These are installed to lower the water table and consists of underground pipes which collect water and remove it through a network of such pipes. These pipes are usually made of porous earthenware and circular in section and the diameter varies from 10 to 30 cm. for installation of these drains, trenches are dug in the ground and these pipe sections are butted against each other with open joints which help in allowing ground water to enter into the pipes. The trenches are then backfilled with sand and excavated material. The water drained by the tile drain is discharged into a bigger drain or into a deep surface drain. If the tile drain network is buried quite deep into the ground, it may be necessary to discharge the water of the drains into an underground sump and remove the water from the sump by pumping. Generally, an area is under laid with a network of tile drains, it is essential to calculate the spacing of these drains based on the depth through which the water table in the region is to be lowered.