Methods of drainage
There are generally two methods that are employed for drainage of crop fields:
A. Open drainage
The open drains are easy to construct and maintain. The open drains are generally employed for drainage of surface water in our country. Open drains are very suitable for an area where rainfall is heavy and drainage of more water is needed within a short period of time. Drainage channels are laid on the lowest contours of the land and they are utilized for draining of excess water from the land. The soils, having impervious lower layer are best suited for open drainage. But this system has the following disadvantages:
i. Wastage of land is the main disadvantage of this method as the drains occupy considerable land.
ii. Drains within the land hamper the cultivation practices.
iii. Soil erosion is more.
iv. The drains need repairing every year. As a result, cost of cultivation is increased.
v. Weeds grow on the side of drainage channels and their seeds are disposed off everywhere.
vi. Drainage of sub-surface water is not possible.
Arrangement of open drains
a. Parallel system: This system is used where surface is almost level and soil is uniform. The main drain may run at the centre, fed by laterals in right angles. It is the most desirable open drain system.
b. Mixed and random system: This system is used where land is not sufficiently lavelled; there are series of ridges and depressions. The land is undulating here and there. Topography determines the location of main drain and the laterals. Depending on the situation there may be more than one main drain located at different points. At depressions small collecting ditches and from them small drains leading to laterals or mains may be constructed. In the crop fields drainage type depends on the type of crop. Several furrows may be confined into one grid and drained. If the plots are basins they can also be grouped and drained to laterals and main drains.
B. Closed drainage
The closed drains are laid underground. The closed drains are desirable for a place where the land is costly. The closed drains save the land, but it is very costly. Generally tile drains, mole drains, pole drains and stone drains etc. are used as sub-surface drains.
1. Tile drains
The tile made of clay or concrete pipe or perforated steel pipes are mostly used for tile drains. The tile is about 30 to 50 cm in length and 7 to 12 cm in diameter. The tiles are installed end to end in the field after digging trenches with 2-3 millimeter spacing in the joint. The drains are usually spaced 1.5-45 m apart according to the type of soil. Excess water enters the system through the space in between the two tiles and conveyed along the gradient. Tile drains are grouped in classes depending on the arrangement of tiles:
a. Parallel system
b. Grouping system
c. Diagonal system
a. Parallel system: In the parallel system, the tiles are placed in underground soil by the following methods:
i. Single main:One main line is provided in a depression and lateral lines enter the main from both sides.
ii. Double main: Two mains are provided on either side of the depression. Laterals are joined to the mains to catch the water.
iii. Grid iron method: The grid iron method must be used where Ole’ land is practically level. It is a good method when the entire area is to be drained. Laterals enter the submain line from one side only to minimize double drainage.
iv. Herringbone or fishbone method: Herringbone pattern is used if the submain line is laid in a depression. The laterals join from each side alternatively and collect the water from either side.
b. Grouping system: If the field is vast, grades are different at different points and there are depressions here and there, this system is adopted. It consists of having individual system of drainage suitable for each area and finally joining all to a common outlet, if possible.
i. Mixed system: If the land is vast and terrain is rolling with scattered wet patches, the main drain is located in the natural drain-line avoiding making of several cuts here and there to make lines straight. To provide adequate drainage parallel system and/or other combinations may be used.
ii. Intercepting system: If the subsoil is impervious and seepage component is high this system is used. The tiles are laid at suitable depths to intercept the seepage water. The tiles are laid at seepage plains located by trenching. The tile line is so located that there remains at least 60 cm soil layer above the tile line.
c. Diagonal system: The field is divided into smaller units. Tiles are set in diagonally dug out drains.
Fig.Diagonal tile drain
2. Mole drains
Mole drains are unlined, circular earthen channels in the subsoil made with a tractor drawn mole plough. The channels are made 3.0-3.5 metres apart. The channels receive and discharge the water to drainage ditches at the edge of the field. Mole drains are suitable only on heavy clay soil.
3. Pole drains
In 60-90 cm deep and 30-40 cm wide drains, bamboo poles are placed in triangles and covered with leaves, twigs and soil. The sub lines join to the main.
4. Bush and stone drains
Trenches are made in the field and they are filled up with bush and stone. Water may pass through the open space of bush and stone. Afterwards, the bushes are dried and decomposed and made into a fine channel. Drainage is continued until the drains are closed with soil. There is every possibility of closing the drains with soil.
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