Drainage is of two forms
Surface drainage and
Sub surface drainage or underground drainage.
A) Surface drainage (Natural system of drainage):
It may consist of open ditches that are laid out by eye judgment, leading from one wet spot to another and finally into a nala or river. This is often called natural system.
Open ditch drains: The pattern of ditches is regular. The method is adopted to land that has uniform slope.
Field ditches: Field ditches for surface drains may be either narrow with nearly vertical sides or V shaped with flat side slopes. V shaped ditches have the advantages of being easier to cross with large machinery.
Narrow ditches: Narrow ditches are most common where large farm machinery is not used.
In level areas, a collecting ditch may need to be installed at one side of the field and shallow shaped ditches are constructed to discharge into the collecting ditch. The field ditches should be laid out parallel and spaced 15 to 45 meters or more apart as required by the soil surface conditions and crop to be grown. They should be 30 to 60 cm deep depending upon the depth of the collecting ditch.
Farming operations should be parallel to the field ditches. The care that a ditch will drain satisfactorily depends up on how quickly water runs into the ditch how much rain falls on the land, slope, and the condition of the soil and plant cover.
B) Sub surface or under ground drainage:
A sub surface or underground drainage will remove excess soil water. It percolates in to themselves, just like open drains. These underground drains afford the great advantages that the surface of the field is not cut off, no wastage of lad and do not interfere with farm operations. On the other hand, they are costly to lie and are not effective in slowly permeable clay soils.
Underground drains may be classified as:
Tile or pipe drain
Rubble (coarse stones or gravels filled) drains
Mole drains and
Use of pumps for drainage.
1. Tile drain: It consists of digging a narrow trench, placing short section of tiles at the bottom and covering the tiles with earth. The loose joints between two section of the tiles serve as a place where drainage water may enter into the drainage system. Water moves by gravity into the joins between tiles and through tile walls.
Porous tile gives no better drainage than tiles that water does not percolate and porous tile can easily broken or crushed. the drains are two types of tiles in use. Tile should be always placed at least 75 cm deep to prevent breakage by heavy machinery.
2. Box drains: Instead of pipes, underground drains may be made in V shaped cut or trench, sides of which are reverted with soil, restoring the surface of the field. Depth may be 90 cm below ground.
3. Rubble drains: A somewhat equally substitute for tile drains is made by cutting narrow V shaped drains or rectangular in section, as for box drains, filling them up with rough stones large and small and then covering the whole up with soil level with surface field soil. Depth may be 90 cm.
4. Mole drains: They are often used in clay, clay loam soils. A moling machine is one that draws a bullet nosed cylinder; usually 10-15 cm in diameter is therefore formed. A mole drain should be at least 75 cm below the surface to prevent closing of the holes by compaction from farming operations. Mole drains are extremely used in Europe.
5. Use of pumps for drainage: The pumps are used in U.S.A. and many other countries for drainage. River bottoms, lakes and costal plains, peat lands and irrigated lands are the main types of lands reclaimed by pump drainage. The subsequent must be sufficiently permeable for the ground water to move to the pipes enough for effective pumping.