Contents preface (VII) introduction 1—37

5.15. DRAINAGE OF IRRIGATED LANDS
Drainage is defined as the removal of excess water and salts from adequately irrigated agricultural lands. The deep percolation losses from properly irrigated lands and seepage from reservoirs, canals, and watercourses make drainage necessary to maintain soil productivity.
Irrigation and drainage are complementary to each other. In humid areas, drainage attains much greater importance than in arid regions. Irrigated lands require adequate drainage to remain capable of producing crops. The adequate drainage of fertile lands requires the lowering of a shallow water table, and this forms the first and basic step in the reclamation of waterlogged, saline, and alkali soils. The drainage of farm lands: (i) improves soil structure and increases the soil productivity, (ii) facilitates early ploughing and planting, ( iii) increases the depth of root zone thereby increasing the available soil moisture and plant food, (iv) increases soil ventilation, (v) increases water infiltration into the ground thereby decreasing soil erosion on the surface, (vi) creates favourable conditions for growth of soil bacteria, (vii) leaches excess salts from soil, (viii) maintains favourable soil temperature, and (ix) improves sanitary and health conditions for the residents of the area.
The water table can be lowered by eliminating or controlling sources of excess water. An improvement in the natural drainage system and the provision of an artificial drainage system are of considerable help in the lowering of the water table. A natural drainage system can be properly maintained at low costs and is a feasible method of protecting irrigated lands from excessive percolation. Artificial drainage also aims at lowering the water table and is accomplished by any of the following methods:
(i) Open ditch drains (ii) Subsurface drains (iii) Drainage wells
Open ditch drains (or open drains) are suitable and very often economical for surface and subsurface drainage. They permit easy entry of surface flow into the drains.
Open drains are used to convey excess water to distant outlets. These accelerate the removal of storm water and thus reduce the detention time thereby decreasing the percolation of water into the ground. Open drains can be either shallow surface drains or deep open drains. Shallow surface drains do not affect subsurface drainage. Deep open drains act as outlet drains for a closed drain system and collect surface drainage too.
The alignment of open drains follows the paths of natural drainage and low contours. The drains are not aligned across a pond or marshy land. Every drain has an outlet the elevation of which decides the bed and water surface elevations of the drain at maximum flow. The

220 IRRIGATION AND WATER RESOURCES ENGINEERING
longitudinal slope of drain should be as large as possible and is decided on the basis of non-scouring velocities. The bed slope ranges from 0.0005 to 0.0015. Depths of about 1.5 to 3.5 m are generally adopted for open drains. The side slopes depend largely on the type of embankment soil and may vary from 1/2 H : 1V (in very stiff and compact clays) to 3H : 1V (in loose sandy formations).
The open drains should be designed to carry part of storm runoff also. The cross-section of open drain is decided using the general principles of channel design. The channel will be in cutting and the height of banks will be small. If the drain has to receive both seepage and storm water, it may be desirable to have a small drain in the bed of a large open drain. This will keep the bed of the drain dry for most of the year and maintenance problems will be considerably less. Only the central deeper section will require maintenance.
Open drains have the advantages of: (a) low initial cost, (b) simple construction, and (c) large capacity to handle surface runoff caused by precipitation. However, there are disadvantages too. Besides the cost of land which the open drains occupy and the need of constructing bridges across them, open drains cause: (a) difficulty in farming operations, and (b) constant maintenance problems resulting from silt accumulation due to rapid weed growth in them.
Flow of clear water at low velocities permits considerable weed growth on the channel surface. The open drains have, therefore, to be cleaned frequently. In addition to manual cleaning, chemical weed killers are also used. But, at times the drain water is being used for cattles and the weed poison may be harmful to the cattles. Aquatic life is also adversely affected by the chemical weed killers.
Subsurface drainage (or underdrainage) involves the creation of permanent drainage system consisting of buried pipes (or channels) which remain out of sight and, therefore, do not interfere with the farming operations. The buried drainage system can remove excess water without occupying the land area. Therefore, there is no loss of farming area. Besides, there is no weed growth and no accumulation of rubbish and, therefore, the underdrainage system can remain effective for long periods with little or no need for maintenance. In some situations, however, siltation and blockage may require costly and troublesome maintenance or even complete replacement.
The materials of the buried pipes include clay pipes and concrete pipes in short lengths (permitting water entry at the joints) or long perforated and flexible plastic pipes. In addition, blankets of gravel laid in the soil, fibrous wood materials buried in the soil or such materials which can be covered by the soil and which will remain porous for long time are used for the construction of underdrainage system. If such drains are to be placed in impervious soil, the drains should be surrounded by a filter of coarser material to increase the permeability and prevent migration of soil particles and blocking of drains.
Mole drains are also included as subsurface drains. The mole drains are unlined and unprotected channels of circular cross-section constructed in the subsoil at a depth of about 0.70 m by pulling a mole plough through the soil without digging a trench, Fig. 5.25. The mole plough is a cylindrical metal object (about 300 to 650 mm long and 50 to 80 mm in diameter) with one of its ends bullet-shaped. The mole is attached to a horizontal beam through a thin blade as shown in Fig. 5.26. A short cylindrical metal core or sphere is attached to the rear of the mole by means of a chain. This expander helps in giving a smooth finish to the channel surface. The basic purpose of all these subsurface drains is to collect the water that flows in the subsurface region and to carry this water into an outlet channel or conveyance structure. The

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outlets can be either gravity outlets or pump outlets. The depth and spacing of the subsurface drains (and also deep open drains) are usually decided using Hooghoudt’s equation described in the following.

	Slot left by mole blade
	Mole channel