After reading this article you will learn about:- 1. Types of Land Drainage 2. Advantages of Land Drainage 3. Design.
Types of Land Drainage:
When the water stands on the surface, the problem of surface drainage takes place. Some methods, say a system of open ditches, have to be provided for removing surface water. In other situations both in humid and in arid region the problem is of high water table. This requires sub-surface drainage which can be provided by a system of deep open drains or by a system of buried pipe lengths known as tile drains.
In this system, tiles 30 cm in length and 10 to 15 cm dia. are burried at a suitable depth and placed end to end. Water enters through the tile joints and is led to a suitable outlet. This system has been profitably used in other countries for more than a century.
In our country, the use has been almost negligible. This is because of less emphasis on drainage, lack of know-how and possibly because of advantages being less direct than other practices like irrigation and fertilisation.
Both systems, open and tile drains, have their own advantages and disadvantages. Open ditches are comparatively easy to install. But they occupy land. In western countries, where cultivation is mechanised, the field ditches can be cultivated and properly maintained. In India cultivation is not mechanised because of small holding of farmers. The farmers can afford narrow deep drains in preference to wide shallow drains.
The deeper ditches are difficult to cross with farm machinery. Therefore, culverts become necessary. Maintenance of open ditches is also a big problem especially in humid regions. If not well maintained, these get choked with weeds and would need periodic clearing. Thus, although their initial cost is less, the recurring expenditure is quite heavy.
1. Surface Drainage:
Surface drainage is done by open ditches. These have to be of different sizes. One of these has to be a large size ditch or collection ditch which would take care of the drainage of the area. Others are the small field ditches that collect the water from the fields to the collection ditch.
(a) Field Ditches:
They are shallow ditches which should, as far as possible, be able to be crossed with farm machinery otherwise culverts should be provided at suitable points. They may either be constructed manually or properly spaced dead furrows constructed during ploughing operation may themselves be used to serve the purpose. The field ditches should be spaced 15 to 45 m apart, laid parallel to each other, and 30 to 60 cm deep.
When these are constructed, ploughing operation should be done parallel to field ditches. Generally, field ditches would have the same slope as the maximum slope of the field may range from 0.2 to 0.5%. On dead furrows, the side slope cannot be adhered to strictly. If manually constructed, they should have a side slope of 3 :1.
(b) Collection Ditches:
They may be either V-shaped or trapezoidal. If constructed by a grader, it will be easier to construct it as a V-shaped ditch. When manually constructed, it can be constructed either as a V-ditch or of a trapezoidal shape. For design of a size, design criterion has to be established for a particular area. In the absence of any criterion, 10 cm of removal in 24 hours may be tentatively used.
The side slopes may be kept about 2 : 1 to 1.5 :1 for clay loam soils. The size of ditch should be so designed that velocities do not exceed 75 to 90 cm/sec. Sometimes with higher slopes, it may need drop structures to cut down slopes and limit velocities.
The soil should be kept at a sufficient distance so that it does not fall back in the ditch. If practicable, this should be levelled gradually. The sizes and criteria have been only as an approximate idea.
2. Sub-Surface Drainage:
Drainage research is faced with two different but inter-related problems. The practical one of determining the optimum design for a drainage system on any given piece of land, and more purely scientific one of determining how the system functions and what effect it has on the soil and the crop.
Advantages of Land Drainage:
Drainage offers several advantages:
(1) It permits aeration of the soil, which is essential to root extension and growth.
(2) It makes conditions favourable for the development of beneficial soil micro-organisms. The oxygen of the air as well as suitable moisture conditions are essential for their growth.
(3) On poorly drained soils the yields are increased and the quality of the crop is improved.
(4) The length of the growing season is increased since early planting is possible.
(5) Root growth is increased because there is no interruption of root growth by excess water.
(6) It permits the use of flexible crop rotations and better soil management practices.
(7) Drainage may make available for cultivation of highly productive areas that were not suited for cultivation previously.
Providing a drainage system although has initial investment, but makes the farming profitable.
The Optimum Design for a Drainage System:
The following points need special consideration in designing a system:
(a) The arrangement of the drains with respect to each other and to the topography of the land;
(b) The depth and distance apart from the minors;
(c) The size and quality of tiles;
(d) Method of laying the tiles and refilling the trench, including, if necessary, special precautions to prevent blockage of the drain;
(e) The construction and protection of outfall; and
(f) Provision of inspection pits, setting chambers, etc. where necessary. However, more research has concentrated on depth and distance of the minors because on this aspect information is most urgently required.
The Optimum Drain Depth and Distance:
The optimum drain distance is the distance apart of the minors which give the best return on the money or labour expended in construction, but not necessarily the one which leads to the greatest improvement in the land. From the soil improvement point of view, heavy soils should be over-drained that effectiveness of the system could be diminished by laying the drains too close together.
In light soils, however, the possibility of over draining is a more real one. So optimum drain depth could be considered from economic point of view or maximum effectiveness of a system. Since cost of construction does not vary much with slight change in depth, this may coincide approximately with the optimum economic depth.
Although drainage engineers have worked out in considerable detail the mechanics and hydraulics of tile drainage, it appears that adjustment of the depth and spacing of tile lines to drainage requirement of different soils is still largely a matter of personal experience and judgement. Attempts have been made to determine the proper depth and spacing of tile on the basis of measurable characteristics.
The best known of these is the work by Neal at the University of Minisota who gaged the depth and spacing of tile by the measurement of soil plasticity and clay content. It is known, however, that these parameters are related only in a general way to soil permeability and drainage behaviour, so that the measurements are the best but doubtful indices of what depth and spacing of tile should be for specific soils.
A much more logical approach for tile spacing and depth is based on soil permeability and application of Daray’s law to the condition of lateral flow. Based on the permeability classes as identified by Neal (1949) the following depth and spacing (Table 7.3) has been calculated by Slater (1950).
The effectiveness of a drainage system with a given depth and distance will depend upon:
(a) The nature of soil, including mechanical composition, structure, percentage of calcium carbonate and organic matter;
(b) The crop and, more generally the type of farming practiced, i.e. grassland or arable, extensive or intensive;
(c) The topography of the land;
(d) The weather; and
(e) The cause of unfavourable moisture conditions. Soil condition has received more attention. In practice the life of the system also can only be estimated approximately.