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Here is a compilation of essays on ‘Drainage’ for class 7, 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Drainage’ especially written for school and college students.
Essay on Drainage
Essay Contents:
- Essay on the Meaning of Drainage
- Essay on the Causes for Water-Logging
- Essay on the Effects of Poor Drainage
- Essay on the System of Surface Drainage
- Essay on Subsurface Drainage
- Essay on Drainage Coefficient
- Essay on Drain Depths and Spacing
Essay # 1. Meaning of Drainage:
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The word drainage has multiple meanings. It is used in general sense to denote water outflow from a section of land. More specifically, it can serve to describe the artificial removal of excess water from cropped fields. Drainage aims at maintenance of soil moisture within the range required for optimum crop growth. In humid areas, drainage is needed for the removal of excess rain water.
In arid and semiarid areas, drainage is a necessary complement of irrigation. Drainage removes excess water to ensure a favourable salt balance in the soil and a water table elevation optimum for crop growth and development.
Essay # 2. Causes for Water-Logging:
Water losses resulting from irrigation and leaching will eventually join the ground water reservoir causing the water table to rise. In areas where natural drainage conditions are favourable, such a rise will be temporary. Many irrigation projects are adjacent to rivers. There is little natural drainage to the irrigated lands and the water table will continue to rise.
Causes for waterlogging are:
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1. Poor natural drainage of subsoil.
2. Submergence under floods and deep percolation from rainfall.
3. High intensity of irrigated agriculture, irrespective of soil and subsoil.
4. Heavy seepage losses from unlined canals, distributories and farm water courses.
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5. Enclosing irrigated fields with embankments and choking up natural drainage.
6. Hydraulic pressure from upper saturated areas at higher elevations.
7. Blocking of natural drainage channels by roads and railways.
Essay # 3. Effects of Poor Drainage:
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Lack of proper drainage causing serious damage to crops is not so well appreciated as that of irrigation.
Major points for consideration in this regard are:
1. Poor soil aeration inhibiting normal aerobic respiration and micro-organisms activity.
2. High water table curtail root penetration and inhibit crop growth and production.
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3. Soluble salts, harmful to crops, tend to concentrate in the root zone.
4. Plant nutrients in soluble form are lost through leaching and water flow.
5. Soil structure is destroyed, besides encouraging certain diseases and delaying crop maturity.
6. Wet spots in the field delay timely field operations.
For all these reasons, irrigated lands require drainage. In fact, irrigation without drainage, especially in river valleys prone to high water table conditions can be disastrous. Such conditions of waterlogging and salinisation, following the introduction of irrigation, are the rule rather than exception. To prevent or correct them, a drainage system will be indispensable.
Essay # 4. Systems of Surface Drainage:
Removal of free water tending to accumulate over the soil surface is called surface drainage. It is, generally, carried out by shaping the land so as to direct and dispose of overland flow. There are four types of drainage systems used in flat areas (less than 2 per cent slope) and one system for slopes exceeding 2 per cent.
a. Random System:
It is used where small scattered depressions to be drained occur over the area. Drains are usually designed to connect one depression to another and water is conveyed to an outlet. Open drains besides occupying land area are likely to interfere in farming operations.
b. Parallel System:
It is the most effective surface drainage system well suited both for irrigated and rainfed areas. Individual fields are properly graded for discharge into field drains. The field drain may discharge into field laterals bordering the fields and the laterals in turn lead to the mains. Laterals and mains should be deeper than field ditches to provide free outfall. This is also referred to as field ditch system.
c. Parallel Open Ditch System:
It is applicable in soils that requires both surface and subsurface drainage. It is similar to parallel field drain system, except that the drains are replaced by open ditches which are relatively deeper with steeper side slopes than the field drains. This is also known as diversion ditch system.
d. Bedding System:
It is adopted when the slope of land does not exceed 0.5 per cent and in slowly permeable soils. Parallel beds are developed by shaping and smoothing the land surface so that runoff drain laterally from the beds into dead furrows created in the same location each year by ploughing. Dead furrows lead runoff into cross slope collection and finally into an outlet ditch.
e. Interception System:
It is adopted in sloping areas (slopes exceeding 2 per cent). It consists of a series of shallow open drains across the slope with a mild grade to intercept and remove surface runoff. It is also known as cross slope ditch system.
The functioning of open drains depends on their proper maintenance.
If not maintained properly, the functioning of open drains may deteriorate due to:
1. Sedimentation.
2. Growth of vegetation.
3. Bank erosion.
4. Damage to side slopes.
Sedimentation of drains is mainly due to sediment moving into drains along with surface runoff. Sediment inflow can be minimised by watershed erosion control. Construction of silting basins and low check dams are recommended to reduce the rate of sedimentation. Weed growth reduces drainage channel capacity.
Depending on the situation, manual, mechanical or chemical weed control methods are to be followed. Damage by bank erosion and side slope damage occur due to topographical conditions through which the drain has to pass and also due to cattle and human trespass. Timely control measures are necessary to maintain the open drains for effective drainage.
Essay # 5. Subsurface Drainage:
Subsurface drainage refers to the natural outflow or to the artificial disposal of excess water within the soil or subsoil and it, generally, involves lowering the water table or preventing its rise. It is also known as groundwater drainage. Soils with high water table in the profile need subsurface drainage.
While surface drainage removes excess rain water in the root zone before it affects the crop, subsurface lowers the water table to provide better environment in the root zone. In subsurface drainage, water moves to outlets under the influence of gravity. Pipe or tile drains, mole drains, deep open drains and combination of tile and open drains are commonly used in subsurface drainage.
a. Pipe or Tile Drainage:
Continuous lire of pipes or tiles laid at specified spacing, depth and grade remove excess water from the soil profile. Perforated PVC pipes have replaced the tiles. Tiles or pipes of 360 to 600 cm length having 10 to 30 cm diameter are used.
Grades vary from 0.15 per cent for 10 cm diameter pipe to 0.05 per cent for 30 cm diameter pipe. The flow velocity of 30 to 45 cm s-1 is maintained to carry the sediment getting into the pipe lines. Pipes or tiles should be placed at 60 to 70 cm below the soil surface.
b. Mole Drainage:
Mole drains are cylindrical channels formed at a desired depth in the soil profile. There is no lining material and the inherent stability of soil gives stability to the mole drains. Water entering throughout the mole drains is guided to the outlet.
Layout of pipes or drains can be of several ways. Random or natural system is used for draining isolated patches and when entire area does not need drainage. The herringbone system is used in areas where a main line or sub-main could be laid in low area and laterals could be drawn from both sides.
The gridiron system is similar to herringbone system, except that the laterals enter the main only from one side. It uses less number of junctions and hence economical than herringbone system. Layouts such as double main, diagonal, interceptor, mixed, etc are followed depending on topography of land to be drained.
c. Drainage Well:
Water table can also be lowered by drainage well. A deep and uniform soil without any obstruction for downward movement of water is ideal for drainage well. It may be a gravity well or pumped well to remove water directly from root zone. It should be away from canals.
Essay # 6. Drainage Coefficient:
The flow rate from soil to drains depends on:
1. Hydraulic conductivity (permeability) of the soil.
2. Configuration of the water table (constant depth or falling water table).
3. Depth of the drain.
4. Horizontal spacing between drains.
5. Character (open ditches or tubes) of the drains.
6. Nature of the gravel used to increase the seepage surface and to prevent scouring of soil and clogging of drains.
7. Diameter and slope of the drains.
8. Rate of seepage (recharge) to the groundwater due to excess infiltration over ET.
A drainage coefficient which varies from place to place is used in determining the flow rate in drainage channel. It is defined as the depth of water (cm) to be drained in 24 hours period from the entire drainage area. Drainage of one ha cm (105l) in 24 hours equals drainage of 1.157 lps. It is also expressed as the flow rate per unit area (cumecs km-3).
The coefficient helps to determine size of the drain. Drainage coefficient is decided such that no appreciable damage is caused to the crops grown in the area. For open ditches in small areas, the value ranges from 0.6 to 2.5 cm. Intensity of rain and its duration are inversely proportional to the time allowed for removal of water. A time interval of 40 hours for removal of storm runoff for field crops and 72 hr for wet season rice are adopted.
Essay # 7. Drain Depths and Spacing:
Two types of soil profile are considered to illustrate the influence of drain depth, spacing and other factors on the quantity of flow of ground water towards and into drains (Hansen et al 1980). In highly permeable sandy soils, underlain by compact clay of low permeability of 1.7 to 3.0 m below the land surface (Fig. 7.43), the ground water flow is essentially horizonal towards the drains.
The water surface is maintained in the reservoir and adjoining soil at a distance of H m above the clay. Flow from reservoir to drain is steady, it being assumed that the reservoir is the only source of water.
Groundwater actually flows to the drain from both sides with similar conditions existing on both sides of the drain. Let 2q represent the flow into a drain in length L. Then the groundwater flow from one side to the drain is
q = A x V
where, A = cross sectional area
V = velocity of flow.
From Darcy law,
V = k x (hf/R) = k x [H – h/R]
Consider the depth of saturated sand about midway between the reservoir and the drain as average. Then the average area of saturated soil in drain length L, through which the groundwater flow is
And the quality of flow from reservoir to the drain
The quantity of flow to the drain from reservoir on both sides would be
From which
Flow, mid-way between reservoir and drain has been assumed to be q. However, with uniform vertical flow, the discharge past this mid-section will be only q/2. Hence, equation (1) becomes
and the quantity of water flow into the drain receiving percolating water will be
From which
For soils of great depth and approximately uniform permeability, groundwater flows radially towards the drain. Under such situations it is essential to use the calculus to derive the rationale equation for radial flow.
in which all the symbols except S and d have the same meaning as equation (2). S is the spacing between drains and d is the diameter of the drain.
Ranges of depth and spacing, generally, used for placement of drains in the field practice, based on mean hydraulic conductivity for different soils are given in Table 7.41.
Example 1:
Calculate the capacity required at the outlet end of the discharge ditch draining a watershed of 500 ha with a drainage coefficient of 5 mm?
Solution:
Example 2:
A ditch discharges 0.50 cumecs and drains 300 ha. Compute the drainage coefficient of the land?
Solution:
Total water discharged in 24 hours = 0.5 x 60 x 60 x 24
= 43,200 cumec
Example 3:
A watershed of 1,000 ha is discharging through a drain at 2.0 cumecs. Calculate the drainage coefficient? If the drainage coefficient should be 2.5, what should be the discharge through the drain?
Solution:
Discharge in 24 hours = 2 x 60 x 60 x 24 = 1,72,800 cumec
If the drainage coefficient is 2.5 cm, flow rate through drain
Example 4:
A drainage system is draining 20 ha flow at a design capacity for 3 days. If the drainage coefficient is 2.5 cm, how much water is drained in 3 days?
Solution:
Volume of water entering the drain per day = Drainage coefficient x area
Volume of water flowing for 3 days = 3 x 5000 = 15000 m3
Example 5:
Find out the spacing between drains when a waterlogged area, 6 m in depth, has to be drained with a drain of 6 m depth. Water should not flow above 1.0 m height in the drain. Length of the drain is 1,000 m and should carry a discharge of 0.05 cumecs. Soil permeability is 5 x 10-4 m s-1. Water table should be maintained beyond 2.0 m from soil surface?
Solution:
K = 5 x 10-4 ms-1
L = 1,000 m
H = 6 – 2 = 4 m
h = 1.0 m
Q = 0.05 cumecs
Spacing between drains
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