Here is an essay on the ‘Drainage System’ for class 9, 10, 11 and 12. Find paragraphs, long and short essays on the ‘Drainage System’ especially written for school and college students.
Essay # 1. Introduction to Drainage System:
Irrigation and drainage are two face of the same coin. Surface irrigation is a boon only if it is practiced with great care. As stated earlier, only optimum amount of water should be applied to the crop, as per the requirement of the crop and the properties of the soil. In fact, the root zone of the soil fails to absorb excess water which may percolate and help in raising the water table. If this gravity water encounter an impervious stratum and is not drained up properly.
This excess water is harmful to crop yield. It becomes necessary to remove excess water by draining it out from below the soil. The drained water may be discharged back either into a river or a canal or some other safe place. Hence, while designing a canal irrigation network, it is desirable to provide a suitable drainage system, for removing the excess irrigation water. Thus irrigation and drainage go together. Drainage system is also required for draining out the rain water and ensure its easy disposal to prevent its percolation.
Essay # 2. Drainage Coefficient:
The rate at which the water is removed by a drain is called the drainage coefficient. It is expressed as the depth of water in cm or metres, to be removed in 24 hours from the drainage area. The drainage coefficient largely depends upon the rainfall but varies with the soil type, crop, and degree of surface drainage. Its recommended value is 1 % of the average annual rainfall to be removed per day. Values of Drainage Coefficient.
A suitable value of drainage coefficient (DC) may be taken for the calculations, depending upon the local recommendations. Values of 1 to 2.5 cm/day for mineral soils and 1.25 to 10 cm/day for organic soils for different crops, have been suggested for humid regions, by U.S. Soil Conservation Service.
Essay # 3. Classification of Drainage System:
Drainage system can be classified as:
A. Surface Drainage.
B. Sub-Surface Drainage.
Surface drainage is also called open drainage system while sub-surface drainage as tile drainage or underground drainage.
A. Surface Drainage or Open Drainage:
Surface drainage is the removal of excess rainwater falling on the fields or the excess irrigation water applied to the fields, by constructing open ditches, field drains, and other related structures. In this process the land is sloped towards these ditches or drains, as to make the excess water flow in to these drains. In fact, land grading, which results in a continuous land slope towards the field drains, is an important part of a surface drainage system. Land grading or land leveling is also necessary for surface irrigation.
Types of Surface Drain:
Surface drains are needed for removing the storm water and excess farm water, for most of the cultivated crops on flat or undulating topography However, if no impervious layer occurs below the farm land and the water table is sufficiently lower, internal soil drainage are sufficient and no additional drainage facility is required. But for maximum productivity of most of the crops drainage facility becomes essential, particularly in waterlogged areas. Surface drains are two types shallow and deep surface drains.
i. Shallow Surface Drains:
The open drains, which are constructed to remove the excess irrigation water collected in the depressions on the fields, as well as the storm (rain) water, are broad and shallow, and are called shallow surface drains. These drains carry the runoff to the outlet drains. They are trapezoidal in cross-section. If designed properly, they should carry the normal storm water from the fields, plus the excess irrigation water.
Many a times, the excess irrigation water is neglected and these drains are designed only for the runoff resulting from the average storms which is neither economical nor desirable. Manning’s equations may be used to obtain design velocity of these drains, keeping the velocity within the critical velocity, and thereby avoiding silting or scouring. Proper shape is selected based on available information. Manning’s equation may however be used for the design of shallow as well as deep surface drains.
ii. Deep Surface Drains:
The drains, which are large enough to carry the flood water of the catchment area from the shallow surface drains, and are of sufficient depths to provide outlets even for the underground tile drains, if provided are called deep surface drains. These drains carry the storm water discharge, drains, shallow surface drains, and the seepage water coming from the underground tile drains. They are, therefore, designed for the combined discharge of the shallow surface drains as well as that of the tile drains.
Sub-surface drains are required for soils with poor internal drainage and a high water table Generally, a cunnette of about 0.6 m depth with steeper slope is provided in the center of the drain bed, so as to carry the seepage water of the underground tile drains. Cunnette is lined so as to withstand higher flow velocities.
The full section of drain is used only during the rainy season when the cunnette is not able to handle the flow will Manning’s equation may be used for estimating velocity and based on that flow capacity can be determined while designing of deep surface drains.
B. Sub-Surface Drainage or Tile Drainage:
Tile drains are required for high water table conditions in the areas where the soils are having poor internal drainage ability. If no impervious layer occurs below the farm land and the water table is low (lower than about 3m from the ground), internal soil drainage may be sufficient and no tile drains needed. For maximum productivity of most of the crops, both surface as well as sub-surface drains may sometimes, however, become, essential, particularly in areas of higher water table.
Types of Sub-Surface Drains:
The different forms of sub-surface drains are:
i. Surface Inlet:
A surface inlet is intake structure constructed to carry the pit water into the sub-surface or tile drain. A cast iron pipe or a manhole constructed of brick or monolithic concrete, is sufficient and satisfactory. Basically, it is the facility to remove the surface water from the pot holes depressions, road ditches and farmstead. This may also be accomplished by connecting them with the shallow surface drains called random field drains.
ii. French Drain:
When the quantity of water to be removed from the pits or depression is small, a blind inlet may be installed over the tile drain which is also called French drain. These are constructed by back filling the trench of the tile drain with graded materials, such as gravel and coarse sand, or with corn cobs, straw and similar substances.
Such inlets are not permanently effective. The voids in the backfill of the blind inlet become filled up with the passage of time, thereby reducing its effectiveness. Even though they are not permanently effective, they are economical to be installed and do not interfere with the farming operations.
iii. Bedding:
Bedding is a method of surface drainage which makes use of dead furrows. The area between the two adjacent furrows is known as a bed. The depth of the bed depends on the soil characteristics and tillage practices. In the bedded area, the direction of fanning may be parallel or normal to dead harrows, Tillage practices, parallel to the beds, retardh water movement to the dead furrows.
Ploughing is always parallel to the dead furrows. Bedding is most practicable on flat slopes of less than 15%, where the soils are slowly permeable and the drainage IS not economical.
Advantages of Tile Drains:
Tile drainage helps in increasing crop yields by draining the water or by lowering the water table in the following manner:
(a) Removes the free gravity water, not available directly to the plants, thus, increases the volume of root zone soil from which roots can obtain nutrient.
(b) Increases air circulation and bacterial activity in the soil, thus improving soil structure and making the plant food more readily available.
(c) Reduces soil erosion as a well-drained soil has more capacity to hold rainfall, resulting in reduced runoff and soil erosion.
(d) Helps removing toxic substances such as sodium, their excess amount may retard plant growth.
(e) Lowering the water table during rainy seasons thus checking water logging and salinity and other soil and water problems.
(f) Easy and timely cultural practices.
Limitations:
(a) Providing underground tile drains is a costly affair and may be required only in areas of high water table, and where the ground soil has a poor internal drainage capacity. In other words where it is absolutely desired.
(b) Without proper maintenance and care it becomes un-functional.
Laying Tile Drains:
Tile drains are usually, pipe drains made up of porous earthenware and are circular in section. The diameters may vary from 10 to 30 cm or so. These drains are laid below the ground level, butting each other with open joints. The trenches in which they are laid are back filled with sand and excavated material to form an envelope.
Envelope Filters:
It facilitates flow of drained water to tile drainage system. Tile drains are laid below the ground level in the trenches. The trenches in which they are laid are back filled with porous material, as shown in Fig. 3.26. As far as possible, the tile drains should not be placed below less pervious strata without envelop filter as the water will not be able to reach the drain. Thus, in absence of filter they may remain dry even-though the land above the impervious strata may be waterlogged.
When tile drains are placed in less pervious soils, they are generally surrounded by graded gravel filters, called envelope filters. The filter consists of different gradations, such as gravel, coarse sand, bajri, and other coarse material. The coarsest material is placed immediately over the tile, and the size is gradually reduced towards the surface.
The minimum thickness of the filter is about 7.5 cm. The graded filter may sometimes be substituted by a single gradation, depending upon the availability and cost considerations. Corrugated metal pipe with a flap shutter to prevent entry of rodents and back flow from the outlet into the tile drain is generally provided at the outfall point as shown in Fig. 3.27.
We can summaries the function of envelop filters as follows:
(a) It prevents the inflow of the soil into the drain.
(b) It increases the effective tile diameter, and thus increases the inflow rate.
Provision of Pump Outlet:
A pump outlet system needs to be installed, if the bed level of the outlet drain is higher than that of the discharging tile drain. It consists of an automatic controlled pump with a small sump for storage as shown in Fig. 3.28. However, pump outlets are costly and require technical expertise in installation and operation. We should collate the cost of deepening the outlet drain with the cost of installing and maintaining a pump outlet before finally deciding the course of action.
Depth and Spacing of the Tile Drains:
The rate of drop of water table mainly depends upon the soil permeability and spacing of the drains. Here, horizontal permeability of the soil is more important as the water has to travel more distance horizontally than vertically before it reaches the drain. As the permeability of most of the soils decrease with depth which affects the shape of the flow lines and the rate of the fall of water table. Let ‘S’ be spacing between the drains, and ‘a’ be the depth of impervious stratum from the centre of the drains, as shown in Figs. 3.29 and 3.30.
The tile spacing (S) is given by the following formula where, K = Hydraulic conductivity:
i = Difference between center of drain and minimum level of water level,
de = Equivalent depth of water conducting layer below the drain. It can be determined using Nomograp and
b = Height of water table above impervious layer.
The closed drains are generally spaced at such a distance as to be capable of lowering the water table sufficiently below the root zone of the plants. For most of the plants, the top point of the water table must be at least 1.0 to 1.5 metres below the ground level; although this distance may vary from 0.7 to 2.5 m, depending upon the soil and the crop.
The tile drains may be placed at about 0.3 metre below the desired highest level of the water table. A fair idea of the spacing between the tile drains can be obtained based on the above theory.
Essay # 4. Maintenance of the Drainage System:
For work reliability and long life the drainage system should be periodically maintained.
The following need to be done:
(a) In case of surface drainage regular cleaning and removal of weed, etc., must be taken care of. Particularly before start of rainy season special care should be taken.
(b) Any damage to the side wall should be corrected as and when required.
(c) In case of subsurface drain all different parts should be maintained periodically.
(d) All the damaged components should be replaced as and when required.
