Here is a compilation of essays on ‘Wells’ for class 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Wells’ especially written for school and college students.
Essay on Wells
Essay Contents:
- Essay on the Introduction to Wells
- Essay on the Types of Wells
- Essay on the Design of Wells for Water Supply
- Essay on the Installation of Tube Wells
- Essay on the Installation of Well Screens
- Essay on the Gravel Packing of Well and Well Development
- Essay on the Well Interference
- Essay on the Classification of Groundwater Movement into a Well
- Essay on the Steady Flow and Unsteady Flows to a Well
Essay # 1. Introduction to Wells:
Evidence of extraction of water from dug wells has been found in the archeological remnants of Mohenjodaro. In many of the cities established during the medieval ages in India, the main source of water was dug wells, though people were dependent on surface water bodies like rivers or, lakes, or ponds, if that happened to be nearby It is only during the past century that tube wells became popular as an easily operatable source of extraction of ground water. A well is a hole or a shaft, normally vertical, excavated in the ground for bringing water to the surface.
Gradually with easy access to electricity deep tube wells have become a common source of water. Establishment of tube well extraction of water involves knowledge about the movement of water through the geological formations. Water may have to be extracted from formations ranging from sand, silt, clay, fractured rocks of different compositions etc., A well may be dug to extract water from a confined or an unconfined aquifer. Digging of more than one well in close vicinity affects each others’ yield as the drawdown of one influences the other. This may be quantitatively estimated by theories of ground water flow applied to the radial flow of water to each well.
Essay # 2. Types of Wells:
A well is a hole or a shaft, normally vertical, excavated in the ground for bringing water to the surface.
Wells are generally classified as:
A. Open wells, and
B. Tube wells.
A. Open Well:
An open well is a well 2-13 m in diameter, dug into the ground to tap water from the tap pervious stratum. The depth is normally limited to 30 m. Large quantity of water is stored in the open wells.
Normally, these wells are either square or circular of uniform cross-section. Open well is also known as a dug well.
Open wells are further classified as:
(a) Shallow well, and
(b) Deep well.
(a) Shallow Well:
An open well that is constructed to tap water from the topmost water-bearing stratum, i.e., from the unconfined aquifer, is called shallow well. The bottom of such a well will not rest on an impervious layer. The water level in such a well will be equal to the level of the water table. These wells are also known as water table wells, unconfined wells or gravity wells.
(b) Deep Well:
A deep well is constructed to rest on an impervious layer, drawing water from the aquifer below it. The impervious layer provides a support to the wall of the well. The yield from a deep well is comparatively more than a shallow well and is relatively pure. The water level in such wells is equal to the piezometric head in the water bearing strata.
This classification of wells is purely technical and has nothing to do with the actual depth of the wells. A shallow well may be deeper than a deep well.
Based on lining, open wells can also be classified as follows:
(1) Unlined wells,
(2) Wells with pervious lining, and
(3) Wells with impervious lining.
B. Tube Wells:
A tube well is a long pipe sunk deep into the ground intercepting more than one stratum. Its diameter varies from 80 mm to 600 mm. The depth may vary from 50 to 150 m. Blind pipes are used for the impervious strata, and strainers are used against the previous strata. These strainers allow passage of water through them but prevent sand from coming in with the help of designed filters.
Tube well of water through s classified as:
1. Strainer well,
2. Cavity well, and
3. Slotted well.
1. Strainer Well:
This is most common and widely used type. The term ‘tube well’ is generally referred to this type of well. In this type, a special designed with mesh filter pack covering the water-bearing strata is used to draw water free from silt and sand. The pipe in this portion is perforated. In such type of wells, the inflow is radial. Fig. 5.28 shows a strainer well.
2. Cavity Well:
This is a special type of tube well in which water is not drawn through the sides but is drawn through the bottom of the well where a cavity is formed as shown in Fig. 5.29. Normally this type in this case is used when a clay layer is met with. The zone of the flow in this case is spherical.
3. Slotted Tube Well:
This type of well is used when a strainer type or a cavity type cannot be used. In this case, a slotted tube is installed to penetrate into water bearing confined stratum. A slotted tube is used at the bottom portion only as compared to a strainer type where strainers are used for several aquifers sandwiched between impervious layers. Figure 5.30 shows a slotted type tube well.
Essay # 3. Design of Wells for Water Supply:
A well is an intake structure dug on the ground to draw water from the reservoirs of water stored within. The water from the well could be used to meet domestic, agricultural, industrial, or other uses. The structure may be an open dug well, or as is common these days, may be tube-wells. The well may be shallow, tapping an unconfined reservoir or could be deep, penetrating further inside the ground to tap a confined aquifer located within aquicludes. In this lesson, we shall discuss the design of tube wells, a typical installation of which is given in Fig. 5.42.
Design of a well involves selecting appropriate dimensions of various components and choosing proper materials to be used for its construction. A good design of tube well should aim at efficient utilisation of the aquifer, which it is supposed to tap, have a long and useful life, should have a low initial cost, and low maintenance and operation cost.
The parameters that need to be designed for a well include the following:
i. Well diameter of the well must be chosen to give the desired percentage of open area in the screen (15 to 18 percent) so that the entrance velocities near the screen do no exceed 3 to 6 cm/s so as to reduce the well losses and hence, the draw down. The velocity should be reasonably low as indicated, such that the five particles within the sand should not migrate towards the well strainer slots.
ii. Well depth.
iii. Selection of strata to be tapped.
The samples during drilling are collected from various depths and a bore log is prepared. This log describes the soil material type, size distribution, uniformity coefficient etc., for the material available at different depths.
iv. Well screen design.
This includes fixing the following parameters for a well:
i. Well screen length.
ii. Well screen slot size.
iii. Well screen diameter.
iv. Well screen material.
In case of unconfined aquifers, where too thick and homogeneous aquifer is met, it is desirable to provide screen in the lower one third thickness. In case of confined aquifers where thick and nearly homogeneous aquifer is met, about 80 to 90 percent of the depth at the centre of the aquifer is advised to be screened.
Where too thick and homogeneous aquifers are encountered it is common practice to place screen opposite the more permeable beds leaving about 0.3 m depth both at the top and bottom of the aquifer, so that finer material in the transition zone does not move into the well.
The size of the well screen slots depends upon the gradation, and size of the formation material, so that there is no migration of fines near the slots. In case of naturally developed wells the slot size is taken as around 40 to 70 percent of the size of the formation material. If the slot size selected on this basis comes to less than 0.75 mm, then an artificial ground pack is used. An artificial gravel pack is required when the aquifer material is homogeneous with a uniformity coefficient less than 3 and effective grain size less than 0.25 mm.
The screen diameter is determined so that the entrance velocity near the well screen does not exceed 3 to 6 cm/sec.
The screen material should be resistant to incrustation and corrosion and should have the strength to withstand the weight of the well pipe. The selection of the screen material also depends on the quality of ground water, diameter and depth of the well and type of strata encountered.
Essay # 4. Installation of Tube Wells:
The entire process of installation of tube wells include drilling of a hole, installing the screen and housing pipes, gravel packing and development of the well to insure sand free water. Depending on the size of the tubewell, depth and formation to be drilled, available facility and technical know-how, different methods are used for the construction of tubewells.
Methods for Construction of Tubewells:
Two methods that are commonly used are explained below:
i. Cable-Tool Percussion Drilling:
A rig consists of a mast, lines of hoist for operating the drilling tool and a sand pump (Fig. 5.43).
The cutting tool is suspended from a cable and the drilling is accomplished by up and down movement (percussion) of the tool.
A full string of drilling tool consists of four components:
i. Drill bit.
ii. Drill stem.
iii. Drilling jars.
iv. Rope socket.
The drill bit is used to loosen the formation material and its reciprocating action breaks it down to smaller particles or muck. Water injected from the top converts the muck into slurry. For this purpose water is added as long as drilling continues in dry formations. The slurry flows up due to the pressure of water.
The drill stem fixed just above the bit provides additional tools in order to maintain a straight line. The drilling jars consist of a pair of linked steel bars and can be moved in a vertical direction relative to each other. The rope socket connects the string of tools to the cable.
ii. Rotary Drilling Method:
There are two main types of rotary drilling methods:
a. Direct rotary methods, and
b. Reverse rotary method.
In either case, a rotating bit is used as a drilling bit. The major difference is in the direction of the flowing fluid (Fig. 5.44).
The rotary drilling method, also sometimes called the hydraulic rotary method of drilling, uses continuously circulating pumped fluid. The power to the drill bit is delivered to the bit by a rotating hallow steel pipe or drill pipe. The drilling fluid or bentonite slurry is pumped down through the drill pipe and out through a nozzle in the drill bit.
The mud then rises to the surface through the hole and also removes the drilled formation material or muck. At the surface the fluid is led to a setting pit and then to a storage pit from where it is pumped back into the hole. Water and clay are added to the storage in to maintain quantity and consistency.
Essay # 5. Installation of Well Screens:
For installation of well screens, different methods are used depending upon the design of the well, the type of well, locally available facility and the type of problems encountered in drilling operation. The Pull-back method is generally used with the cable-tool percussion method of well drilling. After the casing pipe has reached to the depth where the bottom of the screen is to be located, the sand that might have flowed into the pipe is removed.
The well assembly consisting of screen and blind pipe lengths is lowered into the well. A heavy plate bail handle is provided at the bottom of the screen. The lowering of the assembly may be accomplished by suspending it by the bail handle using a flat hook attached to the sand line to engage the bail. After lowering the complete well screen assembly inside the casing pipe, the casing rip is pulled back.
For rotary drilled wells generally the Open-Hole method of screen installation used, though the Pull-Back method can also be used in this case too. In the open-hole method, after drilling the hole below the well caring, the drill stem is withdrawn and a telescope-size screen is lowered into the hole by any suitable method. The depth of the hole should be checked such that when the screen rests on the bottom of the hole, the lead packer should remain inside the lower end of the casing.
Essay # 6. Gravel Packing of Well and Well Development:
Well can either manually ground packed or artificially ground packed. Natural ground packed condition is created by removing the fine sand from the formation either by pumping or by surging. An artificially gravel packed well has a envelop of specially grand sand or gravel placed around the well screen. Ground pack is designed on the basis of sieve analysis of the aquifer materials obtained during drilling. Aquifer consisting of coarse materials of less uniform sizes may not require any gravel pack.
Well Development:
This process is used to remove sand, silt and other fine materials from a zone immediately surrounding the well screen. This is done by flow reversal through the screen openings so as to rearrange the formation particles in a naturally developed well and form a graded filter with materials of increasing porosity and permeability towards the well in an artificially gravel packed well, so that ultimately the well will yield clear sand free water.
Essay # 7. Well Interference:
When two wells are located close to each other, and when pumping is started in both the wells, a cone of depression will be developed for each well. If these cones of depression overlap each other, the yield from each of the wells will naturally be rte. When two or more cones of depression of the pumping wells overlap, affecting their yield, the phenomenon is known as well interference and is shown in Figure 5.45.
Essay # 8. Classification of Groundwater Movement into a Well:
The groundwater movement into a well is classified as under:
1. Steady radial flow in an unconfined, aquifer.
2. Steady radial flow in a confined aquifer.
In both the cases, the following assumptions are made:
i. The flow is horizontal and uniformly distributed in a vertical section.
ii. The velocity of flow is proportional to the tangent of the hydraulic gradient.
iii. The well fully penetrates the aquifer.
iv. The discharge taken from the well is constant.
v. The aquifer is homogeneous and isotropic.
Essay # 9. Steady Flow and Unsteady Flows to a Well:
Imagine a farmer using a deep tube or a dug well as a source of water for irrigating his field. The well may be fitted with a submersible pump or a centrifugal pump to draw out water and discharge at the head of a channel leading to the fields. As long as the pump is not in operation, the water in the well remains at a steady at a level, at that of the water table (Fig. 5.32).
When the pump is just started, it starts drawing out water from the well and the level of water in the well decreases. The water table surrounding the well also gets lowered (Fig. 5.33).
The water table gets lowered and forms a conical depression much like that shown in Fig. 5.34.
It may be observed that the surface of the water table, shaped now in the form of a cone, is steepest where it meets the well. Farther away from the well, the surface is flatter and beyond a certain distance, called the radius of influence, the surface of the cone is almost as flat as the original water table.
As pumping continues (at the rated capacity of the pump), the water table gets lowered further until it becomes steady. At this position the water surface is called the draw down curve (Fig. 5.35).
In must be observed that the water that is being pumped up from the well is being replenished by water traveling through the saturated formation towards the well. Further, if the capacity of the pump amount of water being in thrown from aware would be a lowered still.
Figures 5.32 to 5.35 depict a well that is drawing water from an unconfined aquifer. Corresponding figure of a steady state draw down curve in a confined aquifer would be as shown in Fig. 5.36.
We will now study the mathematical relation between the water pumped and the location of the draw down. It must be remembered that the flow towards the wells is actually taking place radially. Hence, we shall be predominantly using the ground water flow equations using the cylindrical coordinates (r, θ, z, w) in contrast to the ones using Cartesian coordinates (x, y, z).
Steady and unsteady flow situations may further be classified as being confined or unconfined, depending on the relative positions of ground water conveying strata and the water table.
Steady Flow to a Well:
a. Confined Aquifer:
Consider the case of a pumped well completely penetrating a confined aquifer (Fig. 5.37). The corresponding steady state piezometric draw down surface is also shown for the assumed constant pumped discharge Q.
The well is assumed to have a radius rw and the radius of influence is thought to be R where the potentiometric surface is nearly equal to the original undisturbed value H, measured from a datum.
At the well, the depth of water is designated by hw, which is also measured from a common datum. In general, at a certain radius r measured from the center of the well, the potentiometric surface stands at a height ‘A’ measured above the datum.
The yield from the well Q may be expressed in terms of Darcy’s law as:
Q = KiA
Where K is the coefficient of permeability of the formation, i is the hydraulic gradient that is, the slope of the potentiometric surface at the well found and A is the surface area of the well through which the flow is converging into the well from the aquifer.
Thus,
In the above equation, b is the thickness of the aquifer.
Naturally, the same amount of water also travels through the aquifer at a radial distance r from the center of the well.
Thus, yield would also be:
The above expression is true if the aquifer thickness b is assumed to be constant throughout. The above equations give us a value of the yield, Q, of the well but for that a measure of the gradient of the potentiometric surface is essential. This may be done by inserting a piezometer penetrating into the aquifer and noting the water level there (Fig. 5.38).
Integrating the equation between the limits of rw and r1, one obtains the following expressions:
Where T = Kb denotes the transmissibility of the aquifer.
This equation is known as equilibrium equation and can be used to determine variation of the potentiometric head radially outward from the well. The drawdown S at a radial distance r from the well (Fig. 5.39).
Where H is the undisturbed initial potentiometric surface and R is the radius of influence. If the drawdown S at distance r from the well is known it is possible to work out T or K as;
In case two peizometer are inserted near a well (Figure 5.40) and the peizometeric head at these two places are given as h1 and h2 then the following expression is arrived as:
This method of determining the permeability of an aquifer is known as Thiems method.
b. Unconfined Aquifer:
For the case of a pumped well located in an unconfined aquifer (Fig. 5.41) the steady state discharge conditions are similar to that of confined aquifer.
The flow at radial distance r from the well is given by the following equation under the simplifying assumptions made by Dupuit:
Where h denotes the height of the water take at a distance r above a datum, which may be the bedrock. Integrating between the limits of rw and r1.
By knowing the values of the water table at two places located at distances r1 and r2 from the centre of the well with corresponding heads h1 and h2 the value of the coefficient of permeability K can be worked out from the equation.
The water table head at any radial distance r can also be expressed in terms of H, the head at undisturbed initial water table before pumping as:
In the above expression, R is the radius of influence of the radial distances where the water table head is nearly equal to H.
The actual free surface will be slightly higher than the predicted free surface. This is because the gradient of the cone of depression is larger towards the well where the curvature of streamlines is most marked. The free surface of water table will actually meet the periphery of the well at some height above the water level in the well as shown in Fig. 5.41.
Unsteady Flow to a Well:
a. Confined Aquifer:
When a well starts pumping out water at constant rate, the potentiometric surface gradually gets lowered.
The unsteady state representation of the potential head in such a case is given by the following expression:
Where, h is the potential head at a distance from the well at a time t; S is the Storativity and T is the Transmissivity.
The boundary and initial conditions are defined as follows:
i. The potential head is equal to H, the undisturbed potential head at r ≥ R, times, and for all that is for t ≥ 0.
ii. At the well face, that is at r = rw the flux (or water getting discharged), Q, s related to the gradient of the potentiometric surface as,
iii. The initial condition that is at time t = 0, the following condition holds:
A solution of these equation based on the boundary and initial conditions, was given by Theims as in the following equation.
The unsteady state representation of the piezometric head, when the piezometric surface gradually gets lowered during of the well is given as:
In above equation H-h is the drawdown at any radial distance r, measured from the centre of the well. The infinite series term in the equation is generally designated as W(u), and in textbooks on well hydraulics as Raghunath (1998), these are tabulated for ease of calculation. However, with the help of calculators, it is easy to evaluate the first three terms and for practical calculations, only the first three or four terms may be considered.
If only the first two terms are taken, then the expression simplifies to:
b. Unconfined Aquifer:
As with confined aquifers, the decline in pressure in the aquifer yields water because of the elastic storage of the aquifer Storativity (Ss). The declining water table also yields water as it drains under gravity from the sediments. This is termed as specific yield (sy). The flow equation has been solved for radial flow in compressible unconfined aquifers under a number of different conditions and by use of a variety of mathematical methods.