Well Hydraulics MCQ Quiz in தமிழ் - Objective Question with Answer for Well Hydraulics - இலவச PDF ஐப் பதிவிறக்கவும்

Explanation:

1. Specific Retention (Sr):

Specific retention of a rock or soil is the ratio of the volume of water a rock can retain against gravity to the total volume of the rock.

\(\rm\left( {{S_R}} \right) = \dfrac{{{Volume\;of\;water\;retain\;against\;gravity\;(V_{RG})}}}{Volume\;of\;soil} \times 100\)

In coarse gravels, the size of the pores is large, due to the large size of pores, water holding capacity against gravity will be less. Hence the specific retention will also be less.

2. Specific yield: It is defined as the volume of water released from storage by an unconfined aquifer per unit surface area of aquifer per unit decline of the water table.

3. Specific Capacity: It is defined as the rate of flow through the well per unit drawdown that has determined for initial first meter fall of water in well.

4. Specific storage: It is the amount of water that a portion of an aquifer releases from storage, per unit mass or volume of aquifer, per unit change in hydraulic head, while remaining fully saturated.

The discharge corresponding to the critical head is called as critical or maximum yield. Allowing a factor of safety a working head is specified and the corresponding yield is from well is known as safe yield.

Explanation:

Water-table surface: The surface at which pressure head and atmospheric head are equal.

Capillary fringe: It is the layer above the groundwater-surface in which water seeps at some extent due to capillary action.

Piezometric surface of the aquifer: A confined aquifer also known as artesian aquifer is confined between two impervious beds such as aquicludes or aquifuges. The water in the confined aquifer will be under pressure and hence the piezometric level which is the surface joining the static water levels will be much higher than top level of the aquifer.

Cone of depression: A depressed cone appears when groundwater is pumped out from well.

Concept:

Groundwater flow through wells:

Radius of influence:

• It is the maximum horizontal extend of the cone of depression when the well is in equilibrium with inflows.
• It is the distance from the Center of a pumped well to the point, where the drawdown is zero or is inappreciable.

Drawdown (s):

• It is defined as the drop of the WT elevation from the original WT level in a well due to pumping.

Cone of depression:

• It is the conical-shaped depression of the WT around a pumping well caused by the withdrawal of water.

• It is the areal extend of the cone of depression.

Drawdown curve:

• It is the piezometric surface under pumping.

Explanation:

Specific yield (Sy): 

It is the ratio of the volume of water that is extracted from saturated soil (due to gravity) to the total volume of the soil.

\({{\rm{S}}_{\rm{y}}} = \frac{{{\rm{Volume\;of\;water\;extracted}}}}{{{\rm{Volume\;of\;aquifer}}}}\)

Specific Retention (Sr):

Specific retention of a rock or soil is the ratio of the volume of water a soil can retain against gravity to the total volume of the soil.

Important Points
The relation between porosity (n), specific yield (Sy), and surface retention (Sr):

n = Sy + Sr

Explanation:

Thiem equation simplifies the actual geometry and boundary conditions and assumes steady-state flow in a fully confined, horizontal, and homogeneous aquifer.

Dupuit’s equation is used to compute the discharge through an unconfined aquifer.

For confined aquifers,

(a) Theim's theory,  \(Q = \frac{{2\pi bK\left( {{h_2} - {h_1}} \right)}}{{2.303{{\log }_{10}}\left( {\frac{{{r_2}}}{{{r_1}}}} \right)}}\)

(b) Dupit's theory  \(Q = \frac{{2\pi bK\left( {H - h_w} \right)}}{{2.303{{\log }_{10}}\left( {\frac{R}{r_w}} \right)}}\)

For unconfined aquifers,

(a) Theim's theory, \(Q = \pi K\frac{{(h_1^2 - h_{_2}^2)}}{{\ln \frac{{{r_1}}}{{{r_2}}}}}\)

(b)  Dupits theory, \(Q = \pi K\frac{{{H^2} - h_w^2}}{{ln\frac{R}{{{r_w}}}}}\)

Where

Q = Discharge

b = width

K = permeability of aquifer material

R = Radius of influence

rw = Radius of well

Explanation:

Specific yield:

It is the ratio of the volume of water that is extracted from a saturated rock (due to gravity) to the total volume of the rock.

\({{\rm{S}}_{\rm{y}}} = \frac{{{\rm{Volume\;of\;water\;extracted}}}}{{{\rm{Volume\;of\;aquifer}}}}\)

Specific Retention (Sr):

Specific retention of a rock or soil is the ratio of the volume of water a rock can retain against gravity to the total volume of the rock.

\(\rm S_R = \frac{{{Volume\;of\;water\;retain\;against\;gravity\;}}}{Volume\;of\;aquifer} \times 100\)

Therefore, the total porosity(n) is equal to the volume of water that a rock will yield by gravity drainage (Sy) and the volume held by surface tension (Sr) and is given as:

\(\rm {S_y} + {S_r} = n\)

Specific Capacity: 

• It is defined as the rate of flow through the well per unit drawdown that has been determined the initial first-meter fall of water in the well.

Specific storage: 

• It is the amount of water that a portion of an aquifer releases from storage, per unit mass or volume of the aquifer, per unit change in hydraulic head, while remaining fully saturated.

Concept:

Confined aquifer:

• When some amount of water is present in between two impermeable layers, it is known as a confined aquifer.
• It is also known as the Artesian aquifer.
• The pressure in this aquifer is greater than hydrostatic pressure.

• Groundwater in a confined aquifer is under pressure and will rise up inside a borehole drilled into the aquifer. The level to which the water rises is called the piezometric surface.

Hence, A confined aquifer is NOT associated with the phreatic zone because it is associated with an unconfined aquifer.

Concept:

The piezometric surface is an imaginary surface to which groundwater rises under hydrostatic pressure in wells or springs.

Confined aquifers are permeable rock units that are usually deeper under the ground than unconfined aquifers. They are overlain by relatively impermeable rock or clay that limits groundwater movement into, or out of, the confined aquifer. Groundwater in a confined aquifer is under pressure and will rise up inside a borehole drilled into the aquifer. The level to which the water rises is called the piezometric surface.

Isopiestic lines are the contours drawn to represent same piezometric surface.

Note:

The line joining the points of equal rainfall depth called isohyetal lines.

Explanation:

For confined aquifer,

(a) Theim's theory,  \(Q = \frac{{2\pi bK\left( {{h_2} - {h_1}} \right)}}{{2.303{{\log }_{10}}\left( {\frac{{{r_2}}}{{{r_1}}}} \right)}}\)

(b) Dupit's theory  \(Q = \frac{{2\pi bK\left( {H - h_w} \right)}}{{2.303{{\log }_{10}}\left( {\frac{R}{r_w}} \right)}}\)

For unconfined aquifer,

(a) Theim's theory, \(Q = \pi K\frac{{(h_1^2 - h_{_2}^2)}}{{\ln \frac{{{r_1}}}{{{r_2}}}}}\)

(b)  Dupits theory, \(Q = \pi K\frac{{{H^2} - h_w^2}}{{ln\frac{R}{{{r_w}}}}}\)

Where

Q = Discharge

b = width

K = permeability of aquifer material

R = Radius of influence

r_w = Radius of well

According to the above formulas, the correct answer is option 1) 1-B 2-C 3-A.

Concept:

Specific yield (Sy) 

It is the ratio of the volume of water that drains from a saturated rock (due to gravity) to the total volume of the rock.

The Specific Retention (Sr) 

It is the ratio of the volume of water a rock can retain against gravity to the total volume of the rock.

Therefore, the total porosity is equal to the volume or water that a rock will yield by gravity drainage (Sy) and the volume held by surface tension (Sr) and is given as:

\({S_y} + {S_r} = \eta\)

Calculations:

Given data

Sy = 25%

η = 45%

The total porosity is equal to the volume or water that a rock will yield by gravity drainage (Sy) and the volume held by surface tension (Sr) and is given as:

\({S_y} + {S_r} = \eta\)

\(25\% + {S_r} = 45\%\)

\({S_r} = 20\%\)

Hence, specific retention (S_r) is 20%.