Ground Water Hydrology

2. HYDROLOGY
(Ground Water Hydrology)
Second Term, Final Year
B.E. Civil Engineering

3. Ground Water:
 Ground water is the water below the ground
surface occupying the pore spaces in rocks
and soils.
 Ground water is present every where beneath
land surface and ocean bottom and it is
always in motion.
 This ground water originates from precipitation
and surface water.

4. Sub-surface Zones:
The sub-surface can be divided into two zones.
1.Saturated zone
2.Unsaturated zone

5.  Saturated zone:
 It is also called “phreatic zone or zone of saturation.
 In this zone all the void spaces within the rock or
sediments are filled with water.
 Some part of rainfall is absorbed by the soil, the
amount of which depends upon the soil moisture
condition at the time of percolation.
 Unsaturated zone:
 It is also known as “ zone of aeration. In this zone all
the pore spaces in the rock and sediments are not filled
with water.

6.  Capillarity Fringe:
 It is the region above water table where the
water table when the water rises due to
capillary force in the porous medium.
 Interstices:
 Interstices is the portion of a rock or soil which
is not occupied by the solid mineral matter but
may be occupied by ground water.
 They are characterized by their size, shape,
irregularity and distribution.

7.  Types of Interstices w.r.to size:
1. Capillarity interstices ( sufficiently small)
2. Sub-capillarity interstices (very small)
3. Super capillarity interstices
 Types of interstices w.r.to connection:
1. Communicating interstices
2. Isolated interstices
Original Interstices:
They are formed by geological process governing the
origin of geological formation. They are formed in
sedimentary and igneous rocks.
Secondary interstices:
They are developed after rock formation. For example
joints, fractures, openings, etc.

8.  Porosity:
 Ratio of pore volume to total volume of a soil or rock.
 Permeability:
 It is defined as the ability of the formation to transmit or
pass water through it.
 The low permeable formation will produce less water
quantity where as high permeable formation will
produce large quantity of water.
 For permeability total formation is taken into
consideration. For example
Area = b x d (full width x full depth of formation)

9.  Specific Yield:
 The volume of water that a unit volume of
aquifer give up when drained by gravity is
called specific yield.
 If 1 ft3
of coarse sand aquifer produce 0.3 ft3
of
water, it means the specific yield of that aquifer
is 30%.
 Specific Retention:
 The portion of ground water which is retained
as a film on rock/soil surface or in small
openings is called specific retention.
Porosity = specific yield x specific retention
n = Sy x Sr

11.  Hydraulic Conductivity:
 The volume of water which will move in unit time under
unit hydraulic gradient through a unit area of porous
medium, which is at right angle to the direction of flow
is called “Hydraulic conductivity”.
 Transmissibility
 It represents the same meaning of permeability
physically but mathematically it differs from
permeability.
For transmissibility instead of full width, it is taken as
unity, but width is taken full.
Area = b x d = 1 x d
Area = d m2

12.  Transmissivity
The rate of flow of water through a vertical strip
of aquifer of unit width and extending to full
width saturation height under unit hydraulic
gradient at 600
F is called “Transmissivity”.
It shows the ease with which water can flow
through an aquifer.
T = b k
Where,
b = length of strip
k = co-efficient of permeability

13.  Stortivity:
 The volume of water that a confined aquifer will absorb
or expel from storage per unit area, per unit change in
head.
 Aquifer:
 It is defined as a geological formation which permits
storage of water and transmit appreciable quantity on
ordinary gravity condition. It consists mostly high
permeable material.

14.  Aquitard:
It is defined as a formation of semi-
impervious material which permits
storage of water but does not transmit it
freely.
 Aquiclude:
This formation consists of relatively
impermeable material which permits
layer of water but does not transmit it
freely.

15.  Aquifuge:
 This is an impermeable formation which neither
contains water nor transmits any water.
 Perched aquifer:

16.  Tube well:
 It is a longitudinal pipe driven in the soil profile for intercepting
one or more water bearing stratas.
 The discharge from an open well generally limited to 3.6
liters/sec. But in the tube wells, larger discharges can be
obtained by getting a large velocity as well as a large cross-
sectional area of water bearing stratum.
 Purpose of tube wells:
 To have a effective lift irrigation.
 To have a effective draw down of underground water table.
 To supplement the canal irrigation system in tail reaches.
 It is one’s own property of water source, therefore one can
grow any crop.
 Being private property, one can sell water on quantity or on
duty basis to other area.
 Being one’s own property, the wear and tear is less, hence
effectively serve the purpose.
 To have draw down of water table.

17.  Types of Tube wells:
Tube wells may be of four types.
o Strainer type tube well
o Cavity type tube well
o Slotted type tube well
o Perforated type tube well

18. 1. A Slotted type gravel pack tube well:
After placing the assembly of the plain and slotted pipes in the
bore hole, a mixture of gravel and bajri (called gravel
shrouding) is poured into the bore hole.
Shrouding is a process or a mechanism by which sediment
particles are obstructed by some filter material.
Design procedure:
First of all, a casing pipe of 36 cm diameter is lowered and soil
is excavated out, and the water bearing strata is penetrated by
a depth of about 5 m length. The perforated pipe some times
known as education pipe of 15 cm diameter is then lowered ,
the slotted portion being only 5 m and the rest of the length
being of plain pipe. Gravel is then pored from the top up to 3 to
4 m higher than the top level of perforated portion of the pipe.

19. 1. A Slotted type gravel pack tube well:
Continued…
oThe casing pipe is then with drawn 5 cm at a time and a
well is developed with the help of compressed air pumped
into the perforated pipe. Finally when the casing pipe is
fully withdrawn , the annular space between the casing
pipe and the perforated pipe is suitably plugged . By
developing the well with the help of compressed air, the
sand surrounding the gravel filter is freed of finer particles
and the chances of filter chocked are reduced. Due to the
provision of gravel shrouding , a larger area of radial flow
is obtained.

20. 2. Cavity type tube well:
oThis is a special type of tube well in which water is not drawn
through the strainer but it is drawn through the bottom of the
well where a cavity is formed. The tube well pipe penetrates a
strong clay layer which acts as a strong roof.
oThe essential condition for a cavity tube well to function
effectively is to have confined aquifer of good specific yield and
the aquifer should have a strong impervious material above it.
In the initial stage of pumping fine sand comes with water and
consequently a hollow cavity is formed. In the cavity an
equilibrium is established and clean water continues to enter
the well on further pumping.
oIn cavity type tube well flow is not radial but flow is spherical.
In the tube well, the area of flow is increased by enlarging the
size of the cavity. The cavity formed due to particular discharge
Q1 increases if an increased discharge Q2 is pumped out.

21. 3. Perforated type tube well:
oIn this type of tube well, total perorated
pipes are used in order to get more water.
oThe perforated pipe is made to have the
cross-sectional of its opening to that in the
wire mesh.
oInstead of perforated pipes, some times
this type of tube well may be blocked due
to entry of water because without entry of
entry of air water is difficult to take out.

22. 4. Strainer type tube well:
oStrainer type tube well is very common and widely
used tube well. In this type of tube well, a strainer,
which is a special type of wire mesh, is wrapped round
the main tube of the well.
oA strainer well may draw water either from an
unconfined aquifer of unlimited extent or from one or
more confined aquifer layers. The strainers are
provided only in that length of the pipe where it crosses
the aquifer. The pipe in the aquifer portion is kept
perforated but in the rest of the portion, plain or blind
pipe is provided. At the bottom, a short blind pipe is
provided to permit settlement of any sand if passed
through the strainer. The well is generally plugged at
the bottom.

23. Design procedure of Strainer type tube well:
oSize of the tube:
The size (diameter) of the tube is decided from the
permissible flow velocity through the tube. The flow
inside the tube is not constant but it increases to wards
upward direction. Hence it is suggested to fix the
velocity limit, knowing the discharge, the diameter will
be easily calculated and then insert the uniform
diameter pipe.

24. Design procedure of Strainer type tube well:
oDiameter (Size) of the Bore Hole:
Normally the size (diameter) of the bore hole should
be at least 5cm greater than the tube, so as to facilitate
the safe sinking of the pipe.

27. Design procedure of Strainer type tube well:
oType of Pumping:
Mostly following two types are commonly adopted
depending upon depth of water table.
Centrifugal Pump
Bore Hole Pump
Centrifugal Pump:
The centrifugal pump is also known as mono block
pump. It lifts water from lower level to higher level by
creating pressure with the help of centrifugal action.
The maximum head under which it can effectively work
is from 6 m to 8 m.

28. Design procedure of Strainer type tube well:
oType of Pumping:
Bore Hole Pump:
It has got two types:
i.Submerssible Pump
ii.Turbine type Pump
In submerssible, motor and pump both are attached
together, where as turbine type pump is driven by direct
coupled electric motor. These pumps are used for
higher values.

31. Quality of Irrigation Water
Generally irrigation water is not intended to provide
any nutrient, trace element or any other elements
needed for the plant growth. However, irrigation water
contains some silt content as well as certain salts
dissolved in it. The availability of some of these
ingredients in irrigation water would supplement the
nutrients and some ingredients may be injurious to
the plant growth. As such quality of irrigation water
depends primarily on its silt contents and salt
constituents. The effect of each of these factors on
the quality of irrigation water is discussed below.

32. Effect of Silt:
The effect of silt depends on silt material and
characteristics of the soil receiving the water. If
silt contains a larger content of plant nutrients
then it is quite beneficial because it has low
water holding capacity. However, in many
cases it may be injurious. This is specially so
when the silt is not rich in plant nutrients and it
is deposited on the surface. Moreover, the
accumulation of silt may reduce the
permeability of soil and make irrigation more
difficult.

33. Effect of Salts:
Salts of heavy elements such as lead, zinc,
selenium etc are injurious to plant growth even
of low concentration. However, under normal
conditions the solubilities of these salts are
quite low and hence the concentration of these
salts are usually less than those which may be
harmful for plants. Some slats like chlorides,
sulphates and borates of sodium, potassium,
calcium and magnesium have solubilities
significantly larger than the tolerance limit of the
plants.

35. The quality of irrigation water will also depend on the
amount of exchangeable sodium ions available in water.
As such in addition to the assessment of the total salts
content of irrigation water, its quality should also be
determined by assessing the exchangeable sodium ions
present in water. The assessment of exchangeable
sodium ions is made by determining the exchangeable
sodium ratio (ESR) which is defined as the
concentration of exchangeable sodium ions divided by
the sum of the concentrations of exchangeable calcium,
magnesium, sodium and potassium ions that is:

37. Classification of Irrigation Water:
On the basis of suitability of water for irrigation,
it may be classified in three categories as class
I,II and III.

38. Standards for Irrigation Water
Class
of
water
Electrical
conductivi
ty
In micro
mohos/cm
Total
dissolv
ed salts
(TDS) in
ppm
ESR
Chlorid
es
In ppm
Sulphat
es in
ppm
Boran
in ppm
Remark
s
I 0-1000 0-700 0-60 0-142 0-192 0-0.5
Excellen
t to good
for
irrigation
II 1000-3000
700-
2000
60-75 142-355 192-480 0.5-2.0
Good to
injurious
suitable
only with
permea
ble soil
and
moderat
e
leaching
.
Harmful
to more
sensitive
crops
III Over 3000
Over
2000
Over 75
Over
355
Over 480
Over
2.0
Unfit for
irrigation

39.  Advantages of Tube well Irrigation over Canal
Irrigation:
Following are the advantages of Tube well
Irrigation over Canal Irrigation:
 The well is under the direct control of the owner, hence
they may be sunk and equipped as required.
 Isolated area can also be irrigated by a well and wells
may be sited to command any desired land.
 The supply obtained from a well can be fairly control,
wells can be turned off at any moment, taking
advantage of rainfall.
Comparison of Tube well Irrigation with Canal
Irrigation:

40. As the well is centrally located therefore transient
losses are fairly reduced. The duty of water in well
irrigation is generally highest.
Volumetric assessment is possible.
Well irrigation helps in lowering sub-soil water level and
water logged lands can be reclaimed, but in canal
irrigation there are more chances of water logging.
Comparison of Tube well Irrigation with Canal
Irrigation:

41.  Unless drought conditions for several years, the well
irrigation does not fail, while canal irrigation may fail
during drought.
 More than one crop can be grown with the help of well
irrigation.
 The well water is warmer in cold weather and colder in
warm weather and it is more favorable to crop.
 The cost of construction of well is low and irrigation in a
locality can be introduced in stages.
 Since water is to be lifted from the well, therefore the
working expenses are very high in comparison to canal
irrigation.
Comparison of Tube well Irrigation with Canal
Irrigation:

42.  Water may not be available for the crop at right time
due to mechanical defects in pump or due to
interruption caused by electricity breakdown.
 The well water is clear and free from silt therefore it
does not have manuaring value that a canal water
supplies have.
 The tube well strainer is subject to progressive
deterioration to mechanical and chemical action. Thus
it requires replacement after frequent interval of time.
 The mechanical and electrical machines maintenance
also require more funds.
Disadvantages of Tube well Irrigation over
Canal Irrigation: