Mod-01 Lec-07 Aquifer Classification (Contd.), Ground water Basins and Springs; Dorcy’s Law;

Mod-01 Lec-07 Aquifer Classification (Contd.), Ground water Basins and Springs; Dorcy’s Law;


Welcome to this lecture number seven on continuation
of this aquifer classification so we had in the previous lecture we were discussing about
the aquifer characteristics and classification and specifically we discussed about it say
two types of aquifer the first one is the unconfined or table aquifer and we also discussed
about confined aquifer which is also known as artesian aquifer or pressure aquifer this
unconfined is having a impervious stratum only at the bottom and it has a variable top
it does not have any confining layer and water table which is variable depending upon various
factors so that forms the upper flexible boundary of the unconfined aquifers on the other hand
so this confined aquifer it is known as artesian aquifer or pressure aquifer it has two confining
layer one at the top as well as the one at the bottom as shown here and so here this
water in the ground water will be under pressure much above the atmospheric pressure that why
it is also known as pressure aquifer or artesian aquifer now let us also learn about say two
more aquifers so the first one is the perched aquifer so this perched means a raised aquifers
essentially it is a locally raised aquifer suppose in a particular area below the ground
there is a small saucer like impervious layer for a limited extent and over this this impervious
layer some amount of ground water gets stored so this represents the water table so here
this represents a zone of aeration and below that so this represent the zone of saturation
so this one represents the perched aquifer so the perched actually say this is the perched
water table and this one is the perched aquifer so here we should bear in mind that the impervious
layer let me write it as a localized impervious layer as a very limited areal extent and therefore
the amount of ground water stored in a perched aquifer is also limited and so therefore it
is available only for a small period of time and of course because so above the perched
water table there is zone of variation and below the water table there is zone of saturation
so it also is included as one of the maybe here you can call it as localized aquifer
localized unconventional aquifer so that is what the perched aquifer represents so this
forms the third type of aquifer and the last the fourth and last type of aquifer that we
are going to discuss is the leaky aquifer which can also be called semi confined aquifer
in this case it has it is a confined aquifer the only difference is one of the layers either
the the bottom layer and in this case the top layer
so this is a impermeable top layer say this is a semi permeable
bottom layer or it can be either other way also and of course so this is the the other
name for this semi permeable bottom layer is a aqui clude i am sorry aqui tard
and this is the leaky aquifer it can be the other way also that is the top layer maybe
semi confined semi permeable or aqui tard and the bottom layer so this case so this is the top aqui tard that is semi
permeable layer and this is the bottom
semi permeable or semi pervious impervious layer and here this is the
the leaky aquifer also known as semi confined aquifer so in this case so since the top layer
is semi pervious permeable or say pervious so from the top layer there will be a slow
contribution of ground water into this leaky aquifer slow ground water contribution from
top likewise as mentioning here the semi permeable or semi-pervious bottom
layer in this case there will be
so this represents slow ground water leakage at the bottom
so it is for this reason so these are known as semi confined aquifer so basically here
you can say it is ahh one end either one at the end or at the bottom there is a confined
there is a confining layer which is fully impervious and at the other end the either
at in this case here it is at the bottom and in this case here it is at the top so the
layer is an aqui tard which is semi pervious okay so these the unconfined aquifer or the
water table aquifer the confined aquifer or the artesian or the pressure aquifer and thirdly
the perched aquifer or raised aquifer we may also refer to as a localized raised unconfined
and frouthly it is leaky aquifer or say semi confined aquifer so these are the four types
of aquifers and each one of them will be here will yield ground water depending upon the
various factors and among this one of the important factors which determines the ahhh
this the ground water yield from aquifer is known as the storage co efficient also known
as storativity and this storativity is the the volume of water you can say volume of
ground water that an aquifer releases or absorbs per unit surface area per unit change in in
a head obviously head is ahh measured perpendicular to the the surface area so this is known as
the storage coefficient or storativity and generally so the for confined aquifer
and this denoted by the letter s generally for confined aquifer it is a unitless basically
because it represent ratio of two volumes for confined aquifers so this is a this storativity
will be in the range of 5 to the power 5 into 10 to the power of – 5 to 5 into 10 to the
power – 3 so this is the the storage coefficient and it the specific yield or ground water
yield from an aquifer very much depends upon this ahhh parameters and this so this storage
coefficient is considered as one of the three important formation constants of aquifer the
other two being the hydraulic conductivity or permeability and transmissivity or transmissibility
which of course will discuss sometime later in this lecture so now we will discuss little
bit about the ground water basins and springs
so just like the water basin which is which essentially represents a particular area on
the surface of earth which holds water and there will be say there will be specific drainage
pattern so in this case the ground water basin is also some kind of physical entity which
has a certain aerial extent and this this ground water basin consists of one large aquifer
as well as the number of small aquifer just like in the surface water basin there will
be a main course main stream channel or main course and it is there will be number of other
courses so in this case also in case also of ground water so there will be a main aquifer
and a number of inter related aquifer so here we can this ground water basin so we may define
it has a hydro hydrologic or hydro geologic unit containing a large aquifer and
a number of other inter connected aquifers inter connected or say connected aquifers
and like this surface water basin a catchment which is also referred to as watershed so
this ground water basin also has storage and this transport ground water transport both
are involved and many times so this ground water basins ahhh consist of large area aerial
extent as well as depth which can ahh yield a significant amount of ground water now let
us come to this springs so this spring is it is basically water gushing out from the
ground surface is generally known as the spring so we can write this is as concentrated discharge
of ground water appearing it ground surface
as a current of flowing water so here you can say
spring it is basically at the ground surface it represents an interface and upstream of
the spring the water is in ground water form and downstream of the spring the isn’t water
comes on earth so it is a surface water form and these springs may be of different types
such as here we can say this is types of springs and
of course this this springs in this the water may come come out as a current either in the
normal temperature or it may come out as a current of ahh hot or higher temperature so
like this so this spring can be a ahh either a normal water spring or a hot water spring
so in this in the normal water spring the temperature is the normal temperature and
which are generally refer to as a simply springs so these are let me write here so this is
the normal temperature springs so the various types us discuss few of them among the normal
temperature springs is the depression springs followed by contact springs followed by artesian
springs or say fraction artesian springs then there is a tubular spring
so these are the four important types of springs now let us discuss briefly about ahhh each
one of them so here let us consider say suppose suppose this is a ground water sub surface
layer and in this say let us say so this is the ground level and then this is the water table so here at
this table the water table meets the ground level and so therefore here this we have what
is known as depression spring? Through which the water gushes out from suppose there is
some normal even if we create very small depression so the water gushes out through that so this
is the first type of spring next we have what is known as the the contact spring and in
this contact spring suppose we have a a ground water a mount and with this is an impervious
layer and here
it is overlaid by a pervious layer and in this so this is a and this represents the
water table and this water table is in contact with the ground profile at at these two points
where we have will get a contact springs so this is again a
so essentially here what happens so the water slowly little get released from the contact
spring and then it flows along the impervious layer of this ahhh soil or rock mount and
eventually so it may join the stream or it may join a lake or any other surface storage
area like that so essentially this contact spring is found and of course in this case
unlike the the the depression spring so generally the water the the the current velocity will
be slightly less now the third one let us is the fracture artesian spring wherein we
have suppose a pervious layer which is over laid by
an impervious layer so this is impervious layer again this also an impervious layer
in between the pervious layer there is an aquifer so this is a and there is a fracture
so here this we can say this is a fractured rock over the impervious layer which is lying
above the pervious layer and through this fracture the water gushes through what is
known as fracture artesian spring so essentially so this is a the part of this
there is a fracture in the impervious layer which is over and above the aquifer and through
this fracture so the water gushes out through what is known as the artesian spring and the
last one so this are the three types of springs the last one let us discuss is the the tubular
spring and in this tubular spring we have suppose this is the fractured rock so these
are the fractures in the rock and suppose this is the level up to which
the ground water is stored and obviously the same level is maintained here also and here
at this point so this is a tubular spring and here we can say so these are the fractures
saturated with ground water and of course so this is the ground level so because here
the fracture continues upto the ground and then the ground water stored in these fractured
the level of ground water stored in these structures is above this the level of tubular
spring so the water gushes out as a spring so these are four types of springs and also
let us also discuss about the high temperature springs which are known as say thermal springs
they are also known as geothermal springs so in this case what happens is so the because
of the the large temperature the hot temperature through this colling magma chamber so this
is the heat getting released and what this does is suppose this is the
so here the surface water gets released so this is the descending
cool surface water and here of course we have a
so this descending cool surface water comes in contact with the heat release through the
cooling magma and then here so this is the so this is the level of water and this is
the rising hot water so when this descending cool water cold water
sold surface water comes in contact with the heat released by the cooling magma chamber
so it is it is temperature gets increased and then it comes as a rising hot water so
here so this is the hot spring or geezer and obviously so this the there will be many so
this cooling magma chamber is great depth something like three thousand meter or so
and because of that so the thermal springs ad many times so they will also have say other
mineral ingedirants and these cooling springs are available in are con be these hot water
springs or say thermal springs or geothermal springs can be found in different parts of
the world in such as say new zealand there are ample amount of sun and many times so
this geo thermal springs area also used in generating electricity so these some of the
springs so initially we saw the four normal temperature springs or simply springs followed
by we also discussed briefly about the geo thermal or a thermal spring or hot water spring
now so this is a so far we discuss about the ground water storage or occurrence part of
it now let us discuss a little about the the darcy’s law which essentially represents
the basic equation or basic law governing the ground water moment so here so this darcy’s
law we can state it as the discharge q= so firstly it is stated as the discharge is
proportional to i into a so this q is the discharge so here this is the ground water
discharge and this i is known as the hydraulic gradient
hydraulic gradient and this a is known as the is the cross sectional area of flow of
ground water flow s when q is proportional to i into a product of i and a so we can as
well write this q=kia where k is proportionality constant which also known as permeability
it is also known as the coefficient of permeability or it is also known as hydraulic conductivity
so here we can write this down as q over a which is equal to k into i so this q over
a q over a is also equal to the velocity and in this case it is the here we can take it
as the apparent ground water seapage velocity and here left hand side the dimensions of
velocity so for this equations to be dimensionally homogeneous so the dimensions of this k which
is the coefficient of permeability or it is also simply known as permeability or hydraulic
conductivity as equal to the dimensions of velocity divided by the dimensions of this
hydraulic gradient and this hydraulic gradient is a pure number with no units so therefore
this hydraulic conductivity or permeability or coefficient of permeability has the units
of velocity has the dimensions as well as units of velocity so this is darcy’s law
which is ah ah stated by the french hydraulic engineer henry darcy in say nineteen sorry
eighteen fifty six so here this i which is the the hydraulic gradient so this is equal
to – dh by dl so there is a negative sign to indicate that as the travel distance the
ground water travel distance l increases the head the ground water head or h decreases
so therefore this i will have a negative so therefore here we can mention we can state
here so this is v=- k into dh by dl so this is which is=simply k into i so this relationship
is popularly known as darcy’ s law which is applicable for say ground water flow and
as we can see so this ground water flow it is laminar flow it is highly laminar flow
the velocity is a very small so therefore so the ground water velocity for which the
darcy’s law is applicable so it depends upon the parameter which governs the laminar
flow that is the reynolds number here we can denote this as re so this
for the so the darcy’s law is valid for reynolds number
is let me say perfectly valid for reynolds number less than one and here we know this
reynolds is defined as the ratio of inertia force to viscous force and mathematically
=rho vd by mu where rho is the density v is the velocity d is the characteristic dimension
and mu is the viscosity of the coefficient of viscosity and so for reynolds number
greater than 1 and less than 10 so the darcy’s law is
more or less applicable more or less that means it is almost valid so only when the
reynolds number exceeds this 10 which is very rare in case of ground water then so this
the darcy’s law will not be valid and also we know that as per the hagen poisel equation
for the hydrali conductivity so this coefficient of permeability can be expressed by c dm square
into gamma by mu so this is by enology with hegan posial flow equation for laminar flow
so this coefficient of permeability so here the c is the shape factor and this dm is the
the mean particle size and this gamma is the specific weight of water
fluid and of course in case of ground water it is a water ok and obviously so this mu
is the viscosity and so this is=rho into g where rho is the density of water okay now
here we can conclude that so this k=cdm square and this gamma which is row into g
and g if we express it as the denominator of the denominator so this will be so this
is g i am sorry this gamma if i express is at denominator or denominator so then so this
is the kinematic viscosity which is denoted by this small letter mu therefore the hydraulic
conductivity or the coefficient of permeability or permeability is inversely proportional
to the kinematic viscosity of water so as the kinematic viscosity changes so the hydraulic
conductivity there is a inversely so here this term is cdm square which is denoted by
k0 which is the which is known as the intrinsic permeability
so obviously so this cdm square nothing to do with the fluid properties it has only to
do with the properties of the flow medium which is the soil or rock in this case so
here so this intrinsic of course this intrinsic permeability this is also known as the specific permeability is related with the permeability by the equation
k=k0 into g divided by mu so this dimensions of this intrinsic permeability as the dimensions
of this dimensionless constant as well as dm square it has a dimensions of say length
square so this is the the intrinsic permeability which has a so this is a a function of the
flow medium only okay and the next class we will discuss about the transmissibility or
transmissivity as well as other so this transmissibility along with this hydraulic conductivity and
storativity and forms what are known as a formation constants of fact refers so in the
next lecture we will discuss further about this hydraulic conductivity tranmissivity
and its determination and other related topics thank you

One thought on “Mod-01 Lec-07 Aquifer Classification (Contd.), Ground water Basins and Springs; Dorcy’s Law;

  1. Really nice initiative….in such densely populated country lacking institution like iit's can be compensated by providing education by same faculty on e – platforms will create educational equality….hats off..!

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