Mod-01 Lec-40 Modeling and Management of Ground Water : Ground Water – Surface Water Interaction

Mod-01 Lec-40 Modeling and Management of Ground Water : Ground Water – Surface Water Interaction


Welcome to lecture number forty of ahh ground
water hydrology course ahh today we will talk about this modeling and management of ground
water under this ahh the topic that we will cover in this particular lecture is ground
water surface water interaction in lecture thirty nine in we have talked about this conjunctive
subsurface modeling ahhh with surface water a for over length flow case and we have already
talked about this waddle zone flow so in this particular lecture we will talk about ahh
ground water surface water interaction so ahh let us consider one shallow aquifer shallow
aquifer and we have
ground surface and then we have one stream there so this is a direction
of flow for the stream so flow and let us say that this is our water table and ahh this
is the stage height in your stream and this is the water level and the nearby aquifer
which is again shallow aquifer or unconfined aquifer in this case there will be flow from
your ground water table towards the stream and the aquifer will contribute some amount
of water in stream so again we have unsaturated zone here and this is basically ahh top cover
so grass cover or any other kind of cover is there on both the sides so this is a situation
where or aquifer is contributing in the stream so we can say that this is basically gaining
stream that means stream is gaining from our aquifer now if we see the water table contour
for this particular aquifer then ahh it will be ahh some kind of interesting thing because
there will be deviation of water table contour if there is a ahh stream in between ahhh a
particular aquifer region so let us say that this is my old domain now I have contour of
different ahh hydraulic head level so let us say this is level seventy sixty this is
fifty now ahh we have this stream here which is starting from this point now if you have
that contour near to this stream there will be some deviation so this is ahh corresponding to thirty this
is corresponding to twenty then now if we draw one obituary line here so we will see
that in aquifer ahh region your ahh ground water table is not deviating this is ahh groundwater
flow line which will be perpendicular to this so this is basically ground water flow
line and these are water table contours so in this case let us say we have two points
a and b so starting from a ahh if we move towards this stream then we will find that
there is variation and this contour is pointing towards the upstream direction so ahh in line
cross section the water table is at lower elevation ahh at the interest section point
and in this case we can say that our ground water contours will point upstream
in a gaining stream similarly if we see thr thing for loosing
stream where things are similar but only difference is
the water level in our aquifer and
stream so this is our ground surface now this is the water level and water table in the
aquifer is lower than the aquifer ahh level of water in the stream so this is our water
table so there will be movement of water from stream
towards the aquifer so we can say that this is our losing stream
like gaining stream if we draw the contour levels we will see the opposite thing compared
to our ahhh gaining stream so let us say this is our stream direction then this is our contour
level so we have this hundred ninety eighty seventy
ahh water table contours so in this case if we draw straight line as
we have drawn in for gaining stream we will find that this contour will ahh point towards
the downstream direction and that is at the intersection point with the stream so we can
say that in this case our groundwater flow is like this which is away from our stream
flow direction this is groundwater flow line so in this case we can say that if the contour
point downstream the point ahh the contour point downstream so ahh this is losing stream
so difference between losing and gaining stream is that the groundwater flow line in case
of our gaining stream because towards the stream but in case of our ahh this losing
stream this is away from the stream so another special case ahh of losing stream may be there
ahh which is ahh some kind of extreme situation for the
stream aquifer interaction so in this case this is the flow direction
and we have again that shallow water aquifer and water table is below ahh the bed level
of the stream so in this case water table is below so there will be contribution of
flow from our stream and this kind of situation will be there due to excessive pumping in
ahh aquifer region so this is basically disconnected stream
so in disconnected streams we have groundwater table ahh which is below the ahh stream this
is our stream and this is our stream bed level so this is always below the stream bed level
so these are called as disconnected streams now we need to see of what kind of fluid dynamics
is there near to stream and that is in our aquifer part
so in this one if we draw one obituary ahh cross section cross section so in this one we can divide
this whole region this is near to stream this is pointing towards stream and this is towards
our land now if we divide this the whole thing into three regions that is a b c so in this
region the behavior of the ahh total head is different
let us first draw the water table for this cross section so near to this land
there will be sharp change in the contours let us say this is one twenty this is one
ten and this is near to that sixty again some seventy eighty this is ninety then hundred
so in this region there is vertical flow almost vertical flow in this part of the ahh cross
section this is zone c or we can say this as zone so next part there will be almost
vertical contours so these are forty thirty and fron there let us say again there is change
in the pattern and now this region a things are not that vertical there is again curve
kind of contours which will be available so in region b there will be horizontal movement
of water and in this region there will be movement of water in upward direction now
if we install picometers in this three different regions then we will see what is the difference
in hydraulic head that will absorb so let us say that we are installing picometers heads
here then it will correspond to water level here
then if you are installing picometers here so it will exactly level ahh with the water
table and if we further go down then there will be again lowering of the picometric level
so lets say this is our region c prime so with zone c this c prime is nested monitoring
locations and in this nested monitoring locations we can see that for a monitoring will in this
region there will be ahh equivalent water surface which will correspond to this level
again for monitoring will ahh in this region there will be equivalent water surface which
will be corresponding to this level and this is at the top surface so this will correspond
to ahh our ahh original water level but interesting part is that almost ahh in two cases we have
found that for a particular region or picometric head is lower than the water table and the
location in this case if we try to draw the thing then we will see that this is almost
static even if we go down there will not be that much change in the water ahh level in
the picometers so this is almost same because vertically no variation in contours in this
region that is region a so this thing we can denote it as b prime now in c prime region
or else we can denote it when picometers with the straight thing
so in this region a let us say that we have one nested network of a prime so if we install
picometers here what will observe observation is that our water surface our picometric head
will be above water table so we can say that there will be ahh in first
case this is ahh lower than our water table this is almost similar to water table ther
is not much deviation but in this case it is higher so if we can install picometers
here so there will be spontaneous water flow from these picometers and if we have deep
viols in demographic depressions ahh particularly in river valleys then ahh the water is spontaneously
come out and that is ahh which is known as parting chambers when water is discharged
naturally to the surface and discharge point is called as springs so artesian wells if
we install wells and water comes out spontaneously then we say that these are artesian wells
if water comes out naturally ahh to the surface then we say that these are spring another
most important ahh point that is bank storage what is the effect of ahh bank storage on
groundwater surface water interaction bank storage effect so if we draw it you will see
that ahh with our shallow groundwater aquifer these are our ground surface in this case water level which is
this and interestingly this is our water table
during base flow so points a and b are there so what is this
bank storage we have flow direction and we have high stage here
bank storage occurs when the water level elevation in a surface water body increase
beyond the groundwater elevation in the adjacent banks so ahh we have flow direction then we
have high stage so there will be movement of water in this direction through banks there
will be movement of water towards the aquifer so this is called as ahh ahh bank storage
this is water table at high stage interestingly ahh there will be movement of
water from bank ahh from stream towards this bank and there will be elevation of water
table and ahh after some time when it reaches ahh when the balance equilibrium reaches in
the system there will be ahh higher water table near to stream compared to this lower
ahh groundwater table ahhh during base flow and in that equilibrium condition ahh still
beyond this point a and b there will be movement of water from ahh these ahh beyond these two
points in the this direction this is very complex in nature and water near the stream
in that case water near the stream ahh in that case water near the stream ahh moves
towards the stream but ahh it is beyond this a and b points which are intersection points
during high stage ahh levels and streams there will be movement of water towards the aquifer
so these are the direction of water flow situation so if you want to model this thing in ahh
practical situation then we need to idealize this particular system with some ahh simplified
ahh assumptions now let us say that we have some aquifer and in that one we have some
rectangular ahh stream channel so in that rectangular stream channel let us say this is groundwater level and here
is one region near to this channel that is river weight sediment on both the sides
and let us say this is or water level so with respect to out datum which is below this system
let us say this is our x coordinate system this is y coordinate system and this is in
the direction of that stream this is l and the saturated thickness
in this one is taken as m ahh h m and water table from pre defined datum is h and z zero
this is elevation of the stream bed level from ahh the datum and this is width of the
channel b and this is height of water in the channel and we have thickness del z prime
ahh is the thickness of bottom sediment allowing the weighted perimeter of the channel so with
this configuration ahh we can write our governing equation for ahh stream flow we can write
the equation as z del v by del l plus v del z by del l plus del z by del t equals to q
l and q v divided by b so here q v is the flow into the channel per unit width per unit
length through its weighted perimeter and q l is the lateral in flow per unit length
over the channel banks ahh and from tributaries and again ahh we need to have the momentum
equation so this s not is the bed slope and sf is the
friction slope so this is for stream flow no we need to write the equation for groundwater
flow so groundwater in unconfined aquifer ahh will have different thing in that one
dot t del h equals to s del h by del t plus qv b plus two z which is weighted perimeter
ahh of the channel now ahh this is ahh valid for the lower part of the channel and governing
equation for the other part that will be k which is hydrolic conductivity this is h m
this is saturated thickness of the aquifer h m then del h this is s y or specific ail
in this case this is ahh storage coefficient and this is specific ail for the aquifer and
there will be coupling between this ahh equation this equation this one and this one that is
continuing momentum for ahh saturated ahh confining portion and unconfined portion ahh
that will be coupled by darreys law and this is qv b two z this is torsion flex
q vis the flow into the ahh channel per unit length through its weighted perimeter so we
can say that this is the torsion flex and torsion flex ahh left hand side ahh torsion
flex right hand side we should have hydraulic conductivity and ahh this is hydraulic radiant
what is hydraulic radiant here z plus z not this is the total ahh head for stream and
we have head here that is age for any arbitrary location then we have this del z prime that
is the ahh difference between these two so we can say that this is our hydraulic radiant
and it is a coupling our all the equation then to solve it we need certain boundary
condition for open channel flow or river flow or stream flow we can either specify stage
or discharge at upstream location and we need to specify stage discharge relationship for
downstream location so this is for stream ahh thing and for aquifers
we will consider the whole region as impermeable so we have stream here which is flowing like
this and this parts this is our x and this is our y axis so this is basically del h by
del y this is zero and this case this is del h by del x is zero here also we have del h
by del x equals to zero so we can choose ahh a very large region and we can put that ahh
hydraulic head change to zero that way we can manage the ahh boundary conditions the
initial condition for stream flow ahh are depth and velocities so depth and velocity
that should be ahh known for initial condition and ahh with this configuration we can get
the variation so variation will be like this where this is our upstream direction
and this is our downstream portion so for any flood wave ahh there will be change
in the ahh hydraulic head in the aquifer with the change in the ahh fix ahh of flood wave
so if we draw one ahh simple figure maybe for some intermediate point here then we will
see that if this is our hydraulic head then for flood hydrograph with no leakage
this is positive direction this is negative and this is change in stream discharge
from steady state flow condition so this is flood hydrograph ahh this is without or no
leakage so there will be change in the ahh change in this lets consider this is with
no leakage so it will merge here and with leakage
there will be reduction this green line nd if we consider the effect of leakage then
we will see this difference ahh will plotted in this
so this will be the same so this is basically the effect of leakage
so the net effect of ahh leakage or net effect of bank storage this is bank storage or we
can say that effect of leakage leakage is or bank storage now we can have
ahh other situations where our pumping ahh will influence pumping will influence the whole thing so
let us say that we have region now this is our ground surface
and this is our water table ahhh which is matching here this is ahh confined bed water table so there
will be flow from this direction towards the stream this is actually our stream and this
is ground surface or land surface so there will be movement towards this stream now if
place some well here interestingly with small amount of pumping
there will water divivde and again there will be movement this side also this side but it
will be in normal direction but if we have some amount of heavy pumping
then it will be directly connected with the it will be directly connected with the stream
ahh stage and we will see a different ahh water ahh drow down in the pumping well this
is pumping so these are the effects ahh of aquifer on the stream and there is reverse
effect of stream on aquifer so amount of pumping also dictates the water divides so in ahh
the second case where we can have some amount of water divide for low pumping valve but
for high pumping valve this will be directly connected with the stream level so this aspect
is important because our aquifers can directly influence the stream water level so there
is always interaction between ahh streams and aquifers most importantly if we have two
reservoirs let us say that one reservoir is leaving water at certain rate another reservoir
is leaving ahhh at at different rate so in between if there is too much extraction so
in stream there will not be much water available so this is a total effect of stream water
ahh aquifer interaction ahh this ends ahh this lecture number forty

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