Lesson 4: Principles of Groundwater flow Flashcards
Groundwater possesses energy in 3 forms, what are they?
mechanical, thermal, and chemical forms
There are three (3) external forces acting on groundwater, what are they?
gravity, external pressure, and molecular attraction.
When water in ground is flowing through a porous medium, there are 3 forces resisting the fluid movement, what are they?
- shear stresses
- normal stress acting
- internal molecular attraction of the fluid itself
device used to measure liquid pressure in a system by measuring the height to which a column of the liquid rises against gravity
Piezometer
a device which measures the pressure of groundwater.
piezometer
pressure of groundwater is called
piezometric head of groundwater
describe the velocity of groundwater flowing in
porous medium under natural hydraulic
gradient.
very low
average hydraulic gradient of streams during floods
10 m/s
The total hydraulic head is given by the formula ?
h = z + hp
the sum of the elevation head and the pressure head.
flow direction points to what value of head
to lower hydraulic head
what causes density variation to the hydraulic head?
salinity
water level in well filled with water coming from a point in an aquifer and which is just enough to balance the pressure in the aquifer at that point
Point-water head
the height of a column of fresh water in a well that is just sufficient to balance the pressure in the aquifer at that point
Fresh-water head
how to calculate for fresh water head
add the elevation head and fresh water pressure head
Total mechanical energy per unit mass
Force Potential
Force Potential is sum of what?
sum of the kinetic energy, elevation energy, and pressure.
driving impetus behind groundwater flow and is equal to the product of hydraulic head and the acceleration due to gravity.
Force potential (Φ)
Force potential (Φ) formula
Φ = gh
force potential is equal to what
Total mechanical energy
Hydraulic head _______ in the direction of the flow.
decreases
As groundwater moves, it encounters ________ between the fluid and the porous media
frictional resistance
Groundwater is _____ slightly as it flows and ______ energy is converted to _______ energy.
warmed;
mechanical;
thermal
when groundwater moves, is the change in temp measurable?
not under most circumstances
Slowly moving fluids are dominated by ?
viscous forces
when fluids are dominated by viscous forces, there is a low energy level and the resulting fluid flow is ?
laminar
In laminar flow, molecules of water follow smooth lines, called ?
streamlines
As the velocity of flow increases, the moving fluid gains _____.
kinetic energy
relates the four factors that determine whether the flow will be laminar or turbulent, quantifies turbulence
Reynolds number
Reynolds number formula
R = pqd/μ
where: R is the Reynolds number (dimensionless)
p is the fluid density (kg/m3)
q is the discharge velocity (m/s)
d is the diameter of the passageway through which
the fluid moves (m)
μ is the viscosity (kg/s.m)
what does the Reynolds number imply
If R > 2000, then it is turbulent flow
T or F: Turbulence in ground water is easy to detect
False. it is DIFFICULT to detect
______ in ground water is difficult to detect.
Turbulence
T or F: Darcy’s law applies only to slowly moving ground waters.
False: Darcy’s law applies only to VERY SLOWLY moving ground waters
an imaginary line that traces the path that a particle of groundwater would follow as it flows to the aquifer
Flow line
In a/an ________, flow lines will cross equipotential lines at right angles.
isotropic aquifer
Steady-flow conditions may be solved by graphical construction of what?
a flow net
a network of equipotential lines and associated flow lines
flow net
The method of flow net construction is based on 7 assumptions, what are they?
- the aquifer is homogeneous
- the aquifer is fully saturated
- the aquifer is isotropic
4.there is no change in potential field with time - the soil and water are incompressible
- flow is laminar, and Darcy’s law is valid
- all boundary conditions are known
3 types of boundary conditions possible
- no-flow boundary
- constant-head boundary
- water table boundary
If there is recharge or discharge across the water table, flow lines will be ______ to the water table. If there is no recharge across the water table, flow lines can be ______ to it
at an oblique angle; parallel
is a family of equipotential lines with sufficient orthogonal flow lines drawn so that a pattern of square figures results
flow net
resisting force that acts tangentially to the surface of the solid
shear stresses
resisting force that acts perpendicularly to the surface
normal stress
resisting force that resists the movement of fluid molecules past each other
internal molecular attraction of the fluid itself
shearing resistance is aka
viscosity
what must be remembered when measuring water table level
make sure na stable siya inig measure, not directly after gikuhaan (static well level)
how to calculate for hydraulic head
hydraulic head = surface elevation - depth to water
how to calculate for pressure head
pressure head = depth of piezometer - depth to water
how to calculate for the pressure at the tip of piezometer
P = hpg
The pressure at the tip of a piezometer = the height of water standing in the piezometer, multiplied by the density of the water in the piezometer and the gravitational constant
how to calculate elevation head
elevation head = surface elevation - depth of piezometer
how to calculate for vertical hydraulic gradient
vertical hydraulic gradient = total head / vertical distance between two piezometers
As the velocity of flow _____, the moving fluid gains _____. Eventually, the ______ due to movement are more influential than _______, and the fluid particles begin to _______ in an _____ fashion. The result is a _____ flow
As the velocity of flow INCREASES, the moving fluid gains KINETIC ENERGY. Eventually, the INERTIAL FORCES due to movement are more influential than VICOUS FORCES, and the fluid particles begin to RUSH PAST EACH OTHER in an ERRATIC FASHION. The result is a TURBULENT flow.
flow, in which the water molecules no longer move along parallel streamlines.
turbulent flow
The inception of fluid turbulent flow in groundwater has been reported at a Reynolds Number ranging ______
from 60 to 600
Experimentation has shown that Darcy’s Law is valid only when are such that the _____ forces of _____ predominate. These conditions prevail when the Reynolds number is ______
Experimentation has shown that Darcy’s Law is valid only when are such that the RESISTIVE forces of VISCOSITY predominate. These conditions prevail when the Reynolds number is LESS THAN 1 TO 10
when the flow lines will cross the equipotentional lines at an angle
there is anisotropy in the plane of flow
what dictates the angle flow lines will cross equipotential lines in there is anisotropy in the plane of flow?
dictated by
1. the degree of anisotropy and
2. the orientation of gradient (h) to the hydraulic tensor ellipsoid.
rare reynolds number for groundwater
R > 1000
what kinds of waters is turbulent flow common
surface waters
when is groundwater turbulent
R = 60 to 600, in cavernous aquifers with surface inputs and cave outlets, but rare in sand gravel and silt aquifers
boundary where Groundwater cannot pass
no-flow boundary
boundary where the head is the same everywhere, represents an equipotential line
constant-head boundary
boundary for unconfined aquifers
water table boundary
aka (potentiometric) piezometric surface
water table boundary
full NIA
national irrigation authority
