Groundwater Flow
Muhammad Qasim
What happens to groundwater over time?
Does it just sit, unmoving, like the water in a stagnant puddle,
or does it flow and eventually find its way back to the surface?
Countless measurements confirm that groundwater enjoys the latter
fate; groundwater indeed flows, and in some cases it moves great distances
underground.
Let’s examine factors that drive groundwater flow.
In the unsaturated zone the region between the ground surface
and the water table, water percolates straight down, like the water
passing through a drip coffee maker, for this water moves only in response to
the downward pull of gravity.
But in the zone of saturation the region below the water table
water flow is more complex, for in addition to the downward pull of gravity,
water responds to differences in pressure.
Pressure can cause groundwater to flow sideways, or even upward.
(If you've ever watched water spray from a fountain, you've seen pressure
pushing water upward.)
Thus, to understand the nature of groundwater flow, we must first
understand the origin of pressure in groundwater.
For simplicity, we’ll consider only the case of groundwater in
an unconfined aquifer.
 |
| The shape of water table beneath hilly topography. |
Pressure in groundwater at a specific point underground is caused
by the weight of all the overlying water from that point up to the water table.
(The weight of overlying rock does not contribute to the
pressure exerted on groundwater, for the contact points between mineral grains
bear the rock’s weight.)
Thus, a point at a greater depth below the water table feels
more pressure than does a point at lesser depth.
If the water table is horizontal, the pressure acting on an
imaginary horizontal reference plane at a specified depth below the water table
is the same everywhere.
But if the water table is not horizontal, as shown in above, the
pressure at points on a horizontal reference plane at depth changes with
location.
For example, the pressure acting at point p1, which lies below
the hill in figure above, is greater than the pressure acting at point p2,
which lies below the valley, even though both p1 and p2 are at the same
elevation.
Both the elevation of a volume of groundwater and the pressure
within the water provide energy that, if given the chance, will cause the water
to flow.
Physicists refer to such stored energy as potential energy.
The potential energy available to drive the flow of a given
volume of groundwater at a location is called the hydraulic head.
To measure the hydraulic head at a point in an aquifer,
hydrogeologists drill a vertical hole down to the point and then insert a pipe
in the hole.
The height above a reference elevation (for example, sea level)
to which water rises in the pipe represents the hydraulic head water rises
higher in the pipe where the head is higher.
As a rule, groundwater flows from regions where it has higher
hydraulic head to regions where it has lower hydraulic head.
This statement generally implies that groundwater regionally
flows from locations where the water table is higher to locations where the
water table is lower.
Hydrogeologists have calculated how hydraulic head changes with
location underground, by taking into account both the effect of gravity and the
effect of pressure.
 |
| The flow of groundwater. |
These calculations reveal that groundwater flows along concave-up
curved paths, as illustrated in cross section (figure above a, b).
These curved paths eventually take groundwater from regions
where the water table is high (under a hill) to regions where the water table
is low (below a valley), but because of flow-path shape, some groundwater
may flow deep down into the crust along the first part of its path and then may
flow back up, toward the ground surface, along the final part of its path.
The location where water enters the ground (where the flow
direction has a downward trajectory) is called the recharge area, and the
location where groundwater flows back up to the surface is called the discharge
area (see figure above a).
Flowing water in an ocean current moves at up to 3 km per hour,
and water in a steep river channel can reach speeds of up to 30 km per hour.
In contrast, groundwater moves at less than a snail’s pace,
between 0.01 and 1.4 m per day (about 4 to 500 m per year).
Groundwater moves much more slowly than surface water, for two
reasons.
First, groundwater moves by percolating through a complex,
crooked network of tiny conduits, so it must travel a much greater distance
than it would if it could follow a straight path.
Second, friction between groundwater and conduit walls slows
down the water flow.
Simplistically, the velocity of groundwater flow depends on the
slope of the water table and the permeability of the material through which the
groundwater is flowing.
Thus, groundwater flows faster through high-permeability rocks
than it does through low-permeability rocks, and it flows faster in regions
where the water table has a steep slope than it does in regions where the water
table has a gentle slope.
For example, groundwater flows relatively slowly (2 m per year)
through a low-permeability aquifer under the Great Plains, but flows relatively
quickly (30 m per year) through a high-permeability aquifer under a steep
hillslope.
In detail, hydrogeologists use Darcy’s Law to determine flow
rates at a location.
Darcy’s Law for Groundwater Flow
The rate at which groundwater flows at a given location depends
on the permeability of the material containing the groundwater; groundwater
flows faster in a more permeable material than it does in a less permeable
material.
 |
| The level to which water rises in a drill hole is the hydraulic head (h). The hydraulic gradient (HG) is the difference in head divided by the length of the flow path. |
The rate also depends on the hydraulic gradient, the change in
hydraulic head per unit of distance between two locations, as measured along
the flow path.
To calculate the hydraulic gradient, we divide the difference in
hydraulic head between two points by the distance between the two points as
measured along the flow path. This can be written as a formula:
hydraulic gradient = h1 - h2/j
where h1 - h2 is the difference in head (given in meters or
feet, because head can be represented as an elevation) between two points along
the water table, and j is the distance between the two points as measured along
the flow path.
A hydraulic gradient exists anywhere that the water table has a
slope.
Typically, the slope of the water table is so small that the
path length is almost the same as the horizontal distance between two points.
So, in general, the hydraulic gradient is roughly equivalent to
the slope of the water table.
In 1856, a French engineer named Henry Darcy carried out a
series of experiments designed to characterize factors that control the
velocity at which groundwater flows between two locations (1 and 2), each
of which has a different hydraulic head (h1 and h2).
Darcy represented the velocity of flow by a quantity called the
discharge (Q), meaning the volume of water passing through an imaginary
vertical plane perpendicular to the groundwater’s flow path in a given time.
He found that the discharge depends on the hydraulic head (h1-
h2); the area (A) of the imaginary plane through which the groundwater is
passing; and a number called the hydraulic conductivity (K).
The hydraulic conductivity represents the ease with which a fluid
can flow through a material.
This, in turn, depends on many factors (such as the viscosity
and density of the fluid), but mostly it reflects the permeability of the
material.
The relationship that Darcy discovered, now known as Darcy’s
law, can be written in the form of an equation as:
Q = KA(h1 - h2)/j
The equation states that if the hydraulic gradient increases,
discharge increases, and that as conductivity increases, discharge increases.
Put in simpler terms, the flow rate of groundwater increases as the permeability increases and as the slope of the water table gets steeper.
Muhammad
Qasim
Gender MALE
Industry Education
Location Pakistan
Interests Earth Sciences, Hydrogeology, Geoscience
Communication, Geoscience Education and Outreach
Learning
Geology is a science website and a
community of Earth Scientists that shares geology lessons and host live virtual
field tours from all around the world.
With
our postings we aim to reach to our fellows with broader public regardless of
the boundaries. The postings are done in the great interests of the fellow to
learn more and more. The reason for writing is to provide an environment where
we can give the best knowledge present just to be shared in the field of Earth
and space sciences that we present here. Nobody judge anyone so feel free to
feedback and let us know what great ideas you have, be a contributor.
http://geologylearn.blogspot.com/2015/12/groundwater-flow.html
You might also like:
.
Where Did All
That Water Come From?
The "fountains of the great deep" most likely refer to
subterranean sources of water
CLICK HERE . . . to view .
. .
https://puricarechronicles.blogspot.com/2018/05/the-waters-of-noahs-flood-we-dont-know.html
................................................................................................................................................................................................................................................................
pchron
No comments:
Post a Comment