well hydrulics

Instructional Objectives
At the end of this lesson,
the student shall be able to learn the following:
1. The concepts of steady and unsteady ground water flow to wells
2. Mathematical equations for water
flow to wells in confined aquifers
3. Mathematical equations for water fl
ow to wells in unconfined aquifers
4. Determination of the physical pr
operties of confined aquifers
5. Design and installation of tube wells
2.7.0 Introduction
As established in the earlier lessons, an eno
rmous amount of water is stored within the
ground. A small portion of
that is in an unsaturated state
but that below the water table,
also called ground water, can be easily extrac
ted for useful purpose, depending on the
type and location within which the water exists. It has been roughly estimated that of
the global water resources,
about 0.6 percent exists as ground water, out of which
about half can be eco
nomically extracted with the present
drilling technology. In fact,
the ground water is the largest source of
fresh water on earth excluding the polar
icecaps and glaciers. Hence, ground wate
r has been extracted on all regions of the
world for different purposes an
d about nearly one fifth of all
the water used in the world
is obtained from ground water sources.
Evidence of extraction of water from dug
wells has been found in the archeological
remnants of Mohenjodaro. In many
of the cities established dur
ing the medieval ages in
India, the main source of water wa
s dug wells, though peo
ple were dependent on
surface water bodies like rive
rs or, lakes, or ponds, if th
at happened to be nearby. It is
only during the past century that tube wells
became popular as an easily operatable
source of extraction of ground
water. Gradually with easy access to electricity deep
tube wells have become a common
source of water. Howeve
r, establishment of tube
well extraction of water involves knowl
edge about the movement of water through the
geological formations, which has been discu
ssed in Lessons 2.5 and 2.6. Water may
have to be extracted from fo
rmations ranging from sand, silt, clay, fractured rocks of
different compositions etc., A well may be dug
to extract water from a confined or an
unconfined aquifer. Digging of more than one we
ll in close vicinity affects each others’
yield as the drawdown of one influences the ot
her. This may be quantitatively estimated
by theories of ground water flow
applied to the radial flow of water to each well. In this
lesson, these theories are discussed, which would be helpful in designing such wells.
Version 2 CE IIT, Kharagpur
2.7.1 Steady flow and unsteady flows
Imagine a farmer using a deep tube or a dug well
as a source of water for irrigating his
field. The well may be fitted with a
submergible pump
or a
centrifugal pump
to draw
out water and discharge at the head of a channel leading to the fields. As long as the
pump is not in operation, the wa
ter in the well remains at a st
eady at a level, at that of
the water table (Figure 1).