ground water hydrology of Ethiopia. Hydrology means the science of water. The science deals with occurrence, circulation, and distribution of water of the earth and earth’s atmosphere. Practical applications of hydrology are found in such tasks as the design and operation of hydraulic structures, water supply, wastewater treatment and disposal, irrigation, drainage, hydropower generation, flood control, navigation, erosion and sediment control, salinity control, pollution abatement, recreational use of water, and fish and wildlife protection. Hydrology may be considered to encompass all the hydro-sciences, or defined more strictly as the study of the hydrologic cycle, that is, the endless circulation of water between the earth and its atmosphere. Hydrologic knowledge is applied to the use and control of water resources on the land areas of the earth. As the branch of science, hydrology is concerned with the water in streams and lakes, rainfall and snowfall, snow and ice on the land and water accruing below the earth’s surface in the pores of the soil & rocks. In general sense, hydrology is very broad subject of on inter-disciplinary nature drawing support from allied sciences, such as meteorology, geology, statistics, chemistry, physics and fluid mechanics hydrology is basically an applied science. It can be used in irrigation, drainage, flood control, water supply, etc. To further emphasize the degree (extent) of applicability, this subject is sometimes classified as: The three important phases of the hydrologic cycle are:  Evaporation and evapotranspiration  Precipitation and  Runoff and  The globe has one-third land and two-thirds Ocean. Evaporation from the surfaces of ponds, lakes, reservoirs, Ocean surfaces, etc. and transpiration from surface vegetation i.e., from plant leaves of cropped land and forests, etc. take place. These vapors rise to the sky, are condensed at higher altitudes by condensation nuclei, and form clouds, resulting in droplet growth. The clouds melt and sometimes burst to result in precipitation of different forms like rain, snow, hail, sleet, mist, dew, and frost. A part of this precipitation flows over the land called runoff and part infilters into the soil, which builds up the groundwater table. The surface runoff joins the streams and the water is stored in reservoirs. A portion of surface runoff and groundwater flow back to the ocean. Again, evaporation starts from the surfaces of lakes, reservoirs, and ocean, and the cycle repeats. • Of these three phases of the hydrologic cycle, namely, evaporation, precipitation, and runoff, it is the ‘runoff phase’, which is important to a civil engineer since he is concerned with the storage of surface runoff in tanks and reservoirs for the purposes of irrigation, municipal water supply hydroelectric power, etc.

1. Water Wells
 A Water well is a hole or shaft, in most cases vertical,
excavated in the earth, or sunk in to the ground intercepting
one or more water bearing strata, for bringing groundwater
to the surface.
 But we can have also Horizontal wells called- Collector wells
 Can also be called as Bore holes, dug wells etc…

2. • Horizontal Well Example

3. Water Well
The objective:
 To provide water with a good quality (Potable water)
 To provide a sufficient quantity of water
 To provide water for a long time
 To provide water at low cost

4. Classification of water wells
Wells can be classified Based on:
 construction = dug wells and tube wells
 their depth = shallow wells and deep wells
 whether they are vertical or horizontal = vertical wells and
horizontal wells
 Two major categories in practice are: Tube wells and Hand
Dug wells

5. Tube wells
Tube wells are small size holes (usually b/n 10cm – 60cm in dia.)
which penetrate to the strata deeply ( more than 30m).
Advantages:
 Do not require much space.
 Can be constructed quickly. (in days/weeks)
 Fairly sustained yield of water can be obtained in dry years.
 Economical when deep-seated aquifers are encounter
 Generally good quality of water is tapped.

6. Disadvantages:
 Requires costly and complicated drilling equipment and
machinery.
 Requires skilled workers and great care to drill and
complete the tube wells.
 Installation of costly submersible pumps is required.
 Possibility of missing fractures, fissures and joints in
hard rock areas resulting in many dry holes.

7. commonly used methods for SHALLOW tube
wells construction
 Boring Method (using augers)
 This method of excavation consists of shaving or
cutting material from the bottom of the hole by
the rotation of a cylindrical tool with one or more
cutting lips
 The auger is both rotated and raised and lowered
by means of a vertical shaft which extends upward
from the auger to a convenient point above
ground level from which it can be rotated

8. types of earth augers
cylindrical bucket
auger
two-bladed
auger
helical auger tubular auger

9. Driving Method (using well pipe )
In this method the hole is constructed by forcing a
casing (well pipe) equipped with a drive (well)
point into the ground by a series of blows either
manually or machine delivered on the top of the
casing
Jetting method (high water jet)
A jetted well is a well which is constructed by
means of boring equipment using water jetted under
high pressure to facilitate rapid boring.
Jetting is pumping water down the pipe and out
through the well point where the force of the water
losing the surround soil materials.

10. Tube Wells Drilling Methods
 Cable Tool drilling methods
 Rotary Direct Circulation Drilling Method
 Reverse Circulation Rotary Drilling method
 Down the Hole Hammer (DTH) Drilling method

12. Component Parts

14. Drill bits of the method

17. Well completion
Only drilling of the borehole alone does not
complete the construction of an efficient well.
But well completion should follow it to do so.
Well completion involves:-
placement of casing
cementing of casing
placement of well screens
gravel packing

18. Well Casing
 Basically we have two category of well casing: Blind
casing and Well screen
 Reasons for using casing in a water well or
borehole are:
 to prevent the collapse of the walls of the
borehole (i.e. structural support against caving
in) serving as a lining.
 to exclude, along with grouting, pollutants either
from surface or subsurface from entering the
water sources

19. to provide a channel for conveying the water
into the well for injection purpose
to provide a channel for conveying the water to
the surface.
to provide a housing for the pump mechanism
to provide a channel for conveying a cement
grout in the well for cementation purpose
Serving as a reservoir for a gravel pack

20. Types of casing
I. Surface casing
 Surface casing is installed from ground surface
through upper strata of unstable or fractured
materials into a stable
 serves several purposes
supporting unstable materials during drilling,
reducing loss of drilling fluids
facilitating installation or removal of other casing
helping in placing a sanitary seal, and to seal off the
well against the inflow of polluted surface water
serving as a reservoir for a gravel pack

21. II. conductor casing
A high-capacity well design usually specifies that
conductor casing be installed and cemented to a
minimum of 15m, or to the first impervious
formation, in order to prevent well contamination
from the surface.
III. Intermediate casing
Intermediate strings of casing may be required to
facilitate completion in deep boreholes where
difficult drilling conditions are encountered
(hydraulic shale’s, loss of circulation zones, etc.).

23. Well Grouting
• Well grouting involves filling the space
(usually between the casing and the wall
of the well) around the pipe or casing
with a suitable impervious material.
 Concrete
 Sand cement
 Neat cement
 Bentonite clay

24. Reasons for well grouting
– to protect an aquifer, or aquifers, from entry of
contaminating fluids flowing into it
– to prevent undesirable water movement from
one aquifer to another for the purposes of
maintaining quality.
– Protecting the well against the entry of unwanted
water from the surface or a subsurface zone.
– to protect the casing against exterior corrosive
and also to assure structural integrity of casing
against external pressure and buckling.
– To make the casing stay tight in the drilled hole.

25. Water well Screens
 Also called as the heart of a Well.
 Well screen serves as the intake component of a
well and support and stabilizes the aquifer and
filter zone and in the case of gavel envelope wells,
it filters the pack.
 It is usually a pipe with slots or openings along its
wall.
 Wells that obtain water from sand and gravel
formations (unconsolidated formations) require
the use of well screens for proper completion..
That is ….

26. • Well screen permits water to enter the well from
the saturated aquifer, allows a maximum amount of
water to enter the well with a minimum hydraulic
resistance
• Prevents sand movement into the well
• Stabilizes the sides of the hole
• it should be resistant to corrosion and
deterioration,
• it should be strong enough to prevent collapse of a
hole,
• it should offer minimum resistance to the flow of
water, and
• size and shape of each slot
• thickness and material of screen
The basic requirements of a well screen are:

27. During Design of a well screen, the following points
should be Considered.
• Screen Strength
• Screen Material
• Screen Length
• Screen Diameter
• Screen Slot size

28. • Filter(gravel) packed well:-
The screen opening is selected to prevent most of the pack
from passing. The generally recommended range is
between 10% and 20% (occasionally as high as 30%) of the
envelope material to pass.

30. Types of well screens
• Perforated pipe
• Punched and slotted pipe
• Reinforced wire wrapped punch pipe
• Louvered pipe
• Continuous slot wire wound screen
The best type of opening is the V-shaped slot that
widens towards the inside of the screen, i.e.
opening beveled inside.

31. Punched Louvered

32. Continuous slot wire wound screen

33. Torch Cut or Perforated
Perforated Wire wrap

36. Methods of screen Emplacement (Installation of Well
Screens)
• pull back method
• Bail- down method
• Open-hole method
• Wash-down method
• Driving

39. Artificial Gravel Pack
• A gravel pack (or filter pack) consists of clean sand or
gravel of selected grain size and gradation
• It is installed in the annular space between the screen
and the wall of the well bore.
• The gravel pack has a larger average grain size and
usually a smaller coefficient of uniformity than the
aquifer material.

40. • The usefulness of gravel pack in
unconsolidated formation water well
can be summarized as :
• Sand flow in to the well is prevented.
• use of a larger screen slot size
• Increase in effective diameter of the well
• it fills the space between the borehole wall
and lining pipe, and thus prevents formation
slumping
• NB: not all water wells in penetrating aquifer
need Gravel pack.

41. Gravel pack Material
• It is most of the time, either Graded or Uniform
material of gravel or pebbles or otherwise
selected/designed depending up on the characteristics
of the aquifer material.
• The requirement of such a material is mentioned
elsewhere…
• The Design Criteria for gravel pack is shown in the
handout.
• In anyway the design consideration is to permit water
flow to well and detain sand entry to the well.

42. Gravel Pack
Well screen

43. Water Well Development
• A tube well is not completely ready for use just after
construction. The tube well can function successfully only after
proper development.
• Water well development is a process where by the mud cake or
compacted borehole wall, resulting from drilling activity, is
broken down; the mud cake liquefied and drawn with other fines
into the well.
• This material is then removed by bailing or pumping.
• Well development, therefore, stabilizes the walls of a well
adjacent to the screen by a process which removes fine particle
from the formation immediately surrounding the well screen,
leaving coarser particles to contact and surround the screen.

44. • Tube wells are developed to increase their specific capacity,
prevent sanding and obtain maximum economic well life.
• The main objectives of well development are:-
 to correct any damage to or clogging of the water
bearing formation ; i.e., to remove mud or clay particles
which may have blocked the water movement from the
aquifer into the well.
 to increase the porosity and improve the permeability
of the water bearing formation in the vicinity of the
well.
 to stabilize the sand formation (gravel pack) around a
screened well and the formation immediately
 to reduce drawdown in the well during production or
pumping.

45. The benefits which result from well development are:
• sand pumping during well operation will be eliminated
to a greater extent
• the life of the well will be prolonged
• Operation and maintenance costs will be reduced.
• the specific capacity of the well will be improved
(maximum yield at available minimum drawdown)

46. Methods of Water Well development
• Over pumping
• Backwashing:
• Air development
• Water Jetting
• Dispersing agents
• Hydro fracturing
• Over pumping: - Loose sand materials are removed by
pumping the well at a higher rate
• Backwashing: - sand and fine materials are loosened by
reversing the direction of flow through the screen

47. • Air development: - Air lift technique can be used
for surging and pumping.
• Surging is used to loosen sand and fine material in the
screen and filter zone. The surging action is created by
lifting the water near to the surface by injecting air into
the well and then shut off the air to allow the water to
flow back through the well and formation.
• Pumping water with air lift can be used for cleaning a well
from sand and fine material. Using the air lift means no
water, as would be the case if a submersible or turbine
pump is used to clean the well .
• Water Jetting: - High velocity water jetting can be
used to loosen sand and fine material from the filter zone
and the screen.

48. • Hydro fracturing: - High pressure pumps are used to
overcome the pressure of overlying rock and inject fluids
into newly opened fractures.
• Dispersing agents: - Sometimes it is necessary to add
chemical agents to disperse the clay particles in the mud
cake or in the formation to avoid their sticking to sand
grains, and to speed up the development process.
• Development of the well shall be continued until water
pumped from the well at the maximum test pumping rate is
clear and free of sand.
Well Testing for performance
• Following the development of a new well, the well should
be tested to provide information on the potential yield of
the borehole and drawdown

49. • water well may be pump tested for either of two main
purposes:
 The usual objective is to obtain information about the
performance and efficiency of the well being pumped.
The result in such a case is usually reported in terms of
 yield,
the observed drawdown,
and the calculated specific yield
 Another objective of well pumping test is to provide data for
which the principal factors of aquifer performance,
transmissivity and storage coefficient, can be calculated
• In general, the data obtained from pumping test provide
information necessary to determine:-
• capacity of the well
• Aquifer characteristics
• Well efficiency

50. • Pumping rates
• Pump installation depth settings
• Other factors which will be of value in the long term
operation and maintenance of the well
• Well design and construction equipment.