Dams , piping, uplift Pressure, Khosla’s Theory, causes of Failure of Hydraulic structure by piping and uplift pressure what is the importance of reservoir planning and dams? Discuss multipurpose reservoir in detailed, Give Economic height of dam.

1. Mehran University of Engineering
and Technology Shaheed Z.A Bhutto
Campus Khairpur Mir’s
____________________________________________________________
Irrigation Engineering Assignment
Submitted to Dr Kanya Lal Khatri
4/13/2016
Name: LATIF HYDER WADHO
Roll No: K13CE19
Department Of Civil Engineering

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Question No: 1 discuss piping, uplift Pressure Khosla’s Theory. Give
causes of Failure of Hydraulic structure by piping and uplift pressure
Piping
There are many types of piping available – CPVC, PVC, galvanized iron, and polyethylene just to
name a few. The two piping types most commonly used for irrigation systems are white PVC
(polyvinyl chloride) and “black roll pipe” (polyethylene).
PVC Pipe
PVC is generally the piping of choice in the Southern region. It is easy to work with, inexpensive,
and quite common. PVC comes in a number of varieties, but the two used for landscape
irrigation are Schedule 40 and 160 psi “ pressure-rated” (or PR160) piping. Both of these PVC
types will work well for landscape irrigation systems. Schedule 40 piping has a slightly thicker
wall than PR160 in sizes below 6 inches in diameter and can withstand higher pressures, but the
thicker wall also means that Schedule 40 is slightly more expensive and that there will be more
pressure lost to friction. Schedule 40 is somewhat more forgiving if installed in rocky ground.
Either type will work well.
PVC comes in 10 or 20 foot lengths (depending on the supplier) and is glued together with PVC
cement. Most PVC piping has one “belled end” or coupling made into the end of the pipe.
Schedule 40 fittings are used on both PR160 and Schedule 40 PVC pipe. Be careful not to buy
drain, waste and vent (DWV) PVC fittings – they are less expensive, but they are not designed to
handle higher pressures and may fail over time.
Figure 1. PVC pipe.

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Polyethylene or “Black Roll Pipe”
“Black roll pipe” is commonly used for landscape irrigation systems in Northern areas and is
used occasionally in the South. The pipe comes in 300 foot rolls and is connected with insert
fittings and clamps. Black roll pipe is somewhat more difficult to install, but not appreciably so.
Black roll pipe is used in Northern areas because it will expand a small amount, which allows
the water in it to be frozen with little or no damage – an important characteristic in the North.
In the South we typically will not have pipe freezing problems if we install the piping to the
recommended 12 inch depth. Either black roll piping or PVC piping will work well in our climate,
but you may find the PVC piping easier to install and repair.
Figure 2. Black Roll pipe
“Swing” Pipe
Just imagine that your sprinkler system is installed and working nicely. Uncle Bob stops in to
visit, and as he leaves he backs into the yard – right over a sprinkler. The sprinkler is crushed, of
course, but since the sprinkler was screwed directly into the PVC pipe, a good portion of the
pipe below ground is broken, too.
We can’t prevent a sprinkler from being broken in this fashion, but we can protect the piping.
Most manufacturers offer a product called “swing pipe” or “funny pipe.” This piping looks a
great deal like drip tubing, but it has a much thicker wall and can handle higher pressures.
Swing pipe is installed between the PVC piping and the sprinkler to allow some flexibility if the
sprinkler is crushed or driven over. Usually two feet of swing pipe will be used to attach a
sprinkler to the PVC, but three or four feet may be used if needed.

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Swing pipe allows the installer to move the sprinkler around a little during installation just in
case a planned sprinkler location turns out to be right behind a tree or an obstacle.
Figure 3 A spray head installed with swing pipe.
Swing pipe is relatively inexpensive and will certainly pay for itself if even only one repair of this
type is required. Special fittings are sold for the swing pipe to attach it to the sprinkler and the
PVC piping.
Uplift pressure
The term 'Uplift pressure' as it applies to the area of reclamation can be defined as ' See pore-
water pressure’.
OR
Pressure in an upward direction against the bottom of a structure, as a dam, a road slab, or a
Basement floor.
Khosla’s Theory and Concept of FlowNets
Introduction
Many of the important hydraulic structures, such as weirs and barrage, were designed on the
basis of Bligh’s theory between the periods 1910 to 1925. In 1926 – 27, the upper Chenab canal
siphons, designed on Bligh’s theory, started posing undermining troubles. Investigations
started, which ultimately lead to Khosla’s theory.

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The main principles of this theory are summarizedbelow:
(a) The seepage water does not creep along the bottom contour of pucca flood as started by
Bligh, but on the other hand, this water moves along a set of stream-lines. This steady seepage
in a vertical plane for a homogeneous soil can be expressed by Laplacian equation:
(b) The seepage water exerts a force at each point in the direction of flow and tangential to the
streamlines as shown in figure above. This force (F) has an upward component from the point
where the streamlines turns upward. For soil grains to remain stable, the upward component of
this force should be counterbalanced by the submerged weight of the soil grain. This force has
the maximum disturbing tendency at the exit end, because the direction of this force at the exit
point is vertically upward, and hence full force acts as its upward component. For the soil grain
to remain stable, the submerged weight of soil grain should be more than this upward
disturbing force. The disturbing force at any point is proportional to the gradient of pressure of
water at that point (i.e. dp/dt). This gradient of pressure of water at the exit end is called the
exit gradient. In order that the soil particles at exit remain stable, the upward pressure at exit
should be safe. In other words, the exit gradient should be safe.
 Khosla’s Methodof independent variablesfor determinationof pressures
and exit gradient for seepage belowa weir or a barrage
In order to know as to how the seepage below the foundation of a hydraulic structure is
taking place, it is necessary to plot the flow net. In other words, we must solve the
Laplacian equations. This can be accomplished either by mathematical solution of the
Laplacian equations, or by Electrical analogy method, or by graphical sketching by
adjusting the streamlines and equipotential lines with respect to the boundary
conditions. These are complicated methods and are time consuming. Therefore, for
designing hydraulic structures such as weirs or barrage or pervious foundations, Khosla
has evolved a simple, quick and an accurate approach, called Method of Independent
Variables.
In this method, a complex profile like that of a weir is broken into a number of simple profiles;
each of which can be solved mathematically. Mathematical solutions of flow nets for these
simple standard profiles have been presented in the form of equations given in Figure (11.5)
and curves given in Plate (11.1), which can be used for determining the percentage pressures at
the various key points. The simple profiles which hare most useful are:
(i) A straight horizontal floor of negligible thickness with a sheet pile line on the u/s end and d/s
end.
(ii) A straight horizontal floor depressed below the bed but without any vertical cut-offs.
(iii) A straight horizontal floor of negligible thickness with a sheet pile line at some intermediate
point. The key points are the junctions of the floor and the pole lines on either side, and the
bottom point of the pile line, and the bottom corners in the case of a depressed floor.

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The percentage pressures at these key points for the simple forms into which the complex
profile has been broken is valid for the complex profile itself, if corrected for
(a) Correction for the Mutual interference of Piles
(b) Correction for the thickness of floor
(c) Correction for the slope of the floor
Causes of failure of Hydraulic Structure
Common causes of failure include:
• Excessive and progressive downstream erosion, both from within the stream and
through lateral erosion of the banks
• Erosion of inadequately protected abutments
• Hydraulic removal of fines and other support material from downstream protection
(gabions and aprons) resulting in erosion of the apron protection
• Deterioration of the cut-off and subsequent loss of containment
• Additional aspects specific to concrete, rock fill or steel structures.
The main causes are:
1. Piping
Piping is caused by groundwater seeping out of the bank face. Grains are
detached and entrained by the seepage flow and may be transported away
from the bank face by surface runoff generated by the seepage, if there is
sufficient volume of flow.
The exit gradient of water seeping under the base of the weir at the downstream end may
exceed a certain critical value of soil. As a result the surface soil starts boiling and is washed
away by percolating water. The progressive erosion backwash at the upstream results in the
formation of channel (pipe) underneath the floor of weir.
Piping is especially likely in high banks backed by the valley side, a terrace, or some other high
ground. In these locations the high head of water can cause large seepage pressures to occur.
Evidence includes: Pronounced seep lines, especially along sand layers or lenses in the bank;
pipe shaped cavities in the bank; notches in the bank associated with seepage zones and layers;
run-out deposits of eroded material on the lower bank.
Remedies:
• Decrease Hydraulic gradient i.e. increase path of percolation by providing sufficient
length of impervious floor
• Providing curtains or piles at both upstream and downstream

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2. Rupture of floor due to uplift:
If the weight of the floor is insufficient to resist the uplift pressure, the floor may burst. This
bursting of the floor reduces the effective length of the impervious floor, which will resulting
increasing exit gradient, and can cause failure of the weir.
Remedies:
• Providing impervious floor of sufficient length of appropriate thickness.
• Pile at upstream to reduce uplift pressure downstream
.
Question No: 2 what is the importance of reservoir planning and
dams? Discuss multipurpose reservoir in detailed, Give Economic
height of dam.
The Importance of Dams & Reservoirs
Why are dams soimportant?
Dams are important because they help people have water to drink and provide water for
industry, water for irrigation, water for fishing and recreation, water for hydroelectric power
production, water for navigation in rivers, and other needs. Dams also serve people by reducing
or preventing floods.
Water is the vital resource to support all forms of life. Unfortunately, water is not evenly
distributed by location or by the season of the year. Some areas of the country are more arid
and water is a scarce and precious commodity. Other areas of the country receive more than
adequate amounts of rain causing occasional floods and loss of life and property. Throughout
history, dams and reservoirs have been constructed to collect, store and manage the supply of
water to sustain civilization.

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The primary benefit of dams and reservoirs is water supply. Reservoirs also provide benefits
such as flood control, recreation, scenic beauty, fish and wildlife habitat and, at some dams,
hydro-electric power. Currently there are about 45,000 dams higher than 50 feet throughout
the world. While some are more than 2,000 years old, over 70% have been built in the last 50
years.
The Maffitt Dam was constructed by Des Moines Water Works (DMWW) as an emergency
water supply. Construction started in August 1943 and the dam was completed in March 1945.
Water was pumped from the Raccoon River to fill the reservoir. Maffitt Reservoir stores 1.57
billion gallons of water. The original plan was to store water in the reservoir that could be
released during periods of low flow in the Raccoon River. The current plan is to use water from
the reservoir as an emergency raw water source for the L.D. McMullen Water Treatment Plant.
In May of 1982, DMWW entered into a contract with the State of Iowa to purchase storage
capacity in the Saylorville Reservoir. DMWW paid a portion of the Saylorville Reservoir
construction costs and makes annual payments for a portion of the operational costs. These
payments give DMMW access to 3.2 billion gallons of Saylorville Reservoir water that can be
utilized in a drought situation.
Between the Maffitt and Saylorville Reservoirs, DMWW has access to 4.77 billion gallons of
water to meet the water needs of our customers in the event of an emergency or drought
situation.
Intended purposes include providing water for irrigation or town or city water supply,
improving navigation, creating a reservoir of water to supply industrial uses, generating
hydroelectric power, creating recreation areas or habitat for fish and wildlife, flood control and
containing effluent from industrial sites such as mines or factories. Few dams serve all of these
purposes but some multi-purpose dams serve more than one.
Dams generally serve the primary purpose of retaining water, while other structures such as
floodgates, levees, and dikes are used to prevent water flow into specific land regions. The
tallest dam in the world is the 300 meter high Nurek Dam in Tajikistan. [1]

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Multipurpose of Reservoir
Why do people builddams?
A dam is built to control water through placement of a blockage of earth, rock and/or concrete
across a stream or river. Dams are usually constructed to store water in a reservoir, which is
then used for a variety of applications such as irrigation and municipal water supplies.
Many dams are multipurpose and most dams have at least some flood mitigation effect in
addition to their primary purpose. Dams built specifically for flood control may have some of
their storage capacity kept empty during normal river flow conditions so that space is available
to store excess water inflow under flood conditions. The flood mitigation effect of a dam is such
that the downstream river height at the peak of the flood is reduced but, after the peak has
passed, the river levels usually remain high for a longer period than would have been the case if
the dam had not been built. This is because excess flood water is only stored behind the dam
temporarily and is slowly released from the dam in the days and weeks after the flood peak has
passed.
1. Once a dam is constructed, electricity can be produced at a constant rate.
2. If electricity is not needed, the sluice gates can be shut, stopping electricity generation. The
water can be saved for use another time when electricity demand is high.
3. Dams are designed to last many decades and so can contribute to the generation of
electricity for many years / decades.
4. The lake that forms behind the dam can be used for water sports and leisure / pleasure
activities. Often large dams become tourist attractions in their own right.
5. The lake's water can be used for irrigation purposes.
6. The buildup of water in the lake means that energy can be stored until needed, when the
water is released to produce electricity.
7. When in use, electricity produced by dam systems does not produce green house gases. They
do not pollute the atmosphere.
• agriculture- it stores and provides water for agriculture
• Electricity generation - to rotate wings of turbine we are still using the water energy in
terms of kinetic and static or water pressure energy.
• Flood control - by proper maintenance we can make strong enough dam to storage and
prevent the overflow water coming by rivers in Rainy seasons.

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• Dam is oftenly uses as fishing plants it help to create new jobs and improves the
economical.
• Dam is also uses as picnic place so that there will be some shops and hotels which are
again economical.
• Dams also help to supply of drinking water after purification.
Economic Height of Dam
Economic height of a dam is the height corresponding to which cost of the dam per unit of
storage is minimum
QuestionNo:3 Discuss various dams’ construction what are factors governing
the selectionof particular type of dam, Selectionof Dam Site and Environment
Impact assessment of dams.
Types of Problems in Dams Construction
• In flat basins large dams cause flooding of large tracts of land, destroying local animals and
habitats.
• People have to be displaced causing change in life style and customs, even causing
emotional scarring. About 40 to 80 million people have been displaced physically
by dams worldwide.
• Large amounts of plant life are submerged and decay an aerobically (in the absence of
oxygen) generating greenhouse gases like methane. It is estimated that a hydroelectric
power plant produces 3.5 times the amount of green house gases as a thermal power plant
burning fossil fuels.
TheEnvironmentalProblem
Before 1970, studies of the environmental impact of dams were often too limited, as the
environment was of little concern worldwide. Many old reservoirs would now be built
differently and some would not even be built at all.
Environmental studies may identify and quantify the impact of a dam, as well as proposing
ways to mitigate this impact and to improve the project. However, determining the impact of a

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dam is often a subjective matter: creating a lake, for instance, might be considered both as a
welcome development or as a disaster; preventing flash floods might be regarded both as
progress and as an unacceptable modification of an ecosystem. Indeed, some ecologists and
environmentalists are systematically opposed to the construction of any dam whatsoever.
The main direct environmental impacts of dam reservoirs are the inundation of areas and the
modification of river flows. In the year 2000, the total area of dam reservoirs was about 400
000 km2, or one-third of the world’s natural lakes area. Worldwide, this represents only one
percent of the areas modified for agriculture. However, the relative impact of the total area of
dam reservoirs is more important that this figure might suggest, as river valleys are attractive
habitats for many plant and animal species.
Worldwide, there are 2 000 reservoirs over 10 km2 each. They cover 300 000 km2 collectively,
and some individual dams are well over 1 000 km2.
However, 40 000 reservoirs of large dams, and all the reservoirs of small dams, have a unit area
in the range of 1 km2 or less. The scale and nature of impact are thus very different and it is
unjustifiable to generalize about the impressive impact of very large reservoirs.
Dam reservoirs modify the volume and schedule of flow. This impact may be negative, if it
reduces flow in the dry season; it may be positive, if it prevents flash floods and increases the
natural flow during the dry season. Large reservoirs may also affect the existing water quality.
A very positive effect of hydroelectric dams is the saving of fossil fuel, which would be
necessary in thermal plants. In the 21st century, hydroelectric dams built before 2000 and up to
2050 will save over 100 billion tons of fossil fuel (oil, coal, and gas).
SocialProblem
Dam reservoirs naturally have a huge and direct social impact: through the year, they provide
food and power, guarantee water supply, and control floods. They also have an indirect
positive impact: they favor regional agricultural and industrial development and help prevent
the migration of hundreds of millions of rural inhabitants to city slums, particularly in Asia.
However, many very large dams have had a very significant negative social impact: the
resettlement of people from reservoir areas. In the second half of the 20th century over 20
million people have been thus affected, most of them in Southeast Asia. The costs and complex
organization that resettlement demands was often overlooked during the sixties, but at the end
of the 20th century and today, proper organization and fair amounts of money are usually
devoted to this problem.
In certain cases, resettlement receives 20 to 50 percent of a reservoir’s total investment. The
resettlement of hundreds of thousands of people requires long and special study, similar to the
studies needed for the development of large cities. Resettlement may offer the opportunity to
improve living conditions in developing countries, but it also brings conflict – particularly the
reluctance of old people to leave their family homes. In the cases of some very large reservoirs,

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resettlement has raised special problems in regards to tribal peoples whose culture is bound to
local conditions.
For some large river projects changes in flow may have a negative impact downstream,
affecting fisheries or floodplain activities.
Although the impact of dams upon human health worldwide is largely positive (especially in
regards to the water supply) some large reservoirs have provided environments favoring the
development of tropical diseases such as malaria.
It should, however, be emphasized that over 90 percent of large dams have no negative social
impact at all. Further, in the case of most of largest dams, resettlement – if well-studied and
financed accordingly – may be conducted in a fair manner.
Factors Affecting Selection of Type of Dam
Whenever it is decided to construct a dam, the first question that one face is which type of dam
will be most suitable and most economical? Following are the factors affecting selection of dam
site by dam type.
• Topography
• Geology and Foundation Conditions
• Availability of materials
• Spillway size and location
• Earthquake zone
• Height of the Dam
• Other factors such as cost of construction and maintenance, life of dam, aesthetics etc.
Factors Affecting Selection of Dam
These factors are discussed one by one.
Topography
Topography dictates the first choice of the type of dam.
• A narrow U-shaped valley, i.e. a narrow stream flowing between high rocky walls, would
suggest a concrete overflow dam.
• A low plain country, would suggest an earth fill dam with separate spillways.
• A narrow V-shaped valley indicates the choice of an Arch dam
Geological and Foundation Conditions
Geological and Foundation conditions should be thoroughly surveyed because the foundations
have to carry the weight of the dam. Various kind of foundations generally encountered are

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• Solid rock foundations such as granite have strong bearing power and almost every kind of
dam can be built on such foundations.
• Gravel foundations are suitable for earthen and rock fill dams.
• Silt and fine sand foundations suggest construction of earth dams or very low gravity dams.
• Clay foundations are likely to cause enormous settlement of the dam. Constructions of
gravity dams or rock fill dams are not suitable on such foundations. Earthen dams after
special treatments can be built.
Availability of Materials
Availability of materials is another important factor in selecting the type of dam. In order to
achieve economy in dam construction, the materials required must be available locally or at
short distances from the construction site.
Spillway Size and Location
Spillway disposes the surplus river discharge. The capacity of the spillway will depend on the
magnitude of the floods to be by-passed. The spillway is therefore much more important on
rivers and streams with large flood potential.
Earthquake Zone
If dam is situated in an earthquake zone, its design must include earthquake forces. The type of
structure best suited to resist earthquake shocks without danger are earthen dams and
concrete gravity dams.
Height of Dam
Earthen dams are usually not provided for heights more than 30 m or so. For greater heights,
gravity dams are generally preferred.
Hint: The availability of spillway site is very important in selection of a particular type of dam
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