irrigation engineering terms in civil engineering

Mehran University of Engineering and Technology Shaheed Z.A Bhutto Campus Khairpur Mir’s ____________________________________________________________Irrigation Engineering Assignment

Submitted to Dr Kanya Lal Khatri

Dated: - 4/13/2016Name: LATIF HYDER WADHORoll No: K13CE19

Department Of Civil Engineering

2Irrigation Engineering

Question No: 1 discuss piping, uplift Pressure Khosla’s Theory. Give causes of Failure of Hydraulic structure by piping and uplift pressure

PipingThere 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 pressureThe 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 Flow Nets

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 summarized below:

(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 thestreamlines as shown in figure above. This force (F) has an upward component from the pointwhere 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 Method of independent variables for determination of pressures and exit gradient for seepage below a 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 boundaryconditions. 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.

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(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. 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 StructureCommon 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. PipingPiping 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

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• Providing curtains or piles at both upstream and downstream

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

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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 so important?

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

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history, dams and reservoirs have been constructed to collect, store and manage the supply of water to sustain civilization.

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 build dams?

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

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• 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.

• 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 DamEconomic height of a dam is the height corresponding to which cost of the dam per unit of storage is minimum

Question No: 3 Discuss various dams’ construction what are factors governing the selection of particular type of dam, Selection of 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.

The Environmental Problem

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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 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).

Social Problem

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.

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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, 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.

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• 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

• 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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THE-END

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