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Classification of Dams

Illustration of parts of a dam

A dam is a hydraulic structure of fairly impervious material built across a river to create a reservoir on its upstream side for impounding water for various purposes. These purposes may be Irrigation, Hydro-power, Water-supply, Flood Control, Navigation, Fishing and Recreation. Dams may be built to meet the one of the above purposes or they may be constructed fulfilling more than one. As such, it can be classified as: Single-purpose and Multipurpose Dam.

Different parts & terminologies of Dams:

Crest: The top of the dam structure. These may in some cases be used for providing a roadway or walkway over the dam.
Parapet walls: Low Protective walls on either side of the roadway or walkway on the crest.
Heel: Portion of structure in contact with ground or river-bed at upstream side.
Toe: Portion of structure in contact with ground or river-bed at downstream side.
Spillway: It is the arrangement made (kind of passage) near the top of structure for the passage of surplus/ excessive water from the reservoir.
Abutments: The valley slopes on either side of the dam wall to which the left & right end of dam are fixed to.
Gallery: Level or gently sloping tunnel like passage (small room like space) at transverse or longitudinal within the dam with drain on floor for seepage water. These are generally provided for having space for drilling grout holes and drainage holes. These may also be used to accommodate the instrumentation for studying the performance of dam.
Sluice way: Opening in the structure near the base, provided to clear the silt accumulation in the reservoir.
Free board: The space between the highest level of water in the reservoir and the top of the structure.
Dead Storage level: Level of permanent storage below which the water will not be withdrawn.
Diversion Tunnel: Tunnel constructed to divert or change the direction of water to bypass the dam construction site. The hydraulic structures are built while the river flows through the diversion tunnel.

Classification of dams

Dams can be classified in a number of ways. But most usual ways of classification i.e. types of dams are mentioned below.

Classification based on function

Storage dams: They are constructed to store water during the rainy season when there is a large flow in the river. Many small dams impound the spring runoff for later use in dry summers. Storage dams may also provide a water supply, or improved habitat for fish and wildlife. They may store water for hydroelectric power generation, irrigation or for a flood control project. Storage dams are the most common type of dams and in general the dam means a storage dam unless qualified otherwise.
Diversion dams: A diversion dam is constructed for the purpose of diverting water of the river into an off-taking canal (or a conduit). They provide sufficient pressure for pushing water into ditches, canals, or other conveyance systems. Such shorter dams are used for irrigation, and for diversion from a stream to a distant storage reservoir. It is usually of low height and has a small storage reservoir on its upstream. The diversion dam is a sort of storage weir which also diverts water and has a small storage. Sometimes, the terms weirs and diversion dams are used synonymously.
Detention dams: Detention dams are constructed for flood control. A detention dam retards the flow in the river on its downstream during floods by storing some flood water. Thus the effect of sudden floods is reduced to some extent. The water retained in the reservoir is later released gradually at a controlled rate according to the carrying capacity of the channel downstream of the detention dam. Thus the area downstream of the dam is protected against flood.
Debris dams: A debris dam is constructed to retain debris such as sand, gravel, and drift wood flowing in the river with water. The water after passing over a debris dam is relatively clear.
Coffer dams: It is an enclosure constructed around the construction site to exclude water so that the construction can be done in dry. A coffer dam is thus a temporary dam constructed for facilitating construction. These structure are usually constructed on the upstream of the main dam to divert water into a diversion tunnel (or channel) during the construction of the dam. When the flow in the river during construction of hydraulic structures is not much, the site is usually enclosed by the coffer dam and pumped dry. Sometimes a coffer dam on the downstream of the dam is also required.

Types of dams

Classification based on structure and design

Gravity Dams: A gravity dam is a massive sized dam fabricated from concrete or stone masonry. They are designed to hold back large volumes of water. By using concrete, the weight of the dam is actually able to resist the horizontal thrust of water pushing against it. This is why it is called a gravity dam. Gravity essentially holds the dam down to the ground, stopping water from toppling it over.Gravity dams are well suited forblocking rivers in wide valleys or narrow gorge ways. Since gravity dams must rely on their own weight to hold back water, it is necessary that they are built on a solid foundation ofbedrock. Examples of Gravity dam: Grand Coulee Dam (USA), Nagarjuna Sagar (India) and Itaipu Dam (It lies Between Brazil and Paraguay and is the largest in the world).
Earth Dams: An earth dam is made of earth (or soil) built up by compacting successive layersof earth, using the most impervious materials to form a core and placing more permeable substances on the upstream and downstream sides. A facing of crushed stone prevents erosion by wind or rain, and an ample spillway, usually of concrete, protects against catastrophic washout should the water overtop the dam. Earth dam resists the forces exerted upon it mainly due to shear strength of the soil. Although the weight of the this structure also helps in resisting the forces, the structural behavior of an earth dam is entirely different from that of a gravity dam. The earth dams are usually built in wide valleys having flat slopes at flanks (abutments). The foundation requirements are less stringent than those of gravity dams, and hence they can be built at the sites where the foundations are less strong. They can be built on all types of foundations. However, the height of the dam will depend upon the strength of the foundation material. Examples of earthfill dam: Rongunsky dam (Russia) and New Cornelia Dam (USA).
Rockfill Dams: A rockfill dam is built of rock fragments and boulders of large size. An impervious membrane is placed on the rockfill on the upstream side to reduce the seepage through the dam. The membrane is usually made of cement concrete or asphaltic concrete.In early rockfill dams, steel and timber membrane were also used, but now they are obsolete. A dry rubble cushion is placed between the rockfill and the membrane for the distribution of water load and for providing a support to the membrane. Sometimes, the rockfill dams have an impervious earth core in the middle to check the seepage instead of an impervious upstream membrane. The earth core is placed against a dumped rockfill. It is necessary to provide adequate filters between the earth core and the rockfill on the upstream and downstream sides of the core so that the soil particles are not carried by water and piping does not occur. The side slopes of rockfill are usually kept equal to the angle of repose of rock, which is usually taken as 1.4:1 (or 1.3:1). Rockfill dams require foundation stronger than those for earth dams. Examples of rockfill dam: Mica Dam (Canada) and Chicoasen Dam (Mexico).
Arch Dams: An arch dam is curved in plan, with its convexity towards the upstream side. They transfers the water pressure and other forces mainly to the abutments by arch action. An arch dam is quite suitable for narrow canyons with strong flanks which are capable of resisting the thrust produced by the arch action.The section of an arch dam is approximately triangular like a gravity dam but the section is comparatively thinner. The arch dam may have a single curvature or double curvature in the vertical plane. Generally, the arch dams of double curvature are more economical and are used in practice. Examples of Arch dam: Hoover Dam (USA) and Idukki Dam (India).
Buttress Dams: Buttress dams are of three types : (i) Deck type, (ii) Multiple-arch type, and (iii) Massive-head type. A deck type buttress dam consists of a sloping deck supported by buttresses. Buttresses are triangular concrete walls which transmit the water pressure from the deck slab to the foundation. Buttresses are compression members. Buttresses are typically spaced across the dam site every 6 to 30 metre, depending upon the size and design of the dam. Buttress dams are sometimes called hollow dams because the buttresses do not form a solid wall stretching across a river valley.The deck is usually a reinforced concrete slab supported between the buttresses, which are usually equally spaced. In a multiple-arch type buttress dam the deck slab is replaced by horizontal arches supported by buttresses. The arches are usually of small span and made of concrete. In a massive-head type buttress dam, there is no deck slab. Instead of the deck, the upstream edges of the buttresses are flared to form massive heads which span the distance between the buttresses. The buttress dams require less concrete than gravity dams. But they are not necessarily cheaper than the gravity dams because of extra cost of form work, reinforcement and more skilled labor. The foundation requirements of a buttress are usually less stringent than those in a gravity dam. Examples of Buttress type: Bartlett dam (USA) and The Daniel-Johnson Dam (Canada).
Steel Dams: Dams: A steel dam consists of a steel framework, with a steel skin plate on its upstream face. Steel dams are generally of two types: (i) Direct-strutted, and (ii) Cantilever type . In direct strutted steel dams, the water pressure is transmitted directly to the foundation through inclined struts. In a cantilever type steel dam, there is a bent supporting the upper part of the deck, which is formed into a cantilever truss. This arrangement introduces a tensile force in the deck girder which can be taken care of by anchoring it into the foundation at the upstream toe. Hovey suggested that tension at the upstream toe may be reduced by flattening the slopes of the lower struts in the bent. However, it would require heavier sections for struts. Another alternative to reduce tension is to frame together the entire bent rigidly so that the moment due to the weight of the water on the lower part of the deck is utilised to offset the moment induced in the cantilever. This arrangement would, however, require bracing and this will increase the cost. These are quite costly and are subjected to corrosion. These dams are almost obsolete. Steel dams are sometimes used as temporary coffer dams during the construction of the permanent one. Steel coffer dams are supplemented with timber or earthfill on the inner side to make them water tight. The area between the coffer dams is dewatered so that the construction may be done in dry for the permanent dam.
Examples of Steel type: Redridge Steel Dam (USA) and Ashfork-Bainbridge Steel Dam (USA).
Timber Dams: Main load-carrying structural elements of timber dam are made of wood, primarily coniferous varieties such as pine and fir. Timber dams are made for small heads (2-4 m or, rarely, 4-8 m) and usually have sluices; according to the design of the apron they are divided into pile, crib, pile-crib, and buttressed dams. The openings of timber dams are restricted by abutments; where the sluice is very long it is divided into several openings by intermediate supports: piers, buttresses, and posts. The openings are covered by wooden shields, usually several in a row one above the other. Simple hoists—permanent or mobile winches—are used to raise and lower the shields.
Rubber Dams: A symbol of sophistication and simple and efficient design, this most recent type of dam uses huge cylindrical shells made of special synthetic rubber and inflated by either compressed air or pressurized water. Rubber dams offer ease of construction, operation and decommissioning in tight schedules. These can be deflated when pressure is released and hence, even the crest level can be controlled to some extent. Surplus waters would simply overflow the inflated shell. They need extreme care in design and erection and are limited to small projects. Example of Rubber type: Janjhavathi Rubber Dam (India).

Analytical Review of Pakistan Flood 2010 by Raza M. Farrukh

9:24 PM Civil Engineering , Hydrology , Irrigation Engineering

Analytic Review of Pakistan Flood 2010 by Raza M. Farrukh

Pakistan observed one of the worst floods in its history during July‐August 2010, which caused wide spread devastation. The floods caused by unprecedented rains in many of the catchments of major and small rivers washed away large number of public and private infrastructure in the northern parts of the country and inundated millions of hectares of land, thousands of human settlements, large network of communication infrastructure, and irrigation and drainage system in the centre and south, resulting in displacements of about 20 million people across the country.

The flood water originating in the northern steeper valleys gushed through Indus, Swat, Kabul rivers and many small hill torrents with extremely high velocities washing away almost everything on its way before spreading over millions of hectares in relatively flatter plains. The high intensity instantaneous flood peaks at various locations on these rivers surpassed historic peaks recorded in the past. Peak discharge recorded in Indus river upstream Tarbela reservoir, Swat river at Amandar and Munda, and Kabul river at Nowshehra was 60‐200 percent higher than the historic maximum.

The flashy floods in Khyber Pakhtunkhwah (KP) Province gave no response time and caused maximum human casualties. In other provinces, the Indus river carrying cumulative flows of all rivers got breached at seven locations, four in Sindh and three in Punjab.Six of these breaches were reportedly caused by high water pressure and one in Punjab was made by the provincial authorities at designated locations in the Right Marginal Bund (RMB) of Jinnah barrage. Water pressure breached Left Guide Bund (LGB) of Jinnah barrage, Left Marginal Bund (LMB) of Taunsa barrage, LMB of Guddu barrage, Tori bund between Guddu and Sukkur barrage on right bank, SM bund downstream Kotri Barrage on left bank, and PB bund downstream Kotri barrage on the right bank. These breaches occurred while flood water was still below their top levels. The damages incurred in Punjab, Sindh, and Balochistan resulted mainly from breaching of these bunds.

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Panama Canal : How? When? Why?

1:12 PM Civil Engineering , General , Hydraulics , Irrigation Engineering

Figure 1: Cargo ships crossing the great Panama Canal

The end of the 19th century was a time of visionaries who conceived of projects that would change the history of humankind. Since the time Vasco N unez de Balboa crossed Panama and discovered a great ocean, planners had conceived of the idea of a water link between the Atlantic and the Pacific Oceans. Having successfully connected the Mediterranean with the Red Sea at Suez, in 1882 the French began work on a canal across the narrow Isthmus of Panama, which at that time was part of Colombia. After struggling for 9 years, the French were ultimately defeated by the formidable technical difficulties as well as the hostile climate and the scourge of yellow fever.

Theodore Roosevelt became president during this period and his administration decided to take up the canal project and carry it to completion. Using what would he referred to as ‘‘gun-boat’’ diplomacy, Roosevelt precipitated a revolution that led to the formation of the Republic of Panama. Having clarified the political situation with this stratagem, the famous ‘‘Teddy’’ then looked for the right man to actually construct the canal. That right man turned out to be John F. Stevens, a railroad engineer who had made his reputation building the Great Northern Railroad. Stevens proved to be the right man at the right time.

Figure 2: Map showing location and important points on Panama Canal

Figure 3: Marine distance between San Francisco and New York with and without Panama Canal

John F. Stevens, chief engineer of the Panama Canal.
(National Archives,Washington,
D.C.)

Stevens understood the organizational aspects of large projects. He immediately realized that the working conditions of the laborers had to be improved. He also understood that measures had to be taken to eradicate the fear of yellow fever. To address the first problem, he constructed large and functional camps for the workers in which good food was available. To deal with the problem of yellow fever he enlisted the help of an army doctor named William C. Gorgas. Prior to being assigned to Panama, Gorgas had worked with Dr. Walter Reed in wiping out yellow fever in Havana, Cuba. He had come to understand that the key to controlling and eliminating this disease was, as Reed had shown, the control of the mosquitoes that carried the dreaded infection and the elimination of their breeding places (see The Microbe Hunters by Paul DeKruif). Gorgas was successful in effectively controlling the threat of yellow fever, but his success would not have been possible without the total commitment and support of Stevens.

Having established an organizational framework for the project and provided a safe and reasonably comfortable environment for the workers, Stevens addressed the technical problems presented by the project. The French had initially conceived of a canal built at sea level and similar to the Suez Canal. That is, the initial technical concept was to build a canal at one elevation. Because of the high ground and low mountains of the interior portion of the isthmus, it became apparent that this approach would not work. To solve the problem of moving ships over the ‘‘hump’’ of the interior, it was decided that a set of water steps, or locks, would be needed to lift the ships transiting the canal up and over the high ground of Central Panama and down to the elevation of the opposite side. The immense size of a single lock gate is shown in Figure 4.

Figure 4: Work in progress on the Great Gatun lock gates. (The Bettmann Archive)

The construction of this system of locks presented a formidable challenge. Particularly on the Atlantic side of the canal, the situation was complicated by the presence of the wild Chagres River, which flowed in torrents during the rainy season and dropped to a much lower elevation during the dry season. The decision was made to control the Chagres by constructing a great dam that would impound its water and allow for control of its flow. The dam would create a large lake that would become one of the levels in the set of steps used to move ships through the canal. The damming of the Chagres and the creation of Lake Gatun itself was a project of immense proportions, which required concrete and earthwork structures of unprecedented size.

The other major problem had to do with the excavation of a great cut through the highest area of the canal. The Culebra Cut, as this part of the canal was called, required the excavation of earthwork quantities that even by today’s standards stretch the imagination. Stevens viewed this part of the project as the construction of a gigantic railroad system that would operate continuously (24 hours a day) moving earth from the area of the cut to the Chagres dam construction site. The material removed from the cut would provide the fill for the dam. It was an ingenious idea.

To realize this system, Stevens built one of the greatest rail systems of the world at that time. Steam-driven excavators (shovel fronts) worked continuously loading railcars. The excavators worked on flexible rail spurs that could be repositioned by labor crews to maintain contact with the work face. In effect, the shovels worked on sidings that could be moved many times each day to facilitate access to the work face. The railcars passed continuously under these shovels on parallel rail lines.

Stevens’ qualities as a great engineer and leader were on a level with those of the Roebling’s. As an engineer, he understood that planning must be done to provide a climate and environment for success. Based on his railroading experience, he knew that a project of this magnitude could not be accomplished by committing resources in a piecemeal fashion.

He took the required time to organize and mass his forces. He also intuitively understood that the problem of disease had to be confronted and conquered. Some credit for Stevens’ success must go to President Roosevelt and his secretary of war,William Howard Taft. Taft gave Stevens a free hand to make decisions on the spot and, in effect, gave him total control of the project. Stevens was able to be decisive and was not held in check by a committee of bureaucrats located in Washington (i.e., the situation present before he took charge of the job).

Figure 5: How Panama Canal Works?

Having set the course that would ultimately lead to successful completion of the canal. Stevens abruptly resigned. It is not clear why he decided not to carry the project through to completion. President Roosevelt reacted to his resignation by appointing a man who, as Roosevelt would say, ‘‘could not resign.’’ Roosevelt selected an army colonel and West Point graduate named George Washington Goethals to succeed Stevens. Goethals had the managerial and organizational skills needed to push the job to successful completion. Rightfully so, General Goethals received a great deal of credit for the construction of the Panama Canal. However, primary credit for pulling the job ‘‘out of the mud,’’ getting it on track, and developing the technical concept of the canal that ultimately led to success must be given to Stevens—a great engineer and a great construction manager.

Methods of Dewatering of Foundations

12:57 PM Civil Engineering , Geotechnical Engineering , Irrigation Engineering

Construction of footings of various buildings, powerhouses, multistory buildings and many other structures requires excavation below the water table into water-bearing soils. Such excavations require lowering the water table below the slopes and bottom of the excavation to prevent raveling or sloughing of the slope and to ensure dry, firm working conditions for construction operations. De-watering is done to lower the water table to achieve above mentioned goals.

Purpose of Dewatering

Construction sites are dewatered for the following purposes:

To provide suitable working surface at the bottom of the excavation.
To stabilize the banks of the excavation thus avoiding the hazards of slides and sloughing.
To prevent disturbance of the soil at the bottom of excavation caused by boils or piping. Such disturbances may reduce the bearing power of the soil.

Methods of Dewatering

Following methods are used for Dewatering,

Sump pumping
Well point systems with suction pumps
Shallow wells with pumps
Deep wells with pumps
Eductor system
Drainage galleries
Electro-osmosis
Other methods

1) Sumps & Sump Pumping

A sump is merely a hole in the ground from which water is being pumped for the purpose of removing water from the adjoining area. They are used up to 8m with ditches leading to them in large excavations.

For prolonged pumping the sump should be prepared by first driving sheeting around the sump area and installing a cage inside the sump made of wire mesh with internal strutting or a perforating pipe filling the filter material in the space outside the cage and at the bottom of the cage and withdrawing the sheeting.

Small sump

Pumping from sump

2) Well Point Systems

A well point is 5.0-7.5 cm diameter metal or plastic pipe 60 cm – 120 cm long which is perforated and covered with a screen. The lower end of the pipe has a driving head with water holes for jetting. Well points are connected to 5.0-7.5 cm diameter pipes known as riser pipes and are inserted into the ground by driving or jetting. The upper ends of the riser pipes lead to a header pipe which, in turn, connected to a pump. The ground water is drawn by the pump into the well points through the header pipe and then discharged.This type of dewatering system is effective in soils constituted primarily of sand fraction.

Well Point De-watering System

The well points can lower a water level to a maximum of 5.5 m below the centerline of the header pipe. In silty fine sands this limit is 3-4 m. Multiple stage system of well points are used for lowering water level to a greater depth. A single well point handles between 4 and 0.6 m3/hr depending on soil type.

Multi Stage Well Point De-watering System

3) Shallow Wells

Shallow wells comprise surface pumps which draw water through suction pipes installed in bored wells drilled by the most appropriate well drilling and or bored piling equipment. Its limit is 8 m because these are pre bored. These wells are used in very permeable soils when well pointing would be expensive and often at inconveniently close centers. These can extract large quantities of water.

Shallow vs Deep Well

4) Deep Wells/Bedrock Wells

When water has to be extracted from depths greater than 8 m and it is not feasible to lower the type of pump and suction piping used in shallow wells to gain a few extra meters of depth the deep wells are such and submersible pumps installed within them. A cased borehole can be sunk using well drilling or bored piling rigs to a depth lower than the required dewatered level. The diameter will be 150 – 200 mm larger then the well inner casing. These systems are used in gravels to silty fine sands and in water bearing rocks.

Bedrock Well vs Shallow Well

Typical Deep Well

5) Eductor System (Jet Eductor System)

It is similar to well point system. Instead of employing a vacuum to draw water to the well points, it uses high pressure water, each about 30-40 mm in diameter. A high pressure is supplied through a venturi tube creating a reduction in pressure which draws water through the large diameter pipe. The high pressure main feeds off the return water. Its advantage is that in operating many well points from a single pump station, the water table can be lowered in one stage from depths of 10-45 m. This method becomes economically competitive at depth in soils of low permeability.

Eductor System

6) Drainage Galleries

Drainage galleries are used for the removal of large quantities of water for dam abutments, cut-offs, etc. Large quantities of water can be drained into gallery (small diameter tunnel) and disposed of by conventional large – scale pumps.

7) Electro Osmosis

It is used in low permeability soils (silts, silty clays, etc) when no other method is suitable. In this method direct current electricity is applied from anodes (steel rods) to cathodes (well-points, i.e. small diameter filter wells)

8) Other Methods

Ground freezing with ammonium brine or liquid nitrogen. It is used for all types of saturated soils.
Slurry trench cut-off walls with bentonite or native clay. It is used for all types of soils.
Impervious soil barrier. Used for all soils. Relatively shallow applications (5-6m max.)
Sheet piling. It is used for all soils except soils with large boulders.
Compressed air. It is used for all types of saturated soils and rock. Its applications is in tunnels, shafts and caissons.

Introduction to Design and Construction of Earth and Rock-Fill Dams

1:20 PM Civil Engineering , Hydraulics , Hydro Power , Irrigation Engineering

The successful design, construction, and operation of a reservoir project over the full range of loading require a comprehensive site characterization, a detailed design of each feature, construction supervision, measurement and monitoring of the performance, and the continuous evaluation of the project features during operation. The design and construction of earth and rock-fill dams are complex because of the nature of the varying foundation conditions and range of properties of the materials available for use in the embankment. The first step is to conduct detailed geological and subsurface explorations, which characterize the foundation, abutments, and potential borrow areas. The next step is to conduct a study of the type and physical properties of materials to be placed in the embankment. This study should include a determination of quantities and the sequence in which they will become available. The design should include all of the studies, testing, analyses, and evaluations to ensure that the embankment meets all technical criteria and the requirements of a dam as outlined in b below. Construction supervision, management, and monitoring of the embankment and appurtenant structures are a critical part of the overall project management plan. Once the project is placed into operation, observations, surveillance, inspections, and continuing evaluation are required to assure the satisfactory performance of the dam.

Basic requirements of an embankment dam

Dams are a critical and essential part of the Nation’s infrastructure for the storage and management of water in watersheds. To meet the dam safety requirements, the design, construction, operation, and modification of an embankment dam must comply with the following technical and administrative requirements:
(1) Technical requirements

• The dam, foundation, and abutments must be stable under all static and dynamic loading conditions.
• Seepage through the foundation, abutments, and embankment must be controlled and collected to ensure safe operation. The intent is to prevent excessive uplift pressures, piping of materials, sloughing removal of material by solution, or erosion of this material into cracks, joints, and cavities. In addition, the project purpose may impose a limitation on allowable quantity of seepage. The design should include seepage control measures such as foundation cutoffs, adequate and nonbrittle impervious zones, transition zones, drainage material and blankets, upstream impervious blankets, adequate core contact area, and relief wells.
• The freeboard must be sufficient to prevent overtopping by waves and include an allowance for settlement of the foundation and embankment.
• The spillway and outlet capacity must be sufficient to prevent over-topping of the embankment by the reservoir.
(2) Administrative requirements

• Environmental responsibility.
• Operation and maintenance manual.
• Monitoring and surveillance plan.
• Adequate instrumentation to monitor performance.
• Documentation of all the design, construction, and operational records.
• Emergency Action Plan: Identification, notification, and response subplan.
• Schedule for periodic inspections, comprehensive review, evaluation, and modifications as appropriate.

Embankment

Many different trial sections for the zoning of an embankment should be prepared to study utilization of fill materials; the influence of variations in types, quantities, or sequences of availability of various fill materials; and the relative merits of various sections and the influence of foundation condition. Although procedures for stability analyses afford a convenient means for comparing various trial sections and the influence of foundation conditions, final selection of the type of embankment and final design of the embankment are based, to a large extent, upon experience and judgment.

Features of design

Major features of design are required foundation treatment, abutment stability, seepage conditions, stability of slopes adjacent to control structure approach channels and stilling basins, stability of reservoir slopes, and ability of the reservoir to retain the water stored. These features should be studied with reference to field conditions and to various alternatives before initiating detailed stability or seepage analyses.

Other considerations

Other design considerations include the influence of climate, which governs the length of the construction season and affects decisions on the type of fill material to be used, the relationship of the width of the valley and its influence on river diversion and type of dam, the planned utilization of the project (for example, whether the embankment will have a permanent pool or be used for short-term storage), the influence of valley configuration and topographic features on wave action and required slope protection, the seismic activity of the area, and the effect of construction on the environment.

Computation of Runoff by Infiltration Method

9:40 AM Civil Engineering , Hydrology , Irrigation Engineering

One of the methods to compute runoff is "infiltration method" explained below.

Infiltration Capacity

Maximum rate at which water enters the soil in a given condition

Infiltration Rate

Rate at which water actually enters the soil during a storm and is equal to the infiltration capacity or the rainfall rate, whichever is less

Infiltration Index

Average rate of loss such that volume of rainfall in excess of that rate will be equal to direct runoff

Infiltration Indices

—W-Index or Average Infiltration Rate

W = (P-R)/tr cm/hr and k = (i-W)/i

W = Index cm/hr,

P = Precipitation, cm

R = Runoff, cm

tr = duration of infiltration, hr

k = Runoff Coefficient

i = Rainfall intensity, cm/hr

—Phi Index

Rate of rainfall above which rainfall volume equals the runoff volume

Runoff

9:00 AM Civil Engineering , Hydrology , Irrigation Engineering

Runoff is the movement of water along the earth’s surface as a result of precipitation. Runoff occurs when precipitation moves across the land surface eventually reaching streams or lakes.Or drainage or flowing off of precipitation from a catchment area through a surface channel.

Watershed

The land area that contributes surface runoff to any point of interest is called a watershed. A large watershed can contain many smaller sub-watersheds.

Drainage Basin

The tract of land (both surface and subsurface) drained by a river and its tributaries is called a drainage basin. A watershed area supplies surface runoff to a river or stream, whereas a drainage basin is the tract of land drained of both surface runoff and groundwater discharge.


Watershed

Factors Affecting Runoff

Storm or Precipitation Characteristics
Shape and Size of Catchment
Topography
Geological Characteristics
Meteorological Characteristics
Character of Catchment Surface
Storage Characteristics

Storm or Precipitation Characteristics

◦Type or nature of storm and season
◦Intensity, Duration, and Frequency
◦Arial extent or distribution
◦Direction of storm movement

Shape and Size of Catchment

High peak for wider catchment than for narrow catchment.

Topography

More runoff for smooth, steep and windward side catchment.

Geological Characteristics

Greater runoff for impervious, rocky, surface and subsurface strata.

Meteorological Characteristics

Temperature, Humidity, Wind speed, Wind direction, and Pressure variation

Basin or Catchment’s Characteristics

More absorption for catchment with no natural drainage.Less runoff for cultivated, vegetated, and unsaturated surface.

Storage Characteristics

Depressions, Pools and ponds/lakes, Stream Channels, Check dams, Upstream reservoirs or tanks or weirs, Floodplain swamps, Groundwater storage in pervious deposits. All storages tend to reduce the peak flow.

More useful information

Drip and Trickle Irrigation System

6:50 AM Civil Engineering , Hydraulics , Irrigation Engineering

Trickle irrigation, sometimes referred to as drip irrigation, is a low-pressure irrigation method that uses such systems that place water slowly and directly in the root zone of the desired plant, increasing the efficiency of the water applied.

Trickle irrigation can reduce water usage by 30 to 70 percent compared to more traditional means of irrigation, such as overhead sprinklers or hand watering .

History

Drip and Trickle Irrigation was used in Egypt as early as in 300 B.C.

But to the modern world “Drip irrigation”, was first introduced in Germany in the 1860s, but it did not gain widespread popularity until it was introduced in England in the 1940s. The drip/trickle irrigation technology existing today flourished in the 1960s.

Drip irrigation was only implemented over 56,000 hectares worldwide during the 1970s. Over the next thirty years, this had grown to almost 2.0 million hectares. Perhaps in Pakistan, the farming community may not be ready for this concept right now. In the next twenty years, however, with the farmer increasingly progressive and the consistent lack of optimum water resources, drip irrigation in the country may well be a reality.

Methodology

Drip, or micro-irrigation, technology uses a network of plastic pipes to carry a low flow of water under low pressure to plants. Water is applied much more slowly than with sprinkler irrigation system. Drip irrigation exceeds 90 percent efficiency whereas sprinkler systems are 50 to 70 percent efficient. If systems are set to water excessively, any system including drip can waste water. Low volume application of water to plant roots maintains a desirable balance of air and water in the soil. Plants grow better with this favorable air-water balance and even soil moisture. Sprinkler irrigation results in a greater wet-to-dry fluctuation in the soil and may not produce optimal growth results. Slopes are inefficient to irrigate because gravity pulls water downhill, causing runoff and water waste. The slow rate of water applied through drip irrigation is more likely to soak in before it runs off .

Components of a Drip Irrigation System

Water Source
Pumping System
Distribution System
Filtration System
Injection Units-Chemicals/Fertilizer
Systems Controls
Zone Controls
In-field Delivery System
Miscellaneous

Advantages of Drip Irrigation System

Water use is reduced. Plants need the same amount of water no matter what the delivery method. Trickle irrigation places the water at the roots, where plants can use it best.

Precise application of nutrients is possible using drip irrigation. Fertilizer costs and nitrate losses can be reduced. Nutrient applications can be better timed to meet plants' needs.

Fewer weeds germinate. Water is directed to the crop, leaving the area between the rows dry, so weed seeds located there are less likely to germinate.

Fewer leaf diseases occur. Wet leaves encourage fungal and bacterial plant diseases. Trickle irrigation does not wet leaves.

Soil structure is not damaged from water falling on bare soil.

Insecticide and fungicide use is reduced. Trickle irrigation does not wash pesticides from the foliage.

Give more and better output, because these systems maintain the soil moisture at a high enough level to avoid putting plants under water stress;

Use problematic soils and waters.

Require no land leveling.

Irrigate more land with less water.

Irrigate with very high efficiency.

Undulating terrains, sandy, hilly lands can also be brought under productive cultivation.

Uniform application of water in drip irrigation achieves high water efficiency.

Watering only the roots of your plants with drip irrigation cuts down on water-borne pests and fungal diseases that spread by water movement.

Using drip irrigation below ground eliminates the potential risk of disease caused by bacteria and viruses in reclaimed water.

In many instances, a totally new and more favourable root zone environment is created and a relatively constant soil moisture level is maintained.

Fertilizer use efficiency may increase by 30%

Soluble fertilizer and chemicals can be given through the drip irrigation system and delivered to the root zone.

Disadvantages of Drip Irrigation System

Time is required for initial planning and installation.

It is more expensive than most sprinkler systems.

The tiny emission holes can become clogged with soil particles, algae or mineral particles.

Insects and rodents may damage the trickle line emitters.

The initial investment costs are rather high.

Declogging is difficult and time consuming.

Salinity is higher on the soil surface and between two drippers.

These systems wet only a part of the plant root where shrinkages and output losses may occur with insufficient water or nutrients.

Strong winds may fell large trees.

These systems require a well-trained workforce; and

Dust problems may arise in dry strips.

More information on drip irrigation system

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