D A M S

DAMS

DamsTypes of damSelection of dam siteSelection of dam typeDetermination of dam heightInstrumentationInspection of damComponents of earth damDesign criteria for earth dam

Components of a water power schemeEssentials of general plant layoutGeneral types of plant layoutSurge chambersGoverning of an impulse turbineContents

DAMSDAMDam is constructed to create a reservoir (permanent or temporary) Purpose of dams:To divert water from a stream (Diversion dams)For navigation (Navigation dams)For hydropower generation (Power dams) Store water for municipal/industrial use, irrigation, flood control, river regulation, recreation, (Storage dams) A dam serving two or more purposes is a multipurpose dam. The dam should be economical and the material used for construction should be easily available.

DAMSDams are classified based on their purpose, shape, material used and mode of construction

TYPES OF DAM

Embankment Dams Concrete Dams Composite Dams

DAMSEmbankment Dams

Earth fill DamsRock fill Dams

Types of Earth fill Dams

Homogeneous earth fill Modified HomogeneousZoned earth fillHydraulic fill Dams

DAMSTypes of Earth fill Dams

DAMSTypes of Rock fill Dams

Central coreSloping coreDiaphram

DAMSTypes of Concrete Dams

Concrete Arch DamConcrete Buttress DamConcrete Gravity Dam

DAMSCOMPOSITE DAMThis dam is a combination of embankment and concrete dam. It generally consists of concrete gravity or buttress sections in combination with earth fill or rock fill sections. The concrete dam portion helps to pass flood flows over or through the section during construction, and act as the spillway after construction. The earth or rock fill section take advantage of low cost construction and local materials.EARTH DAMSEarth dams have been used for water storage since early civilization.Earth dams may be built of rock, gravel, sand, silt or clay in various combinations.

DAMSEarth dams are constructed with an inner impervious core with upstream and downstream zones or more pervious materials, sometime including rock zones.Earth dams limit the flow of water by the use of fine-grained soils. Where possible, these soils are formed into a relatively impervious core. In sand or gravel foundation, the core may by connected to bedrock by a cutoff trench backfilled with compacted soil. If such cutoffs are uneconomical because of the great depth of pervious foundation, then the central impervious core is connected to a long horizontal upstream impervious blanket that increases the seepage path. The impervious core is encased in pervious zones of sand, gravel, or rock fill for stability.Transition zones prevent the core material from being transported into the pervious zones by seeping water.

DAMSWhen pervious soils are scarce, the entire dam may be a homogeneous fill of relatively impervious soil. Downstream pervious drainage blankets are provided to collect seepage passing through, under, and around the abutments of the dam.Materials can be obtained from excavation for the dam and from borrow area. Rock fill is generally used when large quantities are available or when soil borrow is scarce. Earth fill embankment is placed in layers and compacted by sheep-foot rollers or heavy pneumatic-tire rollers. Moisture content of silt and clay soils is carefully controlled to facilitate optimum compaction. Sand and gravel fills are compacted in slightly thicker layers. Rock fill is placed in layers 1-3 ft deep.

DAMSSELECTION OF DAM SITEThis depends upon hydrologic, topographic, and geologic conditions; storage capacity of reservoir; accessibility; cost of lands and necessary relocations of prior occupants or uses; and proximity of sources of suitable construction materials.For a storage dam the site should have the desired amount of storage to be economically developed. Power dams must be located to develop the desired head and storage. For a diversion dam the site must be in conjunction with the location and elevation of the outlet canal or conduit.Site for navigation dams involves factors such as desired navigable depth, channel width, slope of river channel, natural river flow, amount of bank protection, amount of channel dredging, approach and exit conditions for tows, and locations of other dams.

DAMSSELECTION OF DAM SITEUnless other conditions are satisfactory, hydrological features may need to be subordinated. Topographic characteristics include width of the flood plain, shape and height of valley walls, existence of nearby saddles for spillways, and adequacy of reservoir rim to retain impounded water. Geologic conditions include the depth, classification, engineering properties of soils and bedrock, occurrence of sinks, faults, and major landslides at the site or in the reservoir area. The elevation of ground water table influence the construction operations and suitability of borrow materials. The reservoir water recharges the ground water and have adverse effects on mineral resources.

DAMSSELECTION OF DAM TYPEIt depends on the estimated costs of various types. Important factors are topography, foundation conditions and the accessibility of construction materials. A hard-rock foundation is suitable for any type of dam, provided the rock has no unfavorable joints, no movement in existing faults, and seepage is controlled at reasonable cost.Rock foundations of high quality are essential for arch dam because the abutments receive the full thrust of the water.Rock foundations are necessary for all concrete dams. An earth dam may be built on almost any kind of foundation if properly designed and constructed.

DAMSSELECTION OF DAM TYPEAn embankment dam is most economical if large spillway and outlet capacities are required and topography and foundation are favorable. In a wide valley a combination of an earth embankment dam and a concrete section containing the spillway and outlets is economical. A concrete dam requires adequate quantities of suitable aggregate and availability of cement, while an earth dam requires sufficient quantities of both pervious and impervious earth materials. If enough rock is available, a rock-fill dam with an impervious earth core may be the most economical.

DAMSDETERMINATION OF DAM HEIGHT The dam must be high enough to Store water to the normal full pool elevation Provide for the temporary storage needed to route the spillway design flood through the dam Provide sufficient freeboard to assure an acceptable degree of safety against possible overtopping from waves and run up.Physical characteristics of the dam and reservoir site or existing development within the reservoir area may impose upper limits in selecting the normal full-pool level. Freeboard is the distance between the maximum reservoir level and the top of the dam. Usually 3ft or more of freeboard is provided to avoid overtopping by wind generated waves. Additional freeboard may be provided for possible effects of surges induced by earthquakes, landslides etc,.

DAMSINSTRUMENTATIONInstruments are installed at dams to observe structural behavior and physical conditions to check safety, and for design improvement. In concrete dams instruments are used to measure stresses. Plumb lines are used to measure bending, and clinometers to measure tilting. Contraction joint openings are measured by joint meters. Temperatures are measured either by embedded electrical resistance thermometers or by adopting strain, stress, and joint measuring instruments. Water pressure on the base of a concrete dam is measured by uplift pressure cells.

DAMSMeasurements are made to determine horizontal/vertical movements; strong-motion accelerometers are installed on and near dams in earth-quake regions to record seismic data. Piezometers determine pore water pressure in the soil or bedrock during construction and seepage after reservoir impoundments.Settle mental gages determine settlements of the foundation of the dam under dead load. Vertical and horizontal makers determine movements, especially during construction.Inclinometers determine horizontal movements along a vertical line.

DAMSINSPECTION OF DAM:Failure of dam may result in loss of life or property in the downstream area.

The design of dams should be reviewed to assure competency of the structure and its site, and inspection should be made during construction to ensure that the requirements of the design and specification are incorporated in the structure.

After completion and filling, inspections should be regularly scheduled. The objective is to detect symptoms of possible distress in the dam at earliest time.

DAMSINSPECTION OF DAMThese symptoms include slough or slides in embankments; piping or boils; abnormal changes in flow; increase in seepage quantities; changes in pore water or uplift pressures; movement or cracking of embankments/abutments; cracking of concrete structures; appearance of sinkholes near foundations; excessive deflection, displacement erosion, vibration of concrete structures; movement, deflection or vibration of spillway gates; or any other unusual condition in the structure or surrounding terrain. Detection should be followed by an investigation of the causes, probable effects, and remedial measures required.Systematic monitoring of the instrumentation installed in dams is essential to the inspection program.

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DAMSIt is an impervious element which intends upwards from the base of the dam to the top practically always constructed of impervious soil and may simply be an extension of the cutoff wall upwards to the top of dam. Although core wall is often located on the longitudinal centre line of the dam, which may be located anywhere on the upstream side of the centre.

DAMSIt is a relatively impervious element which extends down wards through the foundation soil from the base of the dam. The function is to reduce the amount of seepage water flowing through the foundation. This cut-off wall should be carried down to solid rock or other very impervious material. Its effectiveness is greatly reduced, if it extends only part way and seepage water may flow beneath the cut off wall. It may be made of steel sheet pile or clay or reinforced cone or fine grained impervious soil.

DAMSIt is made up of pervious material. The u/s shell section, provide protection against rap draw down which causes rotational type of failure of u/s slop. The d/s sheet provides protection against out drop of seepage.

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DAMSIt is placed on the u/s face as protection against wave wash and on the d/s slope for protection against rains. It comprises of a 3-5 ft. thick layer of big stones (rock fragment and boulder).

DAMSTo protect a dam from the effects of seepage drains are provided. The drains provided completely intercept the seepage and the down stream zone is kept free of saturation. The most commonly used methods. Are as follows:-Longitudinal drains and blanketsChimney drains extending upwards into the embankmentToe drainsRelief wells

DAMSDESIGN CRITERIA FOR EARTH DAMThere should be no danger of overtopping. Overtopping quickly erodes the d/s slopes and the failure may occur. The height should be enough that dam is never over topped even during max anticipated flood. The seepage line should be well below the d/s slope surface. If this line is too high is will intersect the d/s slope and the seepage water will break out on the slope and produces a wet marshy condition in the vicinity of the d/s toe of the dam. Provide artificial drainage for seepage water to make the line low. The u/s face slope should be safe against sudden draw down of the reservoir. High stresses are induced in the soil due to removal of hydrostatic pressure. Therefore provide porous media on the u/s face taking these factor into consideration.

DAMSThe u/s & d/s slopes should be flat enough to be stable and should have satisfactory factor of safety under all conditions.The u/s & d/s slopes should be flat enough to make the shearing stresses in the foundation less than the shearing strength of the foundation soil and to provide a satisfactory factor of safety.There should be no free passage of water from the u/s to d/s face. The surface should be scarified before first layer of soil is laid and compacted. Provide baffle wall at comparatively short intervals along the length of conduit which extends through the dam. Piping is a sort of internal erosion by which small channel gown in length from the d/s end. If unchecked it may completely damage the dam. Similarly seepage water may cause boils at points downstream from the dam. Place layers of coarse and gravels over the effected area until the weight of over burden overcomes tendency towards boiling.

DAMSWhen water passes through and under the dam, reaches the discharge surface, its pressure and velocity should be such that not to erode the material of the dam and foundation. Protect u/s & d/s slopes from erosion by rains and wave action. Place about 3 ft. thick layer of rip rap. Provide 1/8 size stone beneath the rip rap in a layer of 9 to 18, to prevent washing away of the soil as water rushes through rip rap stone during wave action.

DAMSCOMPONENTS OF A WATER POWER SCHEMEA water power development is to utilize the available power in the fall of a river, through a portion of its course, by means of hydraulic turbines. The essential features of a water power development are:DamIt is a structure built at a suitable location across the river, both to create head and a reservoir. In many cases the power development is at or close to the dam, utilizing the available head at the dam only, known as concentrated fall development. In some cases, additional head is obtained by carrying the water in a waterway for some distance downstream to the power plant, known as a divided fall development.

DAMSWaterwayMore often the development must utilize, in addition to the head created by the dam, an amount obtained by carrying the water in a waterway, which may be a canal, penstock (closed pipe) or a combination of these, for some distance downstream.

PenstockA penstock is a pipe that conveys water from a fore-bay, reservoir , or the source to a turbine in hydroelectric plant. Pressure rise and speed regulation must be considered in the design of a penstock.

DAMSPressure rise, or water hammer, is the pressure change that occurs when the rate of flow in a pipe or conduit is changed rapidly. One important consideration is that the pressure rise which occurs in a penstock when the turbine wicket gates are closed after the loss of load. The penstock connect the wheel units (Turbine) with main waterway or intake at the Dam. In the case of a short penstock, there could normally be a pipe for each wheel unit. For location of penstock, the economically shortest route is desired. Penstock always sloping towards the powerhouse. To minimize the pressure and cost of pipe, the part of length of penstock is kept on as flat a grade as possible and with sudden pitch to the powerhouse through a relatively short distance.

DAMSThe intake of the penstock at the dam or fore bay of the canal must be at a level low enough to provide an adequate water seal under all conditions, particularly at low water. This will commonly mean that the top of penstock at its intake should be 4 or 5ft. or more below the lowest water level. A gate and usually racks are placed at the entrance of the penstock. An air vent or a stand pipe connecting the top of the penstock with the open air should be provided below and near the gate. This is to permit air to enter the penstock when the head gates are closed, otherwise dangerous collapsing pressure may be exerted on the penstock.The entrance to the penstock should be flared to avoid any loss of head by contraction. Sharp bends in the penstock should be avoided, as they cause loss of head and require special anchorages.

DAMSThe Powerhouse and EquipmentThis includes the hydraulic turbines, generators along with their accessories and the building required for their protection and operation. The TailraceIt is the waterway from the powerhouse back to the river. Mostly the powerhouse is located on the river bank so that no tailrace channel is required, but occasionally, to develop additional fall, a tailrace channel of some distance is used. SpillwaysA spillway releases water in excess to protect the dam and its foundation against erosion and possible failure. Spillway is essential except in small dams where the runoff can be safely stored in the reservoir without danger of overtopping. Ample spillway capacity is important for large earth dams.

DAMSESSENTIALS OF GENERAL PLANT LAYOUT/PLANNING A WATER POWER DEVELOPMENTTwo basic principles to be kept in mind in planning a water power development are economy and safety.

FACTORS AFFECTING ECONOMY OF PLANT:The factors affecting the relative economy of a water power development may be divided into the characteristics of:

Site Use and market

DAMSSite CharacteristicsThe site characteristics affect the construction, operating cost of the plant and the conditions which decide whether a site is worthy of development and, if so, the best manner of making this development include the following:

Geological ConditionsThe geological conditions for a power site is the suitable foundation for structure. The absence of a suitable rock foundation may even prevent the utilization of a power site. Topographical conditionsThey determine the dimensions of the dam, and affect its cost, the relative proportion of the fall/head to be developed by the dam and the manner in which the waterway may be constructed.

DAMSSlope of River:It affects necessary length, cost of waterway, and amount of poundage obtained at the dam. Head to discharge RelationshipFor the power available, the greater the head as compared to discharge, the less costly will be the development owing to the greater capacity required for all the features except the dam, as discharge increases. In general, the higher head developments are always less expensive per horsepower of capacity than those of lower head. Operating costs:A stream subjected to frequent floods may have the power frequently curtailed by back water in the tailrace, and on such a stream flash boards on the dam may require frequent renewal. The presence of ice in streams having numerous falls or stretches of quick water also introduces problems of operation and often adds to its cost.

DAMSUse and MarketLocation of MarketIt includes the conditions affecting the sale price and value of the power being developed. A water power site may be developed at low cost but situated far from any possible market is unworthy of consideration for development.

Load FactorCertain features of the water power development, particularly the power house and equipment, vary in cost nearly inversely to the load factor. It is of advantage, therefore, to keep the load factor at a hydro-electric development as high as possible.

DAMSGENERAL TYPES OF PLANT (WATER POWER) LAYOUTNot two alike water power developments will probably ever be built. However, certain general types of plant layout consistent with the site characteristics e.g., head, available flow, topography of river, vicinity etc., are discussed. These factors are more or less interdependent.Concentrated FallA low-cost development can be made by placing the power house in the river at one end of the dam (Fig. a). This results in an undesirable limitation of length of spillway. To obtain necessary spillway length, the power house must be located as shown in (Figs. b, c, or d). A development utilizing concentrated fall has been made by using a hollow concrete dam of the Ambursen type with power house in the dam.

DAMSDivided FallIn (Fig. e-f) the river banks remain high to afford room for a canal development which provide an additional head due to fall in the river between dam and the tailrace level.In (fig. g) a canal can be used for only a part of the distance. For large flow, it may be necessary to use more than one penstocks, although may result in increased cost, as compared with (Fig. e & f) for a given total length of waterway. In (Fig. h) the manner is similar to that of g but advantage is taken of a bend in the river to utilize a greater head for a given length of waterway. In (Fig. k) the flow is low enough to permit the use of a penstock throughout, until near the power house, where a quick descent is made, usually with individual penstocks to each wheel unit. Here again a curve in the river is utilized to shorten the length of penstock.

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DAMSSURGE CHAMBERSIn a hydroelectric plant, the flow of water to a turbine must be decreased rapidly whenever there is a sudden drop in load. This results in high water-hammer pressures and may need a very strong and hence expensive pipe. Surge chambers are used to handle this situation. A surge chamber is a vertical standpipe connected to the pipeline (Figure). With steady flow in the pipe, the water level zl in the surge chamber is below the static level (z=0). When the valve is suddenly closed, water rises in the surge chamber. The water surface in the tank will fluctuate up and down damped out by fluid friction. Surge chambers are usually open at the top and of sufficient height so that they will not overflow. In some instances they are permitted to overflow if no damage will result. The surge chamber, in the event of a sudden demand for increased flow, provide some excess water, and the entire mass of water in a long pipeline is accelerated.

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DAMSGOVERNING OF AN IMPULSE TURBINE (PELTON WHEEL)The load on the generator is always fluctuating. This has some effect on the turbine. The change of load on the turbine is sure to change its speed and rate of flow. In order to have a high efficiency at different loads, the speed of the turbine must be kept constant. The process of flow is known as governing of the turbine. For an impulse turbine, the Servomotor or Relay cylinder method is commonly.The Servomotor method is a mechanism consisting of the following parts as shown in fig.: Centrifugal governor Control valve Servomotor Gear pump Oil sump Spear or needle A set of pipes, connecting oil sump with control valve, and control valve with relay cylinder.

DAMSThe centrifugal governor is driven from the main shaft of the turbine, either by belt or gear arrangement. The control valve, controls the direction of flow of the liquid (which is pumped by gear pump from the oil sump) either in pipe AA or BB. The servomotor or relay-valve has a piston (whose motion, towards left or right, depends upon the pressure of the liquid flowing through the pipes AA or BB) is connected to a spear or needle, which reciprocates inside the nozzle as shown. When the turbine is running at its normal speed, the positions of piston, control valve and fly balls of centrifugal governor will be in their normal positions (Figure). The oil pumped by the gear pump, into the control valve, will come back to the oil sump as the mouths of both the pipes AA and BB are closed by the two wings of the control valve.The increase of load on the turbine will decrease its speed.

DAMSThis will also decrease the speed of centrifugal governor and the fly balls will come down.This coming down of the fly balls, will also bring down the sleeve, which is connected to the central vertical bar. This downward movement of the sleeve will raise the control valve rod (sleeve is connected to the control valve rod). A slight upward movement of the control valve rod will open the mouth of pipe AA (the mouth of pipe BB closed). The oil will rush from the control valve to the right side of the piston in the servomotor through the pipe AA. This oil, under pressure, will move the piston and spear towards the left, which will open more area of the nozzle controlling the flow to the turbine. This increase in the flow area will increase the rate of flow and the speed of the turbine increases. When the speed of the runner will come up to the normal speed, fly balls will move up and the sleeve as well as the control valve rod will occupy its normal position.

DAMSWhen the load on the turbine decreases its speed will increase. As a result of this, the fly balls will go up (due to increase in centrifugal force) and sleeve will also go up. This will push the control valve downwards. This downward movement of the control valve rod will open the mouth of the pipe BB (still keeping the mouth of the pipe AA closed). Now the oil (under pressure) will rush from the control valve to the left side of the piston in servomotor through the pipe BB. The oil, under pressure, will move the piston and spear towards the right, which will decrease the area of the nozzle and ultimately decrease the rate of flow. This decrease in the rate of flow will decrease the speed of the turbine till the speed, once again, comes down to the normal.

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