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Design of Hydraulic Structures

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Unit-I

Unit-I

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Design of Hydraulic Structures Design of Hydraulic Structures Presentation Transcript

  • Design of Hydraulic Structures BTCI09007
  • Syllabus Unit-I Elements of Dam Engineering: Introduction: • Introduction, classification, comparative study of different types of dams, selection of type of dam, selection of site of dam, preliminary and final investigations of dam sites, fixation of storage capacity, reservoir losses, sedimentation in reservoirs, density currents.
  • Elements of Dam Engineering Dam: A dam is an impervious barrier constructed across a river or stream to store water on its a upstream side. The side of barrier on which water is stored is called upstream side and the other side of the barrier is called downstream side.
  • Purpose of Dam • Dam is generally most suitable in hilly area where deep valleys are available which gives a deep storage of water. The stored water on its upstream side serves various purpose such as: • Flood Mitigation • Irrigation • Water Supply • Navigation • Fishery and wild life Preservation • Hydro-electric Power Generation • Recreation
  • Flood Control
  • Irrigation
  • Water Supply
  • Hydro Electric Power Generation
  • Fisheries
  • Classification of Dams Classification as per function and use Storage dam • This is the most common type of dam normally constructed to store excess flood water which can be utilized later when demand exceeds the flow in river. The Storage dams may be constructed for various purposes such as irrigation, water supply, hydro-power generation etc. they may be made of concrete, stone or earth or rock fill etc.
  • Storage Dam
  • Classification of Dams Detention Dams • This type of dams are mainly constructed to control flood. This type of dam stores water temporarily and releases it gradually at a safe rate when the flood recedes. Detention dam provides safeguard against possible damage due to flood on the downstream side of it. Sometimes a detention dam may also be used as storage dam.
  • Detention Dams
  • Classification of Dams Diversion Dam • The purpose of diversion dam is necessarily different. It is constructed to divert the river water into canal, conduit etc. For this purpose, mostly a weir or low level dam is constructed across the river to raise the water level which can be diverted as per the needs. This type of dam may be used for water supply, irrigation or some other purposes.
  • Diversion Dam
  • Classification of Dams Classification as per hydraulic design Overflow Dam: An overflow dam is built to allow the overflow of surplus discharge above the top of it. They are generally built of masonry or concrete and they are gravity type of dam. Usually dams are not designed as overflow for their entire length. Only few meters of its length is kept as overflow section.
  • Overflow Dam
  • Classification of Dams Non-Overflow Dam: • In this type of dam, water is not allowed to overtop the dam. The top of the dam is fixed at a higher elevation than the expected maximum flood level. Since water is not allowed to overtop, it can be constructed of large variety of materials such as earth, rock fill, masonry, concrete etc.
  • Non-Overflow Dam
  • Classification as per Structural Design Gravity Dam: • It is a solid concrete or masonry dam that all external forces are resisted by its own weight or gravity forces. Arch Dam: • An arch dam is curved masonry or concrete dam which has convex portion facing upstream. It resists major portion of water pressure by arch action. • The self weight of the dam is comparatively lesser than gravity dam.
  • Gravity Dam & Arch Dam
  • Classification as per Structural Design Buttress Dam: • It consists of sloping membrane or deck on upstream which is supported by number of buttress or piers. These buttress are constructed of reinforcements concrete and supported by struts or bracings.
  • Buttress Dam
  • Buttress Dam
  • Classification as per Structural Design Embankment Dams • They are constructed of locally available soils, gravels and sands, which resists all external forces by it shear strength. These types of dams are more suitable up to moderate height. They are generally trapezoidal in section. Earth dam, earth and rock fill dam are the example of this type of dam.
  • Embankment Dams
  • Earthen Dams
  • Rock Fill Dam
  • Classification as per material of construction Rigid Dams • These dams are built of rigid materials such as masonry, concrete, steel or timber. In earlier times dams were mostly built of stone masonry which have been replaced now-adays by concrete. Non-Rigid Dams • These dams are built of non-rigid materials such as earth, rock fill etc. Earth Dam, Rock fill Dam etc are the examples of this type.
  • Rigid Dams & Non-Rigid Dams
  • Embankment Dams • They are the most ancient type of dams that can be build by naturally available materials with minimum of processing. These dams are not suited for sites where good foundations is not available at a reasonable depth for concrete or masonry dam to construct. Earth Dam: • Earth dams are made of locally available soils, sands and gravel with trapezoidal in section. They are economical and suitable for almost all type of available foundation.
  • Embankment Dams
  • Earth Dam Homogeneous Type • This type of dam is constructed of a single kind of materials with stone pitching at upstream side to safeguard against erosion. Also have a rock toe at downstream to drain out the seepage water through the body of the dam.
  • Earth Dam Zoned Type • The zoned type of dam consists of more than one kind of material. I consists of a central impervious core made of clay and outer pervious zone made of mixtures of earth and gravel. It also has rock toe at downstream side and stone pitching on upstream side.
  • Earth Dam Diaphragm Type: • This type of section is used when impervious material is available in lesser quantity at site. It consists of thin impervious core of diaphragm made of clay, cement concrete or bituminous concrete which is surrounded by earth or rock fill. They are also called sometimes as thin core dams. The construction of this type of dam is limited to small dams only. It has upstream stone pitching and downstream stone blanket.
  • Earth Dam
  • Characteristics of Earth Dam • They are normally constructed when huge quantity of material e.g. soils, gravels are available locally. • They are suitable for almost all type of available foundation. • They resist all external forces acting mainly by shear strength of soil. • They can be built rapidly with relatively unskilled labour because they use locally available material in large quantity. • They are comparatively cheaper than concrete and arch dam. • They allow easy increase in their height if needed, without much difficulties.
  • Characteristics of Earth Dam • They require a separate spillway away from the main dam. • They require heavy maintenance cost and constant supervision • They are more susceptible to be damaged by floods than any other type of dam.
  • Rock fill Dam • It is the type of embankment dam which uses various sizes of materials to provide stability. It also has impervious membrane on upstream face to provide water tightness. • Impervious membrane is usually made of concrete. Rock fill dam is preferred when plenty of rocks are available from nearby quarry. • Rock fill dams require foundation which will result in a minimum settlement. The foundation should be free from all foreign materials like silt, clay, sand etc. The upstream and downstream slopes of a rock fill dams depend on the type of impervious membrane and its location. They are cheaper then concrete dam and can be built rapidly if proper rock is available.
  • Rock fill Dam
  • Combined Earth and Rock fill Dam • It is a composite embankment dam. In this upstream consists of soil where as downstream portion is filled with rock. Upstream has a riprap. With cement grouted core wall to check seepage. Riprap makes upstream slope stronger against seepage and damage due to wave action.
  • Combined Earth and Rock fill Dam
  • Concrete Dams • They are Categorized as rigid dams because they are constructed of rigid material like concrete. They may be either straight or curved in plan. These types of dams are normally best suited on solid rock foundations. The construction of such dams requires heavy mechanized plants, concrete, aggregate, cement and sand.
  • Concrete Dams
  • Gravity Dams • It is a solid concrete dam which resists all external forces by its own weight. It needs a sound rock foundation because it transmits all the forces including self weight to the foundation. Most of the gravity dams are provided with an overflow portion known as spillway within the body of the dam
  • Gravity Dams Advantages • It is Stronger and more stable than any other type of dam • It can house an overflow spillway to pass excess flood water safely. • It can built of any height provided suitable foundation is available to bear all the loads coming on it. • The failure of a gravity dam is not sudden at all. It gives sufficient time for evacuation of area downstream of it.
  • Gravity Dams Disadvantages • Its construction is possible only on sound rock foundation. • Initial cost is higher. • It needs skilled labor and mechanized plants for construction. • It may take more time in construction, if manufacturing and transporting equipments are not available.
  • Gravity Dams
  • Arch Dam • It is curved Concrete Dam. The self weight of this dam is quite less compared to gravity dam it transmit major portion of water load to the abutments. Advantages • It is particularly suited in Deep Georges where length is same compared to its height. • Very small portion of water pressure is transmitted to foundation hence it can be built on moderate or weak foundation. • It has less initial cost as compared to Gravity Dam.
  • Arch Dam Disadvantages • It needs skilled labour, sophisticated formwork and specialized design. • Construction time is normally Large. • It needs very strong abutments of solid rock to resist arch thrusts • It is not suitable if solid rocks are not available.
  • Arch Dam
  • Buttress Dams • They may be considered as lightened version of the gravity dam. A buttress dam consists of a continuous inclined upstream face supported by downstream buttresses at regular intervals. They may be of deck slab type. Multiple arch type, multiple dome type or bulk head type.
  • Buttress Dams Advantages • It requires less materials for construction • It can be constructed on even weak foundation as pressure on foundation is quite less. • The water pressure acts normal to the inclined deck. Hence the vertical components of water pressure stabilizes the dam against overturning and sliding.
  • Buttress Dams Disadvantages • It requires more for work than solid concrete dams. • As Thickness of upstream concrete surface is less, it is more liable to get deteriorated • It requires constant maintenance and supervision. • Life of dam is less as compared to other dams.
  • Buttress Dams
  • Selection of Site for Dam (1)Foundation: • Suitable foundations must be available at the selected site for a particular type of dam. The foundation should be free from seams and faults. It is however possible to improve the foundation conditions by adopting suitable foundation treatments. (2) Topography: • Dam site should have a narrow valley to reduce its length it should store maximum volume of water. A major portion of dam should be located on high ground as compared to river basin. This will reduce the cost of dam and facilitates easy drainage of dam section as well.
  • Foundation of Dam
  • Topography
  • Selection of Site for Dam (3) Reservoir: • Dam site should form deep reservoir with small water surface to reduce (i) evaporation loss and (ii) Submergence area (iii)and control on weed growth. The quantity of leakage through the sides and bed of selected site should be minimum. Reservoir site with the presence of permeable rocks reduces the water tightness of the reservoir. • For larger storage in reservoir, dam site should be located at the confluence of two rivers.
  • Reservoir
  • Selection of Site for Dam Catchment Area • The geological conditions of catchment should be such that it yields maximum runoff. It should also have minimum percolation losses. The catchment area should avoid or exclude water from tributaries carrying high percentage of silt in water.
  • Catchment Area
  • Selection of Site for Dam Spillway • A suitable site for spillway should be available near dam site when spillway is to be located separately from dam. E.g. for earth or rock fill dam. There is no special site requirement for the spillway if it is to be built inside the dam.
  • Spillway
  • Selection of Site for Dam Construction Materials • Huge amount of materials is required for the construction of dam. Therefore construction materials should easily be available either locally or near vicinity of the site so as to reduce the transportation cost.
  • Selection of Site for Dam Communication • The dam Site should be easily approachable so that it can economically be connected to the important towns, cities etc. by rails or roads. Environmental Conditions: • Healthy environmental conditions must be available at dam site to set up colonies, residential quarters for labours and other staff members
  • Selection of Site for Dam
  • Factors Governing the selection of Type of Dam • Selection of the kind of dam is the first task. The choice and selection of dam at a particular place in the river may depend on the following factors. Topography: • This is the first factor which governs the choice of dam for a site • (a) A low rolling plain topography gives choice of an earth dam with a separate site of spillway • (b) A low narrow V-shaped valley with sound rock in the abutment suggests an arch dam.
  • Topography
  • Factors Governing the selection of Type of Dam (2) Geology and Foundation Conditions: • Next Important factor for the choice of Dam. • (a) Solid rock foundation with no fault or fissures, any type of dam can be constructed. • Rocks like granite, gneiss and schist provide good foundation for a gravity dam. • Poor rock or gravel foundation suggests choice of an earthen dam, rock fill dam or low concrete gravity dam. • Silt and fine sand foundation pose the problem of seepage settlement etc. Hence such foundations are suitable only for earth dam or low concrete gravity dam. They are not suitable for rock fill dam.
  • Geology and Foundation Conditions
  • Factors Governing the selection of Type of Dam (3)Availability of material for Construction • The Construction materials must be available locally o near the dam site in order to achieve economy in dam construction. The local availability of sand, gravel, crushed stone suggests concrete gravity dam. However, if coarse and fine grained soils are available locally, an earth dam may be suitable. (4) Spillway Size and Location • Spillway is required for safe disposal of of flood water. When separate site of spillway is available earthen dam may be preferred. In case of large capacity spillway, an overflow concrete gravity dam having overflow section in the middle will be the best choice.
  • Factors Governing the selection of Type of Dam (5) Roadway • The provision of roadway at the top of dam requires the choice of earth dam or gravity dam.
  • Spillway • Spillway are important auxiliary works of dams, provided to dispose of surplus floodwater safely which cannot be stored in the reservoir. Spillways are invariably provided in all the dams and often called safety valve for the dam. It is necessary to provide a spillway of sufficient capacity so as to avoid water from overtopping the dam. Overtopping of dam may lead to failure of dam resulting in serious damage to the property.
  • Spillway Requirements of a spillway • A Spillway should fulfill the following requirements • The spillway should have sufficient capacity • The location of spillway should provide safe disposal of water without toe erosion. • Spillway should be hydraulically and structure sufficient • Usually spillway should be accomplished by an energy dissipation work on its downstream side.
  • Components Parts of a Spillway • The various component parts of a spillway are as under (i) Control Structure • Control Structure consists of a weir which may be sharp. It is a major component of a spillway. It regulates and controls the surplus water from the reservoir. It does not allow the discharge of water below from reservoir level and allows the discharge of water below from reservoir level and allows only when water surface in the reservoir rises above that level.
  • Components Parts of a Spillway (ii) Discharge Channel • Discharge Channel It is provided to convey the surplus water released through control structures to the stream bed below the dam safely. The discharge channel may be the downstream face of spillway itself, or open channel excavated along the ground surface or a closed conduit placed through or under a dam.
  • Components Parts of a Spillway
  • Components Parts of a Spillway Energy Dissipaters • They are usually provided on the downstream side of the spillway. High Voltage water coming through Spillway may cause serious damage to the toe of dam and to the adjacent structures. This high energy of flow must be dissipated before it flows back to river. For this, energy dissipaters are provided
  • Components Parts of a Spillway
  • Components Parts of a Spillway Entrance and Outlet Channels • They are not required in case of an overflow spillway. However entrance channels are provided to draw water from reservoir and convey it to control structure. Similarly Outlet channels are provided to carry the spillway flow in river channel below the dam. Similarly, outlet channels are provided to carry the spillway flow to river channels below the dam.
  • Types of Spillways • According to the prominent features related to various components of spillway viz. control Structures, discharge channel etc. The spillways may be classified in the following types. • Free Overfall • Ogee or overflow spillway • Trough Spillway • Shaft Spillway • Side Chanel Spillway • Tunnel Spillway • Siphon Spillway.
  • Types of Spillways Free Over-fall • This is the simplest type of Spillway which consists of a low height narrow crusted Weir having downstream face either vertical or nearly vertical. This type of Spillway is suitable for low earth dam, low concrete masonry gravity dam or low thin arch dam. It is not suitable for high Dam.
  • Free Over-fall
  • Types of Spillways Ogee or Overflow Spillway • It is the modified form of drop spillway suitable for high gravity dam, arch dam and buttress dam. The overflow water is guided smoothly over the crest and profile of spillway. This type of spillway is more preferable on valleys where width of river is more to provide sufficient crest length and river bed can be protected from scouring at reasonable cost.
  • Ogee or Overflow Spillway
  • Types of Spillways Trough Spillway • Trough spillway Is provided when it is not possible to provide an overflow spillway such as in case of embankment dam or due to erodible nature of stream bed in case of concrete masonry dams. It discharges the surplus flood through a steep sloped open channel. The crest of spillway is kept normal to its centre line. It consists of a discharge channel to the river.
  • Trough Spillway
  • Types of Spillways Side Channel Spillway • It is the Spillway in which, the flow after passing over a weir or ogee crest, is carried away by a side channel. It is best suited for non rigid dam like earthen dam.
  • Side Channel Spillway
  • Types of Spillways Siphon Spillway: When available space is limited and surplus discharge is not large siphon spillway is often preferred. It is based on siphon action in the shape of an inverted pipe. Usually siphon spillway is provided in concrete gravity dam through its body.
  • Siphon Spillway
  • Energy Dissipation Below Spillway • Water flowing over a spillway has high potential energy as it glides along spillway. This large kinetic energy rises to high velocity of flow which may cause large scale scour/erosion at the downstream toe, if proper arrangements are not made to dissipate this high energy. The arrangements provided to dissipate this energy are known as energy dissipaters. • In general the dissipation can be achieved in two ways • By developing a Hydraulic jump • By directing the jet of water using a deflector bucket.
  • Energy Dissipation Below Spillway • A. Roller Basin B. Deflector Bucket C. Flip Bucket D. Non-radial spillway and sluice buckets E. Schoklitsch Dissipater
  • Spillway Crust Gates • Gates provided over the crest of a controlled spillway are termed as spillway crest gates. An additional storage can be obtained by providing gates on the spillway crest. However, during floods, these gates are lifted to make full use of spillway capacity. Great care is needed to be taken while operating crest gates of earth dams so as to avoid the overtopping of earth dam.
  • Spillway Crust Gates • The following are some of the common types of crest gates. (i) Dropping shutters or flash boards • They are some sort of temporary gates used only for smaller spillway of minor importance. They consists of wooden panels usually 1.0 to 1.25 m high. They are hinge at bottom and supported by struts to resist water Pressure. These wooden panels can be raised or lowered from an overhead cableway or a bridge.
  • Spillway Crust Gates Radial Gates or Tainter Gates • Radial gate has a curve water supporting face which is made of steel. Its shape is just as sector of a circle properly braced and hinged at the pivot. • The gate is thus made to rotate above a horizontal axis. The load of the gate, water etc. is carried on bearing which are mounted on piers. An operating plateform is provided to lift the the gate by means of ropes and chain
  • Radial Gates or Tainter Gates
  • Drum Gates • This gate is developed by United States Bureau of Reclamation (USBR) this gates is suitable for longer span of the order of 40 or 50 m. The drum gate consists of circular sector in cross section formed by skin plates attached to internal bearing. The entire section may be raised or lowered such that upper surface coincides with the crest of spillway. The buoyant force due to head water pressure underneath the drum assists in its lifting. The drum Gate needs large recess and hence is not suitable for smaller spillway. Some other types of spillway gates are also used namely Vertical lift gates, rolling Gates, Tilting Gate or Flap Gate, Bear Trap gate.
  • Drum Gates
  • Reservoir Sedimentation • Every river carries certain amount of sediment load. The sediment particles try to settle down to the river bottom due to the gravitational force, but may be kept in suspension due to the upward current in the turbulent flow which may overcome the gravity force.
  • Reservoir Sedimentation • Due to these reasons, the river carries fine sediment in the suspension as suspended loads, and larger solids along the river bed as bed load When the silt laden water reaches a reservoir in the vicinity of a dam, the velocity and the turbulence are considerable reduced. The bigger suspended particles and most of the bed load, therefore, gets deposited in the head reaches of the reservoir.
  • Causes of Sedimentation in Reservoir • In the catchment area the soil may get eroded and rocks may get disintegrated due to various reasons. The disintegrated rocks and loose soil form the sediment which is carried by the river and gets deposited on the reservoir bed near the base of the dam. The sediment mainly consists of sand and silt. The process of deposition of sand and silt in the reservoir is designated as reservoir sedimentation.
  • Causes of Sedimentation in Reservoir
  • The following are the causes of sedimentation Characteristics of soil in Catchment area: If the catchment area is composed of loose soil, then it may get easily eroded and get carried away by the river. On the other hand, if the soil of the catchment area is hard and rocky, the river would not be able to carry sediment. Topography of the catchment Area: If the catchment area consists of steep slope, then it will develop high velocity of flow which will cause more erosion of the surface soil thereby making the river carry a lot of sediment. Intensity of Rainfall in Catchment Area: If the intensity of rainfall in catchment area is high, then it will increase the rate of run-off and the river will carry more sediment.
  • The following are the causes of Sedimentation Cultivation in Catchment Area: The intensive cultivation in the catchment area will make the soil loose and rain water will carry a lot of sediment to the river. Vegetation cover in Catchment Area: If the catchment area is covered with vegetation cover like grass, plants, forest area, etc. then the erosion of the soil will be controlled and the river will not carry sediment. If there is no such vegetal apron or cover, the soil may be easily eroded and the river will carry much sediment.
  • Effect of Sedimentation • When the sediment laden water of the river approaches the zone of reservoir, the velocity of flow reduces gradually and thus the heavier particles are settled down at the head of the reservoir, i.e. starting zone of reservoir. This zone is termed as delta.
  • Effect of Sedimentation • Most of the sediments get deposited at this zone. The fine sediments remain in suspension for a considerable time and are carried towards the dam. These sediments are deposited at the foot of the dam. • The very fine particles are carried with water as turbid flow and ultimately discharged through the outlets. The floating debris is collected above delta. The clear water circulates the middle portion of the reservoir. Thus, the life of reservoir depends on the trend of the sedimentation.
  • Effect of Sedimentation
  • Control of Sedimentation • In order to increase the useful life of a reservoir, it is necessary to control the phenomenon of sedimentation. The following steps should be taken to control the deposition of sediment: • Selection of a Reservoir Site The reservoir site should be selected in such a way that so that load of sediment in the flowing water from tributaries is considerably low. If it is found that a tributary carries heavy sediment, the reservoir site should be shifted towards the upstream to avoid the tributary.
  • Control of Sedimentation • Design of Reservoir At the beginning the reservoir should not be constructed to its full capacity. Initially a reservoir of less capacity is constructed. When the reservoir gets silted, gradually the capacity should be increased stage by stage, by increasing the height of the dam. • Sufficient outlets should be provided in the dam at different elevations so that the water carrying heavy sediment may be discharged to the downstream by opening the outlets.
  • Selection of a Reservoir Site
  • Control of Sedimentation • Removal of Sediments The sediment already deposited in the reservoir may be removed through the scouring sluices provided at the bottom of the dam by loosening the sediments by mechanical agitator. • Afforestation New plantations should be planned in the catchment to extend the forest area which serves as a vegetal apron to reduce the soil erosion. • Control of Deforestation The cutting of Trees, i.e. deforestation in the catchment should be restricted by the government so that the vegetal cover is not destroyed The deforestation may cause soil erosion and this may cause soil erosion and this may impart sedimentation load to river water.
  • Removal of Sediments
  • Control of Sedimentation • Control of Cultivation: The intensive cultivation in the catchment makes the soil loose which may then get easily carried by the rainwater to the river. So the cultivation should be done in a planned manner and necessary measures should be taken to avoid the loose soil getting carried away by the rain. • Control of Grazing The grazing of cattle in the catchment should be restricted and they should not be allowed in the area where the soil can get easily eroded by their feet.
  • Control of Sedimentation • Construction of Check Dams: If the tributaries of a river are found to carry heavy sediment load, then check dams of low height are constructed across the tributaries at different stages to arrest the sediments just in the catchment area. Here, the check dams serve the purpose of detention reservoir where the heavy sediments get settled down.
  • Construction of Check Dams
  • Control of Sedimentation • Construction of Contour Bunds: In hilly area contour bunds are constructed on the slope of the catchment at different elevations so that the heavy sediments are arrested at the base of the bunds and comparatively less turbid water passes over the bunds. • Stabilization of Slips: Most of the tributaries of a river obtain the sediments from the land slides which are caused due to heavy rainfall in the hilly area. It is generally composed of loose soil or permeable rock formation. These slips should be stabilized by suitable methods so that the debris may not be carried by flowing water.
  • Construction of Contour Bunds
  • Density Currents • In a Reservoir, the coarser sediment settle down along the bottom of the reservoir, as the muddy flow approaches the reservoir; while the finer sediment usually remains in suspension, and moves in a separate layer than the clear reservoir water.
  • Density Currents • The layer of water, containing the fine sediment, moves below the upper clearer reservoir water, as a density current, since its density is slightly more than the density difference, the water of the density current does not mix easily with the reservoir water, and maintains its identity for a considerable time.
  • Density Currents
  • Density Currents
  • Density Currents • The density current can thus be removed through the dam sluiceway, if they are located properly and at the levels of the density current. A lot of sediment load can, thus be passed out of the reservoir, if it is possible to locate the dam outlets and sluiceways in such a fashion, as to vent out the density currents.
  • Reservoir Losses • Huge Quantity of water is generally lost from an impounding reservoir due to evaporation, absorption, and percolation. Depending upon which, the following losses may occur from such a reservoir. • Evaporation Losses • Absorption Losses • Percolation Losses or Reservoir leakage
  • Reservoir Losses
  • Reservoir Losses • The evaporation losses from a reservoir depends upon several factors as: water surface area, water depth, humidity, wind velocity, temperature, atmospheric pressure and quality of water. The evaporation loss from the reservoir under the given atmospheric condition can easily estimated by measuring the standard pan evaporation and multiplying the same by the pan coefficient. • The evaporation losses becomes very significant in a hot and humid country like India; and realistic estimation of these losses is quite important. These losses in fact vary from place to place and from season to season, and hence monthly values of these losses are usually determined.
  • Reservoir Losses • In order to control such large scale of wastage of water several methods have been devised by engineers and scientists. All these methods are based upon the efforts made to reduce the evaporation rate from the surface of water bodies by physical or chemical means, since the basic meteorological factors affecting evaporation cannot be controlled under normal conditions. The following methods are generally used for evaporation control.
  • Reservoir Losses
  • Reservoir Losses • • • • Wind Breakers Covering of the water surface Reduction of the exposed water surface Use of the underground storage rather than the use of surface storage. • Integrated operations of the reservoir. • Use of Chemicals for retarding the evaporation rate from the reservoir surface.
  • Reservoir Losses Absorption Losses • These losses do not play any significant role in planning, since their amount, though sometimes large in the beginning, falls considerably as the pores get saturated. They certainly depend upon the type of soil forming the reservoir.
  • Reservoir Losses Percolation Losses or Reservoir Leakage • For most of the reservoirs, the banks are permeable but the permeability is so low that the leakage is of no importance. But in certain particular cases, when the walls of the reservoir are made of badly fractured rocks or having continuous seams of porous strata, serious leakage may occur. Sometimes, pressure grouting may have to be used to seal the fractured rocks. The cost of grouting has to be accounted in the economic studies of the project, if the leakage is large.
  • Storage Zones of a Reservoir Normal Pool Level or Maximum Conservation Level • It is the maximum elevation to which the reservoir surface will rise during normal operating conditions. It is equivalent to the elevation of the spillway crest or the top of the spillway gates for most of the cases. Minimum Pool Level • The lowest water surface elevation, which has to be kept under normal operating conditions in a reservoir, is called the minimum pool level. This level may be fixed by the elevation of the lowest outlet in the dam or may be guided by the minimum head required for efficient functioning of turbines.
  • Storage Zones of a Reservoir
  • Storage Zones of a Reservoir Useful and Dead Storage • The volume of water stored in a reservoir between the minimum pool and normal pool is called the useful storage. Water stored in the reservoir below the minimum pool level is known as the Dead Storage, and if it is not of much use in the operation of the reservoirs. The useful storage may be subdivided into conservation storage and flood mitigation storage in a multipurpose reservoir.
  • Storage Zones of a Reservoir
  • Storage Zones of a Reservoir Maximum Pool Level or Full Reservoir Level • During high floods, water is discharged over the spillway, but will cause the water level to rise in the reservoir above the normal pool level. The maximum level to which the water rises during the worst design flood is known as the maximum pool level.
  • Storage Zones of a Reservoir Surcharge Storage • The volume of water stored between the normal pool level and the maximum pool level is called surcharge storage. Surcharge storage is an uncontrolled storage, in the sense that it exists only till the flood is in progress and cannot be retained for later use.
  • Storage Zones of a Reservoir Bank Storage • When the reservoir is filled up, certain amount of water seeps into the permeable reservoir banks. This water comes out as soon as the reservoir gets depleted. This volume of water is known as bank storage. Valley Storage • Even before a dam is constructed, certain variable amount of water is stored in the stream channel, called Valley Storage. • After the reservoir is formed, the storage increases, and the actual net increase in the storage is equal to the storage capacity of the reservoir minus the natural valley storage. The valley storage thus reduces the effective storage capacity of a reservoir.
  • Determining Reservoir Capacity • Long range runoff from a catchment is known as the yield of the catchment, Generally a period of one year is considered for determining yield value. The total yearly runoff, expressed as the volume of water entering/ passing the outlet point of the catchment, is thus known as the catchment yield, and is expressed in Mm3 or M.ha.m • The annual yield of the catchment up to the site of a reservoir, located at the given point along a river, will thus indicate the quantum of water that will annually enter the reservoir, and will thus help in designing the capacity of the reservoir.
  • Determining Reservoir Capacity • After deciding the dependable yield for the proposed reservoir tank, the reservoir capacity is decided as follows • The water demand is computed by estimating the crop water requirement and any other water demand required to meet the water supply needs, or the downstream commitments of water release, if any. Reservoir losses @ about 15 % of the water demand is then added to obtain the live or the net storage required to meet the given demand.
  • Determining Reservoir Capacity • Dead storage is now added to this live storage to obtain the gross storage required to meet the demand. • The reservoir capacity, however, cannot exceed the catchment yield and hence the reservoir capacity is fixed at a value which is lesser of the value of the assessed gross storage required to meet the demand and ii) the assessed dependable yield for the reservoir site. The full tank level or full reservoir level (FRL) is finally computed from the elevation capacity curve.
  • Determining Reservoir Capacity • The dead storage level or dead storage required in the above computation is usually fixed at higher of the following values • Dead storage= rate of silting x Life of the Reservoir. • Dead Storage= 10 % of gross storage or net water demand • Dead Storage level being equal to the full supply level of the off taking canal at the tank Site.
  • References • Modi P.N. (2011), “Irrigation water resources and water power engineering”, Standard Book House • Garg S.K. (2010), “Irrigation Engineering and Hydraulic Structures”, Khanna Publishers • Internet Websites
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