Dam Classification and Spillway PDF

CHAPTER TWO DAM CLASSIFICATION AND SPILLWAY Outlines Introduction Classification of Dam Purpose of dam Selection of Dam Site By: MOHAMMEDSEID.A (MSc) DEC 2025  Introduction  Dams,...

CHAPTER TWO DAM CLASSIFICATION AND SPILLWAY Outlines Introduction Classification of Dam Purpose of dam Selection of Dam Site By: MOHAMMEDSEID.A (MSc) DEC 2025  Introduction  Dams, in different forms, were built by humankind since the earliest days of known history.  Remains of dams older than 2000 years in china, Egypt, Iran, Yemen, ancient Mesopotamia, Rome another places where ancient civilizations used to be.  Therefore, No developed civilization without water management  Con’t  Old-world civilizations developed in the valleys of the big rivers.  If one looks back , one can see that all major settled civilizations were using water supply systems and irrigations.  For a while during mid Twentieth century (especially in Europe) many large dams were built exclusively for power generation.  Con’t  However in most cases in modern practice most of them are built to form multi purpose reservoirs  Introducing certain new goals ( power generation and its transformation to electricity).  Other purposes of dams are common nowadays transportation by water, recreation, fish growth etc.  Dam Structure and Reservoir  Dam are structures controlling the flow of river by completely blocking the valley of the given river or drainage system.  Through the blockage, storage is formed, which can be utilized for various water resources development or water control purposes.  Thus, retained body of water is RESERVOIR, the retaining structure whether earth/rock/concrete.  Cont….. Structure related to dam includes:  Spillway  Bottom outlet  Intake  Gate and Valves  Service road  Reservoirs  Bridge  Energy dissipaters  Diversion Structures  Advantage of Dam The specific main functions /advantages of dams and reservoirs are:  Temporal and quantitative balancing between available water and demand.  Water storage for days, weeks, months, years, and over years for power generation and sediment control  Regulation/balancing of river flow (such as navigation, low flow augmentation)  Water supply and Irrigation purpose  Flood control  Disadvantage of Dam The presence of a dam with its storage reservoir interferes with the natural system. Associated to this, the main disadvantages include:  Distraction of fauna and flora in the dam valley  Interference in the landscape through dams and reservoirs  Change of ground water level  Influence on fish  Strong influence on sediment balancing  Displacement of people from reservoir site  Change in water temperature and quality  Reservoir  A reservoir is a large, artificial lake created by constructing a dam across a river.  Broadly speaking, any water pool or a lake may be termed as reservoir.  However , the term reservoir in water resources Reservoir engineering is used in restricted sense for a comparatively large body of water stored on upstream of dam  Thus, dam and reservoir exist together.  Cont……  The discharge in a river generally varies considerably during different period of year.  If a reservoir serves only for one purpose, it is called single-purpose reservoir.  On the other hand, if it serves for more than one purpose, it is termed as multipurpose reservoir.  Storage component of Reservoir  The reservoir volume is sub-divided in to the following parts, when seen in its vertical section.  Cont…  Dead storage: The portion of the storage capacity which accumulated by silt and sediment.  Active storage: The portion of the storage capacity in which water will normally be stored or withdrawn for beneficial uses.  Surcharge storage: this is required as reserve between full reservoir level and the maximum water level to contain the peaks of flood.  Bottom Outlet: The outlet which is mainly used to discharge the water from the reservoir for the environmental flow and for flushing of sediment. Some times it is also opened to discharge when there is flooding on upstream.  Service outlet: It is used to discharge the required amount of water for downstream beneficiaries as their need.  Classification of Dams They are various ways of classification of dams. Some of them are I. Classification based on function served  Storage Dam: It is a common types of dam constructed to impound surplus flood water for use in dry period.  Diversion Dam: It is a dam of smaller height constructed to raise water level of the river and not for storage or equalization of flow.  Detention Dam: It is constructed to store water during floods and release it gradually at a safe rate when the flood recedes  Debris Dam: Is constructed to retain debris such as sand, gravel and floating woods and the water that passes over the dam is relatively clear.  Coffer Dam: It is constructed to divert the water during a period of construction.  Cont …. II. Classification According to Hydraulic Design  Overflow dam: They are designed to pass the surplus water over their crest. They must be made of material which will not be eroded by such discharge. Example: Concrete, Masonry….  Non overflow Dam: they are those which are not designed to be overtopped. This types of design extends the choice of material includes earth fill and rock fill dam III. Classification According to Construction Material Used  Concrete dam: made of masonry and concrete  Earth Fill Dam: made of earthen material  Rock-Fill Dam: Made of rock  Timber dam: made of timber  Steel dam: made of steel  Cont …. II. Classification According to Hydraulic Design Rigid Dams  It is constructed from rigid material such as concrete, masonry, steel or timber  These dam deflect and deform very little when subjected to water pressure and other forces Non Rigid Dams  It is constructed from non rigid material such as earth fill and rock fill  They are relatively large settlement and deformations in non rigid dams.  Characteristics of Concrete Gravity dam  A types of Dam which is constructed from mass concrete or masonry.  Face slopes are dissimilar.  General steep downstream and Vertical or near vertical upstream slopes.  Concrete dams are hard, none yielding and rigid structures.  Its stability is based on its own weight  It needs imported concrete  It needs sound rock foundation  Advantage of Concrete Gravity Dam  Good for narrow valleys. Except arc or cupola, also suitable for wide valleys.  Smaller construction fill volume (compared to embankments)  Construction can takes place irrespective of weather condition  Smaller free board (1-1.5m)  Can be constructed as overflow dam, i.e. can accommodate spillway, hence cost for separate spillway reduced.  Outlet pipe work , valves and other ancillary works can be provided within the body of the dam.  Dis-Advantage of Concrete Gravity Dam  Sensitive to settlement, demanding sound rock foundation  Sensitive by earth quake  Depending on the dam type, it’s construction is demanding (technical personnel, machine requirement, form work)  Difficult heightening (simplification by pre- stressing)  Bad interference in to natural landscape  Characteristics of Concrete Arch dam  Arch dam is dam with upstream curvature which constructed from concrete material.  Stability is obtained by combination of arch and gravity action.  Utilizes the strength of an arch to resist loads placed upon it by 'arch action’.  The strength of the rock mass at the abutments and immediately down valley of the dam must be strong enough.  It is suited to narrow gorges / V-shaped  High abutment loading  50-80% concrete saved relative to gravity dam  Characteristics of Concrete Buttress dam  Buttress dam require between one third and half of the concrete required for gravity section, hence making it more economical for dams over 14m.  Concrete saved 30-60% relative to gravity dam  There is an elimination of good deal of uplift pressure.  Buttress dams were first developed to conserve water in regions where materials were scarce or expensive but labor was cheap.  Characteristics of Embankment dam  Any dam constructed of naturally excavated material placed without addition of binding material.  It is suited to either rock or compressible soil foundation  It can accept limited different settlement.  Embankment dam are usually referred to be types such as Earth fill, Rock fill and Hydraulic fill  Cont….  Earth fill dam: an embankment dam constructed primarily of compacted earth in either homogeneous or zoned area containing more than 50% earth  Rock fill dam: the term rock fill dam usually represent dams that contain more than 50% compacted or damped pervious rock.  Hydraulic fill dam: An embankment dam constructed from dredged material (earth, sand, gravel or rock) by suspension in flowing water.  Phase of Engineering Activity for site investigation And construction of dam  Dam must be planned, designed and constructed to operate efficiently.  They are five phases for site investigation and construction of dam 1. Reconnaissance survey 2. Preliminary and feasibility studies 3. Final investigation and basic design 4. Construction 5. Operation  Selection of Sites and Type of Dams Influencing factors:  Availability, quality (nature,  Topography (valley form state) of construction materials  Foundation (stability, impermeability)  Construction (Supply, transport,  Geology (layers, fishers) equipment, qualification of  Required height personnel)  Purpose of the dam  Landscape  Climate (ice)  Cost (economics)  Flood spillway  1. Topography Topography dictates the first choice of the types of dam. I. A narrow U-shaped valley, i.e. a narrow stream flowing between high rock walls, would suggest a concrete overflow dam. II. A narrow V-shaped valley indicates the choice of an Arch dam. It is preferable to have the top width of the valley less than one-fourth of its height. But a separate site for the spillway must also be available.  Cont…… III. A low, rolling plain country, would normally suggest an earth fill dam with separate spillway. 2. Geology and Foundation condition I. Solid Rock Foundation: II. Gravel Foundation  Coarse sand and gravel are unable to bear the  solid rock foundation such as granite, gneiss, etc. weight of high concrete gravity dam and suitable have strong bearing power. for earthen and rock fill dam.  They offer high resistance to erosion and percolation.  Low concrete gravity dam up to a height of 15m Almost every kinds of dam be built on such may also be suggested on such foundation. foundation.  sometimes, seams and fractures are present in these rock. They must be grouted and sealed properly.  Cont…… III. Silt and Fine sand foundation  They suggest the adoption of earthen dam V. Non-Uniform Foundation or low gravity dam (up to height of 8m).  At certain places, a uniform foundation  A rock fill dam on a such foundation is not of the types described above may not suitable due to high seepage on the available. foundation and settlement  In such case, a non-uniform foundation IV. Clay Foundation of rock and soft material may have to  Unconsolidated and high moisture clay s be used if the dam is to be built. are likely to Couse enormous settlement of  Such unsatisfactory condition have to the dam. be deal with special design and solution  They are not fit for concrete gravity dam has to be found by experienced of for rock fill dam. engineers  They may accepted for earthen dam, after special treatment.  Cont… For Example (Non uniform foundation)  3.Availability of Material  In order to achieve economy in the dam, the material required for its construction must be available locally or at short distance from the construction site  Some times, good soil is easily available, which naturally calls for an earthen dam  If sand, cement and stone etc… are easily available, one should naturally think of a concrete gravity dam.  If the material has to be transported from far distance then a hollow concrete dam (buttress) is better choice