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DAM, , Adam is a water-retention structure suitably built across a river valley to block the flow of river water, and form an artificial lake in its upstream part called reservoir. Construction of a dam covers the river, valley or channel section and parts of the two sides of the valley called abutments., , Upstream side, , , , Downstream side, , Parapet walls spill way, , (inside dam), , MWL, , Max. level, , Normal Water Level, , Free Board, , Gallery, , pical dam has following parts, , Crest — The top of dam. In some cases, this provides a roadway, or walk way., , Parapet walls — Low protective walls on the either side of the, road way on the crest., , Abutments — The valley slopes on the either s PTT, , Free board — The space between the highest level of water in, the reservoir and crest of the dam.

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1s fo he upstream portion of the dam in contact with the river, bed or foundations, , Toe — The downstream portion of the dam wall for the discharge, of surplus water from the reservoir., , Spillway he passage in the dam wall for the discharge of, , surplus of water from the reservoir., , Gallery — Level or gently sloping tunnel like passage transverse, or longitudinal within the dam wall with drains in the floor for, seepage water, , , , Construction of a dam ensures availability of water in the reservoir throughout the year, which serves, the following purposes:, , (i) The stored water is supplied to the farmers for irrigation of lands even in dry period. An unfertile land, or arid region is irrigated to produce crops by diverting the reservoir water by a system of canals., , (ii) City and town people are supplied water from the reservoir for drinking and domestic use. The, supply remains uninterrupted even during the summer months when there is scarcity of water., , (iii) The artificial lake created by damming of the river is used as a center of recreation such as for, boating and fishing activities. It facilitates growth of tourism and helps navigation., , (iv) Water is needed in industrial units associated with various production works. The demand for water, in industries is mostly met by water from the reservoir., , (vi) Many of the dams with large reservoir capacity release water for generation of hydroelectric power., People in both rural and urban areas and many industries benefits t from the power generated., , (v) Heavy rains in the upstream parts of many rivers cause flooding of downstream areas. Building of, dam stores the excess water in the reservoir and thus controls floods., , When a dam is constructed, the impounded water exerts Pressure on the upstream wall of the dam, tending to push it Downstream whereas the gravity tends to resist it. The larger The dam with respect to, its height, the greater will be the Pressure from reservoir water. The gravitational force of aDam, depends on the density of the dam body., , Dams are broadly classified into the following four groups on the basis of the types of material, Used in their construction:, , Concrete dam Masonry dam — Rock-fill dam Earth dam

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Concrete Dams, , Depending on their bearing strength and stability, concrete dams are subdivided into three Categories,, namely gravity, arch, and buttress dams., , Gravity dams have straight or slightly curved axes and are generally triangular or trapezoidal In cross, section with bed widths about two-thirds their heights. The downstream face of these Dams can be, straight or curved. The thick concrete body of a gravity dam provides the necessary Weight to withstand, the forces due to water thrust and uplift pressure. A narrow valley with Sound rock is the ideal place for, constructing a concrete gravity dam , but It can be successfully built in soft sedimentary strata also after, strengthening the foundation., , Arch dams are arched or curved in their Configuration and hence the name. The 169m-high Idukki dam, in Kerala constructed on massive granite gneiss of a canyon site is an example of large arch gravity dam, In fact, arch dams are curvilinear in plans with the convex side facing headwaters. In cross section, (vertical section), these dams are relatively thin and curved in shape Fig. 14.6 (b). In an arch dam, a part, of the thrust due to reservoir water is distributed to the abutment rock by the arch action. The thickness, and curvature of the body of an arch dam are so designed that more than half of the acting load is, transferred to the abutment. As such, the abutment and foundation rocks should have suffi cient, strength to bear the load. In case more than half the dam load is kept on the foundation, it is to be, called an arched gravity dam. Arch dams are generally constructed in narrow valleys or canyons such, that the heights are generally more than the axial lengths., , Buttress dams are massive, thin, or arched slabs supported by vertical walls or buttresses. The slabs, having upstream slopes bear the water thrust while the buttresses acting normal to the planes of the, slabs take the load of the headwater and transfer it to the foundation. Steep beams or girders spaced, between the buttresses prevent their bending. The structural parts of the buttress dams are made of, concrete or reinforced concrete. . Buttress dams require firm rock in the river bed where they buttress., The interior part being hollow, these dams consume less concrete than other types of dams; hence,, their construction cost is comparatively less., , Full reservoir level, , , , , , , , , , Free board Crest parapet W, Headwat \2\_ Concrete, [structure, a |, Rig v\) GEES vy Left Is, AE stream Yrerim = Oye, , wan Wy rock, , , , (a), , , , , , Heel Toe, , Fig. 14.2 Cross section of a concrete Fig. 14.6 Arch gravity dam: (a) plan view; (b) cross section across the arch, gravity dam illustrating different parts dam (A-A); and (c) cross section along the axis (X-X) of the arch dam

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Plat slab 2 Buttress, , Fullreservoir Gate, level, , , , Stone, blocks, , , , Elevation, , Buttress Elevation, / Buttress, , , , , , , , (, es > EEE Firm rock” Platform, © @ :, Fig. 14.7 Buttress dam: (6) three-dimansional view offal slab type dam Fig. 14.8 Masonry dam made of cut blocks, (b) three-dimensional view of arch type dam; (c) elevation and section of flat of sound rock having gated spillway, , slab type dam; and (d) elevation and section of arch type dam, , Masonry Dams, , Masonry dams (Fig. 14.8) are made of big undressed blocks of rocks including river boulders bound, together by concrete. The rocks selected for construction of masonry dams are non-porous and the, binding among the rock fragments or boulders creates a watertight body. As such, the entire body of the, masonry dam acts as an impermeable barrier against leakage of reservoir water. Only the cemented, joint portions among the blocks must be watertight to avoid seepage. Many old dams in India are stone, masonry structures. The 139 m-high Srisailam dam in Andhra Pradesh is an example of masonry, , construction., Rock-fill Dams, , Rock-fill dams are made up of an admixture of ground spoils, river deposits (e.g., gravels, pebbles, and, boulders), and crushed rocks. They are trapezoidal in cross section with side slope commonly in excess, of 1:1. Depending upon the nature of impermeable membrane provided in the rock-fi Il dams, they are, grouped into the following two types:, , (i) Dams with concrete wall in the upstream face, which creates the impermeable barrier against, leakage, , (ii) Dams with impervious earth or clay (core) in the central part, which acts as the impermeable, membrane, , Earth Dams, , Earth dams are trapezoidal in cross section with Gentle side slopes (Fig. 14.11). An earth dam is, Generally constructed in a broad valley where it Is very expensive to build a concrete or masonry, Structure. The earth dam also does not require Rocky foundation and it can be founded on firm Soil., However, availability of adequate quantity Of homogeneous earth of both pervious and semi-Pervious, types is to be assured for constructing An earth dam.