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  1. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18 1 DAM (GENERAL), SPILLWAYS & ENERGY DISSIPATORS BY SAGAR DODEJA, IES • In order to harness the water potential of a river optimally, it is necessary to construct two types of hydraulic structures: 1. STORAGE STRUCTURE: usually a DAM, which stores excess water of a river during periods of high flows (as during the monsoons) and releases it according to a regulated schedule. 2. DIVERSION STRUCTURE : may be a WEIR or a BARRAGE that raises the water level of the river slightly, not for creating storage, but for allowing the water to get diverted through a canal situated at one or either of its banks. STRUCTURES FOR HARNESSING WATER RESOURCES OF A RIVER
  3. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18 3 • These types of dams have considerable upstream curvature in plan and rely on an arching action on the abutments through which it passes most of the water load on to the walls of the river valley. • This type of dam is structurally more efficient than gravity dams and greatly reduces the volume of concrete required. ARCH AND DOUBLE ARCH(CUPOLA) DAMS
  4. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18 4 • Earth-fill or earthen embankments –These may be classified as dams in which compacted soil is used for constructing the bulk of the dam volume. An earth fill dam is constructed primarily of selected engineering soils compacted uniformly and intensively in the relatively thin layers and at a controlled moisture content. • Rock-fill embankments – In these types of dams, there is an impervious core of compacted earth fill or a slender concrete or bituminous membrane but the bulk of the dam volume is made of coarse grained gravels, crushed rocks or boulders. BUTTRESS DAM
  5. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18 5 • This type of dams consist of a continuous upstream face supported at regular intervals by buttress walls on the down stream side. • These dams are generally lighter than the gravity dam but likely to induce slightly higher stresses at the foundation since most of the load now passes through the buttress walls and not spread uniformly over the foundation as in a solid gravity dam. SPILLWAYS • Passages constructed either within a dam or in the periphery of the reservoir to safely pass this excess of the river during flood flows are called Spillways. • The surface of the spillway should also be such that it is able to withstand erosion or scouring due to the very high velocities generated during the passage of a flood through the spillway. • At the bottom of the channel, where the water rushes out to meet the natural river, is usually provided with an energy dissipation device that kills most of the energy of the flowing water. • These devices, commonly called as Energy Dissipators, are required to prevent the river surface from getting dangerously scoured by the impact of the outfalling water • Usually, spillways are provided with gates, which provides a better control on the discharges passing through. However, in remote areas, where access to the gates by personnel may not be possible during all times as during the rainy season or in the night ungated spillways may have to be provided.
  6. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18 6 TYPES OF SPILLWAYS 1. Free Overfall Spillway 2. Overflow Spillway • It has a crest shaped in the form of an ogee or S-shape
  7. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18 7 3. Chute Spillway • A chute spillway, variously called as open channel or trough spillway, is one whose discharge is conveyed from the reservoir to the downstream river level through an open channel • A side channel spillway is one in which the control weir is placed approximately parallel to the upper portion of the discharge channel 4. Side Channel Spillway 5. Shaft Spillway • A Shaft Spillway is one where water enters over a horizontally positioned lip, drops through a vertical or sloping shaft, and then flows to the downstream river channel through a horizontal or nearly horizontal conduit or tunnel
  8. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18 8 Energy Dissipators • Different types of energy dissipators may be used along with a spillway, alone or in combination of more than one, depending upon the energy to be dissipated and erosion control required downstream of a dam. • Broadly, the energy dissipators are classified under two categories – Stilling basins or Bucket Type Energy Dissipators Stilling basin type energy dissipators • HORIZONTAL APRON TYPE Sloping Apron Type Bucket type energy dissipators SKI JUMP BUCKET TYPE
  9. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18 9 SELECTION OF AN ENERGY DISSIPATOR An energy dissipator is selected based on the following 2 parameters 1. JUMP HEIGHT – fixed for a particular discharge intensity and height of the spillway 2. TAIL WATER DEPTH – calculated by actual discharge observations
  10. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18 10 Now a graph is plotted between 1. Jump Height vs Discharge 2. Tail Water Depth vs Discharge The above graphs are superimposed and the following cases arise -