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DESIGN OF SPILLWAYSSPILLWAY

The SPILLWAYS are openings provided at the body of the dam to discharge safely the excess water or flood water when the water level rises above the normal pool level.

ESSENTIAL?Why is it(a) The height of the dam is always fixed according to the maximum reservoir capacity. The water is never stored in the reservoir above the normal pool level. The dam may fail by over turning so, for the safety of the dam the spillways are essential.

ESSENTIAL?Why is it(b) The top of the dam is generally utilized by making road. The surplus water is not be allowed to over top the dam, so to stop the over topping by the surplus water, the spillways become extremely essential. (c) To protect the downstream base and floor of the dam from the effect of scouring and erosion, the spillways are provided so that the excess water flows smoothly. ESSENTIALWhat are therequirements?It must have adequate discharge capacity It must be hydraulically and structurally safe The surface of the spillway must be erosion resistant It should be provided with some device for the dissipation of excess energy The spillway discharge should not exceed the safe discharge capacity of the downstream channel to avoid its flooding.

ESSENTIALWhat are therequirements?The spillway must be so located that the spillway discharge does not erode or undermine the downstream toe of the dam Generally, the spillways are provided at the following places Provided within the body of the dam. May sometimes be provided at one side or both sides of the dam. Sometimes by-pass spillway is provided which is completely separate from the dam.

SPILLWAYSDesign ofMany parameters need consideration in designing a spillway. These include:The inflow design flood hydro-graphThe type of spillway to be provided and its capacityThe hydraulic and structural design of various components andThe energy dissipation downstream of the spillway.SPILLWAYSDesign ofAFFECTINGFactorsDesign1CONSIDERATIONSafetyMany failures of dams have resulted from improperly designed spillway or spillways of inadequate capacity. Properly designed structure of adequate capacity may be found to be only moderately higher in cost than a structure of inadequate capacity.Consistent withECONOMY2HYDROLOGICALThe spillway design and its capacity depend onInflow discharge, its frequency, and shape of hydrographHeight of damCapacity curveGeological and other site conditionsandSITE CONDITIONSHYDROLOGICALImportant topographical features, which affect spillways design, areSteepness of terrainAmount of excavation and possibility of its use as embankment material.The possibility of scourStability of slopes, safe bearing capacity of soils.Permeability of soils.andSITE CONDITIONS3DAMType ofThe type of dam influences the design flood and spillway. Earth and rockfill dams have to be provided with ample spillway capacity.4PURPOSE of the Dam andOPERATING CONDITIONSThe ungated spillway should be provided, except in special circumstances when gated spillway may be provided.COMPONENTSSpillway

APPROACH CHANNEL1CHANNELSApproachIt is an entrance structure to draw water from the reservoir and convey it to the control structureThe approach channel may be a straight or curved in plan. Its banks may be parallel, convergent, divergent or combination of these and may be vertical or sloping.

INLET STRUCTURE2STRUCTUREInletThis conveys water from the approach channel to the control structure and is intended to improve approach conditions to the control structure

CONTROL STRUCTURE3STRUCTUREControlControl structure consists of a weir which may be sharp. It regulates and controls the surplus water from the reservoir.

It does not allow the discharge of water below from reservoir level and allows only when water surface in the reservoir rises above that level.

CONVEYANCE FEATURE4CARRIERDischargeIt is the waterway provided to convey the flows released from the control structure to the river below the dam

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

TERMINAL STRUCTURE5STRUCTURETerminalAt the downstream end of the discharge carrier when flow from reservoir to downstream river level manifests itself in the form of high velocities enough to cause scour, erosion and subsequent damage to adjacent structures, a terminal structure usually in the form of stilling basin is provided for dissipation of residual energyDISSIPATERSEnergyThey 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.

STILLING BASIN

FLIP BUCKET

BAFFLED BLOCKS

EXIT CHANNEL5CHANNELExitThis conveys water from the terminal structure to the river or the stream.