第1页共20页编号：时间：2021年x月x日书山有路勤为径，学海无涯苦作舟页码：第1页共20页OVERFLOWSPILLWAYAnoverflowspillwayisasectionofdamdesignedtopermitwatertopassoveritscrest.Overflowspillwaysarewidelyusedongravity,arch,andbuttressdams.Someearthdamshaveaconcretegravitysectiondesignedtoserveasaspillway.Thedesignofthespillwayfortowdamsisnotusuallycritical,andavarietyofsimplecrestpatternsareused.Inthecaseoflargedamsitisimportantthattheoverflowingwaterbeguidedsmoothlyoverthecrestwithaminimumofturbulence.Iftheoverflowingwaterbreakscontactwiththespillwaysurface,avacuumwillformatthepointofseparationandcavitationsmayoccur.Cavitationsplusthevibrationfromthealternatesmakingandbreakingofcontactbetweenthewaterandthefaceofthedammayresultinseriousstructuraldamage.Cavitiesfilledwithvapor,air,andothergaseswillforminaliquidwhenevertheabsolutepressureoftheliquidisclosetothevaporpressure.Thisphenomenon,cavitations,islikelytooccurwherehighvelocitiescausereducedpressure.Suchconditionsmayariseifthewallsofapassagearesosharplycurvedastocauseseparationofflowfromtheboundary.Thecavity,onmovingdownstream,mayenteraregionwheretheabsoluteismuchhigher.Thiscausesthevaporinthecavitytocondenseandreturntoliquidwitharesultingimplosion,orcollapse,extremelyhighpressureresult.Someoftheimplosiveactivitywilloccuratthesurfacesofthepassageandinthecrevicesandporesoftheboundarymaterial.Underacontinualbombardmentoftheseimplosions,thesurfaceundergoesfatiguefailureandsmallparticlesarebrokenaway,givingthesurfaceaspongyappearance.Thisdamagingactionofcavitationsiscalledpitting.Theidealspillwaywouldtaketheformoftheundersideofthenappedofasharp-crestedweirwhentheflowratecorrespondstothemaximumdesigncapacityofthespillway.Moreexactprofilesmaybefoundinmoreextensivetreatmentsofthesubject.Thereversecurveonthedownstreamfaceofthespillwayshouldbesmoothandgradual;Aradiusofaboutone-fourthofthespillwayheighthasprovedsatisfactory.Structuraldesignofanogeespillwayisessentiallythesameasthedesignofaconcretegravitysection.Thepressureexertedonthecrestofthespillwaybytheflowingwaterandthedragforcescausedbyfluidfrictionareusuallysmallincomparisonwiththeotherforcesactingonthesection.Thechangeinmomentumoftheflowinthevicinityofthereversecurvemay,however,createaforcewhichmustbeconsidered.Therequirementsoftheogeeshapeusuallynecessitateathickersectionthantheadjacentnooverflowsections.Asavingofconcretecanbeeffectedbyprovidingaprojectingcorbelontheupstreamfacetocontroltheflowinoutletconduitsthroughthedam,acorbelwillinterferewithgateoperation.ThedischargeofanoverflowspillwayisgivenbytheweirequationQ=CωLh32WhereQ=discharge,orm3/secCω=coefficientL=coefficient第2页共20页第1页共20页编号：时间：2021年x月x日书山有路勤为径，学海无涯苦作舟页码：第2页共20页h=headonthespillway(verticaldistancefromthecrestofthespillwaytothereservoirlevel),mThecoefficientCωvarieswiththedesignandhead.Experimentalmodelsareoftenusedtodeterminespillwaycoefficient.EndcontractionsonaspillwayreducetheeffectivelengthbelowtheactuallengthL.Square-corneredpiersdisturbtheflowconsiderablyandreducetheeffectivelengthbythewidthofthepiersplusabout0.2hforeachpier.Streamliningthepiersorflaringthespillwayentranceminimizestheflowdisturbance.Ifthecross-sectionalareaofthereservoirjustupstreamfromthespillwayislessthanfivetimestheareaofflowoverthespillway,theapproachvelocitywithincreasethedischargeanoticeab...
