搜索
TechnicalGuideforAirVentilationAssessmentforDevelopmentsinHongKong1.ThisTechnicalGuideassistsprojectproponenttoundertakeAirVentilationAssessment(AVA)toassesstheimpactsoftheproposalonthepedestrianwindenvironment.TheassessmentshouldfollowthisTechnicalGuideasfaraspossibleandareportshouldbesubmittedtotheproponentdepartments/bureauxorauthoritiesontheassessmentfindings.2.Everysiteisdifferent.Theassessorisstronglyadvisedtoapproachtheassessmentintellectuallyanddiscretionallytakingintoaccountdifferentsiteconditions.Workingwithexperiencedpractisingwindengineersthroughouttheassessmentprocessisstronglyrecommended.Indicator3.WindVelocityRatio(VR)shouldbeusedasanindicatorofwindperformancefortheAVA.Itindicateshowmuchofthewindavailabilityofalocationcouldbeexperiencedandenjoyedbypedestriansongroundtakingintoaccountthesurroundingbuildingsandtopographyandtheproposeddevelopment.GiventhegeneralweakwindconditionsinHongKong,thehigherthewindvelocityratio,thelesslikelywouldbetheimpactoftheproposeddevelopmentonthewindavailability.4.WindVRisdefinedasVp/V∞(Vpedestrian/Vinfinity).V∞capturesthewindvelocityatthetopofthewindboundarylayer(typicallyassumedtobearound400mto600mabovecitycentre,orataheightwindisunaffectedbytheurbanroughnessbelow).V∞istakenasthewindavailabilityofthesite.Vpcapturesthewindvelocityatthepedestrianlevel(2maboveground)aftertakingintoaccounttheeffectsofbuildingsandurbanfeatures.ExpertEvaluation/InitialStudy/DetailedStudy5.ItisalwaysusefulandcosteffectivefortheassessortoconductanearlyroundofExpertEvaluation.Thisprovidesaqualitativeassessmenttothedesignand/ordesignoptionsandfacilitatestheidentificationofproblemsandissues.TheExpertEvaluationisparticularlyusefulforlargePage1sitesand/orsiteswithspecificanduniquewindfeatures,issues,concernsandproblems.ThefollowingtasksmaybeachievedwithExpertEvaluation:(a)Identifiesgooddesignfeatures.(b)Identifiesobviousproblemareasandproposesomemitigationmeasures.(c)Defines“focuses”andmethodologiesoftheInitialand/orDetailedstudies.(d)DeterminesiffurtherstudyshouldbestagedintoInitialStudyandDetailedStudy,orDetailedStudyalone.6.Inexercisingexpertknowledgeandexperience,theassessorshouldrefertothe“UrbanDesignGuidelines”,Chapter11oftheHongKongPlanningStandardsandGuidelinesdownloadablefromthePlanningDepartment’s(PlanD)websiteat(a)Initiallyassessesthecharacteristicsofthewindavailability(V∞)ofthesite.(b)GivesageneralpatternandaroughquantitativeestimateofwindperformanceatthepedestrianlevelreportedusingWindVR.(c)Furtherrefinestheunderstanding(gooddesignfeaturesandproblemareas)oftheExpertEvaluation.(d)Furtherdefinesthe“focuses”,methodologiesandscopeofworkoftheDetailedStudy.8.ItissometimesnecessarytoreiteratetheInitialStudysoastorefinethedesignand/ordesignoptions.Page29.WithorwithouttheInitialStudy,theDetailedStudyconcludestheAVA.WiththeDetailedStudy,theassessorcouldaccuratelyand“quantitatively”comparedesignssothatabetteronecouldbeselected.DetailedStudyisessentialformorecomplexsitesanddevelopments,andwherekeyairventilationconcernshavebeenreviewedandidentifiedintheExpertEvaluation/InitialStudy.ThefollowingtasksmaybeachievedwiththeDetailedStudy:(a)Toassessthecharacteristicsofthewindavailability(V∞)ofthesiteindetail.(b)ToreportallVRoftestpoints.ToreportSiteVR(SVR)andLocalVR(LVR)whenappropriate(asoutlinedinparas27to30).Toreport,ifany,windgustproblems.(c)Toprovideasummaryofhowtheidentifiedproblems,ifany,havebeenresolved.SiteWindAvailabilityData10.Itisnecessarytoaccountforthecharacteristicsofthenaturalwindavailabilityofthesite.Asfaraspossible,thedesignshouldutilizeandoptimizethenaturalwind.11.FortheExpertEvaluation,itisadvisabletomakereferencetotheHongKongObservatoryWaglanIslandwinddata,aswellasreasonablewinddataofnearbyweatherstations.Expertlyinterpreted,itispossibletoqualitativelyestimatetheprevailingwinddirectionsandmagnitudesofthesitenecessaryfortheevaluation.12.FortheInitialStudy,itisnecessarytobemoreprecise.Either“simulated”sitewinddata,or“experimental”sitewinddata,asdescribedinparas.13and15below,respectively,couldbeused.Page313.Usingappropriatemathematicalmodels(e.g.MM5andCALMET),itispossibletosimulateandestimatethesitewindavailabilitydata(V∞).TypicallythesitewindroseofV∞couldbeobtained1.14.FortheDetailedStudy,itisnecessarytobeevenmoreprecise.“Experimental”sitewinddata,asdescribedinpara15below,shouldbeused.15.Usinglargescaletopographicalmodel(typically1:2000to1:4000)testedinaboundarylayerwindtunnel,moreprecisewindavailabilityandcharacteristicsinformationintermsofwindrose,windprofile(s)andwindturbulenceintensityprofile(s)ofthesitecouldbeobtained.HongKongObservatoryWaglanIslandwinddatashouldbereferencedtofortheexperimentalstudy.Tools16.WindtunnelisrecommendedforboththeInitialandtheDetailedStudies,andmostparticularlyfortheDetailedStudy.Theconductofthewindtunneltestsh