amendment 2 31 march 2017 annexure s amended stormwater … · although based on limited...

AMENDMENT 2

31 MARCH 2017

ANNEXURE S

AMENDED STORMWATER MANAGEMENT PLAN

CONRADIE BLMEP

CONRADIEBLMEPSTORMWATERMANAGEMENTPLAN

HHOAfricaInfrastructureEngineers 7293-700-8001CapeTown March2017-RevA

Form QS31-SF8 Rev 3

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STORMWATERMANAGEMENTPLAN

DOCUMENT VERIFICATION Rev Date Prepared by Checked by Approved by Description Status

A March 2017

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F de Villiers

NAME M Woodward

NAME C Avenant

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PROJECT NO: 7293 REPORT NO: REP-HHO-700-8001-A

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ConradieBLMEP:StormwaterManagementPlan March2017-RevAWesternCapeGovernment HHOAfrica

TABLEOFCONTENTSSectionNo Description PageNo1.0 INTRODUCTION 11.1. BACKGROUND 11.2. PREVIOUSFLOODINGSTUDIES 11.3. CITYOFCAPETOWNPOLICYOBJECTIVES 21.4. TERMSOFREFERENCE 31.5. APPROVALPROCESS 42.0 METHODOLOGY 53.0 REVIEWOFELSIESKRAALCANALMODELLING 64.0 PROPOSEDSTORMWATERINFRASTRUCTURE 74.1 DETENTIONPONDS 74.2 SWALES 84.3 INTERNALROADS&SITELEVELS 84.4 ELSIESKRAALCANAL 95.0 HYDROLOGYANDHYDRAULICMODELLING 105.1 WATERQUALITY 115.2 QUANTITYANDRATEOFRUNOFF 125.3 HIGHHAZARDZONESANDFLOODLINES 196.0 BULKEARTHWORKS&COSTESTIMATE 226.1 BULKEARTHWORKS 226.2 COSTESTIMATE 227.0 CONCLUSION 238.0 RECOMMENDATIONS 249.0 REFERENCES 25APPENDICES

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LISTOFFIGURES(TOBEUPDATED)FigureNo Description PageNoFIGURE3.1 CONRADIEBLMEPINFLOW 3FIGURE3.2 CONRADIEBLMEPOUTFLOW 3FIGURE4.1 RATANGAROADOVERFLOWATKINETICWAYEXTENSION 5FIGURE4.2 ANTICIPATEDWATERDEPTHATRATANGAROAD 5FIGURE4.3 FULLDEVELOPMENT:SYSTEMINFLOW 7FIGURE4.4 FULLDEVELOPMENT:SYSTEMOUTFLOW 7 FIGURE4.5 FULLDEVELOPMENT:SYSTEMSTORAGE 8

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LISTOFTABLES(TOBEUPDATED)TableNo Description PageNoTABLE3.1 FINDINGSFROM2011SUDSSTUDY 2TABLE3.2 CONRADIEBLMEPFLOWSFROM2011SUDSMODEL 2TABLE4.1 CONRADIEBLMEPFULLDEVELOPMENTCOMPARISON WITH2011SUDSSTUDY 6TABLE5.1 PROPOSEDWATERBODIES 9

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EXECUTIVESUMMARYTheElsieskraalcanalrepresentsasignificantpublicsafetyhazard,whichneedstobemanagedbytheCityofCapeTown.Ithasbeendemonstratedthatthereisnobenefitinrealigningthecanalandthatfillingthesitedoesnotdetrimentallyaffectthefloodregime,thefloodplainoradjacentproperties.Althoughbasedonlimitedinformationandabasicurbandesign,thisreporthasdemonstratedthatthe StormwaterManagement System (SWMS) proposed for the Conradie BLMEP can achieve theparameters foraSustainableUrbanDrainageSystem(SUDS)asdefinedby theCityofCapeTown.ThefutureConradieBLMEPdeveloperwouldneedtodemonstratetoagreaterlevelofdetail,basedonpreliminarydesign,howtheCity’spolicyrequirementscanbeachieved.

KeyFindings• AlthoughthesiteiscurrentlydesignatedaHighHazardZone(HHZ),the100yearRecurrence

Interval(RI)floodcanbecontainedwithintheElsieskraalcanalreserveandtheriskoffloodingonthesitecanalmostentirelybemitigatedbytheimplementationoftheSWMP.

• The proposed SWMS can effectively reduce flood peaks both for relativelyminor nuisancefloods andmajor, extreme floods. Preliminary results indicate that the requirements of theCity’spoliciescanbeachieved.

• TheproposedSWMSformeffectivesedimentandlittertrapsthataresimpletomaintainandoperate.TheCityofCapeTown’swaterqualityobjectivesareachievable.

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GLOSSARYBLMEP BetterLivingModelExemplarProject

DWS DepartmentofWaterandSanitation

GA GeneralAuthorisation

HEC-RAS HHZ HighHazardZone

HOA HomeOwners’Association

NGL NaturalGroundLevel

POS PublicOpenSpace

PCSWMM

RFP RequestforProposal

RI RecurrenceInterval

SCS SoilConservationSurvey

SS SuspendedSolids

SUDS SustainableUrbanDrainageSystem

SWMM USEPAStormwaterManagementModel

SWMP StormwaterManagementPlan

SWMS StormwaterManagementSystem

TA TechnicalAdvisor

TP TotalPhosphorus

VAT ValueAddedTax

WCG WesternCapeProvincialGovernment

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1.0 INTRODUCTION 1.1. BACKGROUND

TheproposedConradieBetterLivingModelExemplarProject(BLMEP),whichisbeingundertakenbythe Western Cape Provincial Government (WCG), requires the existing Elsieskraal canal to bedeviatedforthefollowingreasons:

• TheentireConradie site isa floodingHighHazardZone (HHZ)andconsequentlycannotbe

developed for housing unless the HHZ can be contained in a controlled manner withoutaffectingdownstreamusers.

• Theurbandevelopmentframework,onwhichtheBLMEPisbased,requiresthecanaltoberealigned.Thiswouldcreatedevelopmentareasoneithersideofthecanal,whilstprovidingarecreationalareaandpublicopenspace.

1.2. PREVIOUSFLOODINGSTUDIES

ThefollowingreportsrelatedtotheBLMEPhavebeencompiledtodate.TheseareattachedintheAppendices,butaresummarisedbelow.

1.2.1 StormwaterConceptDesign,ManagementandRiverineDevelopmentPlan(WSP,August2016)

This BLMEP report presents a construction cost estimate and concept design for the realignedElsieskraalcanalthataddressesthefollowingCityofCapeTown(City)policies:

• ManagementofUrbanStormwaterImpactsPolicy• FloodplainandRiverCorridorManagementPolicy

Basedona2012reportbySRKentitled,“SaltRiverHighLevelStormwaterMasterPlan,”thereportdoesnot take intoaccount surfacewater inflows fromThorntonand states that thepiped inflowsfromthatsuburbareinsignificant.WSPalsoalludestocurrentworkbeingundertakenbytheCityofCape Town to assess the upstream capacity of the Elsieskraal canal, which may have a positiveimpactonthisproject.TheWSPreportcontainsanumberofinconsistenciesandambiguities,someofwhichareaddressedinAurecon’sMarch2017report,referredtobelow.

1.2.2 ElsieskraalRiver-FloodMitigationStudy(Aurecon,October2016)

ThisreportwascommissionedbytheCityofCapeTown,whohadappointedAurecontostudytheElsieskraalRiver.ItconsideredonlytheexistingElsieskraalcanalandnottheproposedrealignment.ThestudyfoundthatthefloodlevelsadoptedbyWSP,whichhadbeendeterminedSRKin2012,arebasedonenergylevelsasopposedtoactualwatersurfacelevels.AlthoughinkeepingwiththeCity’spolicy,theresultsareoverlyconservative.The study also assessed the impact of localminor andmajor stormwater systems on the existingcanal.Importantly,itwasbasedon2Dmodelling,whichprovidesthefollowingadvantagesoverthe1Dmodellingpreviouslyused:

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• 2Dmodelling takes topography into account,whichmeans that overland flow paths andovertoppingcanbeassessed.

• It is based on water surface elevations and not energy levels. As such, more realisticanswersareobtained,althoughnotstrictlyinkeepingwiththeCity’spolicy.

This study reviewed and compared the SRK 2012 PCSWMM (1D) and HEC-RASmodels and foundseveralinconsistenciesandinaccuracies.Thesewerecorrectedtobetterrepresentactualconditions.Subsequently,anumberofPCSWMM2Dmodelswerecreated,whichindicatedthatfloodingontheexistingConradieHospitalsiteislesslikelyandlessseverethanpreviouslythought.

1.2.3 Modelling of the Conradie Hospital’s Stormwater Concept Design, Management and Riverine

DevelopmentPlan(Aurecon,March2017)

SubsequenttotheabovestudyameetingwasheldbetweenWSP,WCG,HHOandAurecontoobtainclarityontheWSPconceptdesignand it’ssupportingcalculationsandPCSWMMmodel. Itbecameclear that furtherdevelopmentof theconceptwasrequiredandAureconwasaskedbytheCity toundertake this work through their current appointment. Aureconwas to assess theWSP conceptdesign to determine whether it would perform appropriately. Furthermore, any areas of possibleconcernweretobeidentified.

Thereportconcludedthat:

• AureconwasunabletoaccuratelymodeltheconceptdesignasintendedbyWSPduetothe

paucityofinformation.

• Further conceptdesigndevelopmentwork is requiredbefore it canbedefinitively statedthattheconceptwillwork.

• Thewidthoftheintendedrealignedcanalreserveappearsadequatetoaccommodatethe

100 year flood, provided that key determinants are adequately addressed in thepreliminarydesignstage.

1.3. CITYOFCAPETOWNPOLICYOBJECTIVESThestormwaterdesignobjectivescanbecategorisedintwosections:

• Externalflows:AccommodatetheElsieskraalcanal,whichpassesthroughthesite,andcontainingtheHHZsuchthatthesitecanbedeveloped.

• Internalflows:Controlthequantityandrateofrunoffemanatingfromthesiteitselfandimprovethequalityifrunoff.

ItisimportanttonotethatthedesignobjectiveisnottoattenuateortreattheincomingElsieskraalcanalflows.However,allrunofforiginatingonsitemustbefullytreatedandattenuatedasrequiredbytheCity’spolicies.Theobjectivesofthesemaybemaybesummarisedasfollows:

• CityofCapeTown’sManagementofUrbanStormwaterImpactsPolicyobjectives:− Improvequalityofrunoff− Controlquantityandrateofrunoff− Encouragegroundwaterrecharge

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• CityofCapeTown’sFloodplainandRiverCorridorManagementPolicyobjectives:− Buildingscateringforresidentialorgeneralbusinessshouldhavefloorsabovethe100year

floodlevelandnon-habitablebasementsfloodproofedagainstthe50yearflood.− Educationalfacilities,publichallsandplacesofworshipshouldbesituatedabovethe100

yearfloodlevel.− Publicopenspacesandsports fieldscouldbesituated lower than the20year flood level

(butnotlowerthanthe2yearfloodlevel)providedthatanyclubhouse(orsimilar)buildingstructureshavefloorsabovethe50yearfloodlevel.

− HighHazard Zones (HHZ) should be identified in terms of the 100 year flood and publicsafetyensuredinsuchareas.Figure1.1illustratesthewaterdepthandvelocityparametersassociatedwithfloodzones.

FIGURE1.1:FLOODHAZARDZONES1.4. TERMSOFREFERENCE

HHO Africawas appointed as Technical Advisor (TA) to theWCG on 18October 2016.Whilst theappointment relatesprimarily to theRequest forProposal (RFP)process, it soonbecameapparentthatfurtherworkwasrequiredontheconceptstormwaterdesign.Followingacollaborativeprocesswiththeauthoritiesandotherconsultingengineers,HHOwereinstructedbyWCGtoprovideanewStormwater Management Plan (SWMP) report containing unambiguous stormwater and drainagerequirements.Thisreportwouldbeappendedtotherezoningapplicationandforwardedtobidders.Furthermore,thecostestimateandphasingplanhadtobeverified,oralternativesprovided.In March 217 it became clear that the concept to realign the canal did not achieve its intendedobjectiveoffloodpeakattenuation.Furthermore,fromanurbandesignperspective,itwasunlikelythat the previously proposed realignment, once geometrically corrected, would permit muchdevelopmentbetweenThorntonand itself. Therewere alsounansweredquestions about the size,depth, etc. of the proposed detention pond and sports field and the amount of additional bulkearthworks required to prevent flooding. The earthworks levels (as adjusted to suit the models)

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needed to be assessed against the surrounding rounds, internal heritage areas and adjacentproperties.Thefollowingsignificantamendmentswereproposed:

1) Tokeeptheexistingcanalinitscurrentposition.

2) Tocreateanadequatefloodplain/publicamenityadjacentthecanal.

3) Torevisetheproposedsitelevelsto:

a. Ideallykeepthedevelopmentabovethe100yearfloodlevelb. Respectheritagebuildings,adjacentroadsandproperties.

4) To separate the canal hydrology and flow from the development itself. An Important

implicationwouldbethatthedevelopmentitselfwouldsatisfytheCity’spolicies,butthatthecanalflowswouldnotbeintegratedwiththedevelopmentasinthepreviousconcept.

1.5. APPROVALPROCESS

The proposal to realign the Elsieskraal Canal has been approved by the National Department ofWater and Sanitation (DWS), who have granted General Authorisation (GA) based on the WSPconceptdesign.ThisletterisattachedasAnnexureA.

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2.0 METHODOLOGYThe WCG commissioned this study in response to the concept design report issued by WSP and thehydraulicmodellingundertakenonbehalfoftheCitybyAurecon.Asthisreportreliesontheworkdonebyothers,thefollowingmethodologywasfollowed:

1) ThepreviousconceptdesignandreportbyWSPwasassessed.2) The Aurecon reports on their 1D and 2D PCSWMM and HEC-RAS hydraulic modelling were

assessed.3) MeetingswereheldbetweentheCityofCapeTown,WCG,AureconandWSP.4) OncethedecisionwasmadenottorealigntheElsieskraalcanal,anewhigh levelconceptdesign

wasrequired.Thiswasachievedasfollows:a. AtopographicsurveyoftheElsieskraalcanalwasprovidedbyAurecon.b. CadastralinformationandbaseinformationwasprovidedbyArGArchitects.c. New site levels were calculated in accordance with the 100 year RI flood water levels

calculatedbyAurecon.d. AbermwasproposedalongtheElsieskraalcanalinaccordancewiththe100yearRIflood

energylevelscalculatedbyAurecon.e. BulkstormwaterinfrastructurewasdesignedtosatisfytheCity’spolicies.f. Asimplisticandconservative1DPCSWMMmodelwascreatedforthedevelopment.g. Policyobjectiveswereassessedusingthemodel.

5) Bulkearthworkcostswereestimated.6) AdraftSWMPreportwaswritten.

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3.0 REVIEWOFELSIESKRAALCANALMODELLINGAureconundertooktwomodellingstudiesoftheElsieskraalcanalonbehalfoftheCityofCapeTown.Theformerstudiedtheexistingcanalalignment,whereasthelatterstudyconsideredtheconceptproposedbyWSP.ThefollowingsalientfindingsaffectthisSWMP,whichproposestokeeptheexistingcanalalignment:

• Floodlevelsarebasedonenergylevels,whichtakeflowvelocityintoaccount.Whilstapplicabletoopen channels and rivers, this approach renders unrealistic flood levels in verywide floodplains,suchasthecurrentConradiesite.

• Thepreviousmodels,whichdefinedthecurrentfloodplain,wereoverlyconservative.Thepotentialforfloodingonsiteduetothecanalislesslikelyandlessseverethanpreviouslyindicated.

• Flowvelocitiesinthe100yearRIfloodevent,areveryhighintheElsieskraalcanal.Velocitiesof4to5m/scanbeexpected.Thisimpliesthatenergy(flood)levelsarebetween0.8mand1.3mhigherthanwatersurfacelevels.

• Actual canal water surface levels during the 100 year RI flood event are likely to peak atapproximately 12.0m adjacent the lower end of the site, upstreamof the railway bridge. At theupperendofthesite,thewater levelwouldbeabout13.0m.Waterdepthinthecanalwouldberoughly3.0m.

• Therailwayculverthasadequatecapacitytoconveythe100yearRIfloodpeakof115m3/s.

• The current flooding of the Conradie site, as previously defined, offers negligible flood peakattenuation.Thisimpliesthatsitelevelscouldberaisedwithoutdetrimenttoadjoiningproperties.

• Itwasrecommendedthatthesitebeinfilledtoatleasttheleveloftheexistinglevee.

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4.0 PROPOSEDSTORMWATERINFRASTRUCTURE TheSWMPentailsthefollowingdesignobjectives:Talkaboutpolicies?

• Tokeeptheexistingcanalinitscurrentposition.• Tocreateanadequatefloodplain/publicamenityadjacentthecanal.• Torevisetheproposedsitelevelsto:

- Ideallykeepthedevelopmentabovethe100yearfloodlevel- Respectheritagebuildings,adjacentroadsandproperties.

• To separate the canalhydrologyand flow from thedevelopment itself.An Important implicationwould be that the development itself would satisfy the City’s policies, but that the canal flowswouldnotbeintegratedwiththedevelopmentasinthepreviousconcept.

HHOAfricahasassessedthestormwaterimplicationsofthefulldevelopmentscenarioinaccordancewiththelatestUrbanDesignFrameworkdatedMarch2017.TheSWMPisincludedinAnnexureB.Thefollowinginfrastructureisproposed:

• Detentionpondswithoverflowweirsanddischargepipes• Overlandchannelsandswales• Undergroundstormwaterpipesandculverts• Roadways,whichactasoverlandstormwaterconduitsformajorstorms• PublicopenspaceenhancementsalongtheElsieskraalcanal

4.1 DETENTIONPONDSTwodetentionpondsareproposed.Thesearesituatedadjacenttheproposed25mElsieskraalcanalPublicOpenSpace(POS)orbufferzoneandeachcollectsroughlyhalfofthesiterunoff.Theponds’mainfunctionistoattenuatefloodpeaksandtotraplitterandsilt.Although flap gates to the outlet pipes are proposed to prevent ingress of floodwater from Elsieskraalcanal,thesystemisdesignedtofunctionwithoutthese.Allhabitablebuildingsaresituatedabovethe100yearRIfloodwaterlevel.Apartfromtheoverflowpipes,weirsareprovidedtothe100yearRIfloodlevel(waterlevel)oftheElsieskraalcanal.For the purposes of this assessment, the following detention ponddesign has been assumed (see Table4.1).Bothdetentionpondswouldbeidenticalinsizeandoperation.Figure4.1illustratestheconcept.TABLE4.1:DETENTIONPONDDESIGNITEM DESCRIPTIONBottomarea 1000m2Sideslopes 1V:2HPondfloorlevel 0.5mbelowswaleentrylevelOutflowpipe DN375Outflowpipeinvertlevel 0.5mabovepondfloorWeirlevel 2.1mabovepondfloorWeirlength 1mTopofbermlevel 3.0mabovepondfloor

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Insert4.1FIGURE4.1:TYPICALDETENTIONPONDDETAILS4.2 SWALESAllsurfacewaterrunofffromtheConradieBLMEPisconveyedtotheproposedoverlandchannels/swales,which collect, convey and treat surfacewater runoff. Two swales are proposed,which traverse the siteroughlyperpendiculartotheElsieskraalcanal.Theseeachdischarge intoadetentionpond,which inturndischarge intotheElsieskraalcanal.Figure4.2 illustrates thetypicalswalecrosssectionthat isproposed.Forthepurposesofthehydraulicmodel,aslopeof1:400(0.25%)hasbeenassumed.Thetwoswaleshaveconsiderablelength,measuring390mand335mlongrespectively.

Insert4.2FIGURE4.2TYPICALSWALECROSSSECTION4.3 INTERNALROADS&SITELEVELSAll internal roads are designed as stormwater carriers in the event of major storms. All roads andunderground pipes discharge into one of two overland channels that traverse the development. Ininstances where direct discharge into the channels or detention ponds is not possible, undergroundconduits are provided to relieve trapped lows. These can accommodate stormevents of up to 100 yearRecurrenceInterval(RI).Allbuildingsareallelevatedabovethe100yRIflood(energy)levelasrequiredbytheCity’sFloodplainandRiverCorridorManagementPolicy:

• Allresidentialfloorsareelevatedabovethe100yearRIflood(energy)level.

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• Parking areas underneath certain buildings are below the 100 year RI flood (energy) level, butabove the 100 year RI flood (water) level. This means that the real risk of flooding is greatlyreduced.

• Thereisasignificantdifferenceinelevationbetweenthewaterandenergy(flood)levelduetotheveryhighwatervelocitiesintheElsieskraalcanal.Typically,thetheoreticalfloodlevel isexpectedtobebetween0.8mand1.3mabovethewatersurfacelevel,duetothevelocityofabout5m/sinthe100yearRIfloodevent.

4.4 ELSIESKRAALCANALAlthough the Elsieskraal canal is not realigned or altered in any way, the following improvements areproposed.Figure4.3illustratesatypicalcrosssection.

• A25mwidePOS/bufferzone,whichabutsthecanal,isproposed.Thiszonewouldbeincorporatedinto theElsieskraal canal floodplain.The responsibility formaintenance,public safety,etc.wouldresort with the Homeowners Association. The space inside the POS/ buffer zone would beconfiguredasfollows:

- Thelevelfloodplainonthedevelopmentsideiswidenedtoapproximately8minordertocreateabetterpublicamenity,similartothedownstreamcanalinPinelands.

- Afurther9misrequiredtoprovide landscaped,gentlyslopedberm(s),whichwouldraiseground levels to the100yearRI flood level (energy level), thuscontaining theElsieskraalcanalfloodwaters.

- Another 6m is provided for a pedestrian walkway and trees, beyondwhich another 2mprovidesspacetoadjustbulkearthworklevelstosuitthedesign.

Insert4.3FIGURE 4.3 TYPICAL CROSS SECTION SHOWING PROPOSED 25MWIDE POS/ BUFFER ZONE AT ELSIESKRAALCANAL

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5.0 HYDROLOGYANDHYDRAULICMODELLINGThesoftwarepackageDesignrainfallofSouthAfricabySmithersandSchulzewasusedtodeterminethedesignrainfallforvariousstormevents,asshowninTable5.1below.Althoughthedesignrainfalldatadidnotcontaindataforstormssmallerthan2-yearRI,rainfallforthe1-yearand½yearRIstormeventswascalculated using logarithmic extrapolation. SCS Type I storms were assumed for all scenarios, as thisdistributionhasbeendemonstratedasbeingrealisticforSouthAfricancoastalregionswithwinterrainfall.TABLE5.1:DESIGNRAINFALL(mm)

DURATIONRECURRENCEINTERVAL(YEARS)

2.0 5.0 10.0 20.0 50.0 100.0 200.05 m 4.5 6.0 7.2 8.3 10.0 11.3 12.7

10 m 6.5 8.7 10.4 12.1 14.4 16.4 18.415 m 8.1 10.9 12.9 15.0 17.9 20.3 22.930 m 10.8 14.6 17.3 20.1 24.0 27.2 30.745 m 12.9 17.3 20.5 23.8 28.5 32.3 36.41 h 14.5 19.5 23.1 26.8 32.1 36.4 41.1

1.5 h 17.2 23.1 27.4 31.8 38.1 43.2 48.72 h 19.4 26.1 30.9 35.9 43.0 48.8 54.94 h 24.2 32.4 38.4 44.7 53.5 60.6 68.36 h 27.4 36.8 43.6 50.7 60.7 68.9 77.68 h 30.0 40.3 47.8 55.6 66.5 75.4 85.0

10 h 32.2 43.2 51.3 59.6 71.3 80.9 91.212 h 34.1 45.8 54.3 63.1 75.5 85.7 96.516 h 37.4 50.1 59.4 69.1 82.7 93.8 105.720 h 40.1 53.8 63.8 74.1 88.7 100.6 113.424 h 42.5 56.9 67.5 78.5 94.0 106.6 120.1HHO Africa calculated the impact of the proposed development by using the Rational Method andmodellingtheproposedmajorstormwaterinfrastructureusingPCSWMM.CalculationsweredonetoverifycompliancewiththeCityofCapeTown’s2009ManagementofUrbanStormwaterImpactsPolicy,bearinginmindtheapproximatefloodstageintheadjacentElsieskraalcanal.Giventhelackofhistoricdataforthesite,certainunverifiableassumptionshavehadtobemaderegardingthe pre-development scenario. Although required for the SUDS evaluation, the veracity of this scenariocannot be ascertained. Other unknown information included geological or geotechnical information, atopographic survey of the site and hydrographs or flood models for the Elsieskraal canal catchment.Assumptions had to be made based on observations and local knowledge. For the purposes of thisevaluation,thefollowingparameterswereassumed:

• Post-developmentrunoffcoefficient,Cpost:0.7• Pre-developmentrunoffcoefficient,Cpre:0.2• ApproximateNaturalGroundLevel(NGL):12.0m• AssumedfloodlevelsintheElsieskraalcanalarestatedintherelevantsectionsthatfollow.

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5.1 WATERQUALITY

The City’s Management of Urban Stormwater Impacts Policy requires on-site reduction of post-development annual stormwater pollutant loadof 80%of Suspended Solids (SS) and 45%of TotalPhosphorus(TP)basedonthe0.5yearRI,24hourstorm.Figure5.1illustratesthatoutflowintotheElsieskraalcanalwouldcommencesome19hoursafterthestartofthestormevent,or7hoursafterthepeak.Thisshouldallowsufficienttimeforpollutantremoval,asdetailedinTable5.2below.ItisassumedthatthepeakfloodlevelintheElsieskraalcanalduringthisstormeventis11.4m.

FIGURE 5.1: FLOOD PEAK ATTENUATION AT A SINGLE DETENTION POND: ½ YEAR RI, 24 HOURSTORMItisproposedthattheCity’swaterqualitytargetsmaybeachievedasfollows:Theveryflatlongitudinalswalegradientof0.25%(1:400)wouldresultinaflowvelocityof0.5m/sforthedesignstorm.AsthisvelocityisnotsufficienttokeepSSinsuspension,SSwouldsettleoutalongthe swale. Any SS that is conveyed the full length of the swale would be discharged into thedetentionpond,which featuresapermanentwetwellwithelevatedoutletpipe.AnyremainingSSwouldthussettleonthepondfloor.Regularmaintenancewouldberequiredtocleanthepondsandswales.Thismechanism isexpectedtoreducesuspendedsolidsbyat least80%asrequiredbythePolicy.It is the designed intention that the required reduction in TPwould be achieved throughnutrientabsorptionbyapprovedplantspeciesalongtheswaleandinthedetentionpond.Astheswalesareunlined,significantgroundwaterinfiltrationwouldbeexpected,whichwouldalsocapturesomeTP.BasedonestablishedbestpracticeitisexpectedthatTPwouldbereducedby45%.

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TABLE5.2:SUDSWATERQUALITYOBJECTIVES

SUDSOBJECTIVE:ON-SITEREDUCTIONOFPOST-DEVELOPMENTPOLLUTANTLOADSUDSTARGET:80%SS&45%TPREDUCTIONOFTHE0.5-YEARRI,24HSTORM

PARAMETER TARGET ACHIEVED RESULTRunoffVolume - 1,900m3 -OutflowVolume - 980m3 -SwaleGradient - 0.25% -SwaleFlowVelocity <0.9m/s 0.5m/s OKPondWetWellVolume - 1,150m3 -Sufficient Time for SS to settle (SS &TP)

SS&TP 7h OK

5.2 QUANTITYANDRATEOFRUNOFF

In order to assess the proposed Conradie BLMEP SWMS for quantity and rate of runoff, theobjectivesbelowweremeasuredagainstthefollowingstormevents:TABLE5.3:SUDSQUANTITY&RATEOFRUNOFFOBJECTIVES

To protect the stability of downstreamchannels

24 hour extended detention of the 1-yearrecurrenceinterval,24hourstormevent.

Toprotectdownstreampropertiesfromfairlyfrequentnuisancefloods

Up to the 10-year recurrence interval peakflowreducedtopre-developmentlevel.

To protect floodplain developments andfloodplains fromadverse impacts of extremefloods

Up to the 50-year recurrence interval peakflowreducedtoexistingdevelopmentlevels.Evaluatetheeffectsofthe100-yearrecurrenceinterval storm event on the stormwatermanagement system, adjacent property, anddownstreamfacilitiesandproperty.Manage the impacts through detentioncontrolsand/orfloodplainmanagement.

Themodelledscenariosarepresentedingreaterdetailbelow.

5.2.1 StabilityofDownstreamChannels

This requirement of the Policywas evaluated using the 1-year recurrence interval, 24 hour stormevent. The inflowhydrographof the SCS Type 1 stormpeaks after 12 hours from the start of thesimulationandrainfall ceasesafter24hours.Figure5.2demonstrates that the totalpeak inflow isroughly0.5m3/s.Thisflowwouldapplyequallytoeitherdetentionpond.Thetotalrunofffromthedevelopmentisconservativelycalculatedasbeing0.09m3/s.It isassumedthatthepeakElsieskraalcanalwaterlevelduringthisstormeventwouldbeapproximately11.5m.

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Figure5.3showsthattheoutflowhydrographpeakisroughly0.09m3/s.Thisisapproximately18%oftheinflowpeak, illustratingsignificantattenuation.TheoutflowhydrographinFigure5.3andTable5.4 illustratethatthedesignstormisattenuatedformorethan24hours, thusachievingthePolicyrequirement.

FIGURE 5.2: FLOOD PEAK ATTENUATION AT A SINGLE DETENTION POND: 1 YEAR RI, 24 HOURSTORM

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FIGURE5.3:EXTENDEDDETENTION:1YEARRI,24HOURSTORM

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TABLE5.4:STABILITYOFDOWNSTREAMCHANNELS

SUDSOBJECTIVE:TOPROTECTTHESTABILITYOFDOWNSTREAMCHANNELSSUDSTARGET:24HEXTENDEDDETENTIONOFTHE1-YEARRI,24HSTORM

PARAMETER TARGET ACHIEVED RESULTRunoffPeakFlow - 1.0m3/s -PeakOutflow - 0.2m3/s -DetentionPeriod 24h >24h OK

5.2.2 FairlyFrequentNuisanceFloods

To protect downstream properties from fairly frequent nuisance floods, the Policy requires thatrunoff be reduced to that which was likely before any human intervention or infrastructuredevelopmenttookplace.Itrequiresuptothe10-yearrecurrenceintervalpeakflowtobereducedtopre-developmentlevel.BeforetheConradieHospitalsitewasfirstdeveloped,itisassumedthattheareawascharacterisedby flat topography, sandy soils andCape fynbos vegetation. Figure 5.4 illustrates a suggestedpre-developmentrunoffhydrographforsuchassumptions,basedonanSCSTypeIdesignstorm.

FIGURE5.4:SUGGESTEDPRE-DEVELOPMENTRUNOFFHYDROGRAPH

The simpleandconservativePCSWMMmodelofhalf theproposeddevelopment,which illustratesthe functioning of a single swale and detention pond combination, reveals that the post-developmentpeakrunoffofapproximately1.1m3/sisreducedtoapeakoutflowofabout0.3m3/s.Thepost-developmenthydrographisshowninFigure5.5.Theseflowsmaybedoubledtoobtainthe

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effectof the fulldevelopment. It isassumedthat thepeakElsieskraalcanalwater levelduring thisstormeventwouldbeapproximately11.7m.FIGURE5.5:FLOODPEAKATTENUATIONATASINGLEDETENTIONPOND:10YEARRI,24HOURSTORMAscanbeseeninTable5.5,theassumedpre-developmentpeakrunoffof0.4m3/smaynotbequitematched by the post-development peak outflow of 0.5m3/s. However, as the post-developmentmodelisconservative,theactualoutflowmaymatchtheobjective.WecanthereforeconcludethattheintentionofthisparticularPolicyobjectiveissatisfied.TABLE5.5:FAIRLYFREQUENTNUISANCEFLOODS

SUDSOBJECTIVE:TOPROTECTDOWNSTREAMPROPERTIESSUDSTARGET:10-YEARPEAKFLOWREDUCEDTOPRE-DEVELOPMENTLEVEL

PARAMETER TARGET ACHIEVED RESULTRunoffPeakFlow - 2.0m3/s -PeakOutflow 0.4m3/s 0.5m3/s OK

5.2.3 ExtremeFloods

To protect floodplain developments and floodplains from adverse impacts of extreme floods, thePolicyrequiresthattheeffectsofseverestormevents(100-yearrecurrenceinterval)beevaluatedinthe context of its impact on the stormwater management system, adjacent property anddownstreaminfrastructure.Also,thepeakflowfroma50-yearrecurrenceintervalstormforthefullydeveloped scenario needs to be restricted to the current peak flow. The modelling results arepresentedbelow.

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50-YearRIStormEventThe Policy requires up to the 50-year RI peak flow to be reduced to existing development levels.Currently the site consists of demolished buildings and derelict roads and gardens. Figure 5.6illustratesasuggestedcurrent-developmentrunoffhydrograph,basedonanSCSTypeIdesignstorm.

FIGURE5.6:CURRENTDEVELOPMENTRUNOFFHYDROGRAPHAs before, the simplified PCSWMMmodel, which illustrates the functioning of a single swale anddetentionpondcombinationforhalftheproposeddevelopment,revealsthatthepost-developmentpeak runoff of approximately 1.5m3/s is reduced to a peak outflow of about 0.5m3/s, of whichroughly40%wouldovertoptheweir.ThehydrographinFigure5.7providesillustration.Theseflowsmaybedoubledtoobtaintheeffectofthefulldevelopment.ItisassumedthatthepeakElsieskraalcanal water level during this storm event would be approximately 11.9m. Table 5.6 provides asummaryofthisscenario.

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FIGURE5.7:FLOODPEAKATTENUATIONATASINGLEDETENTIONPOND:50YEARRI,24HOURSTORMTABLE5.6:REDUCTIONOFEXTREMEFLOODPEAKS

SUDSOBJECTIVE:TOPROTECTFLOODPLAINDEVELOPMENTS&FLOODPLAINSSUDSTARGET:50-YEARPEAKFLOWREDUCEDTOCURRENTDEVELOPMENTLEVEL

PARAMETER TARGET ACHIEVED RESULTRunoffPeakFlow - 3.0m3/s -PeakOutflow 1.3m3/s 1.0m3/s OK

100-YearRIStormEventThe Policy requires evaluation of the effects of the 100-year RI storm event on the stormwatermanagementsystem,adjacentproperty,anddownstreamfacilitiesandproperty.Itsimpactsmustbemanagedthroughdetentioncontrolsand/orfloodplainmanagement.The PCSWMM model, which illustrates the functioning of a single swale and detention pondcombinationforhalf theproposeddevelopment,revealsthatthepost-developmentpeakrunoffofapproximately1.6m3/sisreducedtoapeakoutflowofabout0.8m3/s,ofwhichroughlyhalfwouldovertoptheweir.Figure5.8illustratesinflowsandoutflows.Theseflowsmaybedoubledtoobtaintheeffectof the fulldevelopmentas revealed inTable5.7. It is assumed that thepeakElsieskraalcanalwaterlevelduringthisstormeventwouldbeapproximately12.0m.

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FIGURE5.8:FLOODPEAKATTENUATIONATASINGLEDETENTIONPOND:100YEARRI,24HOURSTORMTABLE5.7:EVALUATIONOFEXTREMEFLOODS

SUDSOBJECTIVE:TOPROTECTFLOODPLAINDEVELOPMENTS&FLOODPLAINSSUDSTARGET:EVALUATEEFFECTSOFTHE100-YEARRISTORMEVENTONSWMS,ADJACENTPROPERTY,DOWNSTREAMFACILITIES&PROPERTY

PARAMETER TARGET ACHIEVED RESULTRunoffPeakFlow - 3.2m3/s -PeakOutflow - 1.6m3/s OK

This significant attenuation of the 100 year RI flood peak demonstrates that the Conradie BLMEPSWMSmorethanadequatelybuffersdownstreampropertiesagainstflooddamage.

5.3 HIGHHAZARDZONESANDFLOODLINESThe entire Elsieskraal canal is a safety hazardwhen in flood. In terms of the definition ofHHZ,which isbasedonwaterdepthandvelocity,itwouldbeimpossibleforthiscanaltonotbeclassifiedsuch.Thecanalpassesthroughthesiteandthisrepresentsdangertothepublic,whichneedstobemanaged.

If implemented,theSWMPwould isolatetheElsieskraalcanal fromthedevelopment, therebycontainingthe HHZ and preventing flooding of the site. Annexure C details the 100 year RI flood lines anddemonstratesthattheHHZinsidethedevelopmentcanbecontainedtothedetentionponds.ThisisshowninFigure5.9below.

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[GETTHISFROMTHEANNEXURECPDF)Figure5.9:FloodLinesandHHZ

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6.0 BULKEARTHWORKS&COSTESTIMATE6.1 BULKEARTHWORKSThe Conradie BLMEP requires significant bulk earthworks to effect the SWMP. In the absence of atopographicsurvey,theCity’sLIDARdatawasusedtoestimatetheNaturalGroundLevelonsite.Generallyspeaking, the NGL has been assumed to be roughly 12.0m. Platform levels range between 12.5m and14.0m,withthebermadjacenttheElsieskraalcanalranginginheightfromapproximately13.0mto14.0m.PreliminarybulkearthworksquantitiesarepresentedinTable6.1.TABLE6.1BULKEARTHWORKS

ITEM UNIT QUANTITYTopsoilStrip m3 22000CuttoFill m3 18500ImportedFill m3 1660006.2 COSTESTIMATEWhilstitisnottheobjectiveofthisSWMPtoprovideanaccurateordetailedcostestimate,thefollowingconstructioncostsmaybeestimatedforthefollowingmajorsystemelements:

• Bulkearthworks• Detentionponds• Swales,includingculvertsatroadcrossings

Table 6.2 summarises the anticipated construction cost. The following costs are not included in thisestimate:

• Plantingtoswalesanddetentionponds• Internalroads• Catchpitsandundergroundstormwaterpipes,whichconstruetheminorsystem• Landscaping,footways,lighting,etc.• Fencing,oranyotherpublicsafetydevices• Professionalfees,escalationandVAT

TABLE6.2CONSTRUCTIONCOSTESTIMATE

SWMPCONSTRUCTIONCOSTESTIMATEITEM DESCRIPTION UNIT QUANTITY AMOUNT1.0 BulkEarthworks

Topsoilcuttospoil m3 22000 R1270000.00

Cuttofill m3 18500 R3966000.00

Importedfill m3 166000 R54736000.00

Overhaulinexcessoffreehaul m3.km 382800 R3156000.002.0 Detentionpond No. 2 R623000.003.0 Swales,includingculverts m 715 R5846000.00

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Total(ExclVAT) R69597000.007.0 CONCLUSION

TheElsieskraalcanalrepresentsasignificantpublicsafetyhazard,whichneedstobemanagedbytheCityofCapeTown.Ithasbeendemonstratedthatthereisnobenefitinrealigningthecanalandthatfillingthesitedoesnotdetrimentallyaffectthefloodregime,thefloodplainoradjacentproperties.Although based on limited information and a basic urban design, this report has demonstrated that theSWMS proposed for the Conradie BLMEP can achieve the parameters for a Sustainable Urban DrainageSystem(SUDS)asdefinedbytheCityofCapeTown.ThefutureConradieBLMEPdeveloperwouldneedtodemonstratetoagreater levelofdetail,basedonpreliminarydesign,howtheCity’spolicyrequirementscanbeachieved.

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8.0 RECOMMENDATIONS• Regularmaintenanceandannualclearingof theswalesanddetentionpondswouldberequiredto

removesiltandsediment.OtherregularmaintenancetaskswouldincludetheremovaloflitterfromthePOSandwaterbodiesandpossiblerepairoftheflapgatestodetentionpondoutletpipes.

• AformalagreementbetweenCityandfuturedevelopershouldbedraftedtoaddresscommonissuessuchasmaintenance,accessandpublicsafety.

• Safetymeasures,suchassignageandsafetyequipmentshouldbeimplemented.GiventheseverityofthepublichazardattheElsieskraalcanal,afloodwarningalertsystemshouldbeconsidered.

• Preliminary design should be completed before any proposals contained in this SWMP areimplementedoractedupon.Thisshouldincludedetailed2Dmodelling.

• FloodwaterlevelsmustbedeterminedintheEsieskraalcanalforthefullrangeofdesignstorms(½,1,2,10,50and100yearRIstormevents).

• AnypedestrianorvehicularbridgesovertheElsieskraalcanalwouldrequirebackwatercalculationstodeterminetheeffectonfloodlevels.

• StormwaterrunofffromThorntonandVikingParkneedstobebetterunderstoodandmodelled.

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9.0 REFERENCES

1. ManagementofUrbanStormwaterImpactsPolicy,Catchment,StormwaterandRiverManagementBranch,Roads&StormwaterDepartment,CityofCapeTown,CapeTownMay2009

2. FloodplainandRiverCorridorManagementPolicy,CityofCapeTown

3. StormwaterConceptDesign,ManagementandRiverineDevelopmentPlanPreparedbyWSP,August2016

4. ElsieskraalRiver-FloodMitigationStudy,PreparedbyAurecon,October2016

5. Modelling of the Conradie Hospital’s Stormwater Concept Design, Management and RiverineDevelopmentPlan,PreparedbyAurecon,March2017

6. WRCReportNo.1060/1/03entitled"DesignRainfallandFloodEstimationinSouthAfrica"PreparedbyJCSmithersandRESchulzeVersion3,July2012

7. MinimumStandardsforCivilEngineeringServicesinTownships(Version1),CityofCapeTownJuly 2013


APPENDICESA:ApprovalB:SWMPC:FloodLinesandHHZ

amendment 2 31 march 2017 annexure s amended stormwater … · although based on limited...

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