SUB‐BASIN PLANNING ADDENDUM COMPREHENSIVE WATER & WASTEWATER MASTER PLAN TECHNICAL MEMORANDUM Union County, North Carolina DECEMBER2014
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©Black&VeatchHoldingCompany2013.Allrightsreserved.
Union County, North Carolina | Sub‐Basin Planning Addendum TABLE OF CONTENTS Introduction ......................................................................................................................................... 1 ProjectDescriptionandObjectives.........................................................................................................................1 InterceptorAlignmentPlanning...............................................................................................................................1 Lake Providence Sub‐Basin ................................................................................................................... 2 HydraulicModelExpansions......................................................................................................................................2 Alignment...........................................................................................................................................................................2 FlowProjections..............................................................................................................................................................3 PeakFlowsandSewerSizes.......................................................................................................................................3 DownstreamCapacityConstraints..........................................................................................................................4 ProjectCosts.....................................................................................................................................................................4 Tarkill Sub‐Basin .................................................................................................................................. 7 HydraulicModelExpansions......................................................................................................................................7 Alignment...........................................................................................................................................................................7 FlowProjections..............................................................................................................................................................9 PeakFlowsandSewerSizes.......................................................................................................................................9 DownstreamCapacityConstraints.......................................................................................................................10 ProjectCosts..................................................................................................................................................................10 Mundy’s Run Sub‐Basin ..................................................................................................................... 13 HydraulicModelExpansions...................................................................................................................................13 Alignment........................................................................................................................................................................13 FlowProjections...........................................................................................................................................................14 PeakFlowsandSewerSizes....................................................................................................................................14 DownstreamCapacityConstraints.......................................................................................................................15 ProjectCosts..................................................................................................................................................................15 Conclusions and Recommendations ................................................................................................... 17 BLACK & VEATCH |TableofContents
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Sub‐Basin Planning Addendum | Union County, North Carolina LIST OF TABLES Table1LakeProvidenceSub‐BasinGravityFlowsandDiameters...................................................................4 Table2UnionCountyMasterPlanGravitySewerUnitCosts..............................................................................5 Table3LakeProvidenceSub‐BasinProjectCostOpinions..................................................................................5 Table4LowerTarkillSub‐BasinGravityFlowsandDiameters......................................................................10 Table5UpperTarkillSub‐BasinGravityFlowsandDiameters.......................................................................10 Table6LowerTarkillSub‐BasinProjectCostOpinions.....................................................................................11 Table7UpperTarkillSub‐BasinProjectCostOpinions......................................................................................11 Table8Mundy'sRunSub‐BasinGravityFlowsandDiameters.......................................................................15 Table9Mundy'sRunSub‐BasinProjectCostOpinions......................................................................................15 LIST OF FIGURES Figure1LakeProvidenceSub‐BasinManholeLocations......................................................................................6 Figure2UpperTarkillSub‐BasinAlternatives..........................................................................................................8 Figure3TarkillSub‐BasinManholeLocations.......................................................................................................12 Figure4Mundy’sRunSub‐basinManholeLocations..........................................................................................16 ATTACHMENTS LakeProvidenceSub‐basinMap
TarkillSub‐BasinMap
Mundy’sRunSub‐basinMap
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JUNE2015
Union County, North Carolina | Sub‐Basin Planning Addendum Introduction
PROJECT DESCRIPTION AND OBJECTIVES UnionCountyPublicWorks(UCPW)requestedthatBlack&Veatch(B&V)determinefuture
collectionsysteminfrastructureneedsinthreesub‐basinsinthenorthwestareaofUnionCounty
thatareexperiencingthegreatestinfluxofnewdevelopment.UnionCounty'sWaterandSewerLine
ExtensionOrdinanceprovidesfordeveloperstoconstructoff‐siteimprovements,necessaryto
servetheirdevelopment,inaccordancewiththeCounty'sadoptedWaterandSewerMasterPlan.
Forsomeofthelargerundevelopedsub‐basins,multipleseweralignmentsarepossibleanditis
UCPW'sdesiretoevaluateoptimumsolutionsforprovidingsewerservicetotheseareaswithina
consistentpolicyframework.Thethreesub‐basinsare:
 Sub‐Basin1:LakeProvidenceSub‐Basin(TwelveMileCreekBasin)
 Sub‐Basin2:TarkillCreekSub‐Basin(SixMileCreekBasin)
 Sub‐Basin3:Mundy’sRunSub‐Basin(TwelveMileCreekBasin)
Thesub‐basinswereevaluatedusingUCPW’sInnovyzeInfoWorksCShydraulicmodel.The
primaryobjectivesofthestudywereto:
 Determineapproximatealignmentandsizeoffuturesewerinterceptorstoserveeachsub‐basin.
 Evaluatethepotentialfutureaveragedayandwetweatherflowsineachsub‐basin.
 Estimatetheconstructioncostofthefuturesewerinterceptorprojects.
INTERCEPTOR ALIGNMENT PLANNING Preliminarygravityseweralignmentsweredevelopedforeachsub‐basinandreviewedwithUCPW.
Ineachsub‐basin,apreferredalignmentwasestablishedthatwouldeventuallyserveallcustomers
inthemostcosteffectivemanner.Thepreferredalignmentswereplannedusingthefollowing
criteria:
1. Thealignmentwillserveanyandallpotentialfuturepublicsewercustomersviagravity
connectionstotheplannedinterceptor.Potentialcustomersanddevelopmentswere
determinedusingtheupdatedparceldataandaerialphotographythatwasaddedtothe
model.
2. Thealignmentwillfollownaturalstreamsandswaleswhereverpossibletomaximizethe
areathatcanconnecttotheinterceptorbygravityflow.Followingthenaturaldrainage
pathswillalsominimizethedepthofthesewers.A3Dlayerofthetopographicground
surfacewasusedinthehydraulicmodeltodeterminethedrainagepatternsandsewer
depthsineachsub‐basin.
3. Alignmentswerealsoplannedtominimizepumping,andtheassociatedlong‐term
maintenanceandpowercosts,wheneverpossible.Wastewaterflowscouldnotbepumped
tootherbasinswheretheexistinginfrastructurewasnotplannedforthoseadditionalflows.
BLACK & VEATCH |Introduction
1
Sub‐Basin Planning Addendum | Union County, North Carolina LakeProvidenceSub‐Basin
TheLakeProvidenceSub‐BasinislocatedontheeastsideofWeddington.Thesub‐basinis
borderedonthewestbyBeulahChurchRdandbyAntiochChurchRdontheeasternside.Thereare
severalexistingsubdivisionsinthissub‐basin.ThePreserveatBrookhavenisservedbygravity
sewerthatflowsbygravitytotheexistingBrookhavenPumpStation.Allotherexisting
neighborhoodsinthesub‐basinusesepticsystems.
HYDRAULIC MODEL EXPANSIONS Theexistingwastewaterhydraulicmodeldevelopedduringthe2011ComprehensiveMasterPlan
wasexpandedtoincludeplannedsewersupstreamanddownstreamofthethreesub‐basinsthat
utilizeinterceptorcapacitycommonwiththestudysub‐basins.AllUCPWinfrastructure
downstreamoftheLakeProvidenceSub‐basinwasincludedinthe12MileBasinmodeland
calibratedduringtheComprehensiveMasterPlan.Themodelcatchmentareaswereadjustedto
routetheprojectedaveragedailydry‐weatherandwetweatherflowswithinthesub‐basinthrough
thenewLakeProvidenceseweralignments.
ALIGNMENT TheLakeProvidenceSub‐Basinisservedbyonemaingravitysewerinterceptor.Theupstreamarea
ofthesub‐basinisservedbyagravityinterceptorthatfollowsatributaryto12MileCreeklocated
inthissub‐basin.Nearthemiddleofthesub‐basin,attheGatewoodneighborhood,theaverage
slopeofthegroundsurfacedecreases.Inordertomaintainreasonablesewerdepths,aminimum
diameterof10inchesisrecommendedtolowertherequiredgravitysewerminimumslope,but
maintainasufficientscourvelocity.
Atthedownstreamendofthesub‐basin,thegravitysewermustbeconstructedoneithertheeast
orwestsideofLakeProvidence.TheLakeProvidenceneighborhoodislocatedonthewestsideof
thelake.MostofthehomesintheLakeProvidenceneighborhoodwerebuiltinthe1990sandare
servedbysepticsystems.Theproximityoftheexistingstructurestotheedgeofthewaterwould
causeconstructabilityissuesanddisruptiontothehomeowners.Inordertoserveallupstream
customersbygravity,thesewersinthisareawouldalsoneedtobeburiedverydeep.Onthe
easternsideofthelake,anexistingpowerlineeasementcanbeutilizedtominimizethedisruption
toexistinghomeowners.TheplannedinterceptorroutedtotheeasternsideofLakeProvidencewill
serveallthepotentialfutureupstreamcustomers.Also,thelengthofsewerontheeastsidetothe
UnionCountyinterceptorisshorter.Theshorterlengthandshallowdepthsmakeroutingthe
interceptortotheeasternsideofthelakethemoreeconomicalchoice.Thesewerwillconnectto
theplanned18‐inchgravitysewer(CIPProjectTM‐G‐07).
Routingtheplannedinterceptortotheeasternsideofthelakewillallowallexistingandfuture
neighborhoodsnorthofHuntingtonDrivetoconnecttotheinterceptorbygravity.However,the
LakeProvidenceNeighborhoodwillnotbeabletoconnecttotheplannedinterceptor.Asthe
neighborhoodwasbuiltinthe1990sandlotsaregreaterthan1acre,individualhomeownersare
unlikelytowanttoconnecttothepublicsewersysteminthenearfuture.Inthefuture,iftheentire
neighborhoodchoosestoconverttopublicsewer,theneighborhoodcouldbeservedbygravityand
connecttotheUnionCounty12MileCreekInterceptor.
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Union County, North Carolina | Sub‐Basin Planning Addendum FLOW PROJECTIONS Average Daily Dry‐Weather Flows AverageDailyDry‐weatherFlows(ADDF)weredevelopedusingtwomethods.Futureflowsfor
2030weredevelopedusingthesamemethodologyasthe2011ComprehensiveMasterPlan.The
2030projectionsincludepopulationgrowththroughtheyear2030,butgrowthwasanticipatedto
continuebeyond2030withinthesub‐basin.
Newplanninglevelprojectionsweredevelopedtotrytodeterminetheadditionalflowsthatwould
becontributedtoeachsub‐basinbeyondthe2030window.Thenewprojectionsweredeveloped
usingtheparceldatafromeachsub‐basin.Theexistingparcelswereclassifiedaseithersepticuse
orundevelopedland.Theundevelopedareaineachsub‐basinwasanticipatedtobedevelopedin
thefutureintooneacreresidentiallotsinaccordancewithcurrentlanduseordinances.30%ofthe
undevelopedareahasbeenreservedforopenspaceforroads,easements,commonsspaces,andfor
existingstreamsandbuffers.25%ofexistingsepticsystemswereanticipatedtoconverttopublic
sewer.Futureloadingswerebasedonassumptionsconsistentwiththe2011Comprehensive
MasterPlanregardingresidentialdevelopmentineachsub‐basin.Thepercapitawastewater
loadingratewasanticipatedtobe80gpcdandtheaverageoccupancywasanticipatedtobe2.7
personsperdwellingunit.
Wet Weather Flows Wetweatherflowsweredevelopedusingthesameprocedureandmethodologyestablishedfor
newdevelopmentsinthe2011ComprehensiveMasterPlan.Theresultingpeakwetweatherflowis
dependentontheindividualcharacteristicsofeachbasin(e.g.timeofconcentration,topography).
PEAK FLOWS AND SEWER SIZES Gravitysewersweresizedbasedonthepeakflowssimulatedbytheupdatedmodelandfollowing
thePerformanceCriteriaestablishedforthe2011ComprehensiveMasterPlan.Theperformance
criteriaestablishedaservicegoalforneworreplacementsewersoflessthan18inchesindiameter
tobesizedbasedonamaximumdepth/Diameter(d/D)ratiooflessthanorequalto0.65.Thelow
ratioistopreserveacapacitybufferinthesesmallerlocalpipes.Also,allplannedgravitysewers
shouldmaintaindiametercontinuity,downstreamsewersshouldhavethesameorlargerdiameter
thanupstreamsewers,andtheminimumslopeforeachsewershouldconformtoNCDENRsystem
minimumdesignguidance.
Thewetweatherflowsandresultingsewerinterceptordiametersforthesub‐basinplanninglevel
projectionswerecomparedtotheresultsfromthe2030planningyearmodel.Overall,theresulting
diameterswereverysimilar.IntheLakeProvidenceSub‐Basin,theplanneddiameterwasgoverned
bytheminimumsloperequirements.Theflowsandtheplanneddiameterforeachsegmentof
sewerareshowninTable1.Thepeakwetweatherflowof0.544MGDintheLakeProvidenceSub‐
Basincouldhavebeencarriedbyan8‐inchsewerattheNCDENRrecommendedminimumslopeof
0.4%.However,thedownstreamendofthesub‐basinismuchflatterthantheuppersections.Using
aslopeof0.4%wouldleadtodeepersewersatthedownstreamendofthesub‐basin.Deepsewers
createmoreexcavationcostsandaremoredifficulttomaintain.Byplanninga10‐inchdiameter
sewerattheminimumslopeof0.28%,theplannedsewerwillfollowthenaturalground
topographyandminimizetherequiredsewerdepths.Theaveragedailyflowattheoutletofthe
BLACK & VEATCH |LakeProvidenceSub‐Basin
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Sub‐Basin Planning Addendum | Union County, North Carolina sub‐basinis0.135MGD.ThesegmentIDsinTable1correspondtoFigure1.Theflowvaluesinthe
tablearetotalflowsincludingallflowsupstreamofeachreferencedManhole.
Table1LakeProvidenceSub‐BasinGravityFlowsandDiameters
TOTAL
TOTALWET
MANHOLE
DOWNSTREAM
AVERAGEDAILY
WEATHER
LOCATION
DIAMETER(IN)
FLOW(MGD)
FLOW(MGD)
LP‐1
0.135
0.544
18
LP‐2
0.124
0.448
10
LP‐3
0.082
0.341
8
LP‐4
0.031
0.205
8
LP‐5
0.037
0.108
8
LP‐5a
0.051
0.150
8
DOWNSTREAM CAPACITY CONSTRAINTS ThemodeledpeakwetweatherflowsfortheLakeProvidencesub‐basindidnothaveasignificant
impactondownstreaminfrastructure.Thesub‐basininterceptorconnectstoaplanned18‐inch
interceptorproject.Theproject,TM‐G‐07,wasslatedforthe2016‐2020planninghorizoninthe
MasterPlan.
PROJECT COSTS Wastewaterunitcostsbypipediameterperlinearfootweredevelopedaspartofthe
ComprehensiveMasterPlanproject.ThecostsforgravitysewerpipeareshowninTable2.
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Union County, North Carolina | Sub‐Basin Planning Addendum Table2UnionCountyMasterPlanGravitySewerUnitCosts
DEPTHTOINVERT
DIAMETER
(IN)
10FT
15FT
20FT
25FT
8
$68
$80
$98
$118
10
$75
$87
$105
$125
12
$85
$96
$114
$134
15
$101
$111
$128
$147
18
$113
$145
$169
$196
21
$127
$157
$181
$208
24
$158
$187
$211
$238
27
$185
$210
$233
$260
30
$213
$236
$259
$286
36
$270
$320
$350
$386
42
$333
$383
$413
$449
48
$421
$466
$496
$532
54
$514
$552
$583
$618
Aconstructioncontingencyof20%wasappliedtotheconstructioncost.Also,engineeringcostsfor
design,planningandpermittingaswellastheengineeringrequiredduringconstructionwere
assumedtobe15%ofthetotalconstructioncostincludingthecontingencies.Theprojectcostsare
showninTable3.ThetotalcostopinionfortheLakeProvidenceSub‐Basinprojectsis
approximately$2.11million.
Table3LakeProvidenceSub‐BasinProjectCostOpinions
UPSTREAM DOWNSTREAM CONSTRUCTION CONSTRUCTION ENGINEERING
TOTAL
MANHOLEID MANHOLEID
COST
CONTINGENCY
COST
PROJECTCOST
LP‐2
LP‐1
$693,000
$138,600
$124,740
$956,340
LP‐3
LP‐2
$208,000
$41,600
$37,440
$287,040
LP‐4
LP‐3
$100,000
$20,000
$18,000
$138,000
LP‐5
LP‐3
$527,000
$105,400
$94,860
$727,260
$1,528,000
$305,600
$275,040
Sub‐BasinTotal
BLACK & VEATCH |LakeProvidenceSub‐Basin
$2,108,640
5
Sub‐Basin Planning Addendum | Union County, North Carolina Figure1LakeProvidenceSub‐BasinManholeLocations
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Union County, North Carolina | Sub‐Basin Planning Addendum TarkillSub‐Basin
TheTarkillSub‐basinislocatedsouthwestofWeddingtonandeastofMarvin.Thesub‐basinis
borderedonthewestbyCraneRdandbySouthProvidenceRdontheeasternside.Thereare
severalexistingsubdivisionsinthissub‐basin.TheReserve,locatedoffofCraneRd,isservedby
gravitysewerintheCowhornBranchSub‐Basin.FlowfromtheCraneValley,DraytonHallEstates,
andTheGardensonProvidencesubdivisionsarepumpedintotheSixMileBasin.TheMarvinRidge
HighSchoolisalsoservedbyapumpstationthatpumpstotheCowhornBranchgravity
interceptor.
HYDRAULIC MODEL EXPANSIONS Theexistingwastewaterhydraulicmodeldevelopedduringthe2011ComprehensiveMasterPlan
wasexpandedtoincludesewersupstreamanddownstreamoftheTarkillsub‐basinthatutilizethe
commondownstreaminterceptorcapacity.Thehydraulicmodelwasexpandedduringthistask
ordertoincludeexistingsewersintheSixMileCreekBasintributarytotheTarkillandMillbridge
PumpingStations.TheTarkillCreekSub‐basinwasthencalibratedinthemodelusingflowmeter
datafromtheMarvinRoadpermanentflowmetercollectedduringthe2011ComprehensiveMaster
Planproject.
ALIGNMENT ThemaindrainagepathintheTarkillsub‐basinfollowstheTarkillstreamfromtheupstreamendof
thesub‐basintotheexisting18‐inchsewernearMarvinRidgeHighSchool.TheMarvinRidge
PumpingStationcanberemovedfromserviceandreplacedbyagravitysewerthatwillconnectto
theexistinggravitysewerdownstream.
Basedontheplanningcriteria,theTarkillinterceptoralignmentshouldfollowtheTarkillCreek
downthesub‐basininordertoserveallthefuturecustomersinthesub‐basin.Aninterceptoralong
TarkillCreekwouldpassthroughtheexistingWaldenPondneighborhoodwhichiscurrently
servedbysepticandhasnolong‐termplanstoconverttopublicsewer.TheWaldenPond
neighborhoodislocatedinthemiddleofthesub‐basinwithmanypotentialfuturecustomers
locatedbothupstreamanddownstreamoftheestablishedneighborhood.
ToavoiddisruptiontotheWaldenPondneighborhood,allupstreamflowswouldhavetobe
pumpedtoanadjacentbasinortheflowwouldneedtobepumpedaroundtheneighborhood.After
analyzingtheavailablecapacityinnearbysewers,fouralternativesweredevelopedtoroutethe
wastewaterflowsfromtheupperTarkillbasintoexistingUnionCountyinfrastructure:
 Alternative1:GravityseweralongTarkillCreekthroughtheWaldenPondneighborhood.The
proposedinterceptorwouldpassbetweentheexistinghomesonYellowPineCtandthepondin
ordertominimizethedepthofthesewers.Theplannedalignmentwouldimpact11property
ownersintheWaldenPondneighborhood.
 Alternative2:PumparoundtheWaldenPondhousesandpondthroughtheneighborhood.The
pumparoundoptionwouldrequireapumpingstationsituatednorthoftheWaldenPond
neighborhoodandaforcemainthatmayberoutedalongYellowPineCtasshowninFigure1.
Theforcemainroutingwouldimpactthesame11homeownersasthegravitylineandsix
BLACK & VEATCH |TarkillSub‐Basin
7
Sub‐Basin Planning Addendum | Union County, North Carolina additionalhomeowners.Unlikeagravitysewer,existinghomeownerswouldnotbeableto
connectaforcemainintheeventofasepticfailure.
 Alternative3:PumptheUpperTarkillsub‐basinflowstothe12‐Mileinterceptor.Apump
stationwouldbelocatedupstreamofWaldenPond.Theproposedpumpstationlocationis500
feetnorthoftheexistingneighborhoodandwouldbeaccessedbyanapproximately700footlong
drivewayfromNewTownRd.ThelocationshouldservemostofthefutureUpperTarkillSub‐
BasincustomersbygravityflowtothePS.Giventheupstreamlocationofthepumpstation,there
isapossibilitythatsomefuturecustomersinbetweenthePSandWaldenPondmightnotbeable
toconnect.TheforcemaincouldberoutedalongNewTownRdandProvidenceRdbefore
headingeasttothe12‐MileInterceptorthroughtheelectricallineeasement.The15,470‐foot
forcemainshouldbesizedat6inches.Thereceivingseweris30inchesindiameter.
 Alternative4:PumptheupperTarkillSub‐BasinflowstotheexistingTarkillforcemain.The
pumpstationcouldbelocatedinthesameplaceasalternative3.Theforcemainwouldextend
northtoNewTownRd,thenfollowtheroadtomeetupwiththeTarkillforcemain.Thetwoforce
mainswillmanifoldattheintersectionofNewTownRdandWaxhaw‐MarvinRd.The11,000‐foot
forcemainshouldbesizedat6inches.
ThefouralternativesareshowninFigure2.
Figure2UpperTarkillSub‐BasinAlternatives
AlthoughAlternatives1and2werelessexpensivethantheotheralternatives,bothsignificantly
impactedhomeownersintheexistingWaldenPondneighborhood.AttheNovember3rd,2014
countycommissionersmeeting,thedecisionwasmadetoavoidtheWaldenPondneighborhood
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JUNE2015
Union County, North Carolina | Sub‐Basin Planning Addendum entirelyandmoveforwardwithevaluatingpumpingsolutionstoothernearbyUnionCounty
infrastructure.
Followingathoroughevaluation,alternative3wasselectedbecausetherearemultipleoptionsfor
treatingthewastewaterflowsfromthesub‐basin.Alternative3dischargestothe12‐MileCreek
Basin.Flowreceivedatthe12‐MileWWTPcanbetreatedonsiteordivertedtothe6‐MileCreek
basintobetreatedatMcAlpineWWTP.IftheupperTarkillsub‐basinflowswerepumpeddirectly
totheTarkillforcemainintheSixMileBasin,asinAlternative4,noneofthefutureflowfromthat
sub‐basincouldbetreatedatthe12‐MileWWTP.Alternative3waschosensinceitprovidesfor
moreoperationalflexibilityintreatingfutureflowsfromthissub‐basin.
FLOW PROJECTIONS Average Daily Dry‐Weather Flows Theprojectedaveragedailydry‐weatherwastewaterflowsfortheTarkillSub‐basinwere
developedusingthesamemethodsasdescribedfortheLakeProvidenceSub‐Basin.
Wet Weather Flows ThepeakwetweatherflowsfortheTarkillSub‐Basinweredevelopedusingthesameprocedure
andmethodologydescribedintheLakeProvidenceSub‐Basinflowprojectionssection.
PEAK FLOWS AND SEWER SIZES Gravitysewersweresizedbasedonthepeakflowssimulatedbytheupdatedmodelandfollowing
thePerformanceCriteriaestablishedforthe2011ComprehensiveMasterPlan.Thewetweather
flowsandresultingsewerinterceptordiametersforthesub‐basinplanninglevelprojectionswere
comparedtotheresultsfromthe2030planningyearmodel.An8‐inchsewerwillconveythe2030
peakflows.Fortheplanninglevelprojections,thesewerhadtobeupsizedto10inchesindiameter
upstreamoftheUpperTarkillPumpStation.However,increasingthesewersizefortherequired
10‐inchsewersdidnotsignificantlyincreasetheprojectcostopinion.Installingthelargerdiameter
initiallywillnegatetheneedforfutureparallel8‐inchsewers.Giventhesmallincreasetothetotal
cost,theplanninglevelprojectionsarerecommendedasthebasisforsizingthefuturesewersin
thissub‐basin.
Theplanninglevelprojectionflowsandtheplanneddiameterforeachsegmentofsewerareshown
inTables4and5.TheUpperandLowerSub‐Basinsareshownseparately.ThesegmentIDs
correspondtothemarkedlocationsinFigure3.Theflowvaluesinthetablesaretotalflows
includingallflowsupstreamofeachreferencedManhole.
BLACK & VEATCH |TarkillSub‐Basin
9
Sub‐Basin Planning Addendum | Union County, North Carolina Table4LowerTarkillSub‐BasinGravityFlowsandDiameters
MANHOLE
LOCATION
TOTAL
AVERAGEDAILY
FLOW(MGD)
TOTALWET
WEATHER
FLOW(MGD)
DOWNSTREAM
DIAMETER(IN)
LT‐1
0.187
1.355
12
LT‐2
0.072
0.263
18
LT‐3
0.025
0.073
18
LT‐4
0.019
0.049
8
LT‐5
0.012
0.027
8
Table5UpperTarkillSub‐BasinGravityFlowsandDiameters
MANHOLE
LOCATION
TOTAL
AVERAGEDAILY
FLOW(MGD)
TOTALWET
WEATHER
FLOW(MGD)
DOWNSTREAM
DIAMETER(IN)
UT‐PS
0.208
0.689
6
UT‐1
0.152
0.582
10
UT‐2
0.114
0.481
10
UT‐3
0.099
0.440
10
UT‐4
0.053
0.310
8
UT‐5
0.014
0.118
8
DOWNSTREAM CAPACITY CONSTRAINTS TheupperTarkillsub‐basinflowswillbepumpedtoanexisting30‐inchinterceptorinthe12‐Mile
basin.Thereisnoadverseimpactonthe30‐inchinterceptorduetotheadditionalflowsfrom
Tarkill.
InthelowerTarkillsub‐basin,theexistingmaingravitysewerinterceptornearMarvinRidgeHigh
Schoolissizedat18inchesindiameter.SincetheflowsfromtheUppersub‐basinwillbepumped
toanadjacentbasin,these18‐inchsewersareoversizedfortheprojectedflows.Further
downstreamofthehighschoolandupstreamoftheTarkillpumpstation,theexistingsewerisonly
12inchesindiameter.WiththelowerbuildoutflowsfromtheLowerTarkillsub‐basin,the12‐inch
sewershouldbeabletoaccommodatethefutureflowswithonlyminorsurcharge(surchargeisless
than1footofdepthatonemanhole).
PROJECT COSTS CostopinionsweredevelopedfortheTarkillSub‐Basinusingtheunitcostsandcontingenciesfrom
the2011ComprehensiveMasterPlan.AsummaryoftheassumptionsisgivenintheLake
ProvidenceSub‐BasinProjectCostssectionofthisreport.TheprojectcostsareshowninTables6
and7.ThetotalcostopinionfortheLowerTarkillSub‐Basinprojectsisapproximately$0.95
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Union County, North Carolina | Sub‐Basin Planning Addendum million.ThetotalcostopinionfortheUpperTarkillSub‐Basinprojectsisapproximately$2.14
million.ThecombinedTarkillSub‐basintotalcostopinionis$3.09million.
Table6LowerTarkillSub‐BasinProjectCostOpinions
UPSTREAM DOWNSTREAM CONSTRUCTION CONSTRUCTION ENGINEERING
TOTAL
MANHOLEID MANHOLEID
COST
CONTINGENCY
COST
PROJECTCOST
LT‐2
LT‐1
$573,000
$114,600
$103,140
$790,740
LT‐4
LT‐3
$53,000
$10,600
$9,540
$73,140
LT‐5
LT‐4
$64,000
$12,800
$11,520
$88,320
$690,000
$138,000
$124,200
$952,200
LowerSub‐BasinTotal
Table7UpperTarkillSub‐BasinProjectCostOpinions
UPSTREAM DOWNSTREAM CONSTRUCTION CONSTRUCTION ENGINEERING
TOTAL
MANHOLEID MANHOLEID
COST
CONTINGENCY
COST
PROJECTCOST
UT‐1
UT‐PS
$53,000
$10,600
$9,540
$73,140
UT‐2
UT‐1
$155,000
$31,000
$27,900
$213,900
UT‐3
UT‐2
$119,000
$23,800
$21,420
$164,220
UT‐4
UT‐3
$247,000
$49,400
$44,460
$340,860
UT‐5
UT‐4
$75,000
$15,000
$13,500
$103,500
$206,700
$41,300
$37,200
$285,200
$696,100
$139,200
$125,300
$960,600
UT‐PS
UT‐PSFM
12‐Mile
Sub‐BasinTotal
BLACK & VEATCH |TarkillSub‐Basin
$2,141,420
11
Sub‐Basin Planning Addendum | Union County, North Carolina Figure3TarkillSub‐BasinManholeLocations
12
JUNE2015
Union County, North Carolina | Sub‐Basin Planning Addendum Mundy’sRunSub‐Basin
TheMundy’sRunSub‐BasinisonthesouthsideofWeddington.Thesub‐basinisborderedonthe
westbyProvidenceRdandbyCoxRdandWeddingtonRdontheeasternside.Thereareseveral
existingsubdivisionsinthissub‐basin.Devonridgeistheonlyexistingneighborhoodservedbythe
publicsewersystem.Allotherexistingneighborhoodsinthesub‐basinhavesepticsystems.
HYDRAULIC MODEL EXPANSIONS Theexistingwastewaterhydraulicmodeldevelopedduringthe2011ComprehensiveMasterPlan
wasexpandedtoincludesewersupstreamanddownstreamofthethreesub‐basinsthatutilize
interceptorcapacitycommonwiththestudysub‐basins.AllUCPWinfrastructuredownstreamof
theMundy’sSub‐basinwasincludedinthe12MileBasinmodelandcalibratedduringthe
ComprehensiveMasterPlan.Themodelcatchmentareaswereadjustedtoroutetheprojected
averagedailydry‐weatherandwetweatherflowswithinthesub‐basinthroughthenewMundy’s
Runseweralignments.
ALIGNMENT IntheMundy’sRunSub‐Basin,allpotentialfuturecustomersareservedbythreemajorgravity
sewerbranchesthatextendthroughoutthesub‐basin.Thethreeseweralignmentsfollowthe
naturalswalesandconvergetoasinglegravityinterceptorjustupstreamoftheAeroPlantation
Lake.Themajorityofthesub‐basindownstreamofthispointbelongstotheAeroPlantation
neighborhood.
TheAeroPlantationneighborhoodisfullydevelopedwithhomeswithsepticsystems.Theproperty
sizesintheAeroPlantationneighborhoodarelargeandtheneedtoconnecttopublicsewerservice
isnotanticipated.Eventhoughsomehomesintheneighborhoodwerebuiltbefore1980,thelots
aregreaterthan2acres,soindividualhomeownersareunlikelytoneedtoconnecttothepublic
sewersystem.Inthefuture,iftheentireneighborhoodchoosestoconverttopublicsewer,the
neighborhoodcouldbeservedbygravityandconnecttotheUnionCounty12MileCreek
Interceptor.
Allpotentialfuturecustomersofthesub‐basincanbeservedbytheplannedinterceptorsupstream
oftheAeroPlantationneighborhood.Theprimarypurposeofanyinfrastructuredownstreamof
thislocationistotransportthewastewaterflowfromtheupstreamsub‐basintotheUnionCounty
12MileCreekInterceptor.Multipleoptionswereconsideredtoconnecttheplannedinterceptorto
theexisting12MileCreekInterceptor.Theflowcaneitherbetransportedbyagravityseweror
pumpedthroughaforcemain.Agravitysewercouldbeextendedalongthebanksofthelake,but
theinterceptorwouldneedtobeconstructedclosetoexistingseptichomesnearthewater’sedge
andwouldbeverydeepinsomeareas.Noexistingorfuturecustomerswillneedtoconnecttothe
gravitysewerdownstreamofAeroPlantation.Theproximityoftheexistingstructurestotheedge
ofthewaterwouldcauseconstructabilityissuesanddisruptiontothehomeowners.Alternatively,a
pumpstationcouldtransfertheflowstoanexistinginterceptorintheadjacentsub‐basin.The
pumpstationsolutionislesscostlyanddisruptivethanthegravityinterceptor.Theincremental
pumpedflowsfromtheMundy’sRunSub‐Basindonotcausecapacityconcernstothereceiving
existinginterceptor.
BLACK & VEATCH |Mundy’sRunSub‐Basin
13
Sub‐Basin Planning Addendum | Union County, North Carolina ThePumpStationalternativewaschosenasthemostcosteffectivewaytoserveallpotentialfuture
customersinthesub‐basin.Thegravityseweralternativecouldcosttwicetheamountofthepump
stationalternative,duetothelengthandalignmentofthegravityinterceptorandeasements
throughlakefrontproperty.TheflowtransferisnotexpectedtonegativelyimpactanyoftheUnion
Countyinfrastructuredownstreamoftheplannedpumpstationandforcemain.
FLOW PROJECTIONS Average Daily Dry‐Weather Flows Theprojectedaveragedailydry‐weatherwastewaterflowsfortheMundy’sRunSub‐Basinwere
developedusingthesamemethodsasdescribedfortheLakeProvidenceSub‐Basin.
Wet Weather Flows TheprojectedpeakwetweatherwastewaterflowsfortheMundy’sRunSub‐Basinweredeveloped
usingthesamemethodsasdescribedfortheLakeProvidenceSub‐Basin.
PEAK FLOWS AND SEWER SIZES Gravitysewersweresizedbasedonthepeakflowssimulatedbytheupdatedmodelandfollowing
thePerformanceCriteriaestablishedforthe2011ComprehensiveMasterPlan.Thewetweather
flowsandresultingsewerinterceptordiametersforthesub‐basinplanninglevelprojectionswere
comparedtotheresultsfromthe2030planningyearmodel.However,sincethesub‐basinis
dividedintothreemajorswales,thetotalflowsineachbranchforbothprojectionsarerelatively
small.Thetotalpeakwetweatherflowatthepumpstationlocation,includingtheflowcontribution
fromallthreemainbranches,wasestimatedtobe0.763mgd.
Overall,theresultingdiameterswereverysimilarbetweentheplanninglevelprojectionand2030
flowprojections.Theonlydifferenceinplanneddiameterwasthesegmentsflowingintothe
plannedMundy’sRunPumpStation.Whilean8‐inchsewercouldaccommodatethe2030peak
flows,a12‐inchsewerisrecommendedfortheplanninglevelprojectionflows.Giventheshort
length(lessthan130feet)andsmalldifferenceinconstructioncostforthissegmentofsewer,a12‐
inchsewerisrecommendedupstreamoftheMundy’sRunPumpStationtopreventtheneedfor
parallel8‐inchsewersinthefuture.Theflowsandtheplanneddiameterforeachsegmentofsewer
areshowninTable8.Theflowvaluesinthetablearetotalflowsincludingallflowsupstreamof
eachreferencedManhole.ThesegmentIDscorrespondtolocationsinFigure4.
14
JUNE2015
Union County, North Carolina | Sub‐Basin Planning Addendum Table8Mundy'sRunSub‐BasinGravityFlowsandDiameters
TOTAL
TOTALWET
MANHOLE
DOWNSTREAM
AVERAGEDAILY
WEATHER
LOCATION
DIAMETER(IN)
FLOW(MGD)
FLOW(MGD)
MR‐1
0.219
0.710
12
MR‐2
0.026
0.084
8
MR‐2a
0.050
0.220
8
MR‐3
0.013
0.051
8
MR‐3a
0.026
0.077
8
MR‐4
0.097
0.367
8
MR‐4a
0.129
0.418
8
MR‐5
0.026
0.092
8
MR‐PS
0.248
0.763
‐
DOWNSTREAM CAPACITY CONSTRAINTS ThemodeledpeakwetweatherflowsfortheMundy’sRunsub‐basindidnothaveasignificant
impactondownstreaminfrastructure.TheMundy’sRunforcemainwasconnectedtoanexisting
15‐inchinterceptor.Shortlydownstream,theinterceptorincreasesto24inchesindiameter.
PROJECT COSTS CostopinionsweredevelopedfortheMundy’sRunSub‐Basinusingtheunitcostsand
contingenciesfromthe2011ComprehensiveMasterplan.Asummaryoftheassumptionsisgivenin
theLakeProvidenceSub‐BasinProjectCostssectionofthisreport.Forthepumpstation,aunitcost
of$0.30/GPDwasusedforthepumpstationandacostof$45/feetwasusedforthe6‐inchforce
main.ThesegmentcostsareshowninTable9.ThetotalcostopinionfortheMundy’sRunSub‐
Basinprojectsisapproximately$2.51million.
Table9Mundy'sRunSub‐BasinProjectCostOpinions
UPSTREAM DOWNSTREAM CONSTRUCTION CONSTRUCTION ENGINEERING
TOTAL
MANHOLEID MANHOLEID
COST
CONTINGENCY
COST
PROJECTCOST
MR‐1
MRPS
$36,000
$7,200
$6,480
$49,680
MR‐2
MR‐2a
$377,000
$75,400
$67,860
$520,260
MR‐3
MR‐1
$214,000
$42,800
$38,520
$295,320
MR‐4
MR‐1
$402,000
$80,400
$72,360
$554,760
MR‐5
MR‐4
$138,000
$27,600
$24,840
$190,440
$228,900
$45,800
$41,200
$315,900
$420,000
$63,000
$96,600
$579,600
$1,815,900
$342,200
$347,860
MRPS
MRFM
FMDischarge
Sub‐BasinTotal
BLACK & VEATCH |Mundy’sRunSub‐Basin
$2,505,960
15
Sub‐Basin Planning Addendum | Union County, North Carolina Figure4Mundy’sRunSub‐basinManholeLocations
16
JUNE2015
Union County, North Carolina | Sub‐Basin Planning Addendum ConclusionsandRecommendations
Theplannedsub‐basininfrastructureshouldaccommodateallfuturewastewaterflowsinthesub‐
basins.Theinfrastructurewassizedbasedontheplanninglevelprojectionswhichassumedthat70
percentofthevacantlandisdevelopedinto1‐acrelotsand25%ofsepticusersconverttopublic
sewer.Insomeareas,theplanninglevelinfrastructureexceedsthe2030needs,butthedifference
incostwasminimalandprovidesthebenefitofavoidingfuturereplacement/parallelingsewers
projects.Thesewerdepthswereeffectivelyminimizedbyfollowingnaturaldrainagepaths.The
averagedepthsinthethreesub‐basinsrangedfrom12to14feetdeep.Theplannedalignmentsin
eachsub‐basinsuccessfullymeettheoutlinedplanningcriteria,exceptintheTarkillSub‐basin
wherethecommissiongavethedirectiontogooutsideofthecriteria.
BLACK & VEATCH |ConclusionsandRecommendations
17