Download - Site development in the coastal region differs ... · Green Growth Guidelines: Chapter 3-2 Stormwater Management Site development in the coastal region differs comparatively to development

Green Growth Guidelines:Chapter3-2 Stormwater Management

Sitedevelopmentinthecoastalregiondifferscomparativelytodevelopmentin

Georgia’sPiedmontarea,wheresitedevelopmentinvolvesmovingvastquantitiesoffill

materialtoflattenhillyterrainforbuilding.Inthecoastalregion,sitesaregenerallyflat

tomildlysloping.Basedonthis,developersinthisareaseekenoughchangeinelevation

topreventthesitefromflooding.Thisisoftenaccomplishedbydiggingpondsto

providealowpointtoreceivestormwater,andusingthefillgenerated(fromthecreation

ofthepond)toelevatebuildingpads.Uplandsoilsinthisregiontendtobesandyand

havehighinfiltrationrates.InthePiedmont,uplandsoilsgenerallyhavehighclay

contentandcharacteristicallylowinfiltrationrates.Thesedifferencesdriveachangein

emphasisinrecommendedLIDpracticesforthecoastalarea.

However,manyissuesaffectingdevelopmentactivityarequitesimilar.Land

developmentinbothregionsinvolvesremovingforestedcanopyandreplacingitwith

imperviouscoversuchasroads,parkinglotsandrooftops.Chapter2discussedthe

importanceofretainingasmuchoftheforestedcanopywithinadevelopmentaspossible,

aswellasreplacingthecanopythatislostwithnewplantings.Whenanareaofforestis

convertedtopavement,theareagenerateseighttimesmorevolumeofrunoffbasedonthe

lossofcoverfactoralone.AccordingtotheEPA,goingfromgrassedareastopavement

generatesatleasttwicetherunoff.Thisrunoffreachesthereceivingstreamsinmuchless

timethanfromforestedareas.

Hydrologistsoftenrefertoahydrographwhendeterminingthepeakflowofa

stormevent.Developmentactivitychangesthispeakflowbyincreasingthevolumeof,

andspeedatwhichrainfallbecomesrunoff.Thehydrographillustratesthechanges

inpeakflowforpre-developmentandpost-developmentconditionsincoastalGeorgia.

(Figure3.1.c)

Figure 3.1c Hydrograph Representing Pre-and Post-development Conditions

Image Courtesy of: State.ga.us

Green Growth Guidelines:Stormwater Management Chapter3-1

low imPAct develoPment And stormwAter mAnAgement

3.1 Introduction

ThisChapterdiscussesthemanagementofstormwateratitssourcebyretaining

wateron-siteratherthandisposingofitaswaste.Thisiscommonlyknownaslow

impactdevelopment(LID)andreferstoagroupofengineeringtechniquesusedtomimic

naturaldrainagesystems.LIDtechniquesmanagerainfallwithpassive,non-structural

practicesthatpromotegroundwaterinfiltrationorevaporationatorneartherainfallsite.

ThisChapteradaptstechniquesfromtheGeorgiaStormwaterManualandothersources.

(Figures3.1.aand3.1.b)

Figure 3.1a Conventional Approach: Manage Stormwater

with Centralized Regional Facility

Image Courtesy of: PGDER

Figure 3.1b Decentralized Approach: Manage Stormwater at source with LID Techniques

Image Courtesy of: PGDER


Changesalsooccurinthequalityoftherunoff.Rainfalllandingonimpervious

areaspicksuppollutantsandmovesthemtoreceivingstreamsandotherwaterbodies.

Dissolvedoxygen(DO)inthewaterisreducedinurbanrunoff,sometimestolevels

fataltofishandotheraquaticlife.Intheprocess,rainfalltemperaturesareelevatedas

thewatermovesacrosshotsurfaces.Thesechangestowaterqualityarereferredtoas

“non-pointsourcepollution”andare“theleadingsourceofwaterqualitydegradationin

Georgia”(GeorgiaStormwaterManagementManual,2001).(Figure3.1.d)

Receivingstreamsaresignificantlyaffectedbythequantityandqualityof

stormwaterrunoff.Runoffleavingthesiteathighervelocityandingreaterquantity

scoursthestreambedandcreateschangestothechannelprofile,erodingthestream

banksanddrasticallychangingaquatichabitat(seeChapter4).Withanadditional

pollutantload,lowerdissolvedoxygen,andelevatedwatertemperatures,habitat

degradationisamplified.Also,sincemorewaterrunsoffthesitesooner,thereisless

waterslowlypercolatingthroughthesystemtosupportbaseflowsinthestream,creating

anotherchallengeforaquaticspecies.

Withalloftheseimpactsinmind,lowimpactdevelopmentstrivestoachievea

naturalhydrologicalsystemthatcanmaintainandevenreducepre-developmentrunoff

rates.Secondarygoalsinclude:

uWaterqualityimprovement,

uRechargeoflocalgroundwateraquifer,

uStreamprotection,

uWetlandpreservation,

Figure 3.1d Algal Bloom in Lake Caused by Increased Levels of Nitrogen and Phosphorus from

Non-Point Source Pollution


uWildlifehabitatcreation,

uReducedurban“heatisland”effects,

uImprovedairquality,and

uEnhancedcommunityaestheticalappearance.

GreenGrowthGuidelines(usingLIDprinciples)balancesdevelopmentwith

inherentnaturalsitefeatureswhileenhancinglotyields,reducingdevelopmentcosts,and

encouragingdevelopmentandeconomicgrowth.

Theseguidelinesseektoeliminate,minimize,and

mitigatetherootcausesofdevelopment-generated

impactsatthesourcebyintegratingstormwater

managementmeasuresthatmakethedevelopment’s

landscapemoreecologicallyandhydrologically

functional.(Figure3.1.e)

3.2 Natural Processes for Stormwater Management

Naturalprocessesthatremovepollutantsfromstormwaterrunoffinclude:

(Figure3.2.a)

u Infiltrationandfiltration,

uSedimentation,

uDetentionandretention,and

u Interactionofstormwater

withvegetation.

Figure 3.1e Healthy Wetland Habitat

Photo Courtesy of: Chere Peterson

Figure 3.2a The Water Cycle

Photo Courtesy of: John Evans, USGS


3.2.1InfiltrationandFiltration

Infiltration,water’sentryintodry,unsaturatedsoil,isthemostnaturaland

directwaytohandlestormwaterincoastalGeorgia.Itallowsrainwatertoquicklyenter

groundwaterbeforeaccumulatingapollutantload.Inarainfallevent,thisunsaturated

areaisashrinkingzonebetweenrain-soakedsoilatthesurfaceandthegroundwater

saturatedsoilatthewatertable.Wherewatertablesareparticularlyhigh,asisthecase

inthecoastalregion,thisunsaturatedgapisoftenmeasuredinafewfeetoreveninches.

(Figure3.2.1.a)

Stormwatertendstopoolonthesurface,ordependingongrade,becomes

runoff.Standingwaterorsheetflowcaninfiltratesoil,saturatingitfromthetopdown.

Soiltypeandcompactiondeterminethespeedatwhichthewaterpercolatesormoves

downwardthroughthesoil.Thereby,soiltypesaffecttheinfiltrationprocess.Infiltration

isdependentonthepresenceofawell-drainedsoilcapableofacceptingwaterand

facilitatingitsentry.Soilswithlargeparticlesizes(sandsandgravels)havegreater

infiltrationrates.Certainclaysandorganicsoils(muchsmallerparticlesize)canalso

playmajorrolesinchemicalactivitiesthatremoveorimmobilizepollutants.Assessing

thesoils,topography,andhydrologyduringsitefingerprinting(seeChapter1)isessential

tofindingsuitableareasforinfiltrationaswellaspreventingordiscouragingitin

unsuitableareas.

Infiltrationismosteffectiveinareaswithhighergroundelevations.However,

lowerelevationscanbeusedforinfiltrationiftheypossesssufficientdepthtothewater

Figure 3.2.1a Illustration of Infiltration Process


tableandadequatesoilpercolationrates.Infiltrationcanonlybeusedinareaswhere

runoffisrelativelyfreeofpollutants(chemicals,harmfulbacteria)thatcanleachinto

andcontaminategroundwater.Greatcaremustbetakentoensurethatthewatertableis

protected.

Variousstormwatertechniquesuseinfiltrationtomanagestormwater.Infiltration

basinandtrenchesworkbetterinthecoastalplainthaninotherpartsofGeorgia.They

arelesssubjecttocloggingfromrunoffandhavehigherpercolationratesduetosandy

soils.Othertechniquesthatcombineinfiltrationwithvegetativefilteringinclude

bioretentionareas,enhancedswales,andraingardens.

3.2.2DetentionandRetention

Thesecondkeymethodofusingnaturalprocessestodelayrunoffisachieved

bytwosimilar,thoughintrinsicallydifferentprocesses:detentionandretention.These

processesmitigatethehydrologicandwaterqualityimpactsofstormwaterrunoff(due

toincreasedimperviousareaandincreasedrunoffvolume)bystoringandreleasing

stormwateratratessimilartoratesexistingpriortositedevelopment.Additionally,these

processescontroltherateofwaterflowthatreducesdownstreamchannelflooddamage

anderosion.(Figure3.2.2.a)

Whilethelinebetweendetentionandretentionmayblur,theoveralldifference

isclear.Detentionslowsthetimingofstormwaterrunofftolessenitsimpact,butdoes

notnecessarilyreducethequantityorimprovethequalityoftherunoffwater.Retention

simplyretainsanddisposesoftherunoffthroughnaturalhydrologicalprocesses.Since

Figure 3.2.2a Natural Detention in Wetlands

Photo Courtesy of: Tara Merrill


manyhydrologicalprocessescanalsoremovepollutants,retentioncanalsoimprove

waterquality.Retentiondelaysthereleaseofrunoffbyincorporatingorenhancingthe

naturalhydrologicalactionsthatpurifyandredistributerainwater.

LIDtechniquesprimarilyuseretentionstrategies.Withtheexceptionofexcess

runoffflowthatmayberoutedaroundtheretentionsystem,runoffisnotreleasedtoa

channel,pipe,orwatercoursebutisessentiallydisposedofonsite.Aretentionsystem

focusesonthereleaseofrunoffthoughevapotranspiration(lossofwaterfromthesoil

byplantsviagasexchange)andinfiltration.Inmanyretentionpractices,mostnotably

infiltrationtrenches,thevoidspacesbetweenstonefillallowsforlocalizedmovementof

waterbelowthesoilsurface,eventuallyassistingthereturnofwatertothesoil.

Certainretentionpracticesplacemoreemphasisonstorageorimpoundmentthan

others.Adryretentionpondisagoodexample.(Figure3.2.2.b)Itisnormallyempty

andisfilledduringarainfallevent.Then,itdisposesofitsvolumethroughnatural

hydrologicalprocesses,includinggroundwaterrecharge,ratherthanbydirectdischarge

intoawatercourse.

Someretentionpracticesimpactwaterqualitymorethanothers.Forexample,in

amultiplecellpond,constructingsmallbermsinasequenceacrossthepondslowsrunoff

flow,promotessedimentremovalandallowsthestormwatertointeractwithvegetation

plantedonthebermsandalongthesides.Thiscombinationoftechniquesimproveswater

qualityinseveralways.Thefirstcellservesasasedimentforebayallowingsoilparticles

Figure 3.2.2b Dry Retention Pond

Photo Courtesy of: Dan Fischer


todropoutofthewatercolumn.Thevegetationshadesthewatersurfaceandprovides

significantsurfaceareapromotingbiologicactivitythatservestocleanupthewater.

Detentionencompassesawiderangeoftechniquesincludingstorageinponds,

swales,andnaturalforestedwetlands.Thewaterthatentersadetentionsystemcanbe

processedforfurthertreatmentbutiseventuallyreleasedtoawatercourse.

Detentionmaycreatesomeimprovementstowaterquality.Forexample,simply

holdingthewaterforadayorsoinareservoirallowssomesedimentationandseparation

tooccur.Inlow-orno-flowconditions,particulatesnaturallydropoutofthewater

columnandsomesubstances,suchashydrocarbons,risetothetop.Somedetention

ponddesignsareintendedtohavemoreimpactonwaterqualityduringtheholding

period.Wetponds,forexample,permanentlyretainwaterandsupportvegetationthat

canassistinwaterqualitytreatment.Pondvolumeisprimarilyreducedbythereleaseto

awatercourse,ratherthanre-integrationintotheenvironment.Wetpondsaregenerally

excavatedtotheseasonalhighwatertable,creatinganadditionalhydrologicalinteraction.

Pondsthatincorporatepermanentpoolsofwateraredesignedtousethebiologicalaction

ofplantsandorganismstotrapandthentreatpollutants.

Forbothdetentionandretention,someformofimpoundmentisrequired.For

example,vegetationandforestfloororganicmatterinabufferzonemayprovide

sufficientchemicalorbiochemicalactiontotreatthewaterduringitsimpoundment.

Simpletemporarysurfacepoolingmayalsocontributetoretention.

3.2.3 Sedimentation

Thesettlingoutordepositionoferodedsoilsisreferredtoassedimentation.By

volume,sedimentmakesupthelargestpercentageofpollutantsenteringstreams,ponds

andlakes.Preventingerosionisthefirstandbestlineofdefenseagainstsedimentation.

Erosionbeginsinarainfalleventwhenwaterstrikesanexposedsoilsurfaceor

washesacrossitastherunoff,looseningsoilparticlesandsuspendingtheminthewater.

Sedimentationbeginswhenrunoffcarryingthisloadoferodedmaterialslows,allowing

particlestofallfromthewatercolumn,anddeposititonthestreambedorbank.

Sedimentationoccursgraduallywithheavierparticlessuchassanddropping


outfirstsincetheyrequirehigherflowvelocitytoremainsuspended.Thelighterthe

soilparticle,themoretimeitneedstodropoutofthewatercolumn.Asflowvelocities

continuetodecrease,thesmallest,lightestparticles(clay)settleoutintheslow-moving

areasofthewater.(Figure3.2.3)

Whilesoillossthrougherosioncansignificantlydamagethelandscape,its

depositionassedimentintostreamscanalsohavethefollowingadverseeffects:

u Increasedfloodingduetoreductionofchannelsizebysedimentdeposits,

uEventualchangeinthepathornatureofthestream,

uFilledsecondarychannels,cutofffromthemainstream,

uCloggedfloodcontroland/ortreatmentstructures,

uReducedfishspawningareasfromchangesinstreamcharacteristics,

uModifiedfishpopulations,ofteninfavoroflessdesirablespecies,

uReducedaquaticinsectcommunities,especiallythemorebeneficialspecies,asa

resultofsedimentation-inducedchanges,

u Impairmentordestructionofterrestrial(shore)habitatsbecausesedimentclogs

anexistingchannelandmakesthestreamspread,causinginundationofsensitive

wildlifehabitatalongthestreambank,

Figure 3.2.3 Extreme Sedimentation of River

Photo Courtesy of: The University of Florida Center for Aquatic and invasive Plants


uReducedrecreationalopportunities,

uAdverseimpactoncommercialfisheries,

uReducedstreamnavigability,and

u Increasedcostsofkeepingharborsandmarinasusable.

InthestormwaterpracticesdiscussedlaterinthisChapterandinthebank

stabilizationtechniquesdiscussedinChapter4,thecaptureofsedimentson-sitebefore

reachingstreams,riversandotherwaterbodiesisemphasized.

3.2.4InteractionofStormwaterwithVegetation

Vegetationplaysanimportantroleinimprovingwaterquality.Whilesoil

microorganismsandsoil-basedchemicalreactionsremoveavarietyofnutrients,

pathogens,andheavymetals,vegetationassiststhisprocessinanumberofways:

(Figure3.2.4.a)

uSlowsrunoffflow,

uFilterslargeparticulates,

uProvidesconsiderableamountsofsurfaceareaonwhichpollutant-removing

microorganismsthrive,

uUseswaterfordirectmetabolicorstoragepurposes,

uDecomposesintoanorganicsoillayeractiveinneutralizingheavymetals,

uReleasesoxygenintothesoil,improvingwaterquality,and

uProvidesshadeforthewatersurface,reducingwatertemperature.

Figure 3.2.4a Buffered Stream Flowing Through Farmland

This buffer will slow nutrient loaded runoff before it reaches the stream.

Photo Courtesy of: Landstudies.com