3 activities includethefollowing Potential hydrologic impactsfrom development to thereceiving watersaswell. conveying theresultantland surface, sedimentload increased canalso runoff result inerosion ofthe these pollutantstothereceiving waterbodies.The and otherpollutantsinitsflow path,andcarries runoff picksupsediment amount oftime.This and thisrunoff reaches rivers andlakesinashorter areas generateagreater amountofstormwater, down therateofflow. Because ofthis,developed stormwater runoff. Vegetation nolongerslows soak intotheground, resulting inanincrease in parking areas. Precipitation areas cannolonger fallingontheseimpervious suchasroads, rooftops, surfaces, driveways,and and introduces impervious movement ofwaterofftheland. Development removes natural vegetation Development activitiescanalterthenaturalhydrologic cycle andthe 3-1. Hydrologic Impacts plan forprotection andrestoration ofwaterquality. and tohelpunderstandtheinterconnectedness ofactivitiesinawatershedto to betterunderstandthebasicconcernsandcausesofstormwaterpollution and traditionalstormwatermanagementconcerns.Thischaptermaybeuseful quantity andquality, sources ofstormwaterpollutants,watershedplanning, management. Amongtheseare therelationships between landuseandwater Chapter 3describesseveral fundamentalconceptsofstormwater Management Fundamentals ofStormwater Chapter 3 Adapted from Minnesota Stormwater Manual, 2006. ● ● ● ● ● ● ● ● ● ● ● ● Increased flooding Increased frequency ofbank-full&nearevents Increased velocities runoff Increased discharges peakrunoff Increased volumes runoff Changes toStream Flow 3 : commercial parking lotduringarainstormin Stormwater flows intoacatchbasinfrom a Concord, New Hampshire.

3-1. Hydrologic Impacts 3-2. Water Quality Impacts Changes toStream Geomorphology quality impactsofthesechanges inthelandscapethrough environmentally discharged toreceiving waters.Fortunately, there are ways tolessenthewater These pollutantsarerunoff and pickedupandcarried by stormwater would notbepresent intheundeveloped landscape (CTDEP, 2004). variety ofotherpollutantsare introduced intotheenvironment, allofwhich household products, includingpetwaste,septicsystemleachate, trash,anda Fertilizerstreat surfaces. stormwaterandincreases theimpervious andother This decreases theamountof vegetated areas thatare available tonaturally roads, highways,andparking lots,whichare saltedandsandedforsafety. for homes and businesses. Transportation infrastructure is expanded including landscape. Forests andother undisturbedlandsare tomakeroom converted as socialbehavior. Asthepopulationgrows, manychangesoccurinthe quality comefrom changesinlanduseandassociatedactivitiesaswell alsoaffectswaterquality.stormwater runoff Thepollutantsaffectingwater In additiontothewaterquantityimpactsfrom development activities, 3-2. Water Quality Impacts Changes toAquatic Habitat ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● Lower baseflowsweatherflows) (dry Decline inabundanceandbiodiversity offishandbenthicorganisms Increased stream temperatures Reduced baseflows Loss ofpool-rifflestructure riparian vegetation loss,sedimentdeposition Degrading ofhabitatstructure -channelscour, streambank erosion, changesinchannelgeometry in further Isolation channelfrom itsnaturalfloodplain, resulting oftheprimary Increased floodelevation Loss ofriparianvegetation &canopy Other changesinstream bedduetosedimentation flows orsedimentload elevation duetosedimentdeposition),resulting from changesin Stream degradation(downcutting) oraggradation(riseinchannel Changes inflow velocities Stream widening&bankerosion water withdrawalandstream baseflows. groundwater recharge, allofwhichmakelesswater available for drinking (alignment, cross-section geometry, streambed composition) andreducing the frequency andmagnitudeoffloods,altering stream channel morphology flooding, ithasproven tobedevastating todownstream waters by increasing Although thisisanefficientwayto remove waterquicklyand prevent on-site detention typeBMPs toreduce dischargerates(CEI,2003). peakrunoff tothe nearestsystems, thatdischargerunoff receiving water, orimplementing implementing managementtechniques,suchascurb andgutterpiping from asiteasquicklypossibletoreduce on-siteflooding.Thishasmeant Conventional stormwatermanagementhasfocusedonremoving stormwater 3-3. Concerns with Conventional Stormwater Management quality. leaving asite,butcanalsoreduce thatcanthreaten pollutantrunoff water management practicescannotonlyreduce thevolume andvelocity ofrunoff sensitive siteplanning.Implementing bettersitedesigntechniquesandbest ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● Swimming beachclosures duetobacterial contamination. Shellfish bedclosures duetobacterial result in: Potential waterqualityimpactsfrom development activitiesmay metals, nitrates,andsalt. Contamination ofgroundwater withsolubleorganicchemicals, as warmwatercanholdlessoxygen thancoldwater. shade from decreased ripariancanopy, leadingtoreduced DOlevels Increased andlossof surfaces temperatures duetowarmimpervious material –leadingtooxygen deprivation ofaquaticorganisms. oxygen demand(BOD)ofthewaterfrom biodegradableorganic Depleted dissolved oxygen (DO)levels duetoincreased biochemical contaminants. compounds, pesticides,andother Toxicity from ammonia,metals,organic in runoff. Nuisance algalgrowth from excess nutrients wildlife waste. in combinedseweroverflows (CSOs), petand Pathogenic frombacteria/viruses fecal material contamination. Portsmouth, New Hampshire. Turbidity inHodgson Brook,

3-3. Concerns with Conventional Stormwater Management 3-3. Concerns with Conventional Stormwater Management Flooding ofNew Hampshire’s Exeter and Squamscott Rivers, April, 2007. Officials. Practices toControl Nonpoint Source Pollution AGuide forCitizensand Town New Hampshire ofEnvironmental Department Services. September, 2006. Minnesota Stormwater Steering Committee. May 4,2006. Nonpoint Source Pollution Management Manual theClean Water Toolkit. Massachusetts ofEnvironmental Department Protection. Massachusetts Stormwater Quality Manual. ofEnvironmentalConnecticut Department Protection. 1. March 2003. Alternative Stormwater Management Methods Planning andGuidance. Part Comprehensive Environmental Inc. 2003. Aquatic Systems. Center for Watershed Protection. 2003. Chapter 3References stormwater, andtocontrol quantityandqualityimpactsclosetoitssource. including low impactdevelopment techniques,tominimize thegenerationof natural undisturbedareas. Chapter6offersguidancetoaddress sitedesign, andtoretain surface includes objectives tolimitincreased impervious January 2004. Watershed Protection Research Monograph No. 1.March

stormwater tomeetwaterqualitystandards Chapter 5,New Hampshire’s approach tomanaging discussion oftheAntidegradationProvisions in stormwater management.Aswillbeseeninthe measures tothisconventional approach to Subsequent chaptersof Volume 1present alternative atthesource.of asiteandmanagingstormwater place by maintainingandworking withthehydrology reduce generated theamountofstormwater inthefirst runoff isto key toeffective managementofstormwater their placeinstormwatercontrol andtreatment, treatment. Althoughend-of-pipepracticesstillhave rely onconveyance efficiencyandend-of-pipe management are largelybecausethemethodsused oftraditionalstormwater These shortcomings 2004.

City ofNashua, New Hampshire Impacts ofImpervious Cover on Minnesota Stormwater Manual. Connecticut Best Management the