listed in Table discussedbelow. 2-1isfurther in Chapter4–Designing Best Management Practices. Eachofthecriteria management practices.Thoseadditionalcriteriaare provided foreachBMP criteria, otherBMP-specificcriteriaalsoapplytothedesignofstormwater descriptions provided inthetextthatfollows. In additiontothesedesign oftherequirementsA summary isincludedin Table 2-1,withdetailed management practices: This Chapteraddresses thefollowing designcriteriaforsizingstormwater projects. 1500. This Manual recommends theseparametersforalldevelopment for sizingBMPs tomeettheserequirements are stipulatedintheEnv-Wq For projects thatmustcomplywiththeAoT Regulations, specificparameters provide forstream channelprotection. stormwater qualitytreatment, usestormwaterforgroundwater recharge, and Best Management Practices (BMPs) rates,provide tocontrol peakrunoff development. Landdevelopment projects shouldemploy sitedesignand New Hampshire toprotect thestate’s watersfrom theadverse impacts of This Chapterpresents designcriteriaforsizingBMPs inthe State of Design Criteria Chapter 2 ● ● ● ● ● ● ● ● ● ● ● ● ● ● Peak Control Channel Protection (CP) Undisturbed Cover (UDC) Effective Cover Impervious (EIC) Groundwater Recharge Volume (GRV) Water Quality Flow (WQF) Water Quality Volume (WQV)

Ch a p t e r 2 2-1. Water Quality Volume (WQV) 1 Appendix Aprovides rainfalldataforNew Hampshire, forusewiththesedesigncriteria. Peak Control (CP) Protection Channel EIC &UDC (GRV) Volume Recharge Groundwater Flow (WQF) Water Quality (WQV) Volume Water Quality Criteria Design

WQV =(P)(Rv)(A) rates forthe10&50-yr, 24-hrstormevents. Post-development development levelortothe1-yr, 24-hrpre-developmentlevel. Control the2-yr, 24-hrpost-developmentpeakflowrateto½ofthe2-year, 24-hrpre- If the2yr, 24-hrpostdevelopmentstormvolume development level. then: controlthe2-year, 24-hourpost-developmentpeakflowratetothe2-yr, 24-hrpre- If the2yr, 24-hrpost-developmentstormvolume %UDC =areaofundisturbedcover/totaldrainagewithinaprojectX100 %EIC =areaofeffective imperviouscover/totaldrainageareawithinaprojectX100 R A A Q=thewaterqualitydepthininchesWQV/A GRV =(A CN=waterqualitydepthcurvenumber S=potentialmaximumretentionininches(1000/CN)–10 Ia=theinitialabstraction0.2S Variables neededforexhibits4-IIand4-III: WQV=waterqualityvolumecalculatedinaccordancewithDesignCriteria q A WQF =(q I=percentimperviouscoverdrainingtothestructureconverteddecimal Rv=unitlessrunoff coefficient=Rv0.05+0.9(I) D C B A Hydrologic Group soilgroup,asfollows: development after P =1”ofrainfall =1000/(10+5P+10Q–10[Q above form P =1”ofrainfall u =totalareadrainingtothedesignstructure =totalsiteareadrainingtothestructure I I d u =thetotalareaofeffective impervioussurfacesthatwillexistonthesite )(R =unitpeakdischargefrom stormwater pollutantson anaverage annualbasis.The recommended WQV a rainfallevent thatshouldbecaptured andtreated toremove themajority of The Water Quality (WQV)Volume runoff from istheamountof stormwater Criteria 2-1. Water Quality Volume (WQV) =thegroundwaterrechargedepthbasedon )(WQV) d )

Table 2-1.SummaryofDesignCriteria

peak

discharge

rates TR-55 exhibits4-IIand4-III Description 2 +1.25(Q)(P)]

can 0.00 0.10 0.25 0.40 Rd (inches)

not

does increase exceed does notincrease 0.5

)

pre-development 1 USDA/NRCShydrologic

duetodevelopmentthen: duetodevelopment

peak discharge

event. foreachrainfall of stormwaterrunoff capturing andtreating thefirstone-inch is onaverage, roughly equivalent to capturing 90percent oftheannualrunoff load. In theNortheastern United States, consequently 90percent ofthepollutant and treat and 90percent ofannualrunoff is equaltothestoragerequired tocapture Rule”, inwhichthewaterqualityvolume removals, includingthe“90Percent developed toachieve higherpollutant Other waterqualitysizingmethodswere increases. drops offconsiderablyassiteimperviousness research hasshown thatpollutantremoval achieved usingthehalf-inchrule criterion throughout mostoftheUnited States. However, more recent runoff wasadoptedasawaterquality volumesizing of thefirsthalf-inch carries 90percent ofthepollutionfrom astorm(Novotny, 1995),treatment Based onearlystudiesinFlorida thatdeterminedthefirstflushgenerally ‘cleansed’ thecatchment. more offlater, pollutedthanthestormwaterthatruns aftertherainfallhas offanarea,that initiallyruns often referred toasthe‘firstflush’ willbe increasing thepollutantloadtoreceiving waters.Usually, thestormwater land useincreases. Thesematerialsare runoff, thenwashedoff byrainand wetlands. Pollutant increase depositsonthelandsurface astheintensityof Development impactsthewaterqualityofstreams, ponds,lakesand Rationale Where: WQV =(P)(Rv)(A) rainfall. WQV shouldbecalculatedusingthefollowing equation: equivalent tocapturingandtreating from the90thpercentile therunoff ofall associatedwiththefirstone-inchofrainfall,whichis is thevolume ofrunoff

A =totalsitearea drainingtothestructure form coverI =thepercent impervious drainingtothestructure, indecimal coefficient,Rv=0.05+0.9(I) Rv =theunitlessrunoff P =1inch

2-1. Water Quality Volume (WQV) 2-1. Water Quality Volume (WQV) Figure 2-1. Exhibits 4-IIand4-III:Unit Peak Discharge forNRCS RainfallDistributions and steps: Discharge Method. WQF shouldbecalculatedusingthefollowing equations the Natural Resource(NRCS), Service Conservation TR-55 Graphical Peak BMP descriptionsinChapter4).The WQF iscalculated usingthe WQV and (e.g., Treatment Swales, Pre-treatment Swales, Flow-Through Devices –see with the WQV, forsizingflow-based treatment andpre-treatment practices The Water Quality Flow(WQF) isusedtodetermineaflow rateassociated Criteria 2-2. Water Quality Flow (WQF) 6. 5. 4. 3. 2. 1. equation: Compute thewaterqualityflow(WQF) usingthefollowing (reproduced below) basedontheproject’s location. Read theunitpeakdischarge(qu)from TR-55 Exhibits 4-II or 4-III Calculate initialabstraction(Ia) usingthefollowing equation: following equation: Calculate potentialmaximumretention (S)ininchesusingthe in Chapter3of TR-55. Compute thetimeofconcentration(tc)usingmethodsdescribed thewaterqualitydepthtoaflowCN usedtoconvert rate. itisarepresentativeselected basedonthelanduseandsoiltype.Rather NOTE thatthisCNisnotthesameassubcatchment’s CNwhichis Where: equation: Compute theNRCS Curve Number (CN)usingthefollowing WQF =(q Ia =0.2S S =(1000/CN)–10 A =totalarea drainingtothedesignstructure section) WQV =waterqualityvolume (calculationsshown inprevious Q =thewaterqualitydepthininches WQV/A P =1inch CN =Runoff Curve Number CN =1000/(10+5P+10Q–10[Q

u )(WQV)

2 +1.25(Q)(P)]

0.5 )

2-2. Water Quality Flow (WQF) 2-3. Groundwater Recharge Volume (GRV)

R Rationale runoff volumes. runoff events andcanalsobeusedtopredict flows associatedwithsmallerstorm can more appropriately estimatepeak water qualitydesignstorm,sinceit the peakflow associatedwiththe preferable methodforcomputing with thewaterqualityvolume isthe Discharge Method inconjunction the NRCS, TR-55 Graphical Peak less storagevolume. Theuseof higher flow rates,thereby requiring since theyare designedtotreat rather thanwaterqualityvolume, designed basedonpeakflow rate, devices are more appropriately treatment swalesandflow-through Some treatment practicessuchas A Where: GRV =(A soils andthefollowing equation: (GRV) shouldbebasedonthesite The Groundwater Recharge Volume a result oftheproposed development. development groundwater recharge as minimizing thelossofannualpre- protect groundwater resources by recharge volume criterionisto The purposeofthegroundwater Criteria Volume (GRV) 2-3. Groundwater Recharge soilgroup, asfollows: USDA/NRCS hydrologic depthbasedonthe development willexistonthesiteafter that surfaces impervious I d =thetotaleffective area of =thegroundwater recharge

I )(R d )

infiltrate stormwaterandthat applicable tosystemsthat Additional requirements runoff). channels thattreat roadway to forested buffers,andgrass sheetflowlot runoff, discharge that treat rooftop orparking reduce (e.g.,filterstrips runoff maximum extentpracticableto should beimplementedtothe practices and non-structural be provided totreat thefull WQV management systemsshouldstill reduced. However, stormwater volume requirement maybe areas. In theseareas, therecharge geology, andurbanredevelopment clays, sitesinareas ofkarst include sitesunderlainby marine recharging groundwater. Examples the useofinfiltrationsystemsfor On somesites,existingsoilsorotherconditionsmayseverely constrain The following criteriashouldalsoapply: than 0.5acre andisnotfrom ahigh-loadarea. protection area, unlesstherecharge systemreceives stormwaterfrom less waterthatdefinesasupplyintake or within100feetofasurface (c) regulations (Env-Wq 1500). should beinaccordance withSection 2-4andtheAlterationof Terrain (b) depth shouldbecomputedbasedonthearea ofeachsoilgroup present. (a) Hydrologic Group No recharge isallowed withinthesetbackareas provided in Table 3-3 Infiltration ratesfordesigning Groundwater recharge practices If more thanonesoiltypeispresent atthesite,aweighted recharge D C B A (inches) R 0.00 0.10 0.25 0.40 d

2-3. Groundwater Recharge Volume (GRV) 2-4. Design Infiltration Rate Rationale descriptions includedinChapter4. would contributetogroundwater recharge are listedwiththespecificBMP procedures forselectingadesigninfiltrationrate. practices, andgroundwater recharge practices).This sectionoutlinesthe that rely onstormwaterinfiltration(e.g.; infiltration practices,filtering Chapter 4presents informationonanumberofBest Management Practices 2-4. Design InfiltrationRate occurring duringpre-development conditions. fromvolume multipleevents ofrunoff istoapproximate theannual recharge Thecumulative effectofcapturingand infiltratingtheinitial surfaces. the systemshouldcapturerunoff new impervious from the andinfiltrateall and roofs; forsmallstormevents thatgeneratelessthan0.4inchesof runoff, should capture runoff from all andinfiltratethefirst0.4inchesof pavements Hydrologic Group stormevent, thestormwatersystem A,thenforevery onanarea surfaces if asitedevelopmentwithsoilsin creates impervious For surface. result from thatimpervious that wouldotherwise example, and infiltratedforeachstorm,inorder tomakeupforthelossof recharge (R For eachsoilshydrologic group, theNHDESconsidersrecharge depth stateswithsimilarclimatesandaverage annualprecipitation.northeastern each hydrologic soilgroup. Thismethodhasbeenadoptedinseveral other NRCS Soil Survey mapsandestimatesofaverage annualinfiltrationratesfor existing soils.Thehydrologic soilgroup approach usesthewidelyavailable andtheprevailingdevelopment infiltrationcapacityof siteimperviousness recommended approach forcalculatingtheGRV isafunctionofpost- average annualrecharge thatoccursonasitebefore itisdeveloped. The The objective ofthegroundwater recharge criterionistomimicthe stormwater managementpractices. protection, andpeakflow control), andthusthe overall size andcostof must bemanagedtomeetotherdesigncriteria(i.e.,waterquality, channel groundwater recharge conditionscanalsoreduce that thevolume ofrunoff filtering mechanismto “clean” water. surface Maintaining pre-development table levels, stream baseflow, and wetland moisture levels andtoprovide a The objective ofthegroundwater recharge criterionistomaintainwater appropriate BMPs. from smallerprecipitationall runoff events, iscaptured andinfiltratedusing storm event. Under from this approach, of runoff larger storms, and a portion foreachindividual surfaces from thepost-developmentof runoff impervious annual The groundwater recharge criterionisintendedtomaintainpre-development d ) the amount of runoff that must be captured from an impervious surface surface thatmustbecaptured) theamountofrunoff from animpervious groundwater recharge volumes by capturingandinfiltratingaportion Chapter: descriptions provided inChapter4,andtheguidelinesdiscussedthis should beobtainedandevaluated relative toapplicableregulations, theBMP development site.Information regarding thefollowing seven parameters screening parameters,toidentifysite-specificcharacteristicsoftheproposed The initialstormwaterinfiltrationscreening evaluation involves seven » verification. the siteandidentifieswhere fieldwork maybeneededforsubsequentfield screening establishesthefeasibilityofinstallationinfiltrationmethodson determines potentiallocationsonthesiteforinfiltrationfacilities. Initial Initial screening identifiesthepotentialforusinginfiltrationmethodsand Site Feasibility Confirmation Testing INITIALSCREENINGPARAMETERS 7. 6. 5. 4. 3. 2. 1. Presence offloodplains. endangered specieshabitat). (including wetlands andthreatened or Presence ofsensitive ecological habitat • • to: contamination, includingbutnotlimited areas withidentifiedsoilorgroundwater Presence ornearby proximity toknown Protection Areas). Areas, and Water Supply Intake Setback areas, Groundwater Protection groundwater areas (Water Supply Well Presence ofpotentiallyvulnerable Potential depthtobedrock andseasonalhighwatertable(SHWT). usefulintheinitialscreening process.this willbevery 2006 (ormostrecent), hasbeencreated forthedeveloped sitearea, Mapping Standards forNew Hampshire and Vermont, December New England (SSSNNE)Special Publication No. 3,Site-Specific Soil defined inaccordance withthe Society of Soil Scientistsof Northern Site hydrologic soilgroups orKsatvalues. If asitespecificsoilmapas Site topographyandslopesgreater than15%. adjacent totheproject parcel. underground storagetankswithinor Existing orclosedremediation sites,or

2-4. Design Infiltration Rate 2-4. Design Infiltration Rate potential sitesasaresult oftheinitialscreening: The following informationshouldbe recorded forfield verificationofthe protocol isdescribedbelow. anddepth tobedrock. Astandardizeddepth totheSHWT testpit/boring toverify soilinfiltrationcapacitycharacteristics andtodetermine performed For existing soils,naturalorman-made,testpitsboringsshouldbe findings from initialscreening. Sites anddepthtobedrock toverify shouldbetestedfordepthtoSHWT infiltration. development sitethathave beenconsidered potentiallysuitablefor screening investigation process ofspecificareas includesfurther ona Field verification ofinformationcollectedduringtheinitialsitefeasibility » FIELD VERIFICATION 2. 1. a. A legiblesiteplan/mapthat: The dateordatesthedata were collected. Is drawn toscale.

infiltration andprovide the required information todesignthefacilities.The consists ofsoilsevaluation toconfirm thatthelocationsare suitablefor At specificlocationsidentifiedfor stormwaterinfiltrationfacilities,thisstep » EVALUATION OFSPECIFIC INFILTRATION AREAS and consistency. for soilcolor, texture, structure prior toconductingevaluations frozen, itshouldbethawed NOTE: If thematerialis 3. i. h. g. f. e. d. c. b. i. h. g. f. e. textural classwithrock d. c. b. a. following informationforeachsoilhorizon orlayer: to thesoildatadeterminedabove, soilprofiles shouldincludethe Sampling Soils” (USDANRCS 2002,ormostrecent). In addition interpretations foundinthe“USDAField BookforDescribing and accordance withthedescriptive procedures, terminology, and It isrecommended thatsoilprofile descriptionsbewrittenin areas ifwithin100feetofthedevelopment site. groundwater protection areas, andwatersupplyintakeprotection Shows thelocationsofwatersupplywells andsetbacks, Shows thelocationsofwetlands asfielddelineatedandsurveyed. Shows thelocationsofalltestpits/boringsincludedinreport. Shows tothesite. allfloodplaininformationthatispertinent contour lines,andhighlightsareas withslopesover 15%. Shows thepercent anddirection oflandslopeforthesiteor Includes andhorizontal apermanentvertical reference point. Shows allareas ofplannedfillingand/orcutting. Illustrates theentire development site. soil. bedrock, ordisturbed soil, groundwater, Occurrence ofsaturated Soil boundary. size. Root abundanceand Soil consistence and shape. Soil structure, grade size, fragment modifiers Using theUSDA contrast. Soil redoximorphic feature color, abundance,size, and Munsell soilcolornotation. Thickness, ininchesordecimalfeet.

Infiltration 2,500 sformore Infiltration Basins Less than2,500sf Infiltration Basins Table 2-2.MinimumNumberofTest Pits/BoringsRequired Textural Triangle, soil

Facility

Trenches

100 LF. 1 200 LF–300=3tests 100 LF–200=2tests 0 LF–100=1test 10,000 sf. 1 30,000 –40,000=4tests 20,000 sf–30,000=3tests 2,500 sf–20,000=2tests 1 test

additional additional Minimum NumberofTest Pits/

Borings Required

test test for for every every additional additional

2-4. Design Infiltration Rate 2-4. Design Infiltration Rate The following informationshouldbe recorded forthisevaluation: infiltration facilityasdiscussedabove. minimum numberoftestpitsand/orboringsshouldbeprovided foreach soil mapanddeterminewhichseriesare atthelocationofpractice. considered conservative. To selectadefaultrate,firstusethe Site Specific easier toobtain,however thedesignershouldnotethatthismethodis Default values maybeusedfornative materialsonly. Default values maybe A. Default Rate (inches) Depth 3. 2. 1. methods usedtodeterminethedesigninfiltrationrate: The informationforoneofthefollowingresults andsupporting d. c. b. a. A legiblesiteplan/mapthat: steps. All theinformationobtainedininitialscreening andfield verification Shows distancetowetlands. Shows thelocationsofalltestpitsandborings; Illustrates thelocationsofproposed infiltrationfacilities; Is drawntoscaleorfullydimensional; (micrometers/second) Ksat

(inches/hour) Ksat

protocol: withthefollowing(Ksat) testshouldbeperformed testing for stormwaterBMPs asaturatedhydraulic conductivity For thepurposesofdeterminingadesigninfiltrationrate B. Field Measured InfiltrationRate of safety2. by area ifmore thanonesoilseriesispresent. Lastly, applyaminimumfactor of values. Select theslowest value forthedefaultrate.Use aweighted average by theUSDANRCS. The Ksatforagiven layerreported typicallyhasarange proposed bottomofthepracticeusingPhysical Soil Properties reported Second, determinethelimitinglayer (slowest Ksat)reported beneaththe ● ● ● ● ● ● 2. 1. ● ● ● ● ● ● Result: designinfiltrationrate=0.06inchesper Apply afactorofsafety Results: Thelimitinglayer, atorbelow 24”,is0.06inchesperhour. Select theslowest value reported below thebottomofpractice: If aGuelph Permeameter See Table 2-4below forthe The testshouldbeconducted the finallocationofinfiltrationfacility. The testlocationshouldbewithinthefootprintof scientist, aprofessional geologist,oranengineer. by soil aqualifiedprofessional suchasacertified and/orsupervised The testshouldbeperformed 2-3 forthetestingprotocol. diameter; oraBorehole Infiltration test,see Table 3385), where theinnerringisatleast12inchesin Permeameter; aDouble-Ring Infiltrometer (ASTM Permeameter; aCompactConstantHead The Ksatshouldbemeasured witha Guelph hour/2 =0.03inchesperhour. test location. minimum of3timesforeach a test shouldbeperformed Permeameter testisused,the or CompactConstantHead locations minimum numberoftesting facility. elevation oftheinfiltration at theproposed bottom

2-4. Design Infiltration Rate 2-4. Design Infiltration Rate Figure 2-2.Borehole Infiltration Test Setup Table 2-3.BoreholeInfiltration Test Protocol Infiltration Testing Requirements be recognized. The following limitationsondischargingstormwater intotheground should » LIMITATIONS &CONSIDERATIONS

section B.above. fill material: be usedforinitialdesignproposed The following protocol shouldonly C. LabMeasured InfiltrationRate ● ● ● run thefieldtestinaccordancerun with the designrate. To confirmtherate, soil shouldbefieldtestedtoconfirm ● design infiltrationrate. Ksat by 2.0andusetheresult asthe safety by dividingtherepresentative ● Mold Permeameter”; Using aRigid-Wall, Compaction- Conductivity ofPorous Material for Measurement ofHydraulic D-5856, “Standard Test Methods Soils (ConstantHead)” orASTM Method forPermeability ofGranular ASTM D-2434,“Standard Test with testmethodsdescribedin ● ● ● ● ● ● ● example below: measured infiltrationrate. See of safety2tothefield the procedure. and reviewer fullyunderstand This istoensure thatthetester the infiltrationtestingmethod. and includeadescriptionof should bedocumented, located onplansby survey. Apply aminimumfactor Final infiltrationtestingdata Test locationsshouldbe Once thefillisinplace, Apply aminimumfactorof The Ksatshouldbemeasured

with, aninfiltrationfacility: The following shouldbeconsidered toenhancetheuseof, oravoid problems Practices shouldnotbeinstalledinthefollowing areas: Infiltration practices, Unlined Filtering Practices, and Groundwater Recharge 1. 8. 7. 6. 5. 4. 3. 2. 1. standards developed pursuanttoEnv-Or 600; Within areas havingsoilabove site-specificsoil established inEnv-Or 603.03 above theambientgroundwater qualitystandards Within areas thathave contaminantsingroundwater stormwater comesfrom ahigh-loadarea; Within groundwater protection areas, where the need tobeevaluated forthedirection ofgroundwater flow. high watertable.Largesites considered for infiltrationsystemsmay Groundwater monitoringwells canbeusedtodeterminetheseasonal the reduction testing. isconfirmed by further BMP, orthesoilhasbeenamendedto reduce theinfiltration rateand per hour) unless the stormwater has first been treated by an acceptable Within areas havingsoilswithinfiltrationratesgreater than10inches underdrain system. be required ifthesefacilitiesare designedwitha“daylighting” or permeablepavement, nominimuminfiltrationrateshould inches perhour. For filteringpracticessuchasabioretention area infiltration rateislessthan0.50 Within areas where thedesign forces from causinginstability; carefully engineered toprevent seepage than 15%,unlessthesystemhasbeen Within areas havingslopesgreater vehicles; transferred to dispensed orotherwise under RSA146-A,where gasolineis aboveground storagetanksregulated tanks regulated underRSA146-Cor from areas withunderground storage In anyarea, ifthestormwatercomes pursuant toEnv-Or 600; contaminants insoilabove site-specificsoilstandards developed In anyarea, ifthestormwatercomesfrom areas thathave (manmade soilspresent) Infiltration (no manmadesoilspresent) Infiltration (manmade soilspresent) Infiltration Basins (no manmadesoilspresent) Infiltration Basins Table 2-4.MinimumNumberofTest Locations Facility Trenches Trenches

50 LFoftrench 1 testsforeach 100 LFoftrench 1 testsforeach 1,000 sfofbasinarea 1 testforeach 2,500 sfofbasinarea 1 testforeach Minimum Numberof Test Pits/Borings Required

2-4. Design Infiltration Rate 2-5. Effective Impervious Cover (EIC) and Undisturbed Cover (UDC) 1 under theantidegradationrequirements. sites analysis. Thisisinformallycalledthe“1065 Rule.” It isproposed thateligible development sitestobeusedasasurrogate toconductingpollutantloading (%EIC) maximumanda65%undisturbedcover (%UDC)minimumfor coverNHDES hasproposed atargetof10%effective impervious Requirements, asdiscussedin Volume 1. are usedtodeterminetheapplicabilityofproposed Antidegradation of EffectiveCover Impervious and Undisturbed Cover. Theseparameters Volume 1,Chapter5oftheStormwater Manual describestheconcepts 2-5. Effective Cover Impervious (EIC)and Undisturbed Cover The “1065 Rule” pertains to Tier 2 – High Quality Waters that have useable assimilative useable have Quality that High – Waters to 2 Tier pertains Rule” “1065 The 2. 4. 3. 1 thatmeetthe1065Rule aloadinganalysis donothave toperform (UDC) for infiltrationisagoodcandidatedispersedapproach for infiltration.Adevelopment sitewithmanyareas suitable One ormore areas withinadevelopment sitemaybeselected service afterthecontributingareas areservice fullystabilized. device. In addition,infiltrationfacilitiesshouldonlybeplacedinto theinfiltrationrateandextendlifeof This willpreserve off areas selectedforinfiltrationduringgradingandconstruction. subsequent homebuildingandrelated If possible,rope construction. This includessediment resulting from initialsitegradingas well as No construction-related sedimentshouldentertheinfiltrationdevice. e. d. c. b. a. Stormwater infiltrationdevicesmayfailprematurely ifthere is: mounding problems. device thatismore likelytohave maintenanceandgroundwater development are usuallymore sustainablethanasingleregional to infiltration. Smaller infiltrationdevicesdispersed around a pretreatment facilities. Inadequate maintenanceoftheinfiltrationsystemand flows; Inadequate pretreatment ofpost-development stormwater construction; Excessive compactionorsedimentloadingduring An inaccurateestimationoftheseasonalhighwatertable; An inaccurateestimationoftheDesign Infiltration Rate;

this surrogate measure. capacity remaining. Volume 1, Section 5-2 includes more information on project eligibility for satisfy channelprotection requirements: meet oneofthefollowing criteriato channels withintheproject area, must Off-site flows, or flows into receiving based onpre-developed conditions. exceeds anerosion-causing threshold amount oftimethatareceiving stream watershed. Thiscriterionlimitsthetotal resulting from urbanizationwithina erosion andassociatedsedimentation receiving waters,andwetlands from to protect stream channels,downstream The purposeofthisdesigncriterionis Criteria 2-6. ChannelProtection (CP) 2. 1. ● ● ● ● multiplying theresult by 100. area withinaproject area, usingequalunitsofmeasure, andthen the area ofundisturbedcover withinaproject area by thedrainage %UDC –Theundisturbedcover (%UDC)iscomputed by dividing measure, andthenmultiplyingtheresult by 100. area by thedrainagearea withinaproject area, usingequalunitsof by cover dividingthearea ofeffective impervious withinaproject %EIC –Thepercent effectivecover impervious (%EIC)iscomputed rate to50percent ofthe2-year, post-development peakflow control the2-year, 24-hour development volume, then has increased over thepre- development stormvolume If the2year, 24-hourpost- development peakflow rate. rate tothe2-year, 24-hourpre- post-development peakflow control the2-year, 24-hour pre-development volume, then has notincreased over the development stormvolume If the2year, 24-hour post-

2-6. Channel Protection (CP) 2-7. Peak Runoff Control Rationale stormwater treatment systems: to control peakdischargeratesandimprove theoverall effectiveness ofthe of floodingcaused by development. Thefollowing criteriashouldbemet runoff controls istoaddressThe purposeofpeak increases inthemagnitude Criteria 2-7. Peak Runoff Control this condition. and durationofbankfullevents. TheChannel Protection criterionaddresses receiving channelsexperiencegreater erosion duetotheincreased frequency storm comprisetheerosive, channel-formingevents. Thenet result isthat duration issignificant,becauseflows approaching andlargerthanthe2-year which thereceiving waterexperiencestheflow isextended.Theextended development levels. While thiscontrols therate,periodoftimeduring release itover anextendedperiod,inorder tocontrol therelease ratetopre- To control peakrates,attenuationfacilitiesare designedtostore and runoff rate andgreater thanthepre-development volume ofrunoff hydrograph. the uncontrolled post-development hydrograph shows ahigherpeakrunoff and thedeveloped sitewithfacilitiestoattenuatepeakrates.Asexpected, an undeveloped site,thesamesitedeveloped withnocontrol ofpeakrates, This isillustratedin Figure 2-3,whichcompares typicalhydrographs for (MacRae, 1993and1996,McCuen andMoglen, 1988). are exposedtomore frequent andlongerdurationoferosive bankfullevents channels from erosion andmayactuallycontributetoerosion, sincebanks research indicatesthatthismethoddoesnotadequatelyprotect stream with the2-year, 24-hourstormevent topre-development levels. More recent channels involved thecontrol ofpost-development peakflows associated One oftheearliestandmostcommonmethodsdeveloped toprotect stream 3. 2. 1. pre-development peakflow rate. 24-hour pre-development peakflow rateortothe1-year, 24-hour the100-year floodplain; there asaresult isnoimpacttoproperties ofdeveloping within The project shouldprovide informationshowing supporting that flows leavingthesite; exceed the50-year, 24-hourpre-development peakflow rateforall The 50-year, 24-hourpost-development peakflow rateshouldnot flows leavingthesite; exceed the10-year, 24-hourpre-development peakflow rateforall The 10-year, 24-hourpost-development peakflow rateshouldnot 7. 6. 5. 4. this stormwatermustbehandledby theproject’s drainagesystem, On somesites,stormwaterentersthesitefrom adjacentproperty. If directed intothepeakcontrol structure; In thiscase,thedesignshouldensure thatthisoverflow willbe larger stormsfloodingthedistributionsystemandtraveling overland. drainage systemmayonlybesized tohandleaten-year storm,with control structure maybedesignedforthe25-year storm,whilethe event, whilepipesare designedforadifferent event. For example,the events. On somesites,detentionfacilitiesare designedforonestorm direct tothepeakcontrolstorm structure runoff forallpertinent The designmustensure thattheconveyance systemandlandgrading Rate of Runoff than 0.1hours.Allareas shouldbeaccountedforinthepre/post inThe hydrograph(dT) timeinterval TR-20shouldbenogreater hydrographs andpeakflow ratesfortheproposed development site. Use NRCS or (formerlySCS)methods(TR-20 TR-55) to develop Basin inChapter4). discharging formtheimpoundment(seedescriptionofDetention outletcannothandleflowsfailure, intheevent theprimary emergency spillway’s purposeistoprotect againstembankment rate control structure thatrequires anembankment(dam).The The designshouldprovide foranemergencyspillwayanypeak development conditions; for thisconditioneachdesignstorm,inbothpre- andpost- calculationsshouldaccount then thesystemdesignandsupporting Figure 2-3.Pre- andPost-Development Hydrographs Pre and Post Development Hydrographs Post and Development Pre Time Control Post-development Peak Pre-development Post-development

2-7. Peak Runoff Control 2-7. Peak Runoff Control control whenthedevelopment actuallyoccurs. land produces more thanforested runoff land, resulting inalesserdegree of pre-development peakdischargeratemaybeoverestimated, sincecleared development applicationprocess istriggered. Without thisprovision, the to address thatare properties cleared withanintenttodevelop, before the conditionsisincorporated undisturbed woodsforallpre-construction runoff The provision to consider any site that was wooded within the last ten years as peak attenuationstructures. New Hampshire, withsomeaddedprovisions tohelpguidethedesignof This criterionisgenerallyconsistentwithstormdrainagesystemdesignin Rationale pre-development peakflow rateatthatpoint. flow ratefrom thesiteandoff-sitecontributing area doesnot exceed the immediately downstream from theproject site,thepost-development peak for the10-year and50-year, 24-hourstorm,showing thatatapoint waterbody. Thistypicallycanbeshown through off-sitedrainage calculations iftheproject area abutsanddischargestoalargereceivingbe necessary In controls asdescribedin1)and2)above general,peakrunoff maynot 11. 10. 9. 8. drainage or surface runoff from off-sitesources, runoff shouldbeincluded drainage orsurface enteringthesitethroughthe proposed piped site,includingrunoff area thatcontributes calculations.Thetotaltributary flow to runoff post-development conditions. calculations shouldbelimitedtonomore than100feetforpre- and The lengthof overland sheetflow usedintimeofconcentration(tc) and design stormevents forbothpre andpostdevelopment calculations; Off-site areas shouldbemodeledaspresent landuseconditionforall documentation shouldbeobtained. For allareas thatare notmodeledin“good” condition,photo occurred onthesiteduringthatpre-application period; conditions, regardless ofclearingorcuttingactivitiesthatmayhave considered undisturbedwoodsforallpre-construction runoff Any sitethatwaswoodedwithinthelasttenyears shouldbe leavingthesite; total runoff objective isforthedevelopment’s stormdraindesigntoaccountfor doesnotcontributefloweven ifaportion tothesiteBMPs. The