Accounting for Trees in Stormwater Models and Calculators

Transcription

1 Accounting for Trees in Stormwter Models nd Clcultors Trees nd forests hve nturl bility to reduce stormwter runoff. As more nd more communities encourge or even require the use of nturl vegettive systems s prt of their stormwter mngement progrms, municipl plnners nd engineers require technicl tools tht llow them to quntify the stormwter benefits of this green infrstructure in wy tht works semlessly with existing models nd methods. This fct sheet summrizes methods nd tools to ccount for the bility of green infrstructure to reduce runoff nd remove pollutnts. It is orgnized into two ctegories: 1. Methods for incorporting green infrstructure into runoff models 2. Models nd clcultors for estimting the functions, benefits, nd economics of green infrstructure 1. Methods for Incorporting Green Infrstructure into Runoff Models Historiclly, stormwter mngement hs focused on pek runoff rte control, which requires site designer to generte post-development runoff hydrogrph nd pre-development runoff hydrogrph nd mnge the difference between the two. More recently, site designers hve been introduced to wter qulity control criteri tht re intended to mnge the cpture nd tret (e.g. wter qulity) volume. Most recently, communities hve developed stormwter runoff reduction criteri tht specify runoff volume tht must be cptured nd reduced (e.g., reused, evported, utilized by plnts, infiltrted or otherwise retined on site). Green infrstructure prctices, such s conservtion of forests, rin grdens nd green rooftops, cn be used to meet the runoff reduction criteri. A prticulr chllenge is providing credit for these runoff reduction volumes within rinfll/runoff models. In principle, when runoff reduction prctices re used to cpture nd retin or infiltrte runoff, downstrem stormwter mngement prctices should not be required to detin, retin or otherwise tret the volume tht is removed. In other words, runoff reduction should be ccounted for in stormwter runoff computtions. While it is not esy to predict the bsolute hydrogrph modifiction provided by reducing stormwter runoff volumes, it is cler tht reducing runoff volumes will hve n impct on the runoff hydrogrph of development site. The chllenge fcing stormwter mngers nd site designers is developing hydrogrph generting technique tht provides dequte credit for stormwter runoff volumes tht re reduced on site. There re vriety of pproches tht cn be used to djust the runoff hydrogrph to ccount for the effect of runoff reduction prctices in site dringe re. In most cses, the credit received is likely dependent on the storm event nd development intensity. In order to be useful

2 to stormwter mngers nd site designers, the method developed nd used must meet number of objectives: 1. Field performnce solves rel problems (e.g., wter qulity, chnnel protection, long term mintennce/performnce) 2. Greter efficiency does not led to the overbuilding of stormwter best mngement prctices (BMPs) (e.g., size or number of prctices) 3. Incentivizes runoff reduction nd environmentl site design leds to meningful results if the designer pplies mple effort to use runoff reduction prctices 4. Simple esy to understnd & use, fits into spredsheets nd common models (e.g., TR- 55) 5. Allows for rnge of prctices brodens the suite of BMPs to use t site bsins re not utomtic 6. Accountbility for the locl public works stff provides some ssurnce tht tody s pln pprovls will not equl tomorrow s dringe complints 7. Defensible mkes sense with the site hydrology; engineers believe it is relistic nd plusible 8. Accurte reflects ctul site hydrology 9. Adptble to different pollutnts -- Addresses pollutnts of concern for different pplictions 10. Relevnt t the subwtershed scle Cn be tied to stormwter benchmrks for the subwtershed, such s flow, volume, nd pollutnt lod reduction The following section describes five pproches, ll of which use the USDA Nturl Resources Conservtion Service (NRCS) (formerly known s the Soil Conservtion Service) unit hydrogrph method (USDA SCS, 1986) s bseline. For some methods, post-development hydrogrph without runoff reduction prctices is generted for the site, nd is then djusted. Other methods initilly djust the runoff depth tht results from site with runoff reduction prctices, nd then genertes post-development site hydrogrph. Ech pproch is discussed below. 1. Truncted Hydrogrph (Volume Diversion) The truncted hydrogrph pproch pplies runoff reduction in-line t the outlet of dringe re. The philosophy behind this pproch is tht runoff reduction prctices will ccept nd retin portion of the initil runoff during given rin event, which will modify the ultimte volume of runoff from the site, s well s the shpe of the ultimte runoff hydrogrph. For this prticulr option, post-development runoff hydrogrph for the originl site prior to implementing runoff reduction prctices is generted. The volume of runoff reduced by runoff reduction prctices is then subtrcted from the rising limb, or front portion, of the hydrogrph. If the mount of runoff reduced is less thn the volume up to the hydrogrph pek, then no reduction in the pek flow or time to pek is reflected. As result, this pproch often results in conservtive design estimtes of the resulting pek flow, nd ultimtely gives less credit for runoff reduction prctices.

3 q (m 3 /s) time (hours) Storge s Percent of Totl Runoff 0% 25% 50% 75% Grphic source: Pul Koch 2. Hydrogrph Sclr Multipliction Similr to the previous pproch, the hydrogrph sclr pproch begins by generting postdevelopment hydrogrph for the originl site prior to implementing runoff reduction prctices. In this prticulr pproch, the hydrogrph is then multiplied by sclr, which djusts the mgnitude of the originl site hydrogrph. The sclr is simply the rtio of runoff generted from the site with runoff reduction prctices to the runoff generted from the originl site (with no runoff reduction prctices). The effect of runoff reduction prctices is pplied over the entire hydrogrph rther thn t the beginning. As result, the degree to which the pek flow rte would be reduced is decresed, resulting in conservtive pek flow rte estimte, nd giving less credit for runoff reduction prctices. Also, no dely in the time to pek is reflected using this pproch. 4.0 q (m 3 /s) time (hours) Storge s Percent of Totl Runoff 0% 25% 50% 75% Grphic source: Pul Koch

4 3. Precipittion Adjustment- Subtrct Retention from Rinfll This pproch djusts the NRCS runoff depth formul (USDA SCS, 1986) prior to generting hydrogrph, eliminting the need to develop n originl post-development site hydrogrph. For this pproch, the mount of runoff reduced is subtrcted from the rinfll depth (Eqution 1), nd hydrogrph clcultions re subsequently performed. Q = 2 (( P R) I ) (( P R) I ) + S (1) where P=rinfll depth (in), R = Reduced Runoff (in), Q= Runoff (in), I = initil bstrction, S = potentil mximum retention fter runoff begins The problem with this pproch is tht the volume of runoff reduced is never fully ccounted for, s the chnge in runoff volume generted will lwys be less thn the mount of runoff reduced. Further, djusting the rinfll is not truly representtive of wht ctully occurs over the site, nd no dely in the time to pek is reflected using this pproch. q (m 3 /s) time (hours) Storge s Percent of Totl Runoff 0% 25% 50% 75% Grphic source: Pul Koch 4. Adjusted CN The Adjusted CN pproch djusts the NRCS runoff depth formul (USDA SCS, 1986) by chnging the curve number (CN) for the portion of the site drining to runoff reduction prctices. Site runoff is clculted using Equtions 2-4. The CN cn be djusted to n improved site condition; for exmple, to medow in good condition.

5 1000 S = 10 CN (2) I = 0.2S Q = 2 ( P I ) ( P I ) + S (3) (4) where P=rinfll depth (in), Q= Runoff (in), I = initil bstrction, S = potentil mximum retention fter runoff begins CN = curve number This pproch reduces the runoff generted from the site nd the runoff pek flow rte; however, no dely in the time to pek is reflected. Further, the effect of runoff reduction is distributed over the entire course of the storm, s opposed to occurring t the beginning. As result, the degree to which the pek flow rte would be reduced is decresed, resulting in conservtive pek flow rte estimte, nd less credit for runoff reduction prctices. This method is plusible wy to reduce volumes nd pek rtes, nd fits into the models tht re understood by design consultnts nd pln reviewers. q (m 3 /s) Grphic source: Pul Koch time (hours) Storge s Percent of Totl Runoff 0% 25% 50% 75% 5. Runoff Adjustment - Subtrct Retention from Runoff The philosophy behind this pproch is tht runoff reduction prctices will ccept nd retin portion of the initil runoff during given rin event, which will modify the volume of runoff from the site, s well s the shpe of the resulting runoff hydrogrph. The runoff djustment

6 pproch ws developed by Koch (2005), nd djusts the NRCS runoff depth formul (USDA SCS, 1986) prior to generting hydrogrph. The mount of runoff reduced is subtrcted from the clculted site runoff (Eqution 5). In order to generte site hydrogrph for n entire storm event, the storm is divided into discreet time periods. For ech time period, n excess runoff rte is determined bsed upon wtershed chrcteristics nd the mount of rinfll during tht time period. This excess runoff rte is then trnslted into hydrogrph. The site hydrogrph for the entire storm event is creted by summing ech of these hydrogrphs over the durtion of the storm. Insted of mking subtrction from the site hydrogrph, the runoff djustment pproch subtrcts ech individul time period hydrogrph, until the volume of runoff reduction hs been reched. Q = 2 ( P I ) ( P I ) + S R (5) where P=rinfll depth (in), R = Reduced Runoff (in), Q= Runoff (in), I = initil bstrction, S = potentil mximum retention fter runoff begins The runoff djustment pproch not only subtrcts the runoff reduction volume t the beginning of the hydrogrph, but lso tends to reduce the pek flow nd extend the time to pek of the site hydrogrph, ll of which re expected effects of utilizing runoff reduction prctices. This pproch ppers to model the ctul hydrology of runoff reduction prctices most closely, but it is difficult nd time-consuming becuse subtrction of time period hydrogrphs requires tht the time period hydrogrphs be individully clculted throughout storm event cnnot be used to generte the resulting hydrogrph. Existing hydrology progrms, such s TR-55 nd TR-20, do not hve the cpbility to subtrct individul hydrogrphs from the site hydrogrph nd ccount for runoff reduction prctices in this mnner.

7 q (m 3 /s) time (hours) Storge s Percent of Totl Runoff 0% 25% 50% 75% Grphic source: Pul Koch 2. Models nd Clcultors for Estimting the Functions, Benefits, nd Economics of Green Infrstructure This section describes sixteen models nd clcultors tht re vilble to ccount for the functions, benefits, nd economics of green infrstructure. It includes rnge of hydrologic nd hydrulic (H&H) models, wter qulity models, build-out models, nd cost-benefit clcultors nd tools. Web links re provided for dditionl informtion. "Green Build-Out" Model Csey Trees nd LimnoTech developed model (bsed on the STRATUM model) to predict the stormwter benefits of trees nd green roofs for different coverge scenrios in Wshington, DC. The model ws pplied to n intensive greening scenrio nd moderte greening scenrio, both of which demonstrted tht trees nd green roofs cn be used to chieve substntil reductions in stormwter runoff nd sewge dischrges to locl rivers. Specific outputs from the model include city-wide runoff volume reduction, reduction in CSO frequency nd dischrge, nd the cost svings ssocited with these environmentl benefits. Green Roof Life Cycle Cost-Benefit Clcultor Green Roofs for Helthy Cities developed this clcultor to help evlute vrious roofing relted investment scenrios. The Tool focuses on long timefrmes, rel monetry costs nd svings, nd finncil returns ttributed to employing conventionl nd green (vegettive) roofs. It lso provides some guidnce to the users bout how to fctor in finncil informtion from benefits tht my be overlooked in the nlysis. To ccess the clcultor, free user ccount must be creted.

8 "Green Vlues" Stormwter Clcultor A clcultor developed by the Center for Neighborhood Technology tht cn be used to estimte the finncil nd hydrologic impcts tht vrious green infrstructure technologies cn hve on development site. Specific outputs of the clcultor include reduction in pek dischrge, verge nnul groundwter rechrge increse, reduction in totl detention required nd costs ssocited with green infrstructure versus conventionl prctices. Hydrologicl Simultion Progrm FORTRAN (HSPF) EPA s FORTRAN (HSPF) is comprehensive pckge for simultion of wtershed hydrology nd wter qulity for both conventionl nd toxic orgnic pollutnts. This model cn simulte the hydrologic, nd ssocited wter qulity, processes on pervious nd impervious lnd surfces nd in strems nd well-mixed impoundments. It is the only comprehensive model of wtershed hydrology nd wter qulity tht llows the integrted simultion of lnd nd soil contminnt runoff processes with in-strem hydrulic nd sediment-chemicl interctions. The result of this simultion is time history of the runoff flow rte, sediment lod, nd nutrient nd pesticide concentrtions, long with time history of wter quntity nd qulity t ny point in wtershed. HSPF simultes three sediment types (snd, silt nd cly) in ddition to single orgnic chemicl nd trnsformtion products of tht chemicl. Anlysis of stormwter tretment using HSPF cn be cumbersome. Long-Term Hydrologic Impct Assessment Model (L-THIA) The Locl Government Environmentl Assistnce Network's Long-Term Hydrologic Impct Assessment (L-THIA) model ws developed s n ccessible online tool to ssess the wter qulity impcts of lnd use chnge. Bsed on community-specific climte dt, L-THIA estimtes chnges in rechrge, runoff, nd nonpoint source pollution resulting from pst or proposed development. Inputs include lnd use/cover, soils, nd runoff event men concentrtions. The model llows the user to modify inputs of impervious cover, forest nd open spce to reflect the use of green infrstructure prctices. As quick nd esy-to-use pproch, L- THIA's results cn be used to generte community wreness of potentil long-term problems nd to support plnning imed t minimizing disturbnce of criticl res. Low Impct Development Rpid Assessment (LIDRA) of Cost-Effectiveness for CSO Control This pper presents simple model for ssessing the cost-effectiveness of investments in green infrstructure (GI) techniques, including green roofs, porous pvement nd stormwter wetlnds, for reducing combined sewer overflows (CSOs) in urbn wtersheds. The LIDRA model cn be used s policy-plnning tool to compre GI introduced lone or in conjunction with trditionl stormwter mngement techniques, to conventionl pproches focusing wholly on centrlized infrstructure. The potentil reduction in CSOs resulting from vrious levels of GI doption is simulted using modified Rtionl Method. A life-cycle cost nlysis is used to compre GI with other lterntives. The model ssesses GI effectiveness in terms of estimted chnge in nnul CSO hours (n hour during which CSO event occurs) resulting from GI instlltion.

9 Pollutnt Lod nd Reduction Model Comprehensive Environmentl Inc. hs developed Pollutnt Lod nd Reduction Model tht cn be helpful to vriety of users including wtershed groups, municipl lnd use decisionmkers, nd engineers. The simple spredsheet model llows the user to determine how different types of green infrstructure (GI) techniques, including stormwter wetlnds, ponds, infiltrtion fcilities, rin grdens nd swles, cn reduce the pollutnt lods in given wtershed. Model inputs include lnd use, nnul rinfll, rod snding informtion nd BMP informtion. Impervious cover inputs re bsed on lnd use type but cn be chnged mnully to ccount for GI prctices tht reduce impervious cover or conserve nturl res, if desired. Outputs include nnul lods of TSS, TP nd TN nd the mount reduced by using GI techniques. Progrm for Predicting Polluting Prticle Pssge through Pits, Puddles, nd Ponds (P8) P8 is model for predicting the genertion nd trnsport of stormwter pollutnts in urbn wtersheds. Continuous wter blnce nd mss blnce clcultions re performed on userdefined system consisting of wtersheds (divided into pervious nd impervious res), devices (buffer strips, swles, ponds, infiltrtion bsins, pipes, flow splitters nd quifers), prticle clsses, nd wter qulity components. Simultions re driven by continuous hourly rinfll nd dily ir temperture time series dt. The model simultes pollutnt trnsport nd removl in vriety of devices, some of which re green infrstructure prctices. Wter qulity components include totl suspended solids, totl phosphorus, totl Kjeldhl nitrogen, copper, led, zinc, nd hydrocrbons. Outputs for ech device include such fctors s removl efficiency, flow, lods nd concentrtions, wter nd mss blnce, nd sediment ccumultion rtes. RECARGA The University of Wisconsin developed RECARGA s design tool for evluting the performnce of bioretention fcilities, ringrdens, nd infiltrtions bsins. Individul BMPs, with up to 3 distinct soil lyers nd optionl underdrins, cn be modeled under user-specified precipittion nd evportion conditions. The results of this model cn be used to properly size BMPs to meet specific performnce objectives, such s reducing runoff volume or incresing groundwter rechrge, nd for nlyzing the potentil impcts of vrying the design prmeters. Site Evlution Tool (SET) SET ws developed by the Upper Neuse River Bsin Assocition nd Tetr Tech Inc. to help ssess the environmentl impcts nd costs of site's stormwter mngement design. The SET is designed primrily for locl government site review plnners, professionl developers, nd stormwter engineers, but it is useful for nyone with n interest in reducing stormwter runoff impcts. The model predicts totl nnul stormwter volume nd totl nnul TSS, TP nd TN, s well s costs ssocited with ech scenrio. Although the model ws developed for the Upper Neuse River Bsin, it is pplicble to the entire Piedmont region. The model includes wide rnge of green infrstructure prctices, such s green roofs, permeble pvement, ponds, wetlnds, rin brrels/cisterns, bioretention, nd forest buffers.

10 Stormwter Mngement Model (SWMM) EPA s SWMM is dynmic rinfll-runoff simultion model used for single event or long-term (continuous) simultion of runoff quntity nd qulity from primrily urbn res. SWMM ws first developed in 1971, nd hs since undergone severl mjor upgrdes since then. It continues to be widely used throughout the world for plnning, nlysis nd design relted to stormwter runoff, combined sewers, snitry sewers, nd other dringe systems in urbn res, with mny pplictions in non-urbn res s well. Source Loding nd Mngement Model (SLAMM) SLAMM ws originlly developed by USGS to better understnd the reltionships between sources of urbn runoff pollutnts nd runoff qulity. It hs been continully expnded since the lte 1970s nd now includes wide vriety of green infrstructure prctices nd other pollution controls (infiltrtion prctices, wet detention ponds, porous pvement, street clening, ctchbsin clening, nd grss swles). SLAMM is strongly bsed on ctul field observtions, with miniml relince on theoreticl processes tht hve not been dequtely documented or confirmed in the field. SLAMM incorportes unique process descriptions to more ccurtely predict the sources of runoff pollutnts nd flows for the storms of most interest in stormwter qulity nlyses. SLAMM clcultes mss blnces for both prticulte nd dissolved pollutnts nd runoff flow volumes for different development chrcteristics nd rinflls. Its primry cpbilities include predicting flow nd pollutnt dischrges tht reflect brod vriety of development conditions nd the use of mny combintions of common urbn runoff control prctices. Street Tree Mngement Tool for Urbn forest Mngers (STRATUM) STRATUM is street tree mngement nd nlysis tool developed by the Center for Urbn Forest Reserch for urbn forest mngers tht uses tree inventory dt to quntify the dollr vlue of nnul environmentl nd esthetic benefits: energy conservtion, ir qulity improvement, CO 2 reduction, stormwter control, nd property vlue increse. STRATUM quntifies the stormwter volume reduction benefits of trees bsed on cnopy interception. It is n esy-to-use, computer-bsed progrm tht llows ny community to conduct nd nlyze street tree inventory. Bseline dt cn be used to effectively mnge the resource, develop policy nd set priorities. Using smple or n existing inventory of street trees, this softwre llows mngers to evlute current benefits, costs, nd mngement needs. Urbn Forest Effects Model (UFORE) The Urbn Forest Effects Model (UFORE) is computer model tht clcultes the structure, environmentl effects nd vlues of urbn forests. The UFORE model ws developed by reserchers t the USDA Forest Service, Northestern Reserch Sttion in Syrcuse, NY. The current version ws designed only to incorporte dt on urbn forest structure nd crbon storge, nd sequestrtion. This progrms ids in urbn forest ssessments nd smpling, including ssessments for exotic pest infesttions nd urbn forest effects on crbon dioxide, the dominnt greenhouse gs. One component of the model still under development, UFORE- Hydro, is designed to evlute t the wtershed scle, how chnges in impervious surfce nd tree cnopy (nd some dditionl vribles) ffect 1) the totl volume of runoff, 2) the pek

11 storm event volume nd durtion of pek, 3) strem bseflow, 4) the totl nnul pollutnt loding nd 5) the men event pollution lod. These fctors re determined bsed on the cnopy interception, infiltrtion nd evpotrnspirtion provided by individul trees nd forest ptches. Wter Blnce Model (WBM) The Wter Blnce Model (WBM) powered by QUALHYMO is public domin, on-line decision support nd scenrio modeling tool for promoting rinwter mngement nd strem helth protection through implementtion of "green" development prctices. The ppel nd the strength of the tool is tht it is evolving to meet the "needs nd wnts" of prticipting gencies. The British Columbi Inter-Governmentl Prtnership developed the WBM in Initilly, the WBM ws plnning tool tht hd site focus. It enbled users to evlute the effectiveness of source controls --- such s bsorbent lndscping, infiltrtion fcilities, green roofs, nd rinwter hrvesting --- in chieving performnce trgets for rinwter volume cpture nd runoff rte control under vrious combintions of lnd use, soil nd climte conditions. The WBM hs since been integrted with QUALHYMO, rinfll-runoff simultion tool, to provide dringe engineers with suite of nlyticl cpbilities, from site to wtershed. The over-rching gol in integrting these tools is to help locl governments chieve desired urbn strem helth nd environmentl protection outcomes t wtershed scle. Wtershed Tretment Model (WTM) Developed by the Center for Wtershed Protection, the Wtershed Tretment Model (WTM) is simple spredsheet model tht trcks pollutnt sources nd the effectiveness of vrious wtershed tretment options in urbn nd urbnizing wtersheds. A wide rnge of tretment options, including green infrstructure prctices, re contined in the WTM (e.g., impervious cover disconnection, riprin buffers, ponds, wetlnds, swles nd filters). The WTM cn be used to develop TMDLs for nutrients or sediment; direct bcteri detective work in urbnized wtersheds; determine the effectiveness of wtershed eduction progrms; nd trget the future progrm in Phse II community. Specific outputs of the WTM include totl lods of sediment, nutrients nd bcteri from given wtershed. The WTM is currently being revised to provide estimtes of runoff reduction ssocited with vrious wtershed tretment options. References Koch, P.R A Milwukee Model for LID Hydrologic Anlysis. In Mnging Wtersheds for Humn nd Nturl Impcts Prt of Engineering, Ecologicl, nd Economic Chllenges, Wtershed 2005, Glenn E. Moglen, Editor, July 19 22, 2005, Willimsburg, Virgini. Americn Society of Civil Engineers. Reston, VA. U.S. Deprtment of Agriculture (USDA) Soil Conservtion Service (SCS). (1986). Technicl Relese 55: Urbn Hydrology for Smll Wtersheds. Second Edition. USDA SCS, Wshington, DC.

12 Developed by the Center for Wtershed Protection, 8390 Min Street, 2 nd Floor, Ellicott City, MD 21043