Downstream Economic Benefits of Conservation Development

Transcription

1 Downstrem Economic Benefits of Conservtion Development Dougls M. Johnston 1 ; John B. Brden 2 ; nd Thoms H. Price, P.E. 3 Abstrct: This pper evlutes the downstrem hydrologic nd economic impcts of development strtegies tht promote greter on-site storge of storm wter. This pper pplies methodology to specific cse study tht emphsizes flood risk reduction nd dringe infrstructure. The estimtes re t first level of pproximtion. We use widely ccepted simultion models nd vilble dt to compre lterntive development scenrios for the 0.01 nnul probbility storm event. For wtershed in rpidly developing re ner Chicgo, Ill., reduced downstrem flooding with the employment of conservtion design prctices genertes from $0 to 19,400/h $0 7,800/cre in downstrem property vlue benefits over ll ffected res. For comprison purposes, flood-dmge estimtion methods generte n verge of $16,800 $24,200/h $6,700 $9,700/cres present vlue reduction in dmges for the 0.01 probbility flood event lone. The two methods yield conservtive, but mutully reinforcing estimtes. For infrstructure benefits, considering only downstrem rod culverts, the use of conservtion design techniques upstrem voids $3.3 million in costs of culvert replcement or upgrdes. The sum of the downstrem flood mitigtion nd infrstructure benefits mounts to $920 1,440/developed hectres $ /developed cres following conservtion design prctices. DOI: / ASCE :1 35 CE Dtbse subject hedings: Stormwter mngement; Dringe; Runoff; Urbn res; Economic fctors; Floods; Dmge; Infrstructure. Introduction Mnging storm wter is mjor chllenge in most urbn res. Buildings, rods, nd compcted soils reduce bsorptive cpcity. In suburbn res, 20 50% of the lnd is impervious to precipittion. In inner cities nd commercil zones, imperviousness cn exceed 80%. The hydrologic functions of strems chnge with s little s 5 10% imperviousness, nd they chnge profoundly when imperviousness pproches 25%. The incresed runoff excerbtes flooding nd increses conveynce requirements. Less wter is left in the soil to rechrge quifers, replenish wells, nd mintin bse strem flows. Fster runoff increses erosion, scours strem bnks, nd entrins more sediment, lndscpe chemicls, petroleum residues, pet wstes, nd other nthropogenic detritus. A consequence is surfce wter qulity tht is less ble to support beneficil uses. 1 Associte Professor, Dept. of Lndscpe Architecture, Univ. of Illinois, 611 E. Loredo Tft Dr., Chmpign, Il corresponding uthor. E-mil: dmjohnst@uiuc.edu 2 Professor, Dept. of Agriculturl nd Consumer Economics, Univ. of Illinois, 1301 W. Gregory Dr., Rm. 431, Urbn, Illinois E-mil: jbb@uiuc.edu 3 Principl, Director of Wter Resources Engineering, Conservtion Design Forum, 35 W. First St., Elmhurst, IL E-mil: tprice@ cdfinc.com Note. Discussion open until June 1, Seprte discussions must be submitted for individul ppers. To extend the closing dte by one month, written request must be filed with the ASCE Mnging Editor. The mnuscript for this pper ws submitted for review nd possible publiction on Jnury 5, 2004; pproved on Februry 16, This pper is prt of the Journl of Wter Resources Plnning nd Mngement, Vol. 132, No. 1, Jnury 1, ASCE, ISSN /2006/ /$ For severl decdes, detention bsins hve been the customry prescription for mnging storm wter. They hve received criticism becuse while they reduce pek flows, they generlly increse dmging bnk-full flows, nd do not contribute to remedition of wter qulity or groundwter infiltrtion. More recently, low impct or conservtion design principles use mesures such s porous pving, nrrower streets, green roofs, vegetted swles, nd constructed wetlnds to mintin nerly nturl wter budget nd improve wter qulity e.g., Arendt 1994; Wilson et l Residentil conservtion development sites typiclly incorporte more cluster development thn conventionl development to provide the sme gross density of popultion to lnd re. Improved storm wter mngement cn produce the following types of downstrem benefits: 1 reduced frequency, re, nd impct of flooding; 2 less costly public dringe infrstructure; 3 reduced pollution tretment; 4 reduced erosion nd sedimenttion; 5 improved wter qulity; 6 improved in-strem biologicl integrity nd esthetics; nd 7 incresed groundwter rechrge. While mny studies hve considered specific physicl nd biologicl effects of ltered hydrology, there hs been no effort to synthesize those elements into n overll benefit mesure or to fcilitte their trnsfer by scling them to locl conditions. Streiner nd Loomis 1995, p. 268 group the economic effects of strem corridor enhncement into two ctegories: 1 reductions in property dmges, including residentil nd public structures, lndscping nd prks; nd 2 restortion of the nturl vlues of the strem itself, including more stble strem bnks, enhnced qutic hbitt through restortion of pool-riffle sequences, nd more visully ttrctive ecosystems. Brden nd Johnston 2003 develop typology of impcts nd use benefits trnsfer techniques to develop estimtes of the economic JOURNAL OF WATER RESOURCES PLANNING AND MANAGEMENT ASCE / JANUARY/FEBRUARY 2006 / 35

2 significnce. Their estimtes generlly re conditionl on property vlues nd locl fctors. They conclude tht the reduction of flood dmges nd public infrstructure costs of storm wter conveynce re usully the most significnt sources of chnge in property vlue, with most other benefits being cse specific, or smll. In this pper, we tke the next step towrd usefulness by pplying the benefits trnsfer methodology to specific cse study in suburbnizing wtershed in the Chicgo, Ill. region. The cse study emphsizes the effects of flood risk reduction on property vlues, nd the costs of storm wter dringe infrstructure. It combines hydrologic nlysis of storm wter flows, rel estte economics, nd finncil nlysis of public infrstructure. The resulting nlysis is dependent on the specific onsite mesures used to mnge storm wter. The estimtes re t first level of pproximtion nd bsed on generlly vilble dt. We use widely ccepted simultion models to compre lterntive development scenrios. Approch Assessment of storm wter mngement benefits would idelly be bsed on direct observtion. However, becuse conservtion design strtegies hve yet to be employed on lrge scle, tht is, t wtershed scles, it is not possible to drw on monitoring reserch for ssessing downstrem effectiveness. Simultion studies re therefore required to provide comprison between two possible future scenrios: development of the wtershed with nd without the incorportion of conservtion design or other mesures for on-site storm wter retention. Our hydrologic modeling strtegy involves the following steps: 1 perform flood frequency nlysis of the simulted strem flows to estimte the probbility of different mgnitudes of flood events; 2 compute dischrge flow rtes for reches in the wtershed for the specified flood events; 3 clculte the wter surfce elevtion of the strems; 4 estimte the re of different lnd uses contined within the flooded extent; 5 use benefits trnsfer to clculte the economic benefits ttributble to the differences in flooded res; nd 6 use engineering costing methods nd stndrd design protocols to estimte svings ssocited with infrstructure. Lck of vilble dt limits the specificity permitted in estimting impcts. The vlues computed in this pper represent pproximte mesures of benefits. Assessment is first directed to the existence of benefits, followed by n estimte of the mgnitude of benefits. The Blckberry Creek Wtershed west of Chicgo, Ill. serves s our cse study site. Blckberry Creek drins 189 km 2 73 mi 2 wtershed in south-centrl Kne County nd northcentrl Kendll County. It is 52 km 32 mi long nd origintes north of the villge of Elburn in centrl Kne County. It drins to the Fox River ner Yorkville in Kendll County. Tributries to Blckberry Creek include Lke Run, Est Run, nd severl unnmed tributries Fig. 1. Blckberry Creek represents n urbnizing wtershed. It hs been the subject of numerous studies of wtershed mngement nd conservtion design strtegies Kne County 1996; Blckberry Creek Wtershed Committee 1999; CDF A comprehensive development pln hs been prepred for the Kne County portion of the wtershed Kne County Becuse of dt limittions in Kendll County, only the wtershed in Kne County ws included in this nlysis. Flood Frequency Anlysis Simultions of dischrge from ctchments within the wtershed used the Hydrologic Simultion Progrm Fortrn, or HSPF USEPA A recent study CDF 2003 modeled runoff from ctchments using locl hydrologic prmeters e.g., infiltrtion, evpotrnspirtion, velocity. Prmeteriztion of the model occurred in two different wys to cpture the implementtion of either conservtion design prctices or conventionl development ptterns within the context of the pproved pln. The runoff vlues determined in tht study re used s the bsis for comprison here. The model formultion used in this study chrcterizes runoff from ctchments in Blckberry Creek bsed on conventionl development t forecsted popultions, nd conservtion development t the sme forecsted popultions nd bsic lnd use rrngement. Modeling of best mngement prctices ws performed using HSPF CDF Using two-stge pproch, individul best mngement prctices BMPs were modeled, followed by n ggregte development scenrio. We summrize this pproch here. A more complete explntion is given in CDF The HSPF models surfce runoff, interflow, nd groundwter flow. It models soil moisture in soil lyers filled by infiltrtion nd drined by evpotrnspirtion nd grvity. These fetures permit the chrcteriztion of individul BMP performnce. For exmple, green roofs re modeled s thin, well-drined soil horizon, over dringe medium modeled s very porous groundwter lyer with recession constnt determined by the medi s hydrulic conductivity. In conventionl scenrios, vegetted swles used runoff prmeters for turf, while in conservtion scenrios swles used pririe conditions. Modeling prmeters for other BMPs followed similrly. Conventionl scenrios ssumed stndrd dringe tretments with storm sewers for commercil nd higher density residentil development, grss swles for lower density residentil nd detention throughout CDF In ddition, to help ensure comprbility between the conventionl nd conservtion scenrios, common chrcteristics include stndrd 16.2 h 40 cre prcels, stndrd house footprints but not drivewy re, sme runoff coefficients for sme mterils, nd detention to the current Kne County llowble relese requirement of m 3 /s/h 0.1 cfs/cre. For moderte density residentil, roof res were clculted s m 2 3,000 ft 2 for both scenrios. For conventionl development, curb, gutter, nd storm sewers were ssumed to serve the entire templte. For conservtion development, streets drined to bioswles with 3.35 m 11 ft infiltrtion trenches, 0.3 m 12 in. deep. Runoff from the bioswles, roofs, nd lots ws directed to vegetted swles verging 10.6 m 35 ft wide in the bcks of the lots CDF In griculturl res, the conservtion scenrio ssumes 10% of the lnd re is used s pririe filter strips with runoff on the strips clculted s runoff routed from the cultivted res plus direct precipittion. In modeling the wtershed-scle response, the conservtion scenrio used s its bsis the 2020 Resource Pln published by Kne County Kne County 1996 nd for the conventionl scenrio existing locl municipl plns were used. For both scenrios, existing lnd uses were retined. This included residentil res within designted griculturl zones. Strems nd wetlnds were retined under the ssumption of Federl nd Stte protection. For the conventionl scenrio, no dditionl buffering of strems beyond tht specified in the plns were used. For the conservtion scenrio, in ddition to the ppliction of the conservtion lnd use templtes, strems were buffered from 36 / JOURNAL OF WATER RESOURCES PLANNING AND MANAGEMENT ASCE / JANUARY/FEBRUARY 2006

3 Fig. 1. Blckberry Creek study re Tble 1. Lnd Use Ares Used in Scenrios Tretment Lnd use modeling ctegory Conventionl scenrio conventionl Conventionl Conservtion scenrio Conservtion Hectres Acres Hectres Acres Hectres Acres Commercil/trnsport 1,759 4, ,464 Moderte density residentil 1,857 4,585 1,133 2, ,031 Rurl residentil 1,906 4, ,824 1,562 3,856 Estte residentil 1,999 4, , ,891 Agriculture 3,620 8, ,190 7,875 Wter Wetlnd 964 2,380 1,136 2,804 Urbn grss/prk 605 1, Nturl open spce 2,073 5,119 3,696 9,127 JOURNAL OF WATER RESOURCES PLANNING AND MANAGEMENT ASCE / JANUARY/FEBRUARY 2006 / 37

4 Fig. 2. Probbility plot of nnul pek flows for Blckberry Creek downstrem of Auror Tributry development. Modeled lnd res re given in Tble 1. To the extent possible within the constrints of the 2020 Resource Pln, the scenrios re development density neutrl. In relity, the conservtion scenrio hs 12,742 housing units while the conventionl scenrio hs 14,258 units 10% more. However, differences in the results of the models re ttributble to vrition in storm wter mngement prctices rther thn simple vrition in lnd use re, lthough further reserch is needed to explore the loctionl effects of lnd use. Flood frequency nlysis estimtes the probbility distribution tht fits the highest flow recorded for ech yer of record nnul pek flow. Running HSPF t n hourly time step for the period of vilble dt from the nerest recording rinfll guge t Chicgo O Hre Interntionl Airport, Ill. genertes predicted dischrge record. Simulted pek nnul flows for the three scenrios re plotted in Fig. 2 using the Weibull plotting positions Hn et l As recommended by Bulletin 17B of the Intergency Advisory Committee on Wter Dt Hn et l we used the Log Person III prmeter estimtion using Chow s formultion Hn et l with the men nd stndrd devition of the log of dischrge nd with frequency fctors djusting for skewness tken from Bulletin 17B. The estimtion yields predicted dischrge rtes t different probbilities. Tble 2 shows predicted dischrge for the outlet of the wtershed. Fig. 3 shows comprison plot of the predicted dischrges for the sme rech. As would normlly be expected, the modeled dischrges show n increse in pek dischrges throughout the distribution for the conventionl development scenrio. The conservtion scenrio, however, results in dischrges below not only the conventionl but lso the existing levels of development. This result reflects precisely the potentil impcts of incresed storge provided by conservtion design prctices Tble 2. Dischrge Estimtes for Blckberry Creek Below Auror Tributry Return period Dischrge cfs Frequency fctor Exceednce probbility Conventionl 50 2, scenrio 100 3, Conservtion 50 2, scenrio 100 2, Fig. 3. Log Person type III probbility plot of nnul pek flows for Blckberry Creek downstrem of Auror Tributry including permeble pving, green roofs, bioswles, use of ntive vegettion to increse groundwter infiltrtion, nd evpotrnspirtion over conventionl development types. A possible impliction of this is tht new development cn offset impcts from existing development. It should be recognized however, tht in this region, most griculturl lnds re subsurfce drined, ltering its hydrologic response. Becuse the mgnitude of the effects is dependent on the scle of implementtion, ny nlysis must include locl nd ctul development conditions. The U.S. Army Corps of Engineer s Hydrologic Engineering Center River Anlysis System HEC-RAS genertes wter surfce elevtions. The HEC-RAS is one-dimensionl chnnel flow simultion model. Given strem cross-section nd profile geometry, nd boundry conditions of flow, HEC-RAS computes hydrulic prmeters of dischrge, wter surfce elevtion, nd velocity, mong others. These prmeters re clculted t cross sections locted long the strem. Estimted dischrges for the reches of Blckberry Creek computed from the HSPF simultion become the vrible input conditions for the two scenrios modeled in HEC-RAS. To identify the floodplin ssocited with the modeled events, the cross-sectionl surfce wter elevtions re interpolted in geogrphic informtion system using nerest-neighbor method with hrd brek lines forcing no smoothing t the observed loctions to crete tringulr irregulr network TIN of the wter surfce. Estimtion of floodplin extent nd depth used vilble digitl elevtion models. Kne County dt consists of DEM with 3.0 m 10 ft horizontl, nd 0.60 m 2 ft verticl ccurcy. Becuse the focus is on downstrem impcts, only 17.4 km 10.8 mi portion of Blckberry Creek downstrem of its mjor tributries is exmined in the flood nlysis Fig. 1. The impcts of upstrem development ptterns re progressively diluted downstrem s flows from other sources enter the min chnnel. We then tbulted the lnd use types nd res flling within the flood risk res. In the conventionl scenrio, 704 h 1,743 cres of lnd re subject to flooding while the conservtion scenrio results in 616 h 1,525 cres with difference of 20 h 50 cres of residentil property within the nlysis re. Estimtion of Economic Impct Flood Effects on Property Vlues In this section nd the next, we pply two different pproches to determine the downstrem flood mitigtion benefits of storm 38 / JOURNAL OF WATER RESOURCES PLANNING AND MANAGEMENT ASCE / JANUARY/FEBRUARY 2006

5 wter retention. The first pproch uses benefits trnsfer techniques s suggested by Brden nd Johnston The second pplies flood dmge formul-bsed pproches used by the U.S. Army Corps of Engineers USACE 1996; USWRC In both cses, we confine the nlysis to the 0.01 nnul probbility flood event. This is the cutoff for lnd to be included in the Ntionl Flood Insurnce Progrm. While dditionl lnd is subject to flooding from more extreme events, our nlysis essentilly ssumes tht such flooding hs no economic significnce. This imprts downwrd bis in the economic estimtes. At the sme time, we mke ssumptions tht should offset this bis, t lest in prt. In the first cse, we ssume tht the lnd remining in the 100 yer floodplin is subject to reduction in the extent nd height of higher-frequency events, thereby reducing the costs, but by lesser mount thn lnd removed from flood risk below the 0.01 probbility event. In the second cse, we compute dmges only for the 0.01 probbility event nd neglect dmges tht might ccompny lesser events. In ll cses, we view this s very conservtive pproch to estimting benefits s mentioned erlier. Brden nd Johnston 2003 concluded tht residentil propertie exposed to flooding re discounted in the mrket by n verge of 2 5%, nd 0 2% for properties subject to reduced flooding. To pply this observtion to our cse study, we need to know, first, how mny cres would be in the 100 yer floodplin with conventionl development upstrem but outside it with conservtion development nd the number of cres subject to flooding in both scenrios. Second, we need to know the mrket vlues of those properties. U.S. Census Bureu dt provide loclized estimtes of property vlue. Dt from the 2000 Census Summry File 3 for Medin Owner Occupied Housing H re vilble t the block group level. Census dt reflect regionl differences within county, lthough not differences between individul properties. They re lso self reported, which my imprt bis, nd they reflect medins rther thn men vlues, which mens tht they generlly under-represent ggregte housing vlue. For our study site, the re-weighted medin housing vlue ws $175,600 per unit for homes in the census block groups within the flood risk res. Using n verge density of 5.5 units/h 2.2 units/cres, the difference in flooded residentil re between the conservtion nd conventionl scenrios would hve n ggregte housing vlue of $967,600/h $391,600/cres or totl of $19,580,000. Applying the 2 5% pproximtion from Brden nd Johnston 2003, the conservtion scenrio genertes $391, ,000 in totl benefits $19,400 48,400/h, $7,800 19,600/cres, $3,500 8,800/unit for the re tht would be dded to the floodplin if conventionl rther thn conservtion design prctices re pplied upstrem. For properties remining in the floodplin but subject to reduced dmges, we pply the 0 2% pproximtions from Brden nd Johnston 2003, resulting in n dditionl benefit of $0 19,400/h $0 7,800/cres, or $0 3,500/unit, or totl of $0 1,509,500 for 78 h 192 cres. Therefore, the totl benefit bsed on Census dt is $391,600 $2,488,500 over the downstrem rech study re. The development scenrio used in this nlysis envisions the development of pproximtely 4,050 dditionl hectres 10,000 cres by the yer The downstrem rech flood mitigtion benefits discussed bove re equivlent to between $100 nd $620 per developed h $40 nd $250/developed cres bsed on Census medin housing vlue nd the reduction in flood risk. Flood Dmge Estimtion An lterntive method to estimte flood reduction benefits is to clculte the chnge in flood dmge to structures nd contents bsed on stge-dmge curves following procedures used by the U.S. Army Corps of Engineers These estimtes re cost bsed rther thn vlue bsed, mking them theoreticlly less desirble thn property vlue differentils s bsis for estimtes Brden nd Johnston 2003, but hve prgmtic dvntges of stbility nd trnsprency. Stge dmge curves re idelly developed in situ using dt collected from historic flood events. Dvis et l developed brodly pplicble reltionships bsed on surveys of pproximtely 1,000 homes in vrious geogrphic regions within the United Sttes. They considered severl vribles, such s depth, durtion, wrning led time, nd building mteril, but found depth of inundtion to be the only significnt predictor of dmge. This suggests tht estimting structure nd content dmges bsed on modeled flood depths is n pproprite nd dequte method. Dmge equls the present vlue of expected costs bsed on flood depth t prticulr loction. We estimte flood depth from the surfce wter elevtions produced by our hydrologic models nd pply the stge dmge curves described by Dvis et l Becuse the stge dmge curves re derived for different building types, we estimte dmges using both one- nd two-story homes with no bsement. We further use the previously identified verge mrket vlue of $967,634/h $391,600/cres, bsed on census dt. For the conventionl development scenrio, the modeled flood depths rnge from 0 to 3.0 m 0 to10ft over the 97.7 h 242 cres of residentil property within the floodplin. For the conservtion scenrio, modeling resulted in flood depths of m 0 9 ft over 77.5 h 192 cres of residentil property. We compute flood dmge s the representtive vlue of flooded residentil property multiplied by percent loss using the depth dmge reltions given by Dvis et l The difference in flood dmge between the conventionl nd conservtion scenrios equls the difference in totl expected dmge from the conventionl scenrio nd the totl expected dmge from the conservtion scenrio. The 100 yer 0.01/yer probbility flood event, the cutoff event for the Ntionl Flood Insurnce Progrm, provides illustrtive results. For the conventionl development scenrio, structurl flood dmges for two-story homes re $20,948,000 while content dmge is $12,174,000 or totl estimted dmge of $31,122,000. The nnul expected vlue of totl dmge is $331,220. At 5% rel rte of interest, the expected present vlue is $6,624,000 dt Tble 3 or $67,800/h $27,100/cres. For one-story residences, the expected present vlue is $9,752,000 $99,800/h, $39,900/cres Tble 4. Comprble clcultions for the conservtion design scenrio led to estimted structurl dmges of $23,725,000 nd content dmges of $13,213,000 or totl of $36,937,000. The expected present vlue of dmges is $7,387,000 $75,500/h, $30,200/ cres for one-story housing Tble 5. For two-story residences, the expected present vlue of dmges is estimted t $4,979,000 $50,900/h, $20,400/cres see Tble 6. Therefore, using the stge-dmge pproch, the upstrem conservtion mesures produce n expected present vlue of downstrem flood benefit for the 0.01 nnul probbility event rnges from $1,040,000 to $1,526,000 in structurl dmges nd $605, ,000 in content dmges. The combined vlue is JOURNAL OF WATER RESOURCES PLANNING AND MANAGEMENT ASCE / JANUARY/FEBRUARY 2006 / 39

6 Tble 3. Estimted Flood Dmge Benefits for Two Story Residences: Conventionl Scenrio Flood depth Estimted structurl dmge Estimted content dmge Flooded Totl dmges Dmge % /hectre $ Dmge % /hectre $ hectres Structurl $ Content $ Totl $ , , ,968,803 1,058,496 3,027, , , ,286,773 1,308,877 3,595, , , ,659,476 2,136,154 5,795, , , ,791,061 2,234,276 6,025, , , ,203,629 2,476,660 6,680, , , ,086,125 1,815,869 4,901, , , ,019, ,950 1,618, , , , ,211 1,191, , , ,257 72, , , , ,104 31,863 86,967 Totl ,947,736 12,174,251 33,121,988 Property vlue per hectre Property vlue per cre Expected vlue 331,220 $ 967,634 $391,601 Present 6,624,398 PV/hectre 67,764 $16,800 $24,200/h $6,700 $9,700/cre for one- nd two-story housing, respectively, or $ per developed hectres $ per developed cres. Becuse it is bsed on the 0.01 nnul probbility storm event lone, rther thn the entire distribution of events USACE 1996, the flood dmge clcultion produces lower-bound estimte. In prctice, smpled events re sometimes used USACE While desirble, full risk-bsed estimtion ws beyond the scope of methodology iming for first pproximtion of benefits. Notwithstnding this obvious limittion, the resulting ggregte estimte of downstrem benefits is within the rnge of the benefits trnsfer clcultion bsed on property vlues. The re-normlized vlues re comprble cross exmples. Both methods yield conservtive pproximtions tht exclude dmges from lower-frequency, higher-intensity storm events. Infrstructure Benefits Complete estimtion of downstrem infrstructure benefits would entil the ssessment of differences in the number, size, or type of ny conveynce, flood control, or chnnel modifictions between the conservtion nd conventionl scenrios. Effects re highly site specific. To illustrte the principles of infrstructure estimtion nd to provide conservtive, first-order estimte of the potentil benefits, we consider the design requirements for storm wter conveynce through rodwy culverts long Blckberry Creek nd compre the costs ssocited with the different chnnel flow rtes resulting from the conventionl nd conservtion design scenrios. Computtion of culvert sizes nd costs includes prmeters of chnnel dischrge flow rtes, culvert type, criticl depth, nd Tble 4. Estimted Flood Dmge Benefits for One Story Residences: Conventionl Scenrio Estimted structurl dmge Estimted content dmge Totl dmges Flood depth Dmge % /hectre $ Dmge % /hectre $ Flooded hectres Structurl $ Content $ Totl $ , , ,836,771 1,714,764 4,551, , , ,505,383 2,000,927 5,506, , , ,620,535 3,134,193 8,754, , , ,780,287 3,171,230 8,951, , , ,305,444 3,440,550 9,745, , , ,535,410 2,455,260 6,990, , , ,468, ,411 2,257, , , ,057, ,747 1,623, , , ,484 91, , , , ,138 39, ,257 Totl ,356,852 17,402,832 48,759,684 Property vlue per hectre Property vlue per cre Expected vlue 487,597 $ 967,634 $391,601 Present 9,751,937 PV/hectre 99, / JOURNAL OF WATER RESOURCES PLANNING AND MANAGEMENT ASCE / JANUARY/FEBRUARY 2006

7 Tble 5. Estimted Flood Dmge Benefits for Two-Story Residences: Conservtion Scenrio Estimted structurl dmge Estimted content dmge Totl dmges Flood depth Dmge % /hectre $ Dmge % /hectre $ Flooded hectres Structurl $ Content $ Combined $ , , ,336, ,720 2,055, , , ,581,161 1,477,375 4,058, , , ,202,038 1,869,132 5,071, , , ,165,638 1,865,680 5,031, , , ,430,725 2,021,287 5,452, , , ,098, ,486 1,745, , , , ,840 1,183, , , ,428 76, , , , ,503 32,883 89, , , ,748,644 9,146,386 24,895,030 Property vlue per hectre Property vlue per cre Annul expected vlue 248,950 $967,634 $391,601 Present 4,979,006 PV/hectre 50,910 llowble bckwter hed. To compute pipe size requirements, we use Federl Highwy Deprtment design specifictions FHA Within our study re, bridges comprise ll strem crossings in the downstrem reches of Blckberry Creek. The design of bridge structures is complex due to issues of trffic mngement nd esthetics tht rnge well beyond conveynce considertions tht re the focus of this study. Therefore, we nlyze differences in culvert costs s the purest wy to represent differences in the costs of wter conveynce. We smple existing culverts from throughout the bsin to represent the effects. The flow impcts of the two development scenrios re not uniformly cumultive, so this pproch should cpture the vrition in effect throughout the bsin. In ll cses, the culverts re downstrem of modeled ctchments. The benefits ttributble to infrstructure stem from voided costs of infrstructure due to the reduced pek dischrges in the conservtion scenrio resulting, generlly, in smller culverts. Becuse severl intercting vribles ffect the design nd therefore construction costs of culverts, we fix ll design vribles other thn size. The bseline for comprison is the conservtion flow simultion with existing culvert infrstructure. For the smpled culverts, we use the existing type, size, nd other prmeters. Becuse the conservtion scenrio generlly results in dischrges below those of the existing conditions, it is likely tht existing infrstructure is oversized for tht scenrio. Thus, the results should be viewed s conservtive estimte of benefits, rther thn n bsolute difference between conservtion nd conventionl development where new infrstructure would be provided for both scenrios. Our pproch represents developing wtershed tht hs t lest bsic infrstructure lredy in plce. The costs of infrstructure equl the construction costs for mterils nd lbor for the culvert nd its instlltion Illinois Hevy Construction Cost Dt for Relted costs, such s those for excvtion nd grding, nd relted rod construction, re not included due to the site-specific nture of these fctors. Lnd Tble 6. Estimted Flood Dmge Benefits for One Story Residences: Conservtion Scenrio Estimted structurl dmge Estimted content dmge Totl dmges Flood depth Dmge % /hectre $ Dmge % /hectre $ Flooded hectres Structurl $ Content $ Combined $ , , ,926,169 1,164,326 3,090, , , ,956,648 2,258,516 6,215, , , ,917,963 2,742,415 7,660, , , ,826,695 2,648,062 7,474, , , ,146,087 2,807,950 7,954, , , ,614, ,122 2,488, , , ,073, ,069 1,650, , , ,994 98, , , , ,808 41, , , , Totl ,724,593 13,212,732 36,937,325 Property vlue per hectre Property vlue per cre Annul expected Vlue 369,373 $967,634 $391,601 Present 7,387,465 PV/hectre 75,536 JOURNAL OF WATER RESOURCES PLANNING AND MANAGEMENT ASCE / JANUARY/FEBRUARY 2006 / 41

8 Tble 7. Summry Cpitl Costs for Culverts Culvert Shpe Mteril 1 Ellip Metl 2 Box Concrete 3 Box Concrete 4 Box Concrete 5 CMP Metl 6 CMP Metl 7 CPC Concrete 8 Ellip Metl 9 Box Concrete 10 CPC Concrete 11 CMP Metl 12 CPC Concrete cquisition, esements, legl, design, nd other costs re equl between the scenrios. We lso ssume tht mintennce nd project lifetimes re equl. Interest on cpitl costs is not considered. We considered 12 culvert designs representing rnge of flow conditions nd culvert types. In some cses, multiple culverts re required to meet the design flows. In ll cses, the conventionl scenrio resulted in lrger culvert requirements. The difference between the costs of the conventionl solution nd the conservtion solution rnge from $430 for smll corrugted metl culvert in smll tributry to $302,000 for long concrete box culvert on tributry of Blckberry Creek Tble 7. The verge difference between scenrios ws $38,100 per culvert. In the Blckberry Creek wtershed, there re presently 87 culverts. Using the verge cost, the totl benefit due to smller required dringe culverts in the bsin is $3,315,000. With 4,050 h 10,000 cres of new development in the scenrios for the whole wtershed, the downstrem benefit is equivlent to $820 per developed hectre $340/developed cre. As noted before, these estimtes pply only to existing infrstructure, which is presently oversized reltive to the conservtion scenrio. In the cse of new development, which is not fctored in these estimtes, the difference in the size nd number of culverts, nd therefore benefits, would be greter. Conclusions Hedwll type b b c d d b d Length Difference benefit $ , , , , , , , , , , , Averge benefit 38, Averge benefit without No. 4 17, Squre hedwll. b Wingwll flred c Tpered inlet throt. d Pipe projecting from fill. Notes: Cost sources: box culvert mteril costs from the Rio Vlley Pipe Compny. lbor nd equipment ssumed s 40% of mteril costs ; corrugted metl pipe CMP, concrete pipe culvert CPC nd hedwll costs from 2003 Illinois hevy construction costs. Using benefits trnsfer methods s outlined by Brden nd Johnston 2003, conservtion design prctices in suburbn Chicgo, Ill. wtershed generte estimted totl benefits bsed on incresed downstrem property vlues of $391,600 2,488,500 due to reduced flooding. These vlues rnge between 0.4 nd 2.5% of the vlue of ffected properties, depending whether or not they remin in the 0.01 nnul probbility flood zone. These effects mount to between $100 nd $620 per upstrem developed hectres $ per developed cres. In comprison, using flood-dmge estimtion methods, conservtion design prctices generte n verge of $16,800 24,200 per hectres $6,700 9,700 per cres present vlue reduction in dmge for the 100 yer 0.01 nnul probbility flood event. This mounts to % of the verge property vlue throughout the floodplin re nd $ per upstrem developed hectres $ per developed cres. The former methodology is vlue bsed while the ltter is cost bsed. The fct tht they produce comprble estimtes provides some ssurnce of resonbleness. However, the property vlue method ccounts for lesser flood events while the dmge method does not. Neither ccounts for the economic effects of greter thn 0.01 nnul probbility events. In ddition to property vlue benefits, infrstructure design requirements re ssessed. Benefits in voided costs for culverts totled $3,315,000 or $820 per upstrem developed hectres $340/cres. These estimtes re limited to existing culvert structures. The svings ttributble to fewer nd smller new instlltions s development progresses re not included. Here gin, our estimtes of benefits re conservtive. The cse study provides n ppliction of methodology for ssessing economic benefits. It uses widely vilble dt, nd stndrd prctices to exmine the direction nd mgnitude of off-site benefits. To improve the ccurcy of the results, severl dditionl steps could be included. Benefits could be integrted cross the probbility distribution of flood events insted of the single, lrge mgnitude event 0.01 probbility used here for consistency with flood insurnce policy. More ccurte bse property vlues my be obtined from prcel level dt from sle trnsctions or ssessors dt. However, vrition in property chrcteristics could offset ny potentil improvement. Appliction of the methodology t multiple scles of urbn development nd in different wtersheds would provide useful dditionl informtion on the rnge of potentil benefits from flood reduction. Additionl specific study on property vlue chnges due to different development prctices would lso improve the estimtes bsed on relted conditions used in this study. Potentil benefits not considered directly in this nlysis re wter qulity benefits, quifer rechrge benefits, or hbitt vlues. An increse in downstrem storm wter relted costs re often ssumed necessry outcome of urbniztion. The results reported here provide perspective on the extent to which conservtion design of residentil developments cn mnge these costs. The results indicte tht implementtion of upstrem conservtion design prctices should hve substntil off-site benefits in ddition to ny on-site economic benefits. A remining reserch opportunity is to compre these benefits to the incrementl cost of conservtion design prctices. Acknowledgments This study ws supported in prt by the U.S Environmentl Protection Agency through Awrd No. X to the Conservtion Reserch Institute nd by the Illinois Agriculturl Experiment Sttion nd Coopertive Sttes Reserch, Eduction, nd Extension Service, U.S. Deprtment of Agriculture under Project No Any opinions, findings, nd conclusions or recommendtions expressed in this publiction re those of the writers nd do not necessrily reflect the views of the supporting gencies. Without impliction, the writers thnk Tom Brody, 42 / JOURNAL OF WATER RESOURCES PLANNING AND MANAGEMENT ASCE / JANUARY/FEBRUARY 2006

9 John Huglnd, Dvid Soong, nd Jim Vn Der Kloot for their dvice. The comments nd suggestions of the reviewers were lso very helpful for clrifiction. References Arendt, R. G Conservtion design for subdivisions, Islnd, Wshington, D.C. Blckberry Creek Wtershed Committee Blckberry Creek Wtershed Mngement Pln; Kne nd Kendll Counties, Ill. Brden, J. B., nd Johnston, D. M The downstrem economic benefits of storm wter retention. J. Wter Resour. Pln. Mnge., 130 6, Conservtion Design Forum CDF Blckberry Creek lterntive futures nlysis. Proc., Conservtion Design Forum, Elmhurst, Ill. Dvis, S. A., Crlson, B. D., nd Moser, D. A Depth-dmge functions for corps of engineers flood dmge reduction studies, U.S. Army Corps of Engineers, Institute for Wter Resources, Ft. Belvoir, V. Federl Highwy Administrtion Hydrulic design of highwy culverts, Hydrulic Design Series No. 5, Rep. No. FHWA-IP-85-15, Wshington, D.C. Hn, C. T., Brfield, B. J., nd Hyes, J. C Design hydrology nd sedimentology for smll ctchments, Acdemic, Sn Diego. Kne County lnd resource mngement pln, Kne County, Ill. Streiner, C. F., nd Loomis, J. B Estimting the benefits of urbn strem restortion using the hedonic price method. River, 5 4, U.S. Army Corps of Engineers USACE The Gret Flood of 1993 post-flood report: Upper Mississippi River nd Lower Missouri River Bsins. Rep., North Centrl Division, St. Pul, Minn. U.S. Army Corps of Engineers USACE Engineering nd design: Risk-bsed nlysis for flood dmge reduction studies. Mnul No , Deprtment of the Army, U.S. Army Corps of Engineers, Wshington, D.C. U.S. Army Corps of Engineers USACE Plnning guidnce notebook. Engineering Regultion , Deprtment of the Army, U.S. Army Corps of Engineers, Wshington, D.C. U.S. Environmentl Protection Agency USEPA Hydrologic Simultion Progrm Fortrn, Version 12, Office of Reserch nd Development, U.S. Environmentl Protection Agency. Athens, G. U.S. Wter Resources Council USWRC Economic nd environmentl principles for wter nd relted lnd resources implementtion studies. Wilson, A., Sel, J. L., McMnigl, L. A., Lovins, L. H., Cureton, M., nd Browning, W. D Green development: Integrting ecology nd rel estte, Wiley, New York. JOURNAL OF WATER RESOURCES PLANNING AND MANAGEMENT ASCE / JANUARY/FEBRUARY 2006 / 43