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<br />Alternates were developed considering such variabk~ as regional detention facilities, IO-year
<br />neighborhood de1ention facilities, and regional conveyance The ]I)()-year and 50-year alternate
<br />hydrologicmoJelsincorporaleonlyregionaJdeletllionfacililiesand conveyance strategies_ The 10-, 5-,
<br />l-year, andO.4-inch rainfall modcls include, in addilion 10 regional facilities, neighborhood Jetcntion
<br />facilities at selected sub-catchment outlets to determine if the magnitude of IO-year peaks produced
<br />under future developed conditions can be reduced to those produccd un der existing conditions
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<br />Two alternates wefe selected by th.e Project Sponsors for final consideration Drainage facilities
<br />proposed in each alternate are presented on Figures lla and I lb. VDS"'"\.{ flow charts for the alternates
<br />are presented in Appendix A. Alternate 1 assumedexJstlngdetent!onfactlitiescouldbeen[arged and/or
<br />modified to function as regiolJal facilities, enlarging existing de tention facilities, culverts, storm sewers,
<br />and channels and proposing additional faCilities Alternate 2 minimizes improvements to the existing
<br />stormwater infrastNcture and assumes an easterly flow path tbHowing the VPRR for stormwater
<br />conveyance for the area near the Pre-School and High School. Alternate 2A is similar to Alternate 2,
<br />hut assumes a northerly flow pmh for the area near the Pre-School and High School Alternate 2 or 2A
<br />uses less commercial area for stormwater conveyance than Alternate I and is preferred by the Town of
<br />Eric. Thc Town of Erie is currently negotiating land acquisition for properties within the easterly flow
<br />path assumed in Altemate2.
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<br />D. Limitations
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<br />The locations of proposed regional detention and conveyance facilities are ba:>ed on engineering
<br />judgment, constructibility, inp\it fi'om the Advisory Board, practicality, anticipated cost minimization,
<br />and legal requirements Topographic variability within the studv area required grade, and inverhofall
<br />proposed facilities tobe conceptuaily confirmed using topographic maps. Csing these criteria andthc
<br />d~sign strat~gy r~C"lllmendatiu<Is, unen "<Ily on~ aj~lI i, cuno,;~ptually "b~,t.' tor r~gional dc!Clllioll
<br />facility locations
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<br />Limitations of the modeling effort for the alternate anal}sis include:
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<br />. all regional facilities must be comtructed to mitigate the impact of development on SlOrmwatcr
<br />nows,
<br />. normajdepthandinletcuntrolarea5sum~d.
<br />. local and neighborhood detention facilitics. including existing facilities in the City of Lafayetl~, are
<br />not modeled,
<br />. 10-Y(jilr dctelllion facilities in th~ hydrologIC Illodclllre indudcd for dcmonslllltion purpt'>c, onl}'
<br />(locations and number of ponds are incomplete, and developmcnt pIa n, will dictate their locations),
<br />and.
<br />. wMer quality provi,ion~ in the lO-ye,lr and local dl'1ention facilities ,hould redllet' flood peak
<br />magnitudes for morc frcquent events (limitations of the model scale prohibit;; predictlng these ctfects
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<br />liY ma',c.1I1i(
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<br />Re~ults
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<br />flood peaks at selected design points for existing and future percent imperviousness are compared to
<br />alternate storage/C(lflveyance !trategie~ in Table 13 for the 10- and 100-year events, Location of
<br />proposed detention facilities, regional conveyance facilities, and conveyance improvements are shown
<br />on Fi""rcs 12a and 12b Hydrographs for Ihe 100-year event at selecled design points filr existing,
<br />future, and alternate storage/conveyance strategie~arecompared on Figure]O
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<br />1. Coa] Creek Tributaries
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<br />Regional detention facilities are planned upstream of !>.'E County Linc Road for catchmcnts flowing
<br />eastwardly towards Coal Creek. Alternate detention/conveyance Slrategies are planned to combine
<br />catchments upstream of 1'.10 County line Road to minimize the number of culverts below )ffi County
<br />Line Road and to minimize the number of regional channels discharging to Coal Creek. Combining
<br />catchments means flood peak magniludes may exceed existing flood peak magnitudes for some alternate
<br />flow paths, One example is found at Design Point 514. where sub-catchmenb 446 (Alternate I) or ,ub-
<br />catchments 435, 436, and 446 (Alternate 2) are combined and discharge into the regional channel
<br />identified in the model as conveyance element 445, The combination of the sub-catchments eliminate,
<br />at least one channel The proposed regional channel 445 must be designed to accommodate a larger
<br />tlood peak than experienced under existing conditions,
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<br />For the Catchment upstream of the High School and Pre-School, Alternates I and 2 differ in the location
<br />of regional detention facilities Alternate I requires the existing d,,>tention facilities in Orchard Valley
<br />and Canyon Creek be enlarged and the construction of an off-channel rcgional detention facility
<br />upstream of the t;PRR Alternate 2 requires a trans-catchment diver~ion to a new detention facility
<br />upstream of Lcon A. Wurl Parkway, elimination of the Canyon Creek detention facility, and
<br />coJlstru~tion ofa new oll-~hannel detention facility upstream of the UPRR
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<br />The 100-year flood peak from the Catchment up,tream of the High School and Pre.School dischargIng
<br />to the I)PRR (Design Point 532) will bc attcnuatcd to bclow existing magnitude$ for cither alternate
<br />detention/conveyance strategy, as seen on Table 13 and figure 11, In Alternates I and 2: stormwatcrs
<br />from Design Point 532 are discltarged into an open channel running adjacent to and south of the L"PRR.
<br />to NE CounlY Line Road (Design Point 536) and cventually to Coal Creck (Design Point 915) In
<br />Alternate 2A. stormwaten' fnlm Oesign Point 5.12 are discharged along the nistoric flow path bdow tne
<br />UPRR into an open channel which crosse" Jay Road, the Lower Boulder Ditch, and :-iE County Line
<br />Road before discharging into Coal Creek.
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<br />Developments will be required to provide IO-year detention and water quality facililies Since the
<br />location ofthe"e facilitie, is not known. they have gelleraliy not been included in the mudeling e[tort
<br />and their impact is nOI shown on Table 13, However, flood peaks shown on Table 13 indicates regional
<br />pomh will attt'nuate :lood pcab from the Ie-year evcnt ur.dcr f.;llv dcvclopcd p.'rccnt IIT.pcrv:ou>;ncs>
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