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<br />EXECUTIVE Sl.:MMARY
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<br />Rapid development in the Erie Planning Area and adjacent areas, including portions of Town of Erie,
<br />City of Lafayette, Bouldcr County, and Weld County, has resulted in an immediate need for an outfall
<br />~ystem planning slUdy_ The outfall system planning study evaluates the impact of existing and projected
<br />future development on flood peaks and presents conceptual designs of alternate ~tonnwater outfal]
<br />systems which safely convey the projected lOO-year flood under futuredeve]opment through the '1udy
<br />area, The alternate conceptual designs will be presented to the governing entities for their considcration
<br />and selection of the "best" alternate
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<br />The study area encompasses approximately 17.3 sq, mi. and is located near the confluence of Coal
<br />Creek and Boulder Creek It ineludes the lower portions of three major watersheds: Coal Creek,
<br />Bullhead Gulch! Prince Tributary, and Lower Bou]der Creek,
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<br />Thc study area is currently in transition from agricultura] activities to subdivi~ion and commercial
<br />development and is characterized by the lack of storm water conveyance facilities Flooding along major
<br />channels within the study arca isrepol1ed in the historic record and local flooding has recemlybecn
<br />observed Man.ychannels have becn modiliedby agriCI.l]tura] activities andar eundersized, Thereare
<br />no culverts below roads at most channel crossings; even culverts along the recently re-constJ1.lcted
<br />Highway 287 arc substantially undersized !rriga!ion ditches capture, transpol1, and re-direct stormv.'ater
<br />!low paths to undefined areas, Two railroad lines located within the study area impound water and
<br />modify stormwater flow directions Exi:;ting detention facilities are too small to accommodate
<br />anticipated fmurcdeveloprnent and. in some cases, existingdevelo pmenl,
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<br />Exisling and votential fi.Jture predicted flood peah within the stud:.. area arc defined using hydrologic
<br />models developed from recent aerial topographic mnpping and land use and ~oil~ maps Flood peaks are
<br />estimated for the averagc ~tc>rm and the 2-, 5-, 10-, 50-, and lOO-year evenu The hydrologic analysis
<br />indicates more frequent and possibly severe flooding can be expected if development proceeds as
<br />anticipated without improvements to the stormwnter con',.-eyance system
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<br />To accommodate the various needs and planning etTol1s of thc communities within the study area,
<br />drainage de~ign guidelines are developed for detention. conveyan.ce, and road and railroad eros~ings for
<br />each community Regional and neighborhood detention is proposed to be primarily used within the Erie
<br />Planning Area to reduce majO! and minor developd flood peah to historic magn.itudes and improve
<br />surfacc water quality 1\finimal stormwatcr infrastructure is proposed to be constrJcted in !:lege areas
<br />dedicated for open ~pacc within both the Erie Planning Area and Boulder County not severely impacted
<br />by developed /lows, Lands located witbin City of Lafayette will cmploy ~ma\1 detention fadlities in
<br />each ,,Ie,'dopmelll fur ~torlllwater control In both the Eric Planning Area and in Buulder Coun.ty,
<br />detention will be used immediately above road crossings to reduce overall drainagc infrastructure
<br />construction costs by reducing culvel1 Slze~, avoiding repetitive embankmen.t costs, and r<_'llucing the
<br />numb~'T of outfall channels east of l\:ol1heast County Line Road, The 100-\car floodplain along Coal
<br />l'reekand ljoulderCreekwillbeprc'crved
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<br />The drama!!e de~lgn .b'1lldeline~ eliminate pntential outfall ~olmion~ which are unpractical. inetfect;ve.
<br />expen,ive. mun>>.1fe, such as llsmg largl' channels ttl convey undetall ledl!eve]oped ntl""thmughthe
<br />"lIdvar<'a nl<' (v.o (}tufall "It"lnale, cltl''''11 fo, f,na] e\idluali(,1\ arc bvdralllil:alh' efflcicnt and
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<br />minimize the co~t of drainage improvements The first alternate assumes existing detention facilities
<br />could been]arged and/ormodified to function as regional facilities. The second alternate aSSllmes area,
<br />of proposed open space can accommodate new regional detention fa ci]itie~
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<br />Bnth outfall alternates convey the 100.year flood safely through the ~tudy area and require the
<br />construction of new regional det<'11tion facilitics, channel improvements, culvcrtS below roads. and
<br />improvements to irrigation ditches, For the first alternate to be succe~sful, significant amounts of
<br />additional]andadjacenttocxistingdetention facilities in area scommilledtodevelopmenlarercquired
<br />The second allernate can be implementcd primarily using land for detention in area~ designated as open
<br />space
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<br />Two flow paths from the catchment lributary to the Forever Young Day Care and Pre-Schoo] which
<br />recently experienced flooding were examined asa subse10fthe second altemate~ including a northern
<br />flow path and an eastern flow path The cOSt of the eastern flow path is slightly less than the nOl1hern
<br />flowpath,andlandnegotiationsareundenvayfortheea~ternflowpath
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<br />Tbe second alternate with the eastern !low path is the Engineer's recommended outfall sy~tem The
<br />total estimated cost for drainage improvements for this alternate is approximately $65.100,000 Thc
<br />ma.iorityofthisco~tshouldbefundedbyfuturedevelopment,
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<br />The alternate outfall systems evaluation is spon~ored by Urban. Drainage and Flood Control District and
<br />the Town of Erie and engineered by Love & Associates, Inc. The study was conducted with
<br />participation from representatives of the Urban Drainage and Flood Control Di~trict. the Town ofETie
<br />Planning and Engineering Departments. Boulder Cnunty, Co]oradn Department of Tran,portation, and
<br />Cnion Pacific Railroad,
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