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HYUi<OLOGY I I G~r.era 1 The Scope of'.'ork required an analysis to determine runDff rate 5 for the 2, 10, 25, SO, 1 UO ~nd SOO-y('ar flood frequent ies for ex i st i ng bas in eondi t ions and for full development basin eo,,~i~ioES wi~h ,.;"n~l importation. Full development bd- ~ir, cc,1'~i,:j::::<; ','U'~ ~'~'i~I~P"~ ~y prujectiun, ()Dtd';neu frYI ~ld;,~'il,S d'-,~ zu;-,j,:<: C""].s of lcca 1 governments. The hYdro-,cg:' ~e,.ef';i,,~" ir, nis st~:(;y was uSf:d to develop water surface elevations for these flood events throughout the study redch. The Storm '~dter Management Model (SWI1M) was proposed for use III thi s study and wasapprovedbythpTechnicalCorrrnittee. ThiSrrlode1wasdevelopedbytheElI- vironmental Protection Agency and modified by the Cinaha District of the Army Corps of Engineers. The SWMM Model is a kinematic '~ave hydrologic cOOtputer rrodel dt'veloped specifically for use in urban areas. Thisrrodel considers rain- fall losses to soil infiltratior1ilnddepressionstorage,attenuationofflO\1s throughchdllnel stordge, attenuation of flows through reservoirs and routirlg and laggirlg of hydrographs dowrlstredm. lrri.9.d..tion Canal~ Four irrigation canal, era" tr.~ Spring Cree~ b~sin and drainaSewdY. The Plea san t Vall ey and La ke Can a] c ros se s wes t 0 f Taft Hi 11 Roa d, whi 1 € t he r~ew M~rcer Carl~l, and the ldrimer County a C~ndl cross Spring Creek between Shields ::,trcet and C'ra~e Road. lr.e Arthur's Ditch ~ets Spring (r""" ju,t up"t'-~dl" uf College Avenue and the ColoradO & Southern Railroad. t:d<.;h nf thes~ c<lnal s, \oih~ther ly tJeo;ign or in~dv~rtl'n tly. intl'rcerts <;tom dr~inage during rainfall-rulloff ~~~"l,. Ed~1I cdllal hds a structure at its Spring Creek crossing to spill ~ portion of this ('xcess stor'M drainage. ilow- -11- ever, the spill is controlled manually at Spring Creek and Dpeningdheildgate or pulling flash boards during a storm event can be difficult and darlgerous. In addition, the operator may not be able to reach the spill structure in time toreleasepeakf1ows. Therefore, spill of excess drainage flows directly into Spring Creek may not occur but ratherrilndollloverf1ows dlong the 1 engthofthe ditches. (SeeSheetslO, 114nd13). The City of Fort Collins io; considering the importation of drainage flows into the Spring Creek basin in ~n improved Pleasant Valley and lake Canal, New Mercer Canal and l~rimer County #2 Canal (Reference 12). This system would intercept storm runoff for up to a lOO~year flood event in the basin north of Spring Creek ami would del iver these flows to the Spring Creek drain~geway. At Spring Creek, an uncontrolled spill structure would insure delivery of flows. Canallmportatiol'l lmportation of storm water drain~ge from the drainage basin immediately North of the Spring Creek Basin was first proposed in the "Report on Storm Drainage Re~ QUire~lCnts. Prooosed Facil ities" (Referpnc_e 13). The "RPI)Ort on Storm Wi>tl'r Drainage Facll ities for Fort Collins, Colorado" (Reference 14) looked ~t storm wdter importation in greater detail, recOlllllended a 50-year frequency design for the systemanddeternined the design discharges. The "Feasibi1ityStudies for the Diversion of Storm ~ater RunOff through lrri9~tion Canals from Mulberry Str~et to Sprin~ Crf'pk" (R..f..r..no' 15) pVi>l"ilt..o th.. hyrlrolo!JY in fllrth"r ri.._ t~il and recommended canal importation of the lOO-year event. The subsequent "Preliminary Ot>sign" of these facilities by Resource Consultant srefinedthe hydrology further using the SWMM model. and these hydrographs were incorporated into this study for the analysis of future basin conditions. - 12 ~