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fh€ proposid Spring Creek inflo'"" frolT- each of t,'le irrig~ticn canals determined in the pre li mifl;J.ry des1gn of importation facil1ties 1s presented inf able 2. For both existing and future conditions, drainage ~rea tributary to Dixon Reser- voir was considered non~contributing. Calculations showed the maximum outflow Table 2 from this reservoir to be approximately 25-cfs, and this peak would occur much later than flood peah on Spring Creek. Canal Importation, Peak Inflows lOO.YearPeaklnflol<< Canal Ple~sant Valley and Lake Canal 380-cfs For existing conditions, the Spring Creek Basin was divided into 66 sub-basins with 59 channel elements. Future urbanization included theS,[MM model from the LnirnerCounty n and New Mercer Canal. Mulberry Street to Stuart Street 900-cfs canal importation study prepared by Resource Consultants. ~ith this SWMM model feeding directly intotheSpringCreekbasinmodel,a total of 105 sub-basins and 118 channel elements were included in the analysis. Larimer Canal Creek County 12 and Stuart Street New Mercer to Spring 210-cfs those of Spring Creek. Therefore, the inflows frOM Canal Importation ha~e 1 ittle effect on the total discharge for Spring Creek. The sub-basin areas, basin and channel s.lopes, thewidthofoverlandf1ow,lOd the lengths of channels were taken from the U. S. Geological Survey's 7.5 min- ute quadrangl e mapping (Reference 17) and SUilpl~mental infomation 'o'Ias t,1ken 3'i needed from the topographic mapping provided by the City of Fort Collins (Refer- Thehydrographpeaks fer the irrigation canals didnotcombinedir ectlywith ence16). ~drolo.gic Procedure fhe SWMM rnodel was prepared for existing ~nd future basin conditions separately. Exi st ing tondi t ions reflect tas j n urban i l~t ion as of J anu~ry, 1980 and ignore cl1na11mportationplans. The existing condition also ignores the irrigation c~na1S that cross the basin. Future conditions reflect basin development as He percent of impervious co~er was determined by ident ifyin9 land uses throuqh- out the basin from planning and zoning maps of the City of Fort Collins and LarimerCeunty(References17and18). Typical values of the percentage of lm- pervious area for each land use were then assigned and an imper~ious percentage identified on planning and loning maps of the City of Fort collins and Larimer County. In Larimer Countj, ueas currently zoned agricultural were changedtc lightresidel1tial which is tlie expected type of development. Canal importation ....as included in this analysis to reflect the preli~inary design discharges pre- pared by Resource Consultants. In this case, a11 storm wJter drainage abo~e the was esti'!'ated for E'ach ~IJl>-l>~~ in. Thl' pl'rrentagl' of i'lpl'rviolls a rea assumed for each land use is presented in Ta~le 3. C<lna 1 s W"S a~~u;r,ed to be in te !'\':epted by the cana 1 ~, however. some excess flows R~ infd 11 d<lta WdS the Sa'fl2 for both exis ti ng and future conditi ons. The ra i n fa 11 depths werE' Obtained from the Precipitation-Frequency Atla, o' the Western U~ited States (Reference Ig). The tota 1 ra intal1 depth for th~ flood events andly~ed in this study are presented in Table 4. were considered to spil1 across the canals. .13- -14-