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I I A discharge profile througMut thestlJdy reach for each f100devent is shown in Figures3and4forexistingandfullydevelopedconditions. respectively. Some typica\hydrographsforthel00-yeareventarepresentedinFigures5and6. Tile lQwerdischarge:; calculated in our stu<!y can be attributed to th e four natural detention areas previously mentioned. These areas were considered in the present study and not in the Flood Insurance Study. These detention a~as contribute to the redudion of peak discharges east of Taft Hill Road. The lOO-yearundetained flow for existing basin conditions was also examined and found to be within lO~ The importation of flood flows by means of the canals had little eff ectonthe total discharges for this study. Inflows from tl1e l;anals generally did not com- binewiththemain peak flows that were generated on Spring Creek. Therefore, the maximum increase in peak discharge for the lOO-year flood event due to canal importation is estimated as 300 to 400-cfs anywhere along Spring Creek. of the discharges in the FIA study. Amore thorough description of the hydrology prepared for this relXlrt is presented in the Technical Addendum. KYORAULlCS Procedure Discharges were also developed for the 100-year fully developed basin without considering inadvertent detention storage at roadway and railroad embankments. (Undetained flow) These values are shown in Table 7. A hydraulic analysis was completed along the 7.5 mile study reach to determine water surface profiles for the 2~year, lO-year, 25-yedr, 50-year, lOO-year and 500- year flood events for existing and fully developed conditions. These water surface elevations were computed using the HEC-II conputer model for backwater analysis (Reference 21). Water surface profiles for these flood frequency events are present- edonSheets 16 ttrrough 29. The lO-year, 50-year, lOO-year and 500-yeu frequency Discharges forexistingdevelopmentdeterminedinthisstudyascomparedtothose developed in Ule FlOOd Insurdnce Study (Frs) for Spring Creek are listed in Table 7. wdtt'r ,~rraLe el~v~tion, dt ,,~ch of the cross sections are t;;bulated in '[;;ble ~. Table 7 Cross sections used in this study were developed from field ,urveyed information. These cross sections were supplemented as required with CroS5 sections taken direct- ly from topographic mapping. The geometry of all crossing st"~ctures was frf'ilsured iuthefield. FIA Oischdl"lles: Existinlj Development Without Cana 1 Importatio.~ Locdtion 100-Yedr FIA Study lOO~Year This Study lOO-Year Undetilined l,S70-cf, 1.0i)O~d, 3,lOO-cr, 1,870-cfs 1,6S0-cfs 4,500-cfs 2,650-(f<; 1,740_cfs 4,800-cfs 3,300-cfs 2,ISO-ds S,OOO-cfs Manning's roug~ess coefficients ("n" values) were determined through field in- Shields,il,venue College Avenue L"MayAvenue Confluence with Cache La Poudre River spection. Aphotographicsunmary, by reaches, of the Spring Creek floodplain was prepared,andthe"n"valuesusedforchannelandoverbankareaswere presented to the Technicdl Comittee for review prior to completion of the hydraulic .17_ -18-