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within the Basin since the completion of the Gingery study which would affect the basin hydrology. Construction plans, drainage reports and hydraulic analyses for each of these improvements were obtained and reviewed to determine the effects of each on Spring Creek. AS previously discussed, several limall det.ention areas were not inclUded in the final SWMM analysis, as these were deemed to have little impact on the overall basin. other improvements, such as larger detention basins, improved bridges, and channelization projects, were included in the analYBis. necessary to represent future basin conditions. A detailed description of all modifications to the hydrologic model can be found in the TeChnical Addendum. AlsO contained in the Technical Addendum is an analysis of the differences between the modified hydrology with the hydrology of the previous study. 3.4 Existing and Developed Basin Peak Discharges A field reconnaissance of the study reach from Drake Road to stuart street was conducted on April )0, 1986. A reconnaissance of the rem<linder of the study reach from Stuart Street to the confluence with the Cache La Foudre was comlucted on M<lY 28, 1986. Approximately 180 photographs were taken along Spring Creek and at crossing structures for use in developing M<lnning's "n" values for use in the hydraulic analysis. For purposes of comparison with the Gingery data, existing channel conditions throughout the study reach were alsO noted during the reconnaissance. Peak flows determined by this study for existing and developed conditions are identified in Tables 5 and 6. They are gener<llly similar to those previously determined J.n the Gingery Study. However, the dam failure assumption of this study results in approximately 25% higher peak flows downstream of College Avenue for the 100- and 500-year events. lOO-year peak discharges upstream of the BNRR are approximately 15% greater than the Gingery analysis. Peak discharges for events more frequent than the 100- year storm ranged from 1.05 to 1.30 times greater than the Gingery study. Thes.. increases result from increased imperviousness and channel improvements. The original "future conditions" SWMM data tile fcom the Gingery study was modified to reflect the findings of the field and office investigations. Rainfall hyetographs used in this analysis were identical to those used in the Gingery "nalysis. The most significant change to the Gingery analysis is the assumption, based on the Chen embankment stability analysis, that the BNRR embankment will fail during the 100- and 500-year events. The Gingery study assumed inadvertent detention at this location for elll ev<;>nt... This study assumes inadvertent detention for the 50-year and lesser event". Other modifications consisted of changes in percent impervious area and chdnrlOl configurations; C1dditions of subcatChments, dptention areas, and gut_t_<;>rs and pipes; and deletion of two subcatchments. Utilizing the City Zoning Map, ch"nges ....ere made to the percent impervious <1rea Pdrd"l<-'lor 20 HvdrcKtraphs Hydrographs of the 2-.10-,25-,50-,100-, and !)OO-yea.r storm events at various locations are present.,d in the Technical Addendum. The hyd:rographs represent developed baBin, exi.sting Channel conditions. The double peak apparent in the 100-, and 500- year hydrographs downstre"m of College Avenue are a n'J;IJ1t of the modeled darn break. The first sm"ller pei\k represents 10<:,,1 infloW!", while the second larger peak rCFJI-esents the dam brea<:h. As expected, those locations high up in thE' b""in pxppr'ipnce i1 quick time to peak with an associated riipid decre"sing flow, whereas the locations f\lrther downstream deveLoped" later peak which decreases more gradually. ~ 1