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<br />EXISTISG-CONDlTIO;'llS HYDRAlUC A,""ALYSIS <br /> <br />(A"....nrlixc.l. The~areaswillbeevaJuatedinmoredetailduringthefeasibilityphase <br />, H <br /> <br />of the study. <br /> <br />The following is a summary of the detaikd existing-conditions hydraulic analpis for <br />Dry Creek, which is presented as Appendix C. <br /> <br />Data obtained from a previous study indicate that during a lOO-year peak discharge <br />on Dry Creek, an approximate 50-year peak discharge oa:;urs simultaneously on the <br />Cache La poudte River. A sensitivity analysis of water surfaco: profiles on Dry Creek <br />indicated that the only economic subareas affected by coincident flooding would be <br />subareas 1 and 2, which account for about I pcreent of the expected annuaJ damages. <br /> <br />The purpose of the hydraulic analysis was to identify the existing-conditions flood <br />problems, using as much existing information as possible, and to do:velop a feasible <br />design alternative to reduce the flood hazard from Dry Creek for thc City of Fort <br />Colllns, The study was conducted on Dry Creek from its confluence with the Cache La <br />Poudre River to a location approximately 6.5 miles northwest of the confluo:nce where <br />it crosses North Shields Street, <br /> <br />FLOOD OAMAGE ANALYSIS <br /> <br />The Dry Creek study reach is located in the northeast section of the city of Fort <br />Collins, Typically, the overbanks are wide and flat. In general, the channel is well <br />defined exccpt for the reach from the airport upstream to the Larimer and "'''eld Canal, <br />where little or no channel exists. Even in an.'asofa well-defined channel, tile capacity <br />is Iimil<:d by undersized culverts, <br /> <br />Land use information was collected for the500-ycar flood plain during a fi cldsurvey <br />conducted in June 1990. All values used in the analysis arc based on July 1990 price <br />levels. Included in the analysis are flood damages to structures and their contents. <br />These damages were estimated using the Corps' flood damage model. Damages to exter- <br />naJ components inc!uding utilities, streets, and clcanupeosts werebase.d 0 napcreentage <br />of the flood damages computed by the model. <br /> <br />The flood plain for the Dry Creek basin is cxtremdy wi<Jc in tbe lower jXlrtion of <br />thebasin,o:vcn though tile magnitude of the lOO-ycar discharge is relatively small for this <br />size basin. This is due to wide, flatoverbanks. The depth of flooding in theovcrbaflks <br />is seldom greater than 2 feet, and in some areas there is divided flow wbere the water <br />in tlleovcrbanks is nOlhydra\llically connected to thechannd, As a general rule, there <br />is no definable channel downstream from the Larimer and Wdd Canal until the area of <br />the airport is reached. <br /> <br />STUDY AREi\S <br />The 500-year flood plain was subdividcd into 17 subarcas to facihtate data organiza. <br />tinn and analysis, These S\lbarcas are sbown on plates 2 and 3, The subareas hegin at <br />the Dry Creek cnnfluence with the Caehe La roudre River, as shown on plate 2, and cnll <br />near North Shields Strtd, as shown on plale 3. This subdivision was neccs,ary becallsc <br />land use information is l'rgani,cd on a subarea basIs, and the Corps 'lloodllamagenloJd <br />aggregates estimatt:d damages by subarea, The sekction of subareas is also an important <br />component of the innemcntal analysis of flood control alternatives. However, for plan <br /> <br />^ cursory analysis of overlapping 1100d plains between IheCache La Poudrc River <br />and Dry Creek resulted in live areas tH,'ing affectcd, as shown on plates 5 and 6 <br /> <br />4-13 <br /> <br />4-14 <br />