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<br />FUTURE FLOonS <br /> <br />de0ree of accuracy. Peak discharges for the Standard Project Flood at <br />selected locations are also shown In table 7. <br /> <br />Floods of the same or larqer maCinitude as those that have <br />occurred in the past could occur in the future. To de~~rmine the flood <br />potential of the study area, two floods, the Intermediate Re0ional Flood <br />(IPF) and the Standard Project Flood (SPF) were analyzed and the results <br />are presented in this report as a means of demonstratina the effects of <br />larne floods To Illust t th t I fl <br />., .' ra e a arne oods I ike these can haopen, con- <br />sider the Denver area floods of 1965 and the Raoid City, South Dakota <br />flood of June 1972. The ma<1nitude of these floods at various locations <br />was comparable to that of an SPF. <br /> <br />Table 7 <br />Potential Flood Peak Dischar'JAS <br />for the Cache la Poudre River <br />in the Fort Coil ins to Greeley vicinity <br /> <br />Location <br /> <br />Upstream Um it <br /> <br />Upstream from Roxe I der Creek <br /> <br />Downstream from Boxelder Creek <br /> <br />Upstroam from Con so I idated <br />Law Ditch <br /> <br />Oownstreilm from Conso 1 i dated <br />Lmf Ditch <br /> <br />Downstreilm Limit <br /> <br />I NTERr.1ED I ATE REG I ONA L FLOOD <br />The Intermediate Reqional Flood Is defined as one that occurs <br />with an averaqe frequency of occurrence of about once in 100 years, al- <br />thouqh it could occur ISh <br />'n any year. uc a flood is a relatively lar<1e <br />one and is used to define the flood nlain for reoulatory <br />purposes. The <br />peak flows of this flood were developed from statistical analyses of Cache <br />Ie Poudre River stream flow records at the U.S.G.S. stream 'laCes located <br />near Fort Col I ins and Greeley, Colorado, usinq routino techniques in <br />conjunction with reqional analyses to define flows between oaoes. Peak <br />flows thus developed for the Intermediate Reaional Flood at selected <br />locations in the study reach are shawn in table 7. <br /> <br />IRF Dischareo <br />c. f .s. <br /> <br />17,50cr!f <br /> <br />16,61)0 <br /> <br />28,500 <br /> <br />SPF Di scharqe <br />c.f. s. <br /> <br />41,f]1)J! <br /> <br />30,21)0 <br /> <br />75,200 <br /> <br />6,600 <br /> <br />22, I on <br /> <br />11,(\0f) <br />9,000 <br /> <br />37,000 <br />26,400 <br /> <br />1/ The discharge values shown at the location of the downstream limit <br />of the study reilch in table 4 of Volume I are incorrect. The correct <br />values are as shown in the abeve table and represent the actual values <br />used in the hydraulic computations of both the Volume I and the Volume ill <br /> <br />The frequency of recurrenco of a certain size flood, or the <br /> <br /> <br />probability of its happenin0, can be illustrated with known flow data. <br /> <br /> <br />The flood of June 1965, with a peak discharge of 3,4RO cubic feet ner <br /> <br /> <br />second at tho ll. S.G. S. stream 0age near Gree I ey wou I d occur on the averaqe <br /> <br /> <br />once every I I years. The average annual peak discharge at the qaCie is <br /> <br /> <br />813 cubic feet per second. As can bo seen from tables 5, 6, and :, dis- <br /> <br /> <br />charges smaller than either the Intermediate Regional Flood or thn Standard <br /> <br /> <br />Project Flood are much more common. Ibwever, floods even larger than the <br /> <br /> <br />Standard Project Flood could occur hut conditions necessary to bring ahout <br /> <br /> <br />such an event are extremely rarn. Ice conditions can croate SAver" <br /> <br /> <br />flood cunditions with rAlatively little discharCie. <br /> <br />n~ports. <br /> <br />STANDARD PROJECT FLOOD <br />The Standard Project Flood is defined as a major flood which <br />can be expected to occur from a severe COMbination of meteorolo9ical and <br />hydrological conditions that are reasonably characteristic of the qeo- <br />qraphical area in which the study area is located, excludin~ extremely <br />rare events. Storms that would produce this flood are unco~mon, and <br />it is difficult to assi~n frequencies of occurrence with any reasonable <br /> <br />FHEQUCt!CY <br /> <br />25 <br /> <br />20 <br />