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FUTURE FLOonS dC0ree of accuracy. Peak discharges for the Standard Project Flood at selected locations are also shown in table 7. Floods of the same or larger magnitude as those that have occurred in the 'last could occur in the future. To de~~rmine the flood potential of the study area, two floods, the Intermediate Regional Flood (IPF) and the Standard Project Flood (SPF) were analyzed and the results are presented in this report as a means of demonstratinn the effects of larne floods To Illust t th t I fl . . ra e a aroe oods I ike these can haopen, con- sider the Denver area floods of 1965 and the Rapid City, South Dakota flood of June 1972. The mannitude of these floods at various locations was comparable to that of an SPF. Table 7 Potential Flood Peak Dischar'JAS for the Cache la Poudre River in the Fort Coil Ins to Greeley vicinity Location Upstream Um it Upstream from Roxe I der Creek Downstream from Boxelder Creek Upstroa", from Con so I idated Law Ditch Oownstreilm f rom Canso I i dated Lmf Ditch Downstreilm Limit I NTERr.1ED I ATE REG I ONA L FLOOD The Intermediate Regional Flood Is defined as one that occurs with an averaqe freouency of occurrence of about once in 100 years, al- thouqh it could occur ISh ''1 any year. uc a flood is a relatively laroe one and is used to define the flood olain for reoulatory purposes. The peak flows of this flood were develoned from statistical analyses of Cache la Poudre River stream flow records at the U.S.G.S. stream ganes located near Fort Col I ins and Greeley, Colorado, usinq routino techniques in conjunction with regional analyses to define flows between oaoes. Peak flows thus developed for the Intermediate Renional Flood at selected locations in the study reach are shown in table 7. IRF Discharoo c. f.s. 17,50cr!f 16,61)0 28,500 SPF Di scharqe c.f. s. 4I,f]I)J! 30,21)0 75,200 6,600 22,lon 11,(1)') 9,000 37,000 26,400 1/ The discharge values shown at the location of the downstrea", li",it of the study reach in table 4 of Volume I arc incorrect. The correct values are as shown in the above table and represent the actual values used in the hydraulic computations of both the Volume I and the Volume ill The frequency of recurrence of a certain size flood, or the probability of its happenin~, can be illustrated with known flow nata. The flood of June 1965, with a peak discharge of 3,4RO cubic feet oer second at the ll. S.G. S. stream 0aqe near Gree I ey wou I d occur on the average once every I I years. The average annual peak discharge at the qage is 813 cubic feet per second. As can bo seen from tables 5, 6, and :, dis- charges smaller than either .the Intermediate Regional Flood or thn Standard Project Flood are much more common. Ibwever, floods even larger than tho Standard Project Flood could occur hut conditions necessary to hring about such an event are extremely rarn. Ice conditions can create severe flood cunditions with relatively I ittle discharge. n~ports. STANDARD PROJECT FLOnn The Standard Project Flood is defined as a major flood which can be expected to occur from a severe co",bination of meteorological and hydrological conditions that are reasonably characteristic of the geo- graphical area in which the study area is located, excluding extremely rare events. Storms that would produce this flood are uncommon, and it is difficult to assign frequencies of occurrence with any reasonable FHEQUCt!CY 25 20