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<br />I <br />I <br />I <br />i I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />SECTION VI <br />FLOOD FREQUENCY <br /> <br />Rainfall during the storm in the Frijole Creek basin above the <br />Colorado and Southern Railway bridge averaged 11,8 inches, with rainfall <br />varying from less than 8 to in excess of 16 inches, This is approxi- <br />mately 4 times the rainfall from a 100-year precipitation for the area, <br />Based on USGS flood data at the gaging station at U. S. Highway 160, the <br />flood of 28,400 cfs in Frijole Creek exceeded the previous maximum by a <br />factor of 2.1. <br /> <br />The Federal Railroad Administration (FRA) in its report on the <br /> <br /> <br />train accident used an empirical method developed by the USGS and <br /> <br /> <br />Colorado Water Conservation Board to compute various flood recurrence <br /> <br /> <br />intervals for the Colorado and Southern Railway bridge site, The <br /> <br /> <br />recurrence interval is defined as the average number of years between <br /> <br /> <br />events of equal or greater magnitude. A 100-year storm hss a one <br /> <br /> <br />percent chance of occurring in any given year, a 50-year storm has two <br /> <br /> <br />percent chance, etc. The method used by the FRA produced a 100-year <br /> <br />recurrence interval flood of 11,821 cfs and a 500-year flood of 22,274 <br /> <br /> <br />cfs. The peak discharge at the railway bridge of 50,000 cfs was greater <br /> <br /> <br />than the runoff produced by a 100-year recurrence interval storm by a <br /> <br /> <br />factor of approximately four, and greater than the 500-year storm by <br /> <br />slightly more than twice. <br /> <br />COMPARISON WITH PREVIOUS FLOODS <br /> <br />The rainfall and flood discharge which occurred in the Frijole <br /> <br /> <br />Creek basin on July 2-3 was unusually large, although other more severe <br /> <br />storms have occurred elsewhere. Since there was no record of previous <br /> <br /> <br />flood damage at the bridge, the storm evidently produced the highest <br /> <br /> <br />runoff since the Colorado and Southern Railway was constructed in 1909. <br /> <br />There were no active USGS streamflow gaging stations in the Frijole <br />Creek basin at the time of the flood. Records are available for USGS <br />Gaging Station No. 07125100 for the period from March 1958 until it was <br />discontinued in September 1968. This station is located at the <br /> <br />VI-I <br />