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<br />2000 MDT and was reported as being most intense between 2100 and 2200 MDT. <br />Between 2200 on July 31 and 0100 MDT on August I, the part of the St01111 system that <br />caused flooding along the Big Thompson River moved over the Bellvue area and <br />produced flooding in Rist Canyon, on several Cache la Poudre tributaries near Bellvue, <br />and on the downstream tributaries of the North Fork Cacr.e la Poudre River. Fi!'ure 7 <br />" <br />also shows the apparent motion vector ofthe "detached" stonn system, based on satellite <br />imagery, which caused the flooding in the Cache la Poudle basin and Rist Canyon. <br /> <br />3.6 Frijole Creek, Colorado; July 3, 1981 <br /> <br />During the night and early moming hours of July 2..3, I % I, an intense, short-duration <br />thunderstonn occurred over the upper regions ofthe Frijole Creek and San Francisco <br />Creek basins about 7 miles southeast of Trinidad, Colorado. The hydrology of this event <br />is described in a report by Bishop (1982). The Trinidad a.-ea was deluged with 3.66 <br />inches ofrain. Rainfall that exceeded the 4-hour Probable Maximum Precipitation <br />(PMP) occurred over 1 squal'e mile and over 22,7 square mile portions of the 80 <br />square mile Frijole Creek Basin. A flash flood resulted which caused the failure of a <br />Colorado and Southem Railway bridge. Two people in the train engine were killed when <br />it plunged into the creek fonnerly spanned by the bridge. This was the only storm <br />causing damage to this bridge since it was built in 1909. Rainfall over the 22.7 square <br />mile area above the railroad bridge averaged approximately 11.8 inches over 4-hours. <br />This compares with a 100-year stonn precipitation of3.1 inches and a local storm PMP <br />rainfall of 10.9 inches. Peak discharge from this stonn at the failed railway bridge was <br />approximately 50,000 cfs, or 4 times the peak runoff estimated from a 100-year storm <br />(Bishop, ibid). Rainfall from this event exceeded the rain:all produced by the Big <br />Thompson Canyon thunderstorm system. Loss of life was held to a minimum only <br />because the area affected was remote and sparsely populated. <br /> <br />Figure 8 is a map showing the topography in and around tile affected area. Figure 9 is a <br />location map identifying key points affected by the flash flood. Fishers Peak and Raton <br />Mesa (see Figure 8) f01111 the upper limits of the Frijole Creek basin with major streams <br />above the failed railway bridge including El Poso Arroyo, Bill Creek, Capulin Anoyo, <br />and San Miguel Creek. Elev8tions increase quickly from about 5,800 feet msl at the <br />bridge site southwestward to Raton Mesa where elevation!; range from 9,200 to 9,600 feet <br />msl. Average elevation in the 22.7 square mile portion of the basin atIected by the flash <br />flood averages about 7,500 feet msl. Channel slopes in the affected basin average about <br />5% and about 1% in portions of the channel below the railway bridge site. <br /> <br />As reported by Bishop (ibid) there were no weather stations in the flash flood area. <br />Bishop reconstructed stonn rainfall through a series of interviews with local residents and <br />from access to unofficial precipitation measurements. According to these resi dents, the <br />stonn began at approximately 2200 MDT with the most intense rainfall lasting for about <br />4 hours. Very little precipitation preceded or followed this St0I111. For the first two hours <br />of the event, one resident measured 6 inches per hour using a 6-inch raingage but did not <br /> <br />17 <br />