FLOOD02665 - Laserfiche WebLink

Home Browse Search

and adjusted for partial duration series. The curve, shown in figure 3, gives a 100-year flood discharge of 10,700 cubic feet per second. A WRC update of the record from 1916 to 1977 with zero adopted skew and expected probability gives a 100-year flood discharge of 10,400 cubic feet per second. A variation of this update, in which an estimated 1904 ,event of 18,000 cubic feet per second is considered the highest event since 1864, gives 10,700 cubic feet per second for thelOO-year flood dis,charge. The 1864 flood appears to have been about the same magni- tude as the 1904 flood. In describing the 1904 flood, Floods in Colorado states "below the Union Pacific Railroad tracks in Greeley, the lowlands were overflowed, and the houses in that area were sub- merged to their window sills." A cross section of the Cache la Poudre River near this location indicates a channel and overbank capacity of about 1: ,000 cubic feet per second at an elevation that would reach the windowsills of houses in the area. Since the updated 100-year flood discharge remains near 10,700 cubic feet per second, the results of the 1973 study remain valid and are used in this report. MODELING STUDIES GENERAL The l'assachusetts Institute of Technology Catchment Model (MITCAT) computer program was used to model the Cache la Poudre River basin upstream from the USGS stream gage site at the mouth of the canyon. This is a computer model which estimates runoff based on rainfall and basin characteristics. The model was calibrated to the I-percent dis- charge given by the WRC frequency analysis of the annual peaks. The plains portion of the Cache la Poudre River basin was analyzed using synthetic unit hydrographs. This discussion of modeling studies applies to the mountain area, which was analyzed by the MITCAT model. RAINFALL . Rainfall values for the 10-, 50-, and 100-year, 6-hour events were obtained from the Precipitation-Frequency Atlas of the Western United . 10