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I I I I I I I I I I I I I- I I I I I I - 11 - to model the existing channel. Because the flood producing event is a thunderstorm of small areal distribution and high intensity the greatest amount of rainfall was placed in the first half-hour of the synthetic storm. Soil moisture condi- tion II was assumed in the calculation of SCS soil curve numbers. The design storms were assumed to occur during late summer. During this portion of the year a base flow remains in the Blue River derived from a snowmelt and groundwater discharge. The average daily flow for the month of June was added to the flood hydrograph derived from the MITCAT techniques. Each flood hydrograph was routed through the Goose Pasture Tarn. The routed flood hydrograph was added to the base flow in the Blue River and that hydrograph was used in the hydraulic compu- tations to determine the lateral extent of flooding. Other techniques used to reduce the basic precipitation and runoff data in understanding the hydrology of the Breckenridge area include the Pearson Log III, Hazen and snowmelt analysis. The three methods are statistical in nature, differing primarily in their techniques in determining the skew coefficients. The Pearson Log III and the Hazen methods reduce the peak discharg~~ recorded at the Hoosier Pass snow course ~tation and the peak_ discharge for the same year. The Pearson Log III Analysis was -'run on a number of streams -rn the area on which USGS records are maintained. The results are shown in Table 3. Primary drawback to the statistical methods is that the USGS gage is located downstream from Breckenridge and encompasses a much larger area than what would affect the town of Breckenridge. To reduce the statistical flows it would be necessary to derive a valid, area reduction factor. Apllication of the SCS technique which defines an area reduction factor raised to the 0.8 power results in peak flows through the town of Breckenridge for the statistical methods to range from 560 cfs to 890 cfs. DESIGN RUNOFF " Table 4 summarizes the flows at the various design points along the Blue River derived by MITCAT, which modeled the thunderstorm condition, and Pearson Log III an~l~isiS!, wl1ich was the statistical_ method. Also-shown is the runoff from-a-storm centered over -various tributaries below the Tarn summarized from Figure 2. The design flow selected for each frequency interval has been underlined. ~ Leonard Rice Consulting Water Engineers. Inc.