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I I I I I I I I I I I I I I I I I I I Hydrologic analysc;:s were carried out to establish the peak discharge-frequency relationships for floods of the selected recurrence intervals for each stream studied in detail in the community. The discharge-frequency data for the Blue River within Silverthorne is developed in a report entitled "Hydrology Report, Blue River Downstream of Dillon Reservoir, Summit County, Colorado" (Gingery, 1978). In brief, the approach used considers the total inflow to the Dillon Reservoir and its routing through the reservoir to obtain the 10-, 50-, 100-, and 500-year discharges on the Blue River downstream of the Dillon Dam. The contribution of the local inflow or the tributaries' flows to the Blue River discharges is also described. Flood discharges tor the 10-, 50-, 100-, and 500-year recurrence events on Straight Creek and Willow Creek were calculated using regression equations based on a regional analysis of all of the gages within Summit County. A detailed description of the methodology used is provided in a hydrology report prepared by Gingery Associates, Inc. (Gingery, 1978). At each gaging station, the annual flood peaks caused due to a snowmelt event and due to a rainfall event are separated. Using log-Pearson Type III distribution, the 10-,50- , 100, and 500-year recurrence interval discharges are obtained separately for snowmelt events, as well as for rainfall events. The rainfall events were observed to be insignificant in comparison to snowmelt events for larger watersheds. Rainfall events cover a smaller area (less than five square miles). The discharge-drainage area relationships developed for the 10-, 50-, 100-, and 500-year recurrence interval discharges for the snowmelt events provided a correlation coefficient of approximately 97 percent. Peak discharge-drainage area relationships for the Blue River, Straight Creek, and Willow Creek are shmvn in Table 1. 3.2 Hydraulic Analyses Analyses of the hydraulic characteristics of streams in the community were carried out to provide estimates of the elevations of floods of the selected recurrence intervals along each stream studied in the community. Water-surface elevations of floods of the selected recurrence intervals for the Blue River were computed through use ofthe U.S. Army Corps of Engineers HEC-2 step- backwater computer program (Hydrologic Engineering Center, 1990). Water-surface elevations of floods of the selected recurrence intervals for Straight Creek, and Willow Creek were computed through use of the U.S. Army Corps of Engineers HEC-RAS step- backwater computer program (Hydrologic Engineering Center, 1998). HEC-RAS was used to estimate the mixed regime of mild and steep channel conditions that were found in the tributary channels. Cross sections for the backwater computations and flood plain delineation for the Blue River, Straight Creek, and Willow Creek were obtained from topographic maps and cross sections surveys. The Town of Silverthorne Geographic Information System (GIS) coverage for topography (1994) was combined with cross sections obtained by field survey for a complete description of the flood plain.