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I I I I I I I I I I I I I I I I" I I I ENGINEERING ANALYSIS 2.1 Hydrologic Analysis 2.1.1 Description of the Hydrology of the Blue River and Associated Tributaries Blue River be/ow Dillon Dam Releases from the Dillon Dam to the Blue River are the primary source of potential flooding for the community of Silverthorne. Analysis of the effect of Dillon Reservoir on floods was conducted by Gingery Associates, Inc. in conjunction with the 1978 FIS study ("Hydrology Report, Blue River Downstream of Dillon Reservoir, Summit County", 1978a). The FIS used the Gingery analysis for estimating flood flows in the Blue River through Silverthorne. We searched for a copy of this report but neither FEMA nor CWCB files have an existing copy. We did find the companion report ("Hydrology Report, Summit County, Colorado", Gingery Associates, Inc., 1978b) on file at CWCB. This report does not discuss the development of flood flows for the Blue River, but rather develops regional flood frequency analysis using gages on the Blue River and tributaries upstream of Green Mountain Reservoir. To compare the current gage record (through water year 1994) to the 1978 FrS flood frequency value, a statistical analysis of the USGS gage below the dam (09050700) was conducted using the HEC-FFA program (see the attached output). Without knowing the inflow, the influence of Dillon dam can be observed in the record (see Figure 1). Up to a release of about 1600 I;::fs, flood magnitude steadily increases. Above this discharge, flood magnitude is reduced. Flood flow from the dam has been kept below about 2000 cfs. Of the nine peak t10ws above 1600 cfs, several are for recent wet years (1983, 1984, 1985, and 1986). Gingery's regional analysis of flood frequency found an average skew to the log- normal distribution of -0.55. We spot-checked the statistics for the Blue River near Dillon above the dam (0946600) to see if the statistics may have changed in the past 20 years. As shown in the table below, not much changed. Table 1. Statistics for Gage 0946600 - Blue River near Dillon, Colorado Statistie 1978 Results 1994 Results Mean Log 2.600 2.649 Station Skew 0.077 0.080 Standard Deviation 0.259 0.224 The statistics for the gage below the dam show negative skew due to control of large magnitude flows. Table 2 compared the estimated peak inflow to Dillon reservoir, the statistical estimates obtained using HEC-FF A, and the FIS flood frequency.