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<br />• i <br /> Button. Rock Dam was [hen inserted in the calibrated SWhtct model of St. <br /> Vrain Creek at the Lyons gage and rainfall was applied to the 211 square miles <br /> of mountain area. I[ was found that 6-hour rainfall over South St. Vrain <br /> Creek and North St. Vrain Creek below Button Rock Dam generated higher dis- <br /> charges at Lyons than did rainfall over the entire mountain area. The adopted <br /> frequency curve at Lyons therefore does not reflect any contribution [o peak <br /> discharges from the 109 square miles above Button Rock. The adopted frequency <br /> curve at the mouth was modeled by applying b-hour rainfall to the 924 square <br /> miles of contributing basin area. This curve was developed for existing con- <br /> ditions anH iatfi Button-Rock-Dam in place. <br /> Two different 6-hour storms were used in determining the 5-, 10-, 50-, <br /> 100-, and 500-year discharge profiles. The profiles above Boulder Creek were <br /> produced by centering a storm over the St. Vrain basin above Boulder Creek, <br /> excluding the area above Button Rock Dam. From the mouth of Boulder Creek to <br /> the mouth of SE. Vrain Creek, [he discharge profiles were developed by apply- <br />.~ ~ ing rainfallto-the entire contributing area of the basin. <br /> . The resulting peak flows, based on the above described procedure, are <br /> presented in-Table-4 for the existing basin development conditions, and Table- <br /> s for year 2000 Urbanization. The discharge profiles for existing conditions <br /> above Boulder Creek are shown in Figure-3. <br />C. Comparison of the Peak Flows for Uifferent Development Conditions <br />Information related to the peak flows for the 100-year flood for the <br />existing and year 2000 basin development conditions is presented in Tables <br />4 and 5. From this information,~we can see that the percentage increase <br />in peak flows resulting from the year 2000 development conditions is minimal. <br />Such increases ranAinR from none to 1.55% will have a minimal impact on the <br />water surface elevations resulting from the 100-year frequency flood_ Further, <br />the proposed drainage improvements should be adequate to provide the protection <br />needed over the next 20 years. In some cases for the 50-year and 500-year floods, <br />r <br />-38- <br />