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<br />ENGINEERING METHODS <br /> <br />Hvdrololric Analvsis <br /> <br />Hydrology information for streams in the Fairplay area cannot be estimated by using a regression <br />from the CWCB's Guidelines for Determining IOO-Year Flood Flows for Approximate <br />Floodplains (Guidelines). The South Park portion of the Sou Platte River basin, which is where <br />F airplay is located, was specifically designate s an area 0 no Ul me regression. Because no <br />CWCB regression equation has been developed for streams in the ou ark portion of the basin, <br />the CWCB has deemed the regression equations obtained from a draft U.S. Geological Survey <br />publication entitled "Analysis of the Magnitude and Frequency of Floods in Colorado" as the <br />best availabl~rmation for the estimation of hydrologic values in that geographic area. Flows in <br />the FairplaYj\.can, therefore, be estimated by using the following regression equation, QIOO = <br />39.5(A)o.706 X (S + 1.0)"'" , where QUNl is the 100-year peak flow in cubic feet per second, A is the <br />upstream (contributing) drainage area in square miles, and S is the mean drainage basin slope in <br />feet per foot. The regression equation is only valid for tributary streams having drainage areas <br />between 5.5 square miles and 988 square miles. This equation is valid for the Middle Fork of the <br />South Platte River and for Beaver Creek f~,* )( 0a.:&~ C;. fJ.~1 <br />'v- v-<"j,f}!v-- ?) <br />Figure I, presented in the Appendix at the end of this report, displays^the,,~ Platte Rj..velii?~in, <br />including the South Park "region of no regression".)I:The USGS l2!eJiminary regressio~i!;for <br />the Mountain region is also included in the Appendix, on Figure~ Table I below displays the <br />drainage areas for the Middle Fork of the South Platte River and Beaver Creek at the downstream <br />corporate limits. Although the regression equation for estimating 100-year peak flows has been <br />developed by the USGS, the actual estimates of flows have not been developed because mean basin <br />slopes for the two drainage basins have not been calculated by CWCB staff. In the case of Beaver <br />Creek, it is recommended that the detailed hydrologic analysis being performed for the town's <br />master drainage plan be used instead of the USGS approximate technique. <br />( ~\ <br />Table 1 ' ~'D ') <br />Estimated 100-year peak flow for streams in Fairplay <br /> <br />~~ <br />~c4 <br />~, Ii Lx ct.,,..? <br /> <br />Stream Location Drainage Area (sq mi) 100-year Peak Flow (cfs) <br />Middle Fork of the South Platte River at 85 Use USGS regression <br />downstream corporate limit equation <br />Beaver Creek at downstream corporate 13 Use USGS regression <br />limit equation <br /> <br />~ <br /> <br />A procedure for determining hydrology information for approximate floodplains of lakes, ponds, <br />and local drainages has not been established. The hydrologic methodology developed in the <br />Guidelines is appropriate for designation and approval of approximate floodplain information and <br />for use in simple floodplain analyses for development such as open space, agriculture, farm bridges, <br />pole barns, and individual single residential structures. The methodology is not appropriate for <br />