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<br />. <br /> <br />. <br /> <br />. <br /> <br />ENGINEERING METHODS <br /> <br />Hvdroloeic Analvsis <br />Hydrologic information used for the FEMA approximate Zone A floodplain delineations in <br />Jefferson County is not available or was not docwnented by FEMA. However, the Colorado Water <br />Conservation Board (CWCB) using regional regression methodologies for computing 100-year <br />flood-flow values developed acceptable hydrologic information. <br /> <br />This methodology should be utilized for the watersheds in the western portion of the county only, <br />and is fully described in a publication by the CWCB entitled "Guidelines for Determining 100-year <br />Flood Flows for Approximate Floodplains in Colorado". The Foothills Region equation (SPL-3) is <br />used for streams in Jefferson County having watershed areas above an elevation of 7,500 feet. The <br />CWCB does not have an equation for watersheds within located within the Urban Drainage and <br />Flood Control District (UDFCD). Detailed hydrologic methods should be used for streams within <br />the UDFCD. The CWCB regression equation is: <br /> <br />Regional regression equation for streams in Jefferson County <br />with watershed areas above 7,500 feet <br /> <br />QIOO = 762.4(A)o54' (Only for drainage areas between I and 175 square miles) <br /> <br />For the above equation, the QlOo is the 100-year peak flow in cubic feet per second, and A is the <br />upstream (contributing) drainage area in square miles. The regression equation is only valid for the <br />drainage areas between I and 175 square miles. <br /> <br />The 100-year flow for several tributary streams were estimated by utilizing the regional regression <br />equation presented above. Table I presents 100-year peak flow estimates for 4 stream locations in <br />western Jefferson County. <br /> <br />Table I <br />Selected IOO-year Peak Flow Estimates for Jefferson County <br /> <br /> Drainage Area IOO-year <br />Stream Name and Location (sq. mi.) Peak Flow (cfs) <br />Deer Creek at its mouth (confluence with Ralston 4.32 1,695 <br />Creek) <br />Ralston Creek just upstream of its confluence with 28.0 4,700 <br />Deer Creek <br />Blue Creek at its mouth (confluence with Cub 4.27 1.685 <br />Creek) <br />Unnamed Tributary to North Turkey Creek at its 3.0] 1,390 <br />confluence with N. Turkey Creek (near Aspen Park) <br />Tucker Gulch just downstream of its confluence 4.37 1,705 <br />with Crawford Gulch <br /> <br />4 <br />