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<br />. <br /> <br />. <br /> <br />. <br /> <br />A second methodology should be utilized for the watersheds in the remainder of the county (i.e. <br />South Boulder Creek and Ralston Creek watersheds). This methodology is fully described in a <br />publication by the CWCB entitled "Guidelines for Determining 100-year Flood Flows for <br />Approximate Floodplains in Colorado". The Mountains Region equation (SPL.5) is used for <br />streams in Gilpin County having watershed areas above an elevation of 7,500 feet. The CWCB <br />regression equation is: <br /> <br />Table 2 <br />Regional Regression Equations for Gilpin County <br />Outside ofthe North Clear Creek Watershed <br /> <br />QlOO = 39.4(A)o.776 <br /> <br />(Only for drainage areas between 2 and 480 square miles) <br /> <br />For the equations in Tables 1 and 2, the QlOO is the 100.year peak flow in cubic feet per second, and <br />A is the upstream (contributing) drainage area in square miles. The regression equation equations <br />are only valid for the range of drainage areas as shown next to each equation. <br /> <br />The 100-year flow for North Clear Creek and two of its tributaries within the North Clear Creek <br />watershed were estimated by utilizing the regional regression equations presented in Table 1. Table <br />3 presents 100.year flow estimates for 4 locations in the North Clear Creek watershed. <br /> <br />Table 3 <br />Selected tOO-year Peak Flow Estimates for Gilpin County <br /> <br /> , <br /> Drainage Area tOO-year <br />Stream Name and Location (sq. mi.) Peak Flow (cfs) <br />Silver Creek just upstream of its confluence 2.0 250 <br />with North Clear Creek <br />Pine Creek just upstream of its confluence 3.8 1,150 <br />with North Clear Creek <br />North Clear Creek just upstream of its 5.3 1,250 <br />confluence with Pine Creek <br />North Clear Creek just downstream of its 13.1 1,750 <br />confluence with Silver Creek <br /> <br />Gilpin County should regulate the approximate floodplains on a case-by-case basis. FEMA <br />requires that a detailed floodplain analysis must be performed by a professional engineer and <br />submitted by the developer or development proponent for development projects involving more 5 <br />acres or more than 50 units (whichever is more restrictive). <br /> <br />4 <br />