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<br />. <br /> <br />. <br /> <br />. <br /> <br />ENGINEERING METHODS <br /> <br />Hvdrolol!ic Analvsis <br /> <br />Hydrology information for the approximate floodplain delineations shown on the Coal Creek <br />FHBM for Coal Creek has been estimated in accordance with the CWCB's Guidelines for <br />Determining IOO-Year Flood Flows for Approximate Floodplains (Guidelines). The 100-year <br />flow for Coal Creek can be estimated by utilizing the regression equation for Arkansas River Basin <br />Subregion ARK-3. The equation is: <br /> <br />QlOO = 1,089.3(A)o.653 <br /> <br />In that equation Ql00 is the 100-year peak flow in cubic feet per second, and A is the upstream <br />(contributing) drainage area in square miles. The regression equation is only valid for tributary <br />streams having drainage areas between 1 square mile and 930 square miles. This equation is valid <br />for Coal Creek. Because all of the tributaries to Coal Creek all have drainage areas less than 1 <br />square mile, the regression equation is not valid for those drainages. The drainage areas for Coal <br />Creek and all of the mapped tributaries are listed in Table 1 below. <br /> <br />After using the above regression equation to estimate peak flows for Coal Creek, the CWCB staff <br />compared the estimated flows for Coal Creek in the town to the peak flow computed for Coal Creek <br />in the City of Florence using detailed hydrologic methods. That flow was computed as part of the <br />Flood Insurance Study, City of Florence, Colorado, Fremont County, dated Jtme 4,1984. The <br />engineer who prepared the Flood Insurance Study used a rainfall-runoff analysis in computing <br />flows for Coal Creek. The estimated flows and the detailed flows for Coal Creek are listed below <br />in Table 2. <br /> <br />The regression equation for Subregion ARK-3 is not valid for any of the tributaries of Coal Creek <br />mapped on the FHBM. There is no procedure currently endorsed by the CWCB for estimating <br />hydrologic values for streams within Arkansas River Basin Subregion ARK-3 with drainage basins <br />smaller than 1 square mile. In order to assure that flood risks on those tributary drainages are <br />appropriately taken into accotmt, the Town of Coal Creek will have to require a detailed hydrologic <br />analysis for any development activities along those streams. <br /> <br />Figure 1, presented in the Appendix at the end of this report, displays, in map form, the entire <br />Arkansas River Basin with Subregions ARK-I through ARK-6 shown. Figure 2 shows the western <br />portion of the Arkansas River Basin, including Subregions ARK-2 through ARK-6. The regression <br />curve for Subregion ARK-3 is also included in the Appendix as Figure 3. Tables 1 and 2 below <br />provide information about the drainage areas of the various drainages and the estimated 100-year <br />peak flows for Coal Creek at the upstream and downstream corporate limits. <br /> <br />5 <br />