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<br />. <br /> <br />. <br /> <br />. <br /> <br />Previous Studies <br />The initial publication of flood hazards in Gilpin County occurred on June 28,1974. Revisions to <br />the Flood Hazard Boundary Map were published on January 16, 1976, and the Flood Insurance <br />Rate Map became effective June 10, 1977. There have been no revisions to the Flood Insurance <br />Rate Map since that time. The communities of Blackhawk and Central City each have their own <br />floodplain information reports in addition to the information for unincorporated Gilpin County. <br /> <br />Community Response to the CWCB Desilmation and Aoproval Process <br />A letter from the CWCB was transmitted to Gilpin County regarding the Board's proposed action <br />for designation and approval of the Gilpin County FHBM. Local officials confirmed that there <br />have been minor to moderate flooding problems unincorporated portions of the county, with some <br />development pressure along Highway 119 and the Apex area. Floodplain mapping needs are <br />generally limited to the areas receiving development pressure along the natural drainage courses of <br />Pine Creek and North Clear Creek. The County was receptive to the Board's proposed designation <br />action, and sent an official letter requesting that the CWCB designate and approve the floodplain <br />information currently available to the community. The community did not see a need to have a <br />meeting with the CWCB regarding this matter. A copy of the request letter is included in the <br />Appendix of this report. <br /> <br />ENGINEERING METHODS <br /> <br />Hydrolo!!ic Analvsis <br />Hydrologic information used for the FEMA floodplain delineations in Gilpin County is not <br />available or was not documented by FEMA. However, the Colorado Water Conservation Board <br />(CWCB) using regional regression methodologies for computing 100-year flood.flow values <br />developed acceptable hydrologic information. The first method was carried out to specifically <br />address streams within Gilpin County in the North Clear Creek watershed (vicinity of Blackhawk <br />and Central City). This method involved tabulating drainage areas and 100-year peak flow values <br />from previously published drainage and floodplain information reports for Blackhawk and Central <br />City. Two equations were developed: One for the North Clear Creek mainstem (only for drainage <br />areas between 10 and 40 square miles), and one for streams that are tributary to North Clear Creek <br />(only for drainage areas between 1.2 and 4.2 square miles). The CWCB regression equations are: <br /> <br />Table 1 <br />Regional Regression Equations for North Clear Creek Watershed <br /> <br />Tributaries to North Clear Creek: <br />QIOO = 47.3(A),.39 (Only for drainage areas between 1.2 and 4.2 square miles) <br /> <br />North Clear Creek Mainstem: <br />QIOO = 669.9(A~ (Only for drainage areas between 5 and 40 square miles) <br />D'-174 <br /> <br />3 <br />