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<br />. <br /> <br />. <br /> <br />. <br /> <br />ENGINEERING METHODS <br /> <br />Hvdrololrlc Analvsis <br /> <br />Hydrology information for the streams with approximate floodplain delineations on Pueblo <br />County's FIRM panels has been estimated in accordance with the CWCB's Guidelines for <br />Determining lOO-Year Flood Flows for Approximate Floodplains (Guidelines). The lOO-year <br />flow for each of the streams with approximate floodplains can be estimated by utilizing the <br />regression equations for Arkansas River Basin subregions Ark-I, Ark-3, Ark-4b, and Ark-6. A <br />map showing the four hydrologic subregions within Pueblo County is included in the map pocket in <br />the Appenix of this report. The equations for the four subregions are as follows: <br /> <br />Subregion Ark-I: <br />Subregion Ark-3: <br />Subregion Ark-4b: <br />Subregion Ark-6: <br /> <br />QlOO = 1564.98 * (A)O.539 <br />QlOO = 1089.27 * (A)O.635 <br />QlOO = 3089.12 + 198.56(A) <br />QlOO = 45.98 * (A)O.717 <br /> <br />For 25<A<1125 <br />For l<A<930 <br />For 4<A<75 <br />For 4<A<330 <br /> <br />For all of the equations, QlOO is the estimated IOO-year peak flow in cubic feet per second, and A <br />is the upstream (contributing) drainage area in square miles. None of the regression equations are <br />valid for the Arkansas River mainstem within Pueblo County. Regression curves for the four <br />hydrologic subregions within Pueblo County are presented in the Appendix. A detailed study or <br />other hydrologic analysis must be performed for projects involving the Arkansas River mainstem <br />and for streams with drainage areas that fall outside the applicable range of the regression <br />equations. A copy of the Guidelines can be obtained by writing to the Colorado Water <br />Conservation Board, Floodplain Management Section, 1313 Sherman Street, Room 721, Denver, <br />Colorado, 80203. <br /> <br />The use of the U.S. Geological Survey publication, Techniques For Estimating Regional Flood <br />Characteristics of Small Rural Watersheds in the Plains Region of Eastern Colorado, can be <br />used to supplement the Guidelines. The USGS publication can be used for stream reaches with <br />drainage basins less than 25 square miles but greater than one-half (0.5) square mile for Arkansas <br />River Basin Subregion Ark-I. <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 />development involving residential subdivisions greater than 50 lots or occurring on sites greater <br />than 5 acres, commercial construction, industrial construction, construction of public transportation <br />facilities, or construction of flood control facilities. <br /> <br />Pueblo County should regulate the approximate floodplains on a case-by-case basis subject to a <br />detailed hydrologic and hydraulic analysis performed by a professional engineer, in accordance with <br />