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<br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish the pea~ d~sc~arge- <br />frequency relationships for floods of the selected Lecurrence ~~~erva~s <br />for each stream studied in detail in the community. <br /> <br />The 10-, 50-, 100-, and 500-year-recurrence interval discharges for the <br />streams within the Town of Breckenridge have been included In a report <br />entitled "Flood Plain Informatioh, Breckenridge, Colorado," crepared <br />for the Colorado Water Conservation Board (Reference 1). Tr.~ f~equency <br />discharges in this report were determined using the MITCAT ~ycro:ogy <br />model to develop synthetic hydrographs in the various baSlDS <br />(Reference 5). The MITCAT hydrology model is a general purpcse <br />simulation model that uses kinetic wave function to derive runoff for <br />synthetic storms. These discharges were compared with a study, <br />entitled "Hydrology Report, Summit County," prepared for -:he Federal <br />Insurance Administration (Reference 6). This report used a regional <br />analysis of streamflow gaging records to develop regression equations <br />for 10-, 50-, 100-, and SOO-year-recurrence-interval discharges fo~ <br />Summit County. The report used 326 years of records from 14 gage sites <br />within the Blue River basin. The discharge frequency data developed in <br />the Colorado Water Conservation Board report were suppo~ted by the <br />results of the Federal Insurance Administration report and were, <br />therefore, adopted for use in this study. <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of streams in thp. communlty <br />were carried out to provide estimates of the elevations of floods of <br />the selected recurrence intervals along each streant studied in the <br />community. Users should be aware that flood elevations shown on the <br />FIRM represent rounded whole-foot elevations and may not exactly <br />reflect the elevations shown on"the Flood Profiles or in the Floodway <br />Data tables in the FIS report. Flood elevations shown on the FIRM are <br />primarily intended for flood insurance rating purposes. For <br />construction and/or floodplain management purposes, users are cautioned <br />to use the flood elevation data presented in this Frs in conjunction <br />with the data shown on the FIRM. <br /> <br />Water-surface elevations of floods of the selected recurrence intervals <br />for the Blue River, Lehman Gulch, Illinois Gulch, and Sawmill Gulch <br />were computed through use of the u.S. Army Corps of EnginGers HEC-2 <br />step-backwater computer program (Reference 7). <br /> <br />Cross section data for the backwater analysis of the Blue River, Lehman <br />Gulch, Illinois Gulch, and Sawmill Gulch were obtained from topographic <br />maps at a scale of 1:1,200, with a contour interval of 2 feet <br />(Reference 8); from field measurements; and from a previous report <br />(Reference 1). All bridges and culverts were field checked to obtain <br />elevation data and structural geometry. <br /> <br />Flooding from the Blue River in the Town of Breckenridge is reduced by <br />an overflow channel downstream of cross section D, outside the <br />corporate limits. The backwater analysis for the Blue River has been <br />computed for the reduced flow resulting from this overflow. <br /> <br />Locations of selected cross sections used in the hydraulic analyses are <br />shown on the Flood Profiles (Exhibit 1). For stream segments for which <br />a floodway is computed (Section 4.2), selected cross section locations <br />are also shown on the Flood Boundary and Floodway Map (Exhibit 2). <br /> <br />5 <br />