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<br />. <br /> <br /> <br />. <br /> <br />CHAPTER 4 <br />FLOODPLAIN <br />ADMINISTRATION <br />AND <br />.ELlNEATION <br />SECTION 2.0 <br />FLOODPLAIN <br />DELINEATION <br /> <br />COLORADO STATEWIDE <br />DRAINAGE AND FLOODPLAIN MANAGEMENT CRITERIA MANUAL <br /> <br />2.6 <br /> <br />DETAILED FLOODPLAIN DELINEATION METHOD <br /> <br />Before proceeding with a detailed floodplain delineation study, the project engineer <br />should evaluate the applicability of all available hydrologic and hydraulic studies for <br />the subject watershed/drainage-way. The previously approved studies should be <br />used whenever possible, unless the watershed/drainage-way conditions have <br />changed substantially and/or the original analysis methodology was determined <br />inappropriate. <br /> <br />2.6.1 HYDROLOGIC ANALYSIS <br /> <br />The hydrologic analysis shall include, at a minimum, calculations for the 1D-, <br />50-, 100-, and 500-year frequency discharges. It is recommended that the <br />peak discharge for 2- and 5-year flood events be calculated in addition to the <br />other discharges. The 50D-year flow rate may be estimated by multiplying the <br />100-yearflow rate by a factor of 1.7 (FHWA, HEC-18). . <br /> <br />2.6.1.1 METHODOLOGY <br /> <br />Peak discharge estimates for the floodplain delineation and <br />administration purposes shall be determined by one or more of the <br />following methods, depending on the length of systematic records <br />available. <br /> <br />. Statistical analysis of stream gage data. <br />. USGS regression analysis <br />. Flood estimates using hydrologic models and precipitation <br />records {synthetic analysis} <br /> <br />All available stream-flow data from the drainage basin and adjacent <br />basins shall be inventoried and documented. Detailed hydrologic <br />analysis procedures and standards can be found in Chapter 5, <br />Hydrology. The following guidelines should be used to determine the <br />appropriate hydrologic analysis method. <br /> <br />. When at least 50 years of stream-flow gage records are <br />available, a flow frequency statistical analysis should be <br />performed to determine the flood peaks of selected <br />recurrence intervals. <br /> <br />. When 25 to 50 years of stream-flow records are available, the <br />hydrologic analysis shall include a statistical analysis, and a <br />comparison with similar watersheds. Similar watersheds are <br />defined as watersheds that have similar hydrologic <br />characteristic (precipitation depth and distribution, slope, size, <br />elevation, vegetation cover, etc.) as the watershed being <br />studied. <br /> <br />. When 10 to 24 years of stream-flow records are available, the <br />hydrologic analysis shall include a statistical analysis, <br />comparisons with similar watersheds, and flood estimates <br />using synthetic hydrologic models and precipitation records. <br /> <br />AUGUST 2002 <br /> <br />FLOODPLAIN DELINEATION <br /> <br />CH4-206 <br />