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<br />. <br /> <br /> <br />COLORADO STATEWIDE <br />DRAINAGE AND FLOODPLAIN MANAGEMENT CRITERIA MANUAL <br /> <br />2.6.1.3 PREVIOUS STUDIES <br /> <br />Where appropriate, available flow frequency information should be <br />used so that previous work by federal, state, or local agencies is not <br />duplicated. Where such data is not available, where conditions have <br />changed significantly, or where the methodologies or data used in <br />previous studies are not appropriate, a new hydrologic analysis for <br />each stream shall be prepared. <br /> <br />If a new hydrologic analysis is prepared, a comparison of new <br />discharges with all available published or not published discharge <br />data that exist for the study area shall be provided. If the new <br />hydrologic analysis results are significantly different than the <br />previously adopted flows, the following criteria should be used in <br />deciding which flow estimate should be used. However, the site- <br />specific limitations/conditions may warrant a deviation from the <br />evaluation criteria below. The project engineer should coordinate with <br />. the appropriate agencies in deciding which flow estimate should be <br />used. <br /> <br />. <br /> <br />For drainage-ways with at least 50 years of stream-flow gage <br />records, the following general FEMA evaluation criteria <br />should be used. <br /> <br />. <br /> <br />The latest discharges shall be adopted if the previously <br />established discharges do not fall within the 95 and 5 percent <br />confidence limits (90 percent confidence interval) of the most <br />recent estimates; the previously established discharges shall <br />be adopted if they fall within the 75 and 25 percent confidence <br />limits (50 percent confidence interval) of the most recent <br />estimates. <br /> <br />. <br /> <br />For all other cases, the new hydrologic analysis results <br />should be used if the new analysis is proven to be technically <br />superior and if the resulting peak flow rate change is greater <br />than 10%. <br /> <br />2.6.2 HYDRAULIC ANALYSIS <br /> <br />Hydraulic analysis should be performed to define, at a minimum, the water <br />surface profiles for the 10-, 50-, 100-, and 500-year flood frequencies and the <br />100- and 500-year floodplain boundaries. The 10-year floodplain boundaries <br />should also be delineated if the 10-year flows are not confined within the <br />channel. The floodplain boundaries shown on the floodplain delineation maps <br />shall be consistent with the calculated water surface profiles. <br /> <br />CHAPTER 4 <br />FLOODPLAIN <br />ADMINISTRATION <br />AND <br />eELlNEATION <br /> <br />SECTION 2.0 <br />FLOODPLAIN <br />DELINEATION <br /> <br />2.6.2.1 METHODOLOGY <br /> <br />The hydraulic analysis for detailed floodplain information should be <br />based on the following methods, or any other method approved by <br />CWCB and/or a federal agency, as appropriate. <br /> <br />AUGUST 2002 <br /> <br />FLOODPLAIN DELINEATION <br /> <br />CH4-208 <br />