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<br />. <br /> <br /> <br />COLORADO STATEWIDE <br />DRAINAGE AND FLOODPLAIN MANAGEMENT CRITERIA MANUAL <br /> <br />. <br /> <br />CHAPTER 4 <br />FLOODPLAIN <br />ADMINISTRATION <br />AND <br />...ELlNEATION <br /> <br />~ECTION 2.0 <br />FLOODPLAIN <br />DELINEATION <br /> <br />Geological Survey (USGS). Unless the available topographic map has <br />contour interval of 4 feet or less, field surveyed channel and floodplain cross <br />sections should be used in the hydraulic analysis. <br /> <br />2.7.3 <br /> <br />HYDRAULIC ANALYSIS <br /> <br />The following hydraulic analysis method or more detailed methods should be <br />used for delineation of approximate 1 OO-year floodplain boundaries: <br /> <br />. Calculate 1 OO-year water surface elevations for the cross sections <br />that are representative of the stream reach being studied using <br />normal-depth calculations (Manning's Equation). A sufficient amount <br />of cross-sections should be used to adequately represent and <br />analyze the physical features (bridges, levees, etc.) of the <br />drainageway. The published culvert and bridge rating charts may be <br />used. <br /> <br />2.7.4 FLOODPLAIN DELINEATION <br /> <br />Approximate 100-year floodplain boundaries should be delineated based on <br />the calculated 1 OO-year water surface elevations for the representative cross <br />sections in conjunction with the best available topographic map. If surveyed <br />high water marks from a previous flood event, close to a 100-year event, are <br />available for the drainageway, the high water mark elevations may be used to <br />supplement the computed 1 OO-year water surface elevations. <br /> <br />2.8 <br /> <br />AREAS PROTECTED BY LEVEES <br /> <br />In order for a levee system to be recognized as providing <br />flood protections, the levee must be structurally sound and <br />adequately maintained. Certification from a federal or state <br />agency that the levee meets the minimum freeboard criteria <br />and that it appears, on visual inspection, to be structurally <br />sound and adequately maintained will be required. Levees <br />that have obvious structural defects, or that are obviously <br />lacking in proper maintenance, shall not be considered in <br />the hydraulic analysis. <br /> <br />Detailed discussions on the levee freeboard, ownership, <br />design, operations and maintenance, and certification <br />requirements are provided in Chapter 6, Section 4.0. <br /> <br />2.8.1 FLOODPLAIN ANALYSIS <br /> <br />The natural floodplain areas protected from a 100- <br />year event by a levee system that meets the <br />requirements outlined in Chapter 6, Section 4.0 can <br />be designated as 100-year Shallow Floodplain with <br />1 foot depth (FEMA Zone X). However, areas <br />inundated by the interior drainage behind the levees <br />should be defined, and if necessary, the 100-year <br />water surface elevations, flooding limits and depths, <br />special hazard zones should be clearly identified. <br /> <br />AUGUST 2002 <br /> <br />FLOODPLAIN DELINEATION <br /> <br />In order for a <br />levee system to <br />be recognized as <br />providing flood <br />protections, the <br />levee must be <br />structurally <br />sound and <br />adequately <br />maintained. <br />Levees that have <br />obvious <br />structural <br />defects, or that <br />are obviously <br />lacking in proper <br />maintenance, <br />shall not be <br />considered in the <br />hydraulic <br />analysis. <br /> <br />CH4-213 <br />