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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 />eELlNEATION <br /> <br />SECTION 2.0 <br />FLOODPLAIN <br />DELINEATION <br /> <br />AUGUST 2002 <br /> <br />a. Step-Backwater Method <br /> <br />Flood water surface profiles may be calculated by the standard <br />step method employing the Bemoulli energy equation with energy <br />losses due to friction evaluated with the Manning equation. <br />Detailed riverine flood water surface elevations are usually <br />determined utilizing hydraulic computer programs including HEC- <br />2 and HEC-RAS. <br /> <br />b. Altemative Methods <br /> <br />Unusual site-specific conditions may require the use of <br />specialized modeling techniques in order to correctly model and <br />delineate the flood hazard areas. For these cases, an altemative <br />hydraulic methodology may be used, provided it has been <br />recommended for general use by a federal govemmental agency <br />or notable scientific body, is well documented and is available to <br />the general user. In the case of a computer program, <br />documentation shall include a published user's manual and a <br />programmer's manual. <br /> <br />A list of computer hydraulic modeling programs approved by FEMA <br />and CWCB can be founel on the FEMA's and CWCB websites <br />(www.fema.aov, http://cwcb.state.co.usl). The hydraulic modeling <br />results should be calibrated to match the reliable flood data from <br />previous flood events, if available, within 0.5 foot. <br /> <br />Natural riverine flood water surface profile for the purpose of <br />floodplain delineation should be determined using subcritical flow <br />regime calculations. Critical depth should be used for the natural <br />stream reach where supercritical flow occurs. Supercritical flow <br />modeling may be used for man-made channels designed to handle <br />supercritical flows. <br /> <br />For riverine reaches not effected by backwater, the starting water <br />surface elevation should be estimated using normal depth <br />calculations unless a known water surface elevation for the starting <br />cross section can be obtained from an existing model or previous <br />flood events. <br /> <br />Recommended Manning's "n" values for various channel and <br />floodplain conditions can be found in Table CH6-T102. Manning's <br />roughness coefficients should be estimated considering the following <br />factors: <br /> <br />. Channel bed materials <br />. Type, density, and height of existing vegetations <br />. Existing structures in the overbanks <br />. Roughness variations with different flow depths <br />. Channel maintenance operations <br />. Past flood data <br /> <br />Past flood data, if available, should be used to calibrate roughness <br /> <br />FLOODPLAIN DELINEATION <br /> <br />CH4-209 <br />