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<br />. <br /> <br /> <br />COLORADO STATEWIDE <br />DRAINAGE AND FLOODPLAIN MANAGEMENT CRITERIA MANUAL <br /> <br />coefficients, taking into consideration any alteration in the channel <br />subsequent to the floods. The calibrated roughness coefficients <br />should closely match the observed channel and floodplain conditions. <br />Photographs shall be taken of the study reaches of the stream <br />channel and floodplain to support roughness coefficients used for <br />hydraulic computations. <br /> <br />2.6.2.2 CROSS SECTIONS <br /> <br />The riverine cross-section data should be obtained by <br />photogrammetric methods at the time of map compilation, from the <br />map contours and spot elevations, or through field surveys. All cross <br />section points derived photogrammetrically or from the contours shall <br />meet the accuracy standards defined in Chapter 2. All field-surveyed <br />cross section points shall be within :1:0.5 foot of true elevations. <br /> <br />. <br /> <br />Cross sections shall be located perpendicular to the direction of flow <br />at appreciable changes in flow area, roughness, or stream gradient. <br />Additional cross sections shall be located at bridges and culverts, the <br />head and tail of levees, confluences with tributaries, and all flow <br />control structures. Based on computed results, additional cross <br />sections may be required if the slope of the energy grade line <br />between successive cross sections decreases by more than 50 <br />percent or increases by more than 100 percent. The location of all <br />cross-sections shall be shown on the floodplain delineation maps. <br /> <br />2.6.2.3 BLOCKAGE <br /> <br />All culverts and bridges shall be considered for the potential to <br />become blocked by floating debris and sediment loads. In <br />determining the potential for blockage, and subsequent reduction in <br />the flow conveyance capacity, the following factors should be <br />considered: <br /> <br />. Old photographs <br />. History of maintenance during high flows <br />. Ongoing maintenance operations <br />. Watershed characteristics such as erodibility of channel <br />banks <br />. Amount and type of vegetation along stream <br />. Size and characteristics of the waterway <br /> <br />CHAPTER 4 <br />FLOODPLAIN <br />ADMINISTRATION <br />AND <br />.L1NEATION <br />SECTION 2.0 <br />FLOODPLAIN <br />DELINEATION <br /> <br />Blockage may be accounted for in computer runs by increasing width <br />of piers, raising streambed elevation or reducing waterway opening <br />by a percentage. Where the potential for blockage can be shown, <br />human intervention (e.g., snagging) shall be considered only if such <br />flood fighting activities are specifically included in the community's <br />adopted emergency response plan. <br /> <br />AUGUST 2002 <br /> <br />FLOODPLAIN DEliNEATION <br /> <br />CH4-21 0 <br />