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<br />~;TEP 10 (continuill <br /> <br />METHOD 3 . SOMETIMES YOU NEED A STEP ]~ACKWATER ANALYSIS <br /> <br />There are circumstances when ,an analysis at an individual cross-section is insufficient. <br />In those cases a hydraulic analysis that considers the entire stream reach should be <br />performed. Reasons for perfonning a step backwater analysis indude: <br /> <br />1) A backwater effect from a constriction IIike a bridge or culvert or <br />from a confluence with another stream, <br /> <br />2) Friction is a significant factor :in the stream's hydraulics. <br /> <br />3) There is a substantial change in floodplain width, channel shape, <br />channel slope, or hydraulic roughness, <br /> <br />4) There are flow splits where some of the flood flows leave the main <br />channel. <br /> <br />5) The stream reach is IOlmg. <br /> <br />When any of the above circumstances occur, the local floodplain administrator should <br />require the proponent to hire an engineer to perform an analysis with the HEC-2 computer <br />program or a similar Step Backwater program, Unless the local administrator is familiar <br />with such computer programs, he or she should not try to perform the hydraulic analysis, <br />