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<br />Guide For Approximate Zone A Areas <br /> <br />Developing BFEs <br /> <br />versus energy grade elevation. The critical <br />elevation and, therefore, critical depth, can <br />from this graph where the energy (i.e., <br />elevation) is minimum. <br /> <br />water-surface <br />be determined <br />energy grade <br /> <br />step-Backwater Analysis <br /> <br />step-backwater computations are based on the principle of <br />conservation of energy, which states that the energy at the <br />upstream cross section is equal to the energy at the <br />downstream cross section plus the losses between the two cross <br />sections. The losses considered in the step-backwater <br />analysis are the friction loss and the transition loss. <br /> <br />The equations and the procedure used in the step-backwater <br />analysis are explained in the QUICK-2 user's manual in <br />Appendix 6. Although hand computations can be done to perform <br />the step-backwater analysis, it is advisable to use the QUICK- <br />2 program or other Federally approved programs for ease of <br />computation. The QUICK-2 program currently does not model the <br />effects of bridges or culverts or supercritical flow. <br /> <br />The QUICK-2 program uses the default friction slope method, <br />which is the average conveyance method, from the HEC-2 program <br />to compute friction losses. For transition losses, a <br />contraction coefficient of 0.1 and an expansion coefficient of <br />0.3 should be used in the computations. <br /> <br />The reach lengths between the two cross sections for the left <br />overbank, channel, and right overbank are required for step- <br />backwater computations. The distance for the left overbank <br />should be measured between the center of the floodplains of <br />the left overbank at each cross section. The same is true for <br />the right overbank. The channel distance should be measured <br />along the streambed, and therefore will account for the <br />meandering of the stream channel. <br /> <br />In general, starting water-surface elevations are obtained <br />from normal depth computations (slope/area method) at the <br />first cross section. If there is a structure downstream of <br />the study area, the backwater effects of the structure must be <br />considered in determining the starting water-surface <br />elevation. If there is a known 10D-year water-surface <br />elevation at the downstream end of the study area, that water- <br />surface elevation should be used as the starting water-surface <br />elevation. <br /> <br />V-28 <br />