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<br />Guidelines and Spec((icationsfor Flood Hazard Mapping Partners [April 2003] <br /> <br />lesser rise criterion. The Mapping Partner performing the hydraulic analysis shall obtain the <br />written approval of the RPO, through the Assistance Officer, before computing or mapping a <br />second regulatory floodway based on a criterion established by the community. <br /> <br />When flow is in the supercritical regime for marunade channels, or where velocity conditions are <br />such that normal encroachment analyses are not possible or are inappropriate, the encroachment <br />stations may be computed so that the allowable rise in water-surface elevation may match the <br />target water surface without exceeding the target energy. <br /> <br />Surcharge values must be between zero and the maximum allowable value. Negative surcharge <br />values may be caused by excessive encroachment, errors in bridge modeling, or insufficient <br />encroachment at a downstream section. If attempts to eliminate negative surcharges are <br />unsuccessful, the Mapping Partner performing the hydraulic analysis shall contact the RPO and <br />the reviewing Mapping Partner for guidance. <br /> <br />Normally, the Mapping Partner performing the hydraulic analysis shall determine the regulatory <br />floodway using equal reduction of conveyance: on opposite sides of the stream. If equal <br />reduction of conveyance is not technically appropriate, or where unusual flow patterns are <br />encountered (e.g., interbasin flow, divided flow), the Mapping Partner shall coordinate with the <br />RPO in selecting the most appropriate methods. Where the regulatory floodway dt~signation <br />affects contiguous communities on opposite sides of a flooding source, the Mapping Partner <br />must use equal reduction of conveyance. <br /> <br />The Mapping Partner performing the hydraulic analysis shall compute the regulatory floodway <br />on a tributary stream based on the 1-percent-annual-chance flood discharge and elevation of that <br />stream only and normally shall not include consideration of any backwater flooding from the <br />main stream. Therefore, the floodway elevations in the lower reach of a tributary subject to <br />backwater flooding may be lower than those used to plot the Flood Profiles. <br /> <br />The Mapping Partner performing the hydraulic analysis shall achieve the maximum allowable <br />surcharge (e.g., 1.0 foot) at the upstream-most cross section in a downstream commlmity that <br />does not have a regulatory floodway, when performing a floodway analysis for upstream <br />communities. This is necessary to avoid excessive increases that would occur if the downstream <br />community decides to establish a regulatory floodway. In addition, the Mapping Partner shall <br />determine the starting water-surface elevation for a floodway analysis at the fIrst cross section <br />using the same friction slope as the 1-percent-annual-chance natural Flood Profile. The <br />computed total conveyance between the natural Flood Profile and the floodway profile must not <br />differ by more than 1 percent. If they differ, then the Mapping Partner performing the hydraulic <br />analysis shall use the encroachment stations obtained from the equal conveyance reduction <br />method and the same starting friction slope of the natural Flood Profile for the floodway profile <br />to determine the starting floodway water-surface elevation and the surcharge value. The <br />computed surcharge value must not be more than the allowable surcharge value of each State. <br /> <br />If a regulatory floodway has been determined for the downstream community, the Mapping <br />Partner performing the hydraulic analysis shall use the same flood discharges and corresponding <br />flood elevations for different flood frequencies, floodway water-surface elevation, and the <br /> <br />C-19 <br /> <br />Section C.3 <br />