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.~ Guidelines and Specifications for Flood Hazard Mapping Partners [April 2003] Examples of changed physical conditions include the addition of a hydraulic structure or other watershed development that has affected the effective analyses. Regardless of the reason for the revised analyses, the Mapping Partner shall provide detailed documentation of the changes that have been addressed in the revised analyses and why flood discharges developed are more accurate than the effective discharges. If the reason for the revised hydrologic analysis is an improved method, the Mapping Partner shall provide documentation as to why the alternative method is superior and shall obtain RPO approval to use the improved method before performing the analysis. A detailed study of a community's flood hazards may include a flooding source for which FEMA has not established Base (l-percent-annual-chance) Flood Elevations (BFEs). In these cases, the Mapping Partner shall consult Subsection C.l.2.1 for guidance on establishing flood discharges. C.1.2.1 Preliminary Hydrologic Analysis [February 2002] The Mapping Partner performing the hydrologic analysis shall compare the proposed flood discharges to all available floodflow-frequency data that exist for the study area to ensure compatibility. The Mapping Partner also shall inform the RPO, as well as Federal, State, and local agencies involved in water resources programs in the area, of the proposed flood discharges. The Mapping Partner shall resolve any discrepancies between available information and the flood discharges proposed for the Flood Map Project. That Mapping Partner shall bring such discrepancies to the attention of the RPO in an SPR, as flood discharge discrepancies shall not be the cause for delaying the detailed study. In addition, the Mapping Partner shall keep the RPO informed of progress made in resolving such discrepancies. Comparing Proposed and Effective Flood Discharges In determining whether to grant a map revision request or to fund a detailed study of a community's flood hazards, FEMA shall consider making revisions only when a more recent floodflow-frequency analysis yields flood discharge values that are statistically significant from the effective flood discharges, or when flood discharges yield significant differences in BFEs. Determining Statistical Significance The Mapping Partner performing the hydrologic analysis shall base the test for significance on the confidence limits of the more recent analysis. The new flood discharges shall be adopted if the previous flood discharges do not fall within the 95- and 5-percent confidence limits (90- percent confidence interval) of the recent estimates; the previous flood discharges shall be adopted if they fall within the 75- and 25-percent confidence limits (50-percent confidence interval) of the recent estimates. The Mapping Partner shall consult Bulletin 17B (Interagency Advisory Committee on Water Data, 1982) for procedures on computing confidence limits for gaged streams. The computation of confidence and prediction limits for regression estimates is documented in statistical textbooks (Montgomery and Peck, 1982). C-8 Section C.]