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<br />Guidelines and Specifications for Flood Hazard Mapping Partners [April 2003] <br /> <br />Significant Changes in Base Flood Elevations <br /> <br />When the effective flood discharges fall between the 50- and 90-percent confidence limits, the <br />Mapping Partner performing the hydrologic analysis may use the step-backwater computation <br />performed for the effective study to evaluate the effect of the new flood discharges on effective <br />BFEs. If the new flood discharges yield BFEs that differ from the effective BFEs obtained from <br />the effective water-surface profile by more than 0.5 foot, or if, in flat areas, the floodplain <br />boundaries will be significantly changed, a detailed hydrologic analysis shall be conducted. <br />Otherwise, the Mapping Partner shall not perform a revised hydrologic analysis for the selected <br />stream at this time, unless other substantial changes in hydraulic conditions exist, such as <br />channelization and construction of flood-control structures, or unless there are errors in the <br />effective study. <br /> <br />Where significantly different flood discharges are proposed for use, the Mapping Partner shall <br />contact the RPO immediately for approval. Where confidence limit tests are not applicable, the <br />Mapping Partner shall bring unresolved discrepancies to the attention of the RI'O. The <br />determining factor then becomes the effect on the BFE as described above. <br /> <br />Choice of Methodology <br /> <br />The Project Team shall determine the hydrologic methodology to be used during the Project <br />Scoping phase of the Flood Map Project. In selecting a methodology, the Project Team shall <br />consider the complexity of the project and the effective models and methodology. <br /> <br />The Mapping Partner performing the hydrologic analysis shall apply frequency analysi.s of flow <br />data at gaging stations, using procedures provided in Bulletin 17B (Interagency Committee on <br />Water Data, 1982) wherever possible. When the systematic record at a gaging station is less than <br />50 years, the Mapping Partner shall weight the results with estimates from other methods, such <br />as USGS regression equations. The Mapping Partner may use the method developed by <br />Hardison, published in USGS Professional Paper 750-C (USGS, 1971), to estimate the <br />equivalent years of record for regression equations that are needed in the weighting process. <br />Guidance on weighting two estimates of flood discharges is also given in Bulletin 17B <br />(Interagency Advisory Committee on Water Data, 1982), and USGS regression equation reports. <br /> <br />USGS regression equations, adjusted for urbanization if appropriate, are recommended for <br />estimating the existing-conditions base flood discharges for restudied streams if a flood <br />hydro graph is not required and if the regression equations are applicable to the restudied streams. <br />The regression equations are to be applied only to streams having characteristic parameter values <br />that are within the range of values of the gages used to develop the regression equations. <br /> <br />For watersheds with existing hydrologic models, the Mapping Partner performing the hydrologic <br />analysis may use an existing model in lieu of USGS regression equations if that model was <br />calibrated. Such models must, however, be updated to account for any development that has <br />occurred in the watershed since the existing model was created. The Mapping Partner shall <br />exercise caution when selecting a methodology for watersheds that are undergoing or are <br />projected to undergo development. In such cases, the Mapping Partner shall consider developing <br /> <br />C-9 <br /> <br />Section G.l <br />