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Guidelines and Specifications for Flood Hazard Mapping Partners [April2003} At a Gaging Station The Mapping Partner performing the hydrologic analysis shall perform floodflow-frequency analyses in accordance with the guidelines for determining floodflow frequency presented in Bulletin 17B (Interagency Advisory Committee on Water Data, 1982) and subsequent modifications. To use analysis techniques other than those outlined in Bulletin l7B, the Mapping Partner shall obtain approval from the RPO and provide written justification for their use. The basic floodflow-frequency curve for gaged sites on unregulated streams may be obtained from the U.S. Geological Survey (USGS), Water Resources Division; from published USGS reports; or derived using methods described in Bulletin l7B. The annual maximum peak flows used in floodflow-frequency analyses are available on the USGS Web site at . htto://water.usgs.gov/nwis/sw. Computer programs for performing Bulletin 17B analyses are available from the U.S. Army Corps of Engineers (USACE) and the USGS. The programs include HEC-FFA Frequency Analysis (USACE, 1992) and PEAKFQ, Annual Flood Frequency Analysis Using Bulletin 17B Guidelines (USGS, 1998). The Mapping Partner performing the hydrologic analysis shall use the floodflow-frequency curve and adjust it, if necessary, to provide reliable flood discharge estimates for the site under consideration. The Mapping Partner also may use the methodologies outlined in USACE Engineering Manual No. l110-2-l415 (USACE, 1993) to develop floodflow-frequency curves for gaged streams. The Mapping Partner shall document reasons for the modification and procedures that were used to modify the published USGS floodflow-frequency curves. When modeling mixed populations of hydrologic events, the Mapping Partner shall refer to Engineering Manual No. 1110-2-1415 (USACE, 1993) or Appendix F of these Guidelines. Near a Gaging Station Generally, for peak flood discharges for ungaged sites on a gaged stream, the Mapping Partner performing the hydrologic analysis shall consider both the gaged site information and information from an appropriate regional estimate, where available. The Mapping Partner shall select an appropriate transfer technique for establishing flood discharges at the ungaged location, ensuring that the selected transfer technique considers the difference in the drainage areas at the gaged and ungaged sites. The procedures prescribed in most USGS regional floodflow- frequency reports are recommended for this purpose. These transfer procedures usually use information from the gaged site and the regional estimate when the ungaged site is within 50 to 200 percent of the drainage area at the gaging station. In cases where a more specialized local study of a watershed may be more appropriate than one prepared by the USGS, the Mapping Partner shall consult with the RPO before proceeding. For gaged streams with regulated flows, the Mapping Partner performing the hydrologic analysis may obtain peak flood discharges from the agency responsible for regulation. If the effects of regulation on floodflow frequency have not been established, the Mapping Partner shall determine the most appropriate analysis technique and obtain approval from the RPO before proceeding. USACE Engineering Manual No. 1110-2-1415 (USACE, 1993) provides guidance for performing floodflow-frequency analysis of gaged streams with regulated flows. C-4 Section Cl