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Guidelines and Specifications for Flood Hazard Mapping Partners [April 2003 } C.1.1.3 Ungaged Streams [February 2002] Acceptable hydrologic analysis methods that the Mapping Partner performing the hydrologic analysis may use for ungaged streams include regional regression analyses and rainfall-runoff models. These methods are discussed below. Regional Regression Analysis The Mapping Partner performing the hydrologic analysis shall make use of any valid existing floodflow-frequency analysis conducted by a Federal, State, or local agency that has authoritatively established and officially adopted the flood discharges for the ungaged stream under consideration, or the Mapping Partner shall use of the effective flood discharges in published Flood Insurance Study (FIS) reports. In the absence of such an analysis or in cases in which the analysis is outdated, the Mapping Partner shall use, where appropriate, the most recently published USGS report for estimating flood magnitude and frequency that is applicable to the Flood Map Project area. Such reports are generally available on a statewide basis. The Mapping Partner shall exercise caution to ensure that these reports are used only for the conditions and locations for which they are recommend~:d. The USGS has published regression equations for estimating flood discharges for urban watersheds in Alabama, Florida, Georgia, Kentucky (Jefferson County), Missouri, North Carolina, Ohio, Oregon (Portland), South Carolina, Tennessee, Texas, Virginia, Washington (Vancouver), and Wisconsin. Where the statewide reports do not contain procedures to account for }lrbanized conditions, or the statewide equations do not apply to the watershed conditions, the Mapping Partner performing the hydrologic analysis shall adjust the flood discharges determined for the rural condition. The Mapping Partner shall use the techniques described in Flood Characteristics of Urban Watersheds in the United States (USGS, 1983) to adjust the flood discharges. Occasionally, flood discharges computed with urban equations are lower than those computed with rural equations, especially in less-urbanized drainage areas. In these cases, the Mapping Partner shall use the rural discharges. The Mapping Partner performing the hydrologic analysis may use the USGS "National Flood Frequency" program (USGS, 1994) determine flood discharges of different frequenci~:s for the continental United States, Alaska, Hawaii, American Samoa, and Puerto Rico for both rural and urbanized conditions. When a regression equation other than those published by the USGS is proposed, the Mapping Partner shall obtain the approval. of the RPO and shall justify the use of this equation. The USGS also has developed the region-of-influence method to estimate flood discharges, and computer programs have been published for Arkansas, Louisiana, and North Carolina. In the region-of-influence method, regression equations are computed for an ungaged site by selecting from a statewide database of gaging stations a predetermined number of stations having characteristics similar to the Ullgaged site. This method does not involve published r~:gression equations. The Mapping Partner performing the hydrologic analysis may use the region-of- influence method; however, the Mapping Partner shall obtain the approval of the RPO and shall justify the use of this method. C-5 Section C.]