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<br />-' <br /> <br />Guidelines and Specificationsfor Flood Hazard Mapping Partners [April 2003] <br /> <br />APPENDIX C <br /> <br />GUIDANCE FOR RIVERINE FLOODING ANALYSES <br />AND MAPPING <br /> <br />This Appendix documents the study methods and review procedures that assigned Mapping <br />Partners shall use in performing detailed and approximate hydrologic and hydraulic analyses for <br />riverine flooding sources, preparing floodplain mapping to reflect the results of those analyses, <br />and performing hydrologic analyses of closed-basin lakes. <br /> <br />C.1 <br /> <br />Detailed Hydrologic Analyses <br /> <br />[Februa.ry 2002] <br /> <br />As part of the initial scope of work defined by the Federal Emergency Management Agency <br />(FEMA) Regional Project Officer (RPO) and other members of the Flood Map Project <br />Management Team (detailed in Volume 1, Section 1.3 of these Guidelines), the flooding sources <br />for which detailed hydrologic analyses must be conducted will be identified. This section <br />addresses methods and assumptions to be used in performing detailed hydrologic analyses for <br />riverine flooding sources. <br /> <br />C.1.1 <br /> <br />General Guidance <br /> <br />[February 2002] <br /> <br />For detailed hydrologic analyses, the exceedance probability of flood events to be studied must <br />be determined. At a minimum, the Mapping Partner performing the hydrologic analysis shall <br />analyze the 1-percent-annual-chance (100-year) event; however, FEMA often will require <br />determinations of the lO-percent-annual-chance (10-year), 2-percent-annual-chance (50-year), <br />and O.2-percent-annual-chance (500-year) flood discharges as well. Where appropriate, the <br />Mapping Partner shall use all available floodflow-frequency information and shall not duplicate <br />previous work by Federal, State, or local agencies, or work performed as part of a new or revised <br />Flood Map Project for FEMA. Where such data are not available, where conditions have <br />changed invalidating the published information, or where the methodologies or data used in the <br />previous FEMA flood hazard study are not appropriate, a new hydrologic analysis will be <br />required. <br /> <br />The Mapping Partner perfonning the hydrologic analysis shall estimate the flood discharges for <br />existing land-use conditions. However, FEMA and the Mapping Partner may also consult with <br />community officials to detennine whether they want to c:onsider developing hydrology based on <br />future land-use conditions for local floodplain management purposes. If a community decides to <br />include future-conditions hydrology within the scope of work for a Flood Map Project, the <br />technical information shall be developed by the community and provided to FEMA and the <br />Mapping Partner performing the hydrologic and hydraulic analyses for the Flood Map Project (if <br />that Mapping Partner is not the community) in accordance with the requirements in Section e.8. <br /> <br />The Mapping Partner performing the hydrologic analysis shall consider gaged versus ungaged <br />streams and the appropriateness of developing a rainfall-mnoff model. Each of these approaches <br />is briefly discussed later in this section. When an expected probability adjustment (Interagency <br /> <br />C-1 <br /> <br />Section G.l <br />