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<br />Guidelines and Specifications for Flood Hazard Mapping Partners [April 2003] <br /> <br />C.2 <br /> <br />Hydrologic Review <br /> <br />[February 2002] <br /> <br />A Mapping Partner selected by FEMA, identified during the initial Scoping Meeting (see <br />Volume 1, Section 1.3 of these Guidelines), shall review the proposed flood discharges prior to <br />their being used in hydraulic analyses. The intent is to agree on the l-percent-annual-chance <br />flood discharges before the hydraulic analyses are conducted, and to avoid hydraulic and <br />mapping analysis revisions necessitated by subsequent flood discharge revisions. Therefore, the <br />Mapping Partner performing the hydrologic analysis shall work with FEMA to ensure that <br />hydrology issues are identified as early as possible. This early review could reduce the level of <br />effort during both the detailed study and the production of the FIS report and FIRM. <br /> <br />The goal of the hydrologic review is to provide an assessment of the "reasonableness" of the <br />proposed 1-percent-annual-chance flood discharges and, if necessary, to suggest alternative <br />methods that may provide more reasonable flood discharges. The reasonableness of a flood <br />discharge depends on the requirements for the detailed study and the selected methodologies. <br />The Mapping Partner that is reviewing the hydrologic analysis (hereinafter referred to as the <br />reviewing Mapping Partner) shall check all methods for the reasonableness of their specific <br />application and the sources of the data. A comparison of proposed flood discharges against <br />criteria related to the regression equations is a good first screening tool; however, it does not <br />replace the need to review the applied methodology. <br /> <br />In addition to comparing proposed flood discharges to those derived from gaged data and <br />regression equations, the reviewing Mapping Partner shall compare the proposed flood <br />discharges to the effective flood discharges, noting any significant discrepancies and possible <br />reasons for those discrepancies. Also, the reviewing Mapping Partner shall consider the effect <br />on BFEs as a result of different flood discharges (not just changes in flood discharges) as a check <br />on reasonableness. <br /> <br />The procedures detailed below are recommended for preliminary hydrologic reviews of analyses <br />submitted in support of FEMA-contracted Flood Map Projects, map revisions, and appeals of <br />Flood Map Projects and map revisions. They are applicable to hydrologic analyses conducted <br />using gaging station data, regional regression equations, and rainfall-runoff models. <br /> <br />C.2.1 <br /> <br />Hydrologic Analysis Based on Gaging Station Data <br />[February 2002] <br /> <br />Proposed 1-percent-annual-chance flood discharges based on gaging station data are generally <br />reviewed for conformance to the guidelines in Bulletin 17B (Interagency Committee on Water <br />Data, 1982). If procedures other than those outlined in Bulletin 17B were applied, then the <br />reviewing Mapping Partner shall determine whether these procedures are reasonable. At least 10 <br />years of record are needed to define the 1-percent-annual-chance flood discharge. The reviewing <br />Mapping Partner shall compare estimates based on periods of record less than 25 years to flood <br />estimates based on precipitation data and to regional estimates for similar watersheds as <br />described in Bulletin 17B. In more arid regions, there are often many years when the annual <br />peak flow is zero. For these conditions, at least 10 years of nonzero flow are recommended for <br />defining the l-percent-annual-chance flood discharge. <br /> <br />C-12 <br /> <br />Section C.2 <br />