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<br />Guidelines and Specifications for Flood Hazard Mapping Partners [April 2003] <br /> <br />C.3 <br /> <br />Detailed Hydraulic Analyses <br /> <br />[February 2002] <br /> <br />During the initial Scoping Meeting (Volume l, Section 1.3 of these Guidelines), the RPO or <br />other FEMA Lead and other members of the Flood Map Project Management Team will decide <br />which flooding sources within the community will be studied using detailed hydraulic analyses. <br />Guidance for performing these analyses is provided in the subsections that follow. <br /> <br />C.3.1 <br /> <br />General Guidance <br /> <br />[February 2002] <br /> <br />The Mapping Partner performing the hydraulic analysis shall use, to the maximum extent <br />possible, all valid existing flood elevation, survey, and other pertinent information for the study <br />area. Whenever existing l-percent-annual-chance flood elevations are available for the detailed- <br />study area, the Mapping Partner shall assess their validity without undertaking extensive <br />computations or reanalysis. Except where significant changes in flood discharges, floodplain <br />geometry, or flooding characteristics have occurred, or errors in the original computations have <br />been found, such elevations shall be considered valid for use in a Flood Map Project. <br /> <br />If an existing study that contains a valid I-percent-annual-chance Flood Profile does not provide <br />other profiles or a regulatory floodway that may be required for the Flood Map Project, the <br />Mapping Partner performing the hydraulic analysis shall attempt to obtain the original hydraulic <br />model and use it to generate this information. Whenever the original model is unavailable or <br />unusable, the RPO, through the Assistance Officer, may choose to remove the requirement for <br />these additional elevations and floodway data or request that they be determined by a simplified <br />analysis. <br /> <br />In any case, the Mapping Partner performing the hydraulic analysis shall obtain approval from <br />the RPO before performing hydraulic analyses for flooding sources that have previously <br />established 1-percent-annual-chance flood elevations. The Mapping Partner shall not study areas <br />having a drainage area less than 1 square mile unless RPO approval has been obtained in <br />advance. <br /> <br />The Mapping Partner performing the hydraulic analysis shall carefully estimate the roughness <br />coefficients for use in backwater computations. The estimates, prepared by experienced <br />engineers, shall include the consideration that roughness may vary with flood stages, depending <br />on such factors as the width-to-depth ratio of streams, vegetation in the channel and overbanks, <br />and materials of the channel bed. Wherever possible, the Mapping Partner shall calibrate <br />hydraulic models using measured profiles, reliable high-water marks, or reliable stage <br />information at stream gages for past floods. Models must match known high-water marks within <br />0.5 foot. <br /> <br />The Mapping Partner performing the hydraulic analysis shall not calibrate against data that result <br />in roughness coefficients out of the realm of published roughness coefficients for similar <br />observed conditions. If such data are lacking or are out of date, the Mapping Partner shall <br />determine the roughness coefficients using Cowan's method (Federal Highway Administration, <br />1984) based on a field inspection of the channel and floodplain and compare the new roughness <br /> <br />C-16 <br /> <br />Section C3 <br />