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<br />Guidelines and Specifications for Flood Hazard Mapping Partners [April 2003] <br /> <br />flood discharges to those computed using the most up-to-date regression equations. A significant <br />difference would indicate a need for a flood data update, The Mapping Partner may obtain <br />information on the most current regional regression equations for a particular area from the <br />USGS district office closest to the community. Although other agencies may publish regression <br />equations for a region, only the USGS regression equations are typically used for NFIP purposes. <br /> <br />Changes in Design Storm Data <br /> <br />If the effective hydrologic analyses were performed using a rainfall-runoff model (e.g., HEC-l, <br />TR-20), changes in design storms may affect the base flood discharge. Currently, design storm <br />data are obtained from two publications: National Weather Service Technical Paper No. 40, <br />Rainfall Frequency Atlas of the United States, and National Oceanic and Atmospheric <br />Administration Atlas 2, Precipitation-Frequency Atlas of the Western United States. The revised <br />design storms may cause changes in discharge estimates. <br /> <br />Information on updating design storms can be found on the National Weather Service Web site <br />at http://www .nws.noaa.gov/ oh/hdsc/studies/prcpfreq .html. <br /> <br />Increase in Length of Stream Gage Record <br /> <br />An increase in the length of a stream gage record may also affect the flood discharge estimate. If <br />the effectIve dIscharge was estrmated by conducting a frequency analysis of a relatively short <br />record of stream gage data, the base flood discharge estimate may be changed if newly available <br />data are added. If stream gage data with a relatively long record (50 years or more) were used in <br />the effective analyses, however, a few additional years usually will not cause significant changes <br />in the base flood discharge estimate, unless a large-magnitude event occurred since the analyses <br />were conducted. All frequency analyses are to be performed in accordance with the methods <br />specified in Bulletin 17B, Guidelines for Determining Flood Flow Frequency (Interagency <br />Advisory Committee on Water Data, 1982). The effective flood discharge shall be revised only <br />if that discharge is outside the 90-percent confidence interval (higher than 95-percent confidence <br />limit or lower than 5-percent confidence limits) of the newly computed flood discharge. <br /> <br />Construction of Flood-Control Structures <br /> <br />Certain flood-control structures (e.g., reservoirs and detention ponds) are designed to reduce the <br />peak flood discharges. Therefore, the Mapping Partner performing the Mapping Needs <br />Assessment shall evaluate carefully any flood-control structures constructed since the effective <br />hydrologic analyses were performed to determine whether the structures have a significant effect <br />on the base flood discharge. However, not all reservoirs are designed to mitigate flooding. <br />Therefore, the Mapping Partner must evaluate the function(s) of a reservoir to determine whether <br />it affects discharges. Flood-control structures may be built by Federal agencies (e.g., USACE, <br />Natural Resources Conservation Service (NRCS), U.S. Bureau of Reclamation) or local <br />organizations (e.g., water management districts, irrigation districts). The agency or organization <br />that built and/or administers the structure should have the necessary information available. <br /> <br />In addition to evaluating new flood-control structures, the Mapping Partner performing the <br />Mapping Needs Assessment shall evaluate existing structures to determine whether they <br />continue to operate in the same manner as they did when the hydrologic analyses were <br /> <br />1-18 <br /> <br />Section ],2 <br />