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<br />Guidelines and Specifications for Flood Hazard Mapping Partners [April 2003] <br /> <br />. Time of concentration or lag computations must reflect effects of increases in velocities <br />due to channel modifications and urbanization. Methods developed by Federa.l agencies <br />are preferred. Empirical formulas developed or approved by State agencies c,rn only be <br />used within those States. <br /> <br />. Rainfall duration, at a minimum, must exceed the time of concentration for the watershed <br />and must be large enough to capture all excess rainfall as well as provide reasonable <br />runoff and sediment volumes when performing storage analyses. The Mapping Partner <br />may use the critical storm concept to determine the storm duration, or use th\~ duration <br />specified in guidelines developed by state agencies responsible for flood control or <br />floodplain regulation. The critical stonn is a design storm (total amount, duration, <br />temporal distribution) which provides the highest flood discharge/water-surface elevation <br />for the flooding source. The Mapping Partner shall determine the critical storm through a <br />sensitivity analysis of various storm durations to determine which storm duration <br />produces the highest flood discharge/water-surface elevation (e.g., 6-hour vs. 24-hour). <br />Note that for communities that only get short duration storms, the storm durations to be <br />evaluated must be longer than the time of concentration of the watershed, and not the <br />duration of the rainfall. <br /> <br />. Temporal distributions developed or recommended by J'ederal or State agencIes <br />responsible for flood control or regulating floodplains must be used. <br /> <br />. Streamflow routing methods must be able to analyze the attenuation and translation of <br />hydro graphs. <br /> <br />The Mapping Partner performing the hydrologic analysis shall calibrate the parameters in the <br />models against known storms in the study area and, when available data permits, against a <br />floodflow-frequency curve before the model is used to estimate flood discharges. The Mapping <br />Partner shall compare computed peak flood discharges from the hydrologic model to flood <br />discharges from published regional studies (e.g., USGS regression equations) when they are <br />applicable, or to flood discharges developed from gaging station data in watersheds with similar <br />characteristics. If the discharge values are not comparable, the Mapping Partner shall submit a <br />Special Problem Report (SPR) to the RPO to explain the differences before beginning the <br />hydraulic analysis. <br /> <br />C.1.2 <br /> <br />Considerations for Revised Hydrologic Analyses <br />[February 2002] <br /> <br />In general, revised hydrologic analyses could be initiated for any of four reasons: <br /> <br />l. To reflect longer periods of record or revisions in data; <br /> <br />2. To reflect changed physical conditions; <br /> <br />3. To take advantage of improved hydrologic analysis methods; or <br /> <br />4. To correct an error in the hydrologic analysis performed for the effective study. <br /> <br />C-7 <br /> <br />Section C.1 <br />