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<br />Guidelines and Specifications for Flood Hazard Mapping Partners [April 2003] <br /> <br />a rainfall-runoff model in lieu of a gaged analysis with non-homogeneous data or the use of <br />regression equations. <br /> <br />The Mapping Partner performing the hydrologic analysis shall calibrate the parameters in <br />rainfall-runoff models against major known storms that exceed 10-percent-annual-chance events <br />for single-event analysis if the data are available. The data to calibrate the model are to include <br />the following: <br /> <br />. Peak flood discharges developed at gaging stations, computed by indirect methods (e.g., <br />computations at bridge cross sections based on high-water marks), or flood discharge <br />hydro graphs from responsible agencies; <br /> <br />. Rainfall distribution, reported at a minimum of hourly intervals, at rain gages within the <br />storm area and within or near the watershed being studied; <br /> <br />. Total rainfall values at rain gages within the storm area or isohyetal map of the storm, <br />indicating the duration of the storm; and <br /> <br />. Rainfall and soil moisture conditions before the storm for single-event analysis. <br /> <br />Observed high-water marks may also be of value when calibrating both hydrologic and hydraulic <br />models against historical events. <br /> <br />The Mapping Partner performing the hydrologic analysis may calibrate the rainfall-runoff model <br />against the various flood discharges of a floodflow-frequency analysis. Regardless of whether <br />models have been calibrated against historical events, further calibration may be required to <br />produce floodflows from the 10-, 2-, l-, and 0.2-percent-annual-chance rainfall that are <br />comparable to the floodflows from the floodflow-frequency analysis, if records are available. If <br />reasonable matches cannot be reached by maintaining calibration parameters within acceptable <br />ranges, then the Mapping Partner shall review the model methodology and its application to the <br />watershed. <br /> <br />Where models are calibrated against historical events and are applied properly, and where the <br />modeled floodflows and frequency floodflows do not agree, the Mapping Partner performing the <br />hydrologic analysis shall consider adjusting the design rainfall volume and distribution. The <br />design rainfall distribution is typically selected from traditional distributions prepared by the <br />Natural Resources Conservation Service (NRCS) and USACE, but recommendations from State <br />agencies responsible for flood control or floodplain management regarding state or regional <br />distributions also may be accepted. <br /> <br />Where feasible, in coordination with Federal and State agencies, the Mapping Partner performing <br />the hydrologic analysis shall select a reasonable rainfall distribution for the model to best <br />simulate floodflows corresponding to a floodflow-frequency analysis in accordance with the <br />guidance of Bulletin 17B (Interagency Advisory Committee on Water Data, 1982). For flooding <br />sources where the volume of flood discharge is the major concern, such as ponds in a closed <br />basin, the Mapping Partner may determine the rainfall duration by comparing the calculated lake <br />stages with the stage-frequency curve. <br /> <br />C-10 <br /> <br />Section C.l <br />