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<br />,,~ <br /> <br />Guidelines and Specifications for Flood Hazard Mapping Partners [April 2003] <br /> <br />Rainfall-Runoff Models <br /> <br />Where USGS regional regression equations are not applicable due to flow regulation, storage, <br />watershed development, or other unique basin characteristics, the Mapping Partner performing <br />the hydrologic analysis may obtain RPO approval to develop a rainfall-runoff model using a <br />computer program such as HEC-HMS, HEC-1 or TR-20. A list of models accepted by FEMA <br />for this purpose may be found on the FEMA Flood Hazard Mapping Web site at <br />http://www.fema.gov/fhm/en modl.shtm. For communities where a rainfall-runoff model has <br />been developed for floodplain management or permit purposes, the Mapping Partner may obtain <br />RPO approval to use (or revise first if necessary) such a model for the hydrologic analysis. <br /> <br />A wide variety of automation tools have been developed to facilitate hydrologic modeling. <br />These products range from simple graphical user interfaces that help the user input model <br />parameters to highly advanced Geographic Information System (GIS)-based tools that contain <br />state-of-the-art software and modeling approaches with fully integrated data processing, <br />graphics, and visualization capabilities. The tools have been organized into three categories <br />based on their relationship to accepted FEMA models. The FEMA policy for the acceptance of <br />such tools is summarized below. <br /> <br />. Category 1 Tools: These simple tools can be either pre-processing or post-processing <br />independent modules. They function in conjunction with, but separately from, the <br />executable file of a computer model that is on the FEMA accepted models list. These <br />tools are considered acceptable because they are not computer models themselves. <br /> <br />. Category 2 Tools: These software tools are computer models that perform modeling <br />routines that emulate a model on the FEMA accepted model list; however, their source <br />code has been rewritten to perform these tasks, instead of using the accepted model's <br />source code. Category 2 software tools must be reviewed and placed on the accepted <br />models list. <br /> <br />. Category 3 Tools: These software tools use new hydrologic modeling methods and/or <br />models not currently on the FEMA accepted models list. They may add pre- or post- <br />processing functions similar to the other categories of tools as well. Because these are <br />new computer models, Category 3 software tools must be reviewed and placed on the <br />accepted models list. <br /> <br />In developing a rainfall-runoff model, the Mapping Partner performing the hydrologic analysis <br />shall consider the following factors: <br /> <br />. When the unit hydro graph method is used in developing hydro graphs, subwatershed <br />drainage areas shall be appropriately defined within the limit that the unit hydro graph is <br />able to reflect watershed response to changing conditions. <br /> <br />. Loss rates may be varied when computing different frequency floods. The variation must <br />be based on calibration to observed flood hydrographs. Urbanization effects must be <br />reflected in the loss rates. <br /> <br />C-6 <br /> <br />Section C.] <br />