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<br />Guide For Approximate Zone A Areas <br /> <br />Developing BFEs <br /> <br />NRCS' TR-2D and the U.S. Army Corps of Engineers' HEC-l <br />computer programs. <br /> <br />TR-2D and HEC-l provide a very detailed calculation of <br />discharge through the generation, addition, and routing of <br />runoff hydrographs. The effect on peak flood discharges due <br />to dams, road crossings, and large floodplain storage areas is <br />more accurately assessed with these programs. These models <br />require experience on the part of the user if they are to <br />produce realistic determinations of peak discharge. <br /> <br />Limitations - The limitations of these methods are thoroughly <br />described in their manuals. Because these methods involve <br />many variables and assumptions, the potential for error is <br />great. The users of these models must be thoroughly versed in <br />the limitations and assumptions of the computational methods <br />contained in these models. As with any synthetic model <br />depicting rainfall-runoff relationships, extreme care needs to <br />be taken to ensure that the results of the model are <br />reasonable. It is highly recommended that the discharges <br />produced by these hydrograph methods be compared to discharges <br />produced by another hydrologic method of equal accuracy or by <br />calibrating the model to an actual storm event. <br /> <br />Rvdraulics <br /> <br />There are various hydraulic methods that may be used to <br />determine BFEs along riverine flooding sources. The <br />appropriate method to use depends on flow conditions and the <br />size of the area that is being analyzed. For developments of <br />equal to or less than 50 lots or 5 acres, the normal depth <br />method, which is described in greater detail below, is usually <br />adequate for determining BFEs. After normal depth has been <br />computed, flow conditions should be analyzed. If flow is <br />classified as subcritical (i.e., normal depth is greater than <br />critical depth), normal depth is used as the BFE. If flow is <br />classified as supercritical (i.e., normal depth is less than <br />critical depth), then critical depth is used as the BFE for <br />natural channels. For engineered channels, supercritical <br />(normal) depth may be used for the BFE, provided that the <br />backwater from the normal depth of the downstream cross <br />section is considered properly. If more than one cross <br />section is required, step-backwater computations should be <br />used to determine BFEs along riverine flooding sources. <br /> <br />The procedures for computing normal depth, critical depth, and <br />step-backwater by hand are outlined below. As an alternative <br />to hand calculations, the QUICK-2 computer program may be <br /> <br />V-22 <br />