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<br />Guide For Approxima~e Zone A Areas <br /> <br />Developing BFEs <br /> <br />If the site in question is not located immediately upstream of a <br />structure, a normal depth should be computed at the site. The 100- <br />year water-surface elevation at the site should be the higher of <br />the two elevations from the culvert computation and the normal <br />depth computation. <br /> <br />Federal Highway Administration nomographs predict only the energy <br />grade elevation upstream of the culverts. In most applications, <br />the velocity head is assumed to be negligible and, therefore, the <br />energy grade elevation approximates the actual water-surface <br />elevation. If a more accurate water-surface elevation is desired, <br />a hydraulic computer model, such as HEC-2, should be used to <br />determine the BFE. The procedure outlined in the weir flow section <br />to compute a water-surface elevation that corresponds to a certain <br />energy grade elevation may also be used to determine a BFE upstream <br />of a culvert. <br /> <br />Bridges <br /> <br />Although hand computations can be performed by following the <br />procedures for bridge routines in Federal agency computer models, <br />it is recommended that the water-surface elevation upstream of <br />bridges be determined using a computer model. The number of cross <br />sections required at the structure depends upon the type of bridge <br />routine used. Four cross sections are required if the special <br />bridge routine in the HEC-2 program is used, and six cross sections <br />are required if the normal bridge routine in the HEC-2 program is <br />used. Three cross sections are required if the bridge routines in <br />the WSPRO program and the WSP2 program are used. A step-backwater <br />analysis is also required to compute the water-surface elevations <br />with these bridge routines. The following programs are recommended <br />to compute the water-surface elevation upstream of a bridge: <br /> <br />Comouter Proqram <br /> <br />Aqencv <br /> <br />HEC-2 <br />*HEC-RAS <br />WSPRO <br />WSP2 <br /> <br />U.S. Army Corps of Engineers <br />u.s. Army Corps of Engineers <br />u.s. Geological Survey <br />NRCS <br /> <br />*Not available for general use when this guide was published; <br />please contact our Headquarters office for current status. <br /> <br />V-33 <br />