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<br />Guide For Approximate Zone A Areas <br /> <br />Developing' BFEs <br /> <br />where: a = velocity coefficient <br /> Q = 1DO-year flood dischar~e (cfs) <br /> A = cross section area (ft) at the assumed <br /> water-surface elevation <br /> g = Acceleration due to gravity = 32.2 ft/sec2 <br /> <br />An example of a weir flow computation is included in Appendix 9. <br /> <br />Flow Throuah structures <br /> <br />Culverts <br /> <br />~ <br />I <br />I <br />I <br /> <br />At least two cross sections are required to determine the water- <br />surface elevation upstream of a culvert. The first cross section <br />should represent the natural valley cross section downstream of the <br />culvert, and the second cross section should represent the top of <br />the embankment profile and the opening of the structure (refer to <br />Figure 13, "Cross Section Locations at Structures"). The size, <br />type, length, and upstream and downstream invert elevations of the <br />culvert should be obtained by field survey. The wing wall angle <br />and the entrance opening configuration, such as sharp edge or <br />rounded edge, should also be determined from a field survey. The <br />Federal Highway Administration publication "Hydraulic Design of <br />Highway Culverts" (Reference 2) should be referenced before going <br />to the field so that all the necessary information for culvert flow <br />computations can be collected during one field survey. Water- <br />surface elevations upstream of the culvert can then be computed <br />using the nomographs contained in the above-mentioned publication <br />and the procedures outlined below. <br /> <br />The first cross section should be used to determine the normal <br />depth downstream of the culvert, which will be considered as the <br />tailwater (refer to section on normal depth computations). <br /> <br />Two computations are required to determine the headwater when using <br />Federal Highway Administration nomographs. One computation is for <br />inlet control, and the other computation is for outlet control. <br />The headwater elevations from the two computations are then <br />compared. The higher of the two should be selected as the upstream <br />headwater elevation. If this headwater elevation is higher than <br />the top of embankment profile, weir flow will occur. Perform at <br />least three weir flow computations for headwater elevations between <br />the headwater that assumes that all the flow is culvert flow (the <br />first computation) and the minimum top of embankment elevation. <br />For each selected headwater elevation, compute the culvert flow <br />using Federal Highway Administration nomographs. Combine the weir <br />flow and culvert flow for each selected headwater elevation and <br />plot on graph paper. The BFE for the lOa-year flood discharge can <br />then be obtained from this graph. <br /> <br />V-32 <br />