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<br />.~c~ _c. ~ - c~-""!>;t{~~->rd;Rli~~jl~ <br /> <br />Guide For Approximate Zone A Areas <br /> <br />Developing BFEs <br /> <br />Q = @ x C x L X H3/2 <br /> <br />discharge (cfs) <br />submergence factor <br />weir coefficient, varies from 2.6 to 3.0 and can <br />be obtained from hydraulic text books <br />weir length (ft) <br />available head (ft), measured from top of weir to <br />the selected energy grade elevation <br /> <br />Several values for H should be selected and the corresponding <br />discharge computed until the total weir flow is larger than <br />the lOO-year flood discharge. Plot the discharges and the <br />corresponding energy grade elevations on graph paper. The <br />lOO-year flood energy grade elevation can be determined from <br />this graph. For an approximate analysis, the computed energy <br />gradient elevation can be considered the BFE. <br /> <br />where: Q = <br /> @ = <br /> C = <br /> L = <br /> H = <br /> <br />If the structure profile is not horizontal, as shown in Figure <br />23, "Weir Flow - Embankment Profile is Not Horizontal," <br />several structure segments should be used and an average <br />energy depth, H, for that segment should be determined for use <br />in the above equation for selected energy grade elevations. <br />The sum of the weir flow from each segment will then be equal <br />to the total weir flow for the selected energy grade <br />elevation. <br /> <br />ROAD OR EMBANKMENT PROFILE <br /> <br /> <br />_I_ L(3) <br />---- <br /> <br />ENERGY GRADE LINE" <br />+ L(4) ~k~ <br />- ---- --- - <br /> <br />HAVG(4) <br /> <br /> <br />HAVG(2) HAVG(3) <br /> <br />Q; (iii CLCl) HAVG/1I'/z) + (iii CL/Z! HAVG/ZtZ) + (iii CL/J! HAVG/Ji/z) + (iii CL/.! HAVG/.i/z) <br /> <br />Figure 23 - Weir Flow - Embankment Profile is <br />Not Horizontal <br /> <br />V-30 <br />