Laserfiche WebLink
<br />Guide For Approximate Zone A Areas <br /> <br />Developing BFBs <br /> <br />WP = wetted perimeter (ft) <br />R = A/WP <br /> <br />step 7 - Compute the channel bottom slope, S, from the <br />topographic map or from field survey. <br /> <br />step 8 - Compute the discharge, Q, for each segment of the <br />cross section at each elevation by multiplying K by <br />SO,5. <br /> <br />step 9 - Add the discharges from each segment at the same <br />elevation to obtain the total discharge. <br /> <br />step 10 - Plot the total discharges and the corresponding <br />elevations on graph paper. <br /> <br />step 11 - The BFE can be determined from this graph for the <br />lOa-year flood discharge computed in step 2. <br /> <br />An example of a normal depth hand calculation is included in <br />Appendix 8. <br /> <br />critical Depth <br /> <br />After computing normal depth, the type of flow should be <br />checked. If the velocity head from the normal depth <br />computation is equal to or more than half the hydraulic depth, <br />the flow is supercritical and the critical depth should be <br />used to establish the BFE. The velocity head, HV, for an <br />irregular cross section is computed using the following <br />equation: <br /> <br />where: <br /> <br />a = <br />V = <br />QT = <br />AT = <br />g = <br /> <br />HV= lXV' <br />2g <br /> <br />velocity coefficient <br /> <br />mean velocity = QT (fps) <br />AT <br />total discharge (cfs) <br />total flow area (ft2) <br />acceleration due to gravity = 32.2 ft/sec2 <br /> <br />The velocity coefficient, a, is determined using the following <br />equation: <br /> <br />J<Ki/Al> + (~/A~> + (~/A;)l <br />K~/A;) <br /> <br />V-26 <br />