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<br />Guide For Approximate Zone A Areas <br /> <br />Developing BFBs <br /> <br />used. QUICK-2 is a user-friendly computer program developed <br />by FEMA that may be used for normal depth, critical depth, or <br />step-backwater computations for regular or irregular shaped <br />cross sections. To aid the users of this guide in computing <br />BFEs, the QUICK-2 computer program and user's manual are <br />located in Appendix 6. The user's manual contains a tutorial <br />section which leads a new user through the calculation process <br />using "real life" examples. For those not using the QUICK-2 <br />program, the following sections on Normal Depth and critical <br />Depth illustrate how to compute these depths by hand (see <br />Appendix 8 for an example of a Normal Depth hand calculation). <br /> <br />Normal Depth <br /> <br />Normal depth is the depth expected for a stream when the flow <br />is uniform, steady, one-dimensional, and is not affected by <br />downstream obstructions or flow changes. For uniform flow, <br />the channel bottom slope, water-surface slope, and energy <br />slope are parallel and are, therefore, equal. For normal <br />depth computations, the flow is considered steady because the <br />discharge is assumed to be constant; therefore, the depth of <br />flow does not change during the time interval under <br />consideration. <br /> <br />Normal depth calculations (also called the "slope/area <br />method") compute BFEs at a cross section. The standard <br />formula for determining normal depth at a cross section is <br />Manning's formula. The standard Manning's equation is: <br /> <br />Q = 1. 486 x A x (R667) X 5,5 / n <br /> <br />where: Q = discharge (cfs) <br /> A = cross section area (ft2) <br /> R = hydraulic radius (ft) = A/WP <br /> WP = wetted perimeter (ft) <br /> 5 = energy slope (ft/ft) <br /> n = Manning's roughness coefficient <br /> <br />The cross section area refers to the area below the water- <br />surface elevation, and the wetted perimeter refers to the <br />length of the ground surface along the cross section below the <br />water-surface elevation. The channel bottom slope is used in <br />lieu of the energy slope. <br /> <br />As noted earlier, Manning's "n" values vary depending on the <br />physical features of the stream channel and the channel <br />overbanks. The results of normal depth calculations can <br /> <br />V-23 <br />