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SECTION 2: TRAPEZOIDAL AND CIRCULAR CHANNEL ANALYSIS PROGRAMS <br /> a channel segment, because of local inflow to the channel. In such cases, you <br /> must decide whether to base your analysis on the maximum flow rate or the <br /> average flow rate in the channel segment. Local drainage regulations or <br /> practice may provide guidance in this regard. When in doubt, perform the <br /> analysis with each flow rate and compare the results. <br /> 2.4.2.3 Channel Bottom Slope <br /> The channel bottom slope is the average drop in elevation per foot of length <br /> along the channel. For example, if the channel bottom drops 1 foot in a length <br /> of 1002 feet, then the channel bottom slope is 0.001 feet per foot. Channel <br /> bottom slopes are sometimes expressed in percent. A slope of 0.001 feet per <br /> foot is the same as a 0.1% slope. <br /> The slopes of the water surface and the energy grade line are assumed to be <br /> the same as the channel bottom slope for normal flow conditions. Therefore, it <br /> is important to provide the best possible estimate of the channel bottom <br /> slope. <br /> 2.4.2.4 Manning's Roughness Coefficient <br /> This program uses Manning's Equation to analyze open-channel flow. The <br /> roughness of the channel is represented by Manning's Roughness Coefficient, <br /> commonly called the "n-value" . Suggested values for Manning's n-value are <br /> listed in Appendix B of this manual, and in many hydraulics reference books. <br /> Roughness coefficients should be adjusted according to experience in your <br /> geographic area. <br /> 2.4.2.5 Trapezoidal Channel Side Slopes <br /> The slope of each bank of a trapezoidal channel is illustrated in Section 2.1 <br /> of this manual. The program expects the side slopes to be represented as the <br /> "Z-Ratio" , which is the ratio of horizontal distance to vertical rise in the <br /> channel bank. For example, a channel bank which rises 1 foot for each three <br /> feet of horizontal distance would have a side slope of 3:1, and a Z-Ratio of <br /> 3. Z-Ratios of 3 or 4 are common for earthen channels. Concrete-lined channels <br /> may have steeper banks, with Z-Ratios of 1.5 or 2. <br /> The Trapezoidal Channel Analysis program has the capability of analyzing <br /> channels with a different side slope for each channel bank. For example, flow <br /> in a street gutter can be analyzed using this program. The vertical curb would <br /> cause the side slope to be 0 on one side. On the other side, a 6-inch <br /> difference between the pavement crown elevation and the gutter elevation, <br /> divided by a 12-foot lane width, would yield a side slope of 24. <br /> 2.4.2.6 Trapezoidal Channel Bottom Width <br /> Section 2.2 of this manual illustrates the bottom width of the trapezoidal <br /> channel section. The bottom width is measured between the toes of each channel <br /> bank. <br /> DODSON & ASSOCIATES, INC. THE DODSON HYDRAULICS LIBRARY, PAGE 15 <br />