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<br />. <br /> <br />Hydraulics <br /> <br />The water surface elevations for the 100-year frequency <br />flood were computed by hand using a simplified form of the <br />standard step method. The computation of water surface pro- <br />files involves solving the one dimensional energy equation <br />to determine the slope of the energy grade line between control <br />sections where the water surface is known or can be assumed. <br />The procedure is essentially a trial-and-error solution of <br />the Continuity equation, <br /> <br />Q = VA <br /> <br />(1) <br /> <br />and the Manning equation, <br /> <br />V = 1.486 R2/3 S1/2 <br />n <br /> <br />( 2) <br /> <br />where Q is average flow in cubic feet per second, <br />V is average velocity in feet per second, <br />A is the cross-sectional area in square feet, <br />n is a roughness coefficient, <br />R is the hydraulic radius in feet, and <br />S is the slope of the energy grade line. <br /> <br />. <br /> <br />Elevations for four strategically located cross-sections <br />were obtained from the topographic map to model the valley. <br />These cross-sections are shown graphically in the Appendix. <br /> <br />An average roughness coefficient of n=0.045 was assumed <br />for all cross-sections to simplify the calculations. <br /> <br />A field trip taken on May 13, 1981, verified that this <br />assumption is a reasonable average for all sections, but <br />that some refinement may be possible if separate roughness <br />coefficients are assigned for the channel and overbank area <br />of each cross-section. <br /> <br />The following table is a summary of the results of the <br />hydraulic analysis. <br /> <br />Cross- <br />Section <br /> <br />Channel <br />Station <br /> <br />Channel <br />Elevation <br /> <br />. <br /> <br />D <br /> <br />20 + 70 <br /> <br />6574.0 <br /> <br />100-Year Depth Average <br />Water Sur- of Velocity <br />face Eleva- Flood (ft./sec. ) <br />tion (feet) <br />6558.2 8.2 11.1 <br />6567.7 6.7 18.0 <br />6581.6 7.6 11.3 <br />6595.0 10.0 10.2 <br /> <br />B <br /> <br />13 + 50 <br /> <br />6550.0 <br /> <br />C <br /> <br />17 + 30 <br /> <br />6561.0 <br /> <br />E <br /> <br />26 + 50 <br /> <br />6585.0 <br /> <br />-6- <br />