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<br />. <br /> <br />. <br /> <br />. <br /> <br />Hydraulic Analysis <br /> <br />The water surface elevations for the 100-year frequency <br />flood were computed using a modified form of the standard step <br />method. The computation of water surface profiles involves solving <br />the one-dimensional energy equation to determine the slope oi' the <br />energy grade.line between control sections where the water surface <br />is known or can be assumed. The procedure is essentially a trial- <br />and-error solution, utilizing,the continuity equation <br /> <br />Q = VA <br /> <br />where Q is flow in cubic feet per second, V is averag~ velocity in <br />feet per second, "tl i8 :tn9r3.~G ~.!91g(2!it.y ia fGGt:. peL' 8<]S'~na, and A <br />is the cross sectional area in square feet, and the Manning equation <br /> <br />V= 1.~8~2/3s1/2 <br /> <br />where,n is a roughness coefficient, R is the hydraulic radius in <br />feet, S is the energy gradient or slope, and other variables in the <br />. equation are as defined above. <br /> <br />Basic geometric and roughness data were collected in the <br />field by the CWCB staff on July 5, 6, and 7, 1977. Elevations and <br />distances for four strategically located cross sections were obtained <br />by field survey to model the valley topography, because no detailed <br />topographic maps are known to exist. Photographs were also taken to <br />document roughness coefficients chosen for the analysis. Four areas <br />of generally similar roughness characterists were identified and <br />assigned Manning's n values, as summarized in the following table: <br /> <br />Summary of Assumed Roughness Coefficients <br /> <br />Description <br /> <br />Manning's <br />n <br /> <br />Main Channel <br />Cobbled Overbank <br />Forested Overbank <br />Irrigated Fields <br /> <br />0.040 <br />0.060 <br />0.140 <br />0.038 <br /> <br />Once the water surface elevations are calculated for each <br />cross section and plotted, a flood profile can be constructed which <br />graphically depicts flood elevations between the corss sections. <br />The profile also provides a perspective on the reasonabl~ess of ____ <br /> <br />f7 <br />