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<br />Hydraulic Analysis <br /> <br />Most hydraulic calculations of the magnitude and characteristics of flow <br />in channels and overbank areas of flood plains require an evaluation of the <br />roughness characteristics. These calculations are used in flood-plain studies <br />and in instream-flow-requirement studies (minimum flow than preserves the <br />natural environment) to evaluate the flow depth or discharge. <br /> <br />The degree of roughness depends on several factors, the most important of <br />which in open-channel flow are surface roughness of the bed material, cross- <br />section geometry, channel variations, obstruction to flow, type and density of <br />vegetation, and degree of channel meandering. In general, all factors that <br />tend to cause turbulence and retardance of flow, and hence energy losses, <br />increase the roughness coefficient; those that cause smoother flow conditions <br />tend to decrease the roughness coefficient. <br /> <br />Because of the lack of a satisfactory quantitative procedure, the ability <br />to determine the roughness characteristics of flood plains needs to be devel- <br />oped through experience; however, a basic knowledge of the factors affecting <br />the selection of roughness coefficients will greatly aid in the calculation <br />and selection of n values. <br /> <br />Manning Equation <br /> <br />Most commonly, Manning's roughness coefficient, n, is used to describe <br />the relative roughness of a channel or overbank areas, and it appears in the <br />general Manning equation for open channel flow in the following form (Barnes, <br />1967) : <br /> <br />v = 1.486 R2/3s1/2 <br />n f ' <br /> <br />(1) <br /> <br />where V is the mean velocity of flow, in feet per second; <br /> R is the hydraulic radius, in feet; <br /> Sf is the slope of the energy grade line; and <br /> n is the Manning roughness coefficient. <br /> <br />2 <br />