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<br />Determination of Manning's Roughness Coefficient <br /> <br />As part of the design or eva:.uation of channels, it is necessary to estimate <br /> <br />the flow capacity of the channel. All formulas based on continuit:r of flow <br /> <br />that relate to the discharge capacity of a channel to its geometry include an <br /> <br />estimate of friction losses. The three equations in general use for <br /> <br />estimating friction losses alCe referred to as the Chezy, Manning, and Darcy- <br /> <br />Weisbach equations, each of "hich has a resistance coefficient (designated, <br /> <br />respectively as C, n, and f). Each roughness coefficient is a function of the <br /> <br />size of the bed and bank mat..rial and of other flow obstructions. The <br /> <br />.coefficients are related as follows <br /> <br />C - Rl/6 - 10.8 <br />1.486 n fl/2 <br /> <br />To simplify the evaluation -0:: data presented in figure 7, roughness factors <br /> <br />for gravel bed channels were separated from sand bed channels. This <br /> <br />distinction was made by selec:ting the bed material size (D50) for sand bed <br /> <br />.channels to be 0.00328 ft or 1 rom, which is the maximum size of coarse sand <br /> <br />given by Guy (1969). The range in Mannings roughness coefficient, n, for the <br /> <br />data in figure 7 for gravel, cobble, and boulder bed channels is 0.020 to <br /> <br />0.159, and for sand bed channels, the range in Manning's n is 0.013 to 0.046. <br /> <br />Referring to figure 6, curve A is an enveloping curve for upper limits of <br /> <br />roughness factor data for gr'lvel bed channels. The median bed material size <br /> <br />(DSO) is larger than 0.00328 feet (lmm). This curve is arbitrarily truncated <br /> <br /> <br />at a relative roughness valu., (Da/D50) of 35, although the largest ratio <br /> <br /> <br />measured for gravel or cobbll~ beds was about 185. <br /> <br />23 <br />