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1013 with f estimated from the moody friction factor

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Unformatted text preview: rt, Fig. 6.13. Indeed, the open channel research establishment [17] strongly recommends use of the friction factor in all calculations. Since typical channels are large and rough, we would generally use the fully rough turbulent-flow limit of Eq. (6.64): 2.0 log f 14.8Rh 2 (10.15a) A special case, for rocky channel beds, is recommended in Ref. 2: f 1.2 Rh d84% 2.03 log 2 (10.15b) where d84% is the size for which 84 percent of the rocks are smaller (the largest rocks dominate the friction in the channel). Note that d84% and are not equal, being an overall average size. In spite of the attractiveness of this friction-factor approach, most engineers prefer to use a simple (dimensional) correlation published in 1891 by Robert Manning [12], an Irish engineer. In tests with real channels, Manning found that the Chézy coefficient C increased approximately as the sixth root of the channel size. He proposed the simple formula C 8g f 1/6 Rh n 1/2 (10.16) | v v where n is a roughness parameter. Since the formula i...
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