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 turbulentflow 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 frictionfactor 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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 Spring '08
 Sakar
 Fluid Dynamics, Fig, eText Main Menu, subcritical flow

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