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Unformatted text preview: s and roughness, the scatter bands in Table 10.1 should be taken seriously.
Since Manning’s sixthroot size variation is not exact, real channels can have a variable n depending upon the water depth. The Mississippi River near Memphis, Tennessee, has n 0.032 at 40ft flood depth, 0.030 at normal 20ft depth, and 0.040 at
5ft lowstage depth. Seasonal vegetative growth and factors such as bottom erosion
can also affect the value of n. EXAMPLE 10.1
A finishedconcrete 8ftwide rectangular channel has a bed slope of 0.5° and a water depth of
4 ft. Predict the uniform flow rate in ft3/s. Solution
Part (a) From Table 10.1, for finished concrete, n 0.012. The slope S0
y 4 ft and width b 8 ft, the geometric properties are
A by (8 ft)(4 ft)
Rh A
P 32 ft2 P b 2y tan 0.5°
8 2(4) 0.00873. For depth
16 ft 2 32 ft
16 ft 2.0 ft Dh 4Rh 8.0 ft From Manning’s formula (10.19) in BG units, the estimated flow rate is
Q 1.486
2/3 1/2
ARh S0
n 1.486
(32 ft2)(2.0 ft)2/3(0.00873)1/2
0.012 590 ft3/s Ans. *An inter...
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This note was uploaded on 10/27/2009 for the course MAE 101a taught by Professor Sakar during the Spring '08 term at UCSD.
 Spring '08
 Sakar

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