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15 February, 18:20

What is the normal depth in a 4.5-foot wide rectangular channel that conveys a discharge of 185 cfs and has a bed slope of 4% with a Manning's roughness coefficient of 0.016?

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  1. 15 February, 20:52
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    normal depth is 2.7 ft

    Explanation:

    Given data

    wide = 4.5 foot

    discharge Q = 185 cfs

    slope S = 4%

    roughness coefficient n = 0.016

    To find out

    What is the normal depth

    solution

    we consider height is d

    so area A = 4.5 d

    perimeter = 2d + 4.5

    so Rn = 4.5d / 2d+4.5

    and discharge formula is

    Q = 1/n A (Rn) ^2/3 * (S) ^1/2

    185 = 1/0.016 * (4.5d) (4.5d / 2d+4.5) ^2/3 (0.04) ^1/2

    d² = 0.731 (2d + 4.5)

    d² + 1.462 d + 3.289 = 0

    solving this equation we get

    d = 2.7 ft

    normal depth is 2.7 ft
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