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Math Help - Civil engineering formula

  1. #1
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    Civil engineering formula

    Civil engineers use the formula Q = 3.32LH^3/2

    to find the maximum discharge that the dam (a broad-crested weir) shown in
    the figure can pass before the water breaches its abutments (Standard
    Handbook for Civil Engineers, 1968). In the formula Q is the discharge in
    cubic feet per second, L is the length of the spillway in feet, and H is
    the depth of the spillway. Find Q given that L=60 feet and H=5 feet. Find H
    given that Q=3000 cubic feet per second and 70 feet.

    This is what I have come up with thus far.

    Q = 3.32*60*5^3/2

    Q = 12450

    3000 = 3.32*70H^3/2

    L = 2.95555
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  2. #2
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by pashah
    Civil engineers use the formula Q = 3.32LH^3/2

    to find the maximum discharge that the dam (a broad-crested weir) shown in
    the figure can pass before the water breaches its abutments (Standard
    Handbook for Civil Engineers, 1968). In the formula Q is the discharge in
    cubic feet per second, L is the length of the spillway in feet, and H is
    the depth of the spillway. Find Q given that L=60 feet and H=5 feet. Find H
    given that Q=3000 cubic feet per second and 70 feet.

    This is what I have come up with thus far.

    Q = 3.32*60*5^3/2

    Q = 12450

    3000 = 3.32*70H^3/2

    L = 2.95555
    Is your formula Q = 3.32LH^{3/2} or Q = \frac{3.32LH^3}{2}?

    -Dan
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  3. #3
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    Quote Originally Posted by topsquark
    Is your formula Q = 3.32LH^{3/2} or Q = \frac{3.32LH^3}{2}?

    -Dan
    Not both of them do not make no sense to not me.

    See, Q is supposed to be in cubic feet per second.
    But L and H are simplfy units of length. So how can you ever get 'per second' unit? Further the left hand side is a cubic measurement of legth while the right is not. See the problem?
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  4. #4
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    Quote Originally Posted by ThePerfectHacker
    Not both of them do not make no sense to not me.

    See, Q is supposed to be in cubic feet per second.
    But L and H are simplfy units of length. So how can you ever get 'per second' unit? Further the left hand side is a cubic measurement of legth while the right is not. See the problem?
    The constant is not dimensionless.

    RonL
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  5. #5
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    Formula

    This is the formula I am using

    Q = 3.32LH^3/2
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  6. #6
    Grand Panjandrum
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    Quote Originally Posted by pashah
    This is the formula I am using

    Q = 3.32LH^3/2
    Topsquark's question is about whether you mean:

    Q = 3.32LH^(3/2)

    or:

    Q = 3.32L(H^3)/2.

    Given the numerical example you give I guess its is the second of these.

    If this is true your answers look OK to me (except for the second that should
    be H=2.95555)

    RonL
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