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Homework Help: Max distance Between storm water inlets

  1. Mar 30, 2012 #1
    A suburban street receives storm water runoff at rate of 2.0 L/m-s (on each side). The street has a slope of .8 percent and a cross slope of 2 percent; n = .013. If the street is 10 m wide and can be totally covered by water (depth = 0 at the midpoint), what is the max distance between inlets?

    Thinking to use modified Manning's equation for triangular gutter cross section

    Q = K(z/n)(s^.5)[y^(8/3)]

    Q = gutter flow
    z = reciprocal of the cross slope of the gutter
    n = roughness coefficient
    s= gutter slope
    y = water depth at the curb
    K = constant = .38 for metric

    first I found the Q for the gutter flow
    z= 1/.02=50
    n = .013
    s= .008
    y= 5m(.02)=.1m
    K= .38

    so Q = .38(50/.013)(.008^.5)[.1^(8/3)] = .28 m^3/s

    at this point I am not sure how to use the runoff of 2 L/m-s and the Q = .28 m^3/s to get a max distance between inlets?
  2. jcsd
  3. Mar 30, 2012 #2
    may have figured this out. if I convert the L in 2 L/m-s to m^3 I get .002 m^2/s. then if I take (.28 m^3/s)/(.002m^2/s) = 140m. so basically this is a right triangle with a hypotenuse of 2L/m-s and leg1 = .28m^3/s and leg2 = length. Any thoughts or further info is welcome.
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