City of Frisco Engineering Standards

Engineering Standards
y o = depth of water in the curb and gutter cross section (ft or m)
Q = gutter flow rate (cfs)
n = Manning’s roughness coefficient
S = longitudinal slope (ft/ft)
S X = pavement cross slope (ft/ft)
Z = 1.24
3. If the depth of flow in the gutter is greater than the inlet opening height (d>h), determine the
length of inlet required considering orifice control. The equation for interception capacity of
a curb opening as an orifice follows:
Q = C 0 hL2gd e
Q = total flow reaching inlet (cfs)
C 0 = orifice coefficient = 0.70
h = depth of opening (ft) (this depth will vary slightly with the inlet detail used)
L = length of curb opening inlet (ft.)
g = acceleration due to gravity = 32.2 ft/s 2
d e = effective head at the centroid of the orifice (ft) d e =d - h/2
Rearranging equation allows a direct solution for required length:
Q
L =
C 0 h2gd e
4. If both steps 1 and 2 were performed (i.e., h