City of Frisco Engineering Standards

Engineering Standards
S
T
n
= Longitudinal slope (ft/ft)
= Width of flow in roadway (ft)
= Manning’s roughness coefficient
Depth of flow in the gutter can be calculated using the following modified form of the equation
above:
y o = z QnS x
S 1/2 3/8
y o = depth of water in the curb and gutter cross section (ft or m)
Z = 1.24
If the flow in the gutter is still excessive, the storm sewer shall be extended to a point where the
gutter flow can be effectively intercepted by curb inlets.
B. Capacity of Curb Inlet on Grade
To determine the capacity of a curb inlet on grade, first determine the ratio of the flow in the
locally depressed gutter section to the total flow in the road.
E 0 = 1/ 1 + S w
1 +
S 2.67 −1
w/S x
S X (T/W) − 1 − 1
E 0
S W
S x
T
W
= Ratio of flow in the depressed gutter to the total flow
= Gutter cross slope (ft/ft)
= Roadway cross slope (ft/ft)
= Width of flow in roadway (ft)
= Width of depressed gutter section (ft)
Then calculate the equivalent cross slope at the depressed curb inlet opening.
S e = S x + a w E 0
S e
S x
a
W
E 0
= Equivalent cross slope (ft/ft)
= Roadway cross slope (ft/ft)
= Gutter Depression Depth (ft)
= Width of depressed gutter section (ft)
= Ratio of flow in the depressed gutter to the total flow
Then calculate the inlet length required to capture 100% of the gutter flow.
L T = 0.60Q 0.42 S 0.3 1 0.6
nS e
L T
Q
= Required length of inlet (ft)
= Total flow in the roadway (cfs)
Section 4 – Drainage Design Requirements May 2012 Page 4-11