City of Frisco Engineering Standards

Engineering Standards
S
n
S e
= Roadway longitudinal slope (ft/ft)
= Manning’s roughness coefficient
= Equivalent cross slope (ft/ft)
The efficiency of a curb inlet opening shorter than L T is:
E = 1 − 1 − L 1.8
L T
E
L
L T
= Inlet efficiency
= Length of the curb inlet opening (ft)
= Required length of inlet to capture 100% of the roadway flow (ft)
The total flow captured by the curb inlet is:
Q i = EQ
Q i
E
Q
= Flow capture by inlet (cfs)
= Inlet efficiency
= Total flow in the roadway (cfs)
C. Capacity of Curb Inlets in Sag
The capacity of a curb inlet in sag depends on the water depth at the curb opening and the height
of the curb opening. The inlet operates as a weir to a depth equal to the curb opening height and
as an orifice at depths greater than 1.4 times the opening height. At depths between 1.0 and 1.4
times the opening height, flow is in a transition stage and the capacity should be based on the
lesser of the computed weir and orifice capacities.
1. If the depth of flow in the gutter (d) is less than or equal to 1.4 times the inlet opening height
(h), (d≤1.4H), determine the length of inlet required considering weir control. Calculate the
capacity of the inlet when operating under weir conditions with the following equation:
Q = C W (L + 1.8W)d 1.5
2. Rearrange above equation to produce the following relation for curb inlet length required:
Q
L =
C w y
1.5 − 1.8W
o
Q = total flow reaching inlet (cfs)
C w = weir coefficient (3.0)
y o = head at inlet opening (ft)
L = length of curb inlet opening (ft)
W = lateral width of depression (ft)
y o = z QnS x
S 1/2 3/8
Section 4 – Drainage Design Requirements May 2012 Page 4-12