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488 Chapter 13 Hydraulics of Sewer Systems

From Eq. 13.36:
Q i = Q w R f + Q s R s
3
3
Q i = (0.026 m /s) * 0.96 + (0.0283 - 0.026 m /s) * 0.165
3
= 0.025 m /s
25 L/s

13.10.3 Curb Inlets on Grade

Curb inlets are openings within the curb itself (see Fig. 13.14), and they are used in areas
where grate inlets are prone to clogging. The efficiency of a curb inlet is based on the ratio
of the actual inlet length to the inlet length necessary to capture 100% of the total runoff.
Curb inlets are often inset into the pavement to create a local depression. A local depres-
sion, as shown in Fig. 13.15, is a depression of the gutter at the location of the inlet only, as
opposed to a gutter depression, which is continuous along the curb.
The curb opening length, L T , that would be required to intercept 100% of a flow, Q i ,
on a roadway section with a uniform cross-slope is defined as
1 0.6
0.42 0.3
L = K Q S L a b (13.38)
T
c
nS x
where K c is 0.817 (0.60 in U.S. units).

L

Figure 13.14 Curb Inlet.

S x
a S w
aʹ S w’
S Slope of local depression (m/m, ft/ft)
w’
a Gutter depression (mm, inch)

aʹ Total depression (mm, inch)

Figure 13.15 Local Depression at a Curb Inlet.