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492 Chapter 13 Hydraulics of Sewer Systems

d o
h d i

Horizontal throat
d d (h/2)
i
o
θ π /2

d o
θ
h
Inclined throat
d d h/2 (sinθ)
i
o

d o
Vertical throat
d d i
o
θ 0
h
Figure 13.17 Different Curb Inlet Throat Types.

EXAMPLE 13.18 LENGTH OF AN INLET LOCATED IN SAG
3
For a flow of 1.0 ft /s (28.3 L/s), a longitudinal street grade of 2.0%, a mean crosswise street grade
of 5.6%, and an n coefficient of roughness of 0.015, find:
1. The length of a submerged in sag grate inlet required (assume C o 0.60)
2. The required length of unsubmerged in sag grate opening inlet. Assume C w 3 and
W g 1 ft (0.3 m).

Solution 1 (U.S. Customary System):
3
1. For 100% interception in sag, Q w 1 ft /s (28.3 L/s) and C o 0.60. From Example
13.14, d o 2.2 in 0.18 ft (55 mm). From Eq. 13.43,
Q io = C o A(2gd 0 ) 0.5
1 0.60 A(2 32.2 0.18) 0.5
1 0.60 A(3.40) 2.0 A
2
2
A 1>2.0 0.50 ft (0.046 m )
Assuming the open area in the grate is 50%,
2
2
total grate area 2 0.5 1 ft (0.09 m )
Using a 1-ft–(0.30-m-) wide grate inlet, the required inlet length will be 1 ft (0.30 m).
Note on the efficiency of submerged grate inlets in sag: Compare the size of this on-sag
2
2
grate inlet (1 ft in area or 0.09 m ) intercepting 100% of the gutter flow to the on-grade