Page 531 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch13_457-499.qxd 8/7/10 8:49 PM Page 489

13.10 Street Inlets 489
To account for local depressions or gutter depressions, an additional composite or
equivalent slope, S , is necessary:
e
S S S E (13.39)
x
e
w’ 0
where S , is the gutter cross-slope at the inlet location measured from the pavement cross-
w
slope, S (m/m, ft/ft) and E is the ratio of flow in the depressed section to the total gutter
0
x
flow upstream of the inlet (does not account for local depression).
To calculate L with a composite gutter, replace the road cross-slope S with the equiv-
x
T
alent slope, S , in the equation solving for L .
T
e
The efficiency of a curb opening on grade shorter than the required length for total in-
terception is
L 1.8
E = 1 - a1 - b (13.40)
L T
EXAMPLE 13.17 LENGTH OF AN UNDEPRESSED CURB INLET
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 an undepressed curb inlet required to capture 100% of the flow
2. The length of an undepressed curb inlet required to capture 90% of the flow.
Solution 1 (U.S. Customary System):
1. From Eq. 13.38,
1 0.6
0.42 0.3
L T = K c Q S L a b
nS x
0.3
L T (0.60) (1) 0.42 (0.02) [(1>0.015)(0.056)] 0.6
(0.60)(0.31)(70) 13 ft (4 m)
The length of curb inlet required to capture 100% of the flow 13 ft (4 m).
2. From Eq. 13.40,
L 1.8
E = 1 - a1 - b
L T
0.90 1 (1 L>13) 1.8
(0.10) 0.55 1 L>13
L (1 0.28)(13) 9.4 ft (2.87 m)
The length of curb inlet required to capture 90% of the flow 9.4 ft (2.87 m).

Solution 2 (SI System):
1. From Eq. 13.38,
1 0.6
L T = K c Q 0.42 0.3 b
S L a
nS x
0.3
L T = 0.817 (0.0283) 0.42 (0.02) (1>0.015 * 0.056) 0.6
= 0.817 * 0.2237 * 0.3092 * 70.0539
= 4 m
The length of curb inlet required to capture 100% of the flow 4 m.