Erosion and Sediment Pollution Control Program Manual.pdf

EXAMPLE: Calculate the anticipated discharge velocity for a 24” diameter circular concrete pipe with a
0.5% slope and a design discharge of 12.7 cfs.
Full-Flow Discharge
Full-Flow Velocity
Q
f
Q f
0.464
(2)
0.015
8/3
(0.005)
30.93 x6.35
x0.07
13.7 cfs
3.14 ft
Ratio of Part-Full to Full-Flow Discharge:
V
f
d / D
2
12.
7
13.
7
From Figure 9.1, the Velocity ratio is 1.13
V d
4.
4
4.4 fps
. 93
fps x 1.
13
5.
0
363-2134-008 / March 31, 2012 / Page 230
1/2
13.7 cfs
The downstream channel lining should be able to withstand a velocity of 5.0 fps.
For pipes with slopes equal to or greater than 0.05 ft/ft, Manning’s equation should be used.
The velocity is calculated for full flow conditions even though the pipe is typically flowing only partially
full. Partial flows will be very close to the full flow velocity for depths of flow between 30 percent and
100 percent of the pipe diameter in steep slope conditions. Manning’s equation for full pipe flow is as
follows:
V
1.486
n
fps
D
4
where: V = Velocity in fps
D = Pipe diameter in feet
S = Pipe slope in ft/ft
Example: For a 24-inch diameter CMP pipe at a 10% slope.
V
1.486
.024
2
4
2
3
2
3
S
0.10
V = 61.9 X 0.63 X 0.32 = 12.5 fps
For pipes with high anticipated velocities (>13.0 FPS), drop structures or other energy reducing
structures are recommended to reduce velocity prior to discharge.