Page 324 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SITE UTILITY SYSTEMS

6.46 CHAPTER SIX

2. Find ratio:
Q 399
I = = 0 507 (rounded to 0.51)
.
Q O 698
3. Determine VR: From above, 3.28 in of rain is used.
4. Find V : First enter Fig. 6.28 with QQ/ = 051.. Read 0.27 ratio on the left side of
S O I
chart. Therefore:
×
V = 328 027 = 088
.
.
.
S
5. Find basin volume [Eq. (6.6)]:
.
V = 0 88 ×100 = 88 = 73. acre ft
B
12 12
2
EXAMPLE 2 A 04.mi watershed with an adjusted rainfall of 3.28 in is required to
limit discharge to 103 cfm through a piped outlet. What basin volume is required?
1. Find csm [use Eq. (6.8)]
103
= 258 csm
.
04
Therefore, use Fig. 6.27 since this is less than 300 csm.
2. Find V . Entering Fig. 6.27 with V of 3.28, find V of 1.55.
S R S
3. Find V . Use Eq. (6.6).
B
V × A 155 × 640 04 .
×
.
.
V = S = = 33 1 acre ft
B
12 12
To convert from acre feet to cubic feet, multiply acre feet by 43,560 and divide the product
by 9.
The following design criteria must be established to select the size of the retention basin
and the size of the outflow pipe:
1. Allowable cfs to outflow pipe
2. Height from the top of the basin to centerline of outlet pipe
3. Location of the retention basin
4. Completed site drainage system
5. Trial size of outlet pipe
6. Trial slope of outlet pipe

First, the retention basin must be located on the site plan. The size restrictions must be
determined, if any, and then a trial depth found. Next, assuming a trial size pipe, one-half of
the pipe diameter is subtracted from the basin depth. Also, at this time, the rate of discharge
from the site to the outfall must have been definitely established. With the above informa-
tion, the size discharge pipe can be calculated from the following orifice formula. Note that
the allowable discharge will only occur when the design head is reached.
A = Q (6.9)
C × 2 GH

2
where A = area of outlet pipe, ft (refer to Table 6.7)
Q = cfs discharge allowable

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