Page 306 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SITE UTILITY SYSTEMS

6.28 CHAPTER SIX

5. Construction of the design storm will give the final piece of criteria (R) necessary for the
solution of the rational formula. To accomplish this, the duration of the design rainfall
shall be found. The first part is the finding of the overland flow time. This is done by
using Fig. 6.16, which is a direct reading chart that gives the overland flow time. The
time is found by using a concurrent combination of the distance of the furthest point of
the tributary area into the DI, determining the average slope of the ground and finally the
type or combination of surfaces. If the area consists of more than one surface, calculate
the weighed coefficient. Using the longest distance to the DI find it on the left side of the
diagram. Draw a line horizontally until it intersects the average slope. From there, draw
a vertical line until it intersects the curve of the selected imperviousness coefficient.
From this intersection, draw a line to the right to find the number of minutes that would
be the overland flow time. The minimum overland flow time should be 10 min. Rarely
will a lesser time be economically justifiable.
6. Utilizing the rainfall intensity-duration-frequency curves (Fig. 6.15) for the city closest
to the project location, select the rainfall intensity using the return period selected and
the time of concentration previously calculated. The time of concentration will be the
overland flow time (found in Fig. 6.16) plus the time in pipe, if applicable,

800
(244)

700 2.0
(213)
600 80
(183) 2.5% slope 1.5 1.0 0.75 0.5% slope 70
Distance from the most remote point of the tributary to the inlet, ft (m) (122) 0 imperviousness 60 Overland flow time, min
500
(152)
400
0.10
0.20
300
50
0.30
(91)
0.40
200
40
0.50
(61)
0.60
100
(30)
0.70 30
0 0.80 20
0.90
0.95
10

0
FIGURE 6.16 Overland flow time.

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