Page 305 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SITE UTILITY SYSTEMS

SITE UTILITY SYSTEMS 6.27

Rate of Rainfall (Rainfall Intensity). The last figure for inclusion into the rational formula
is “R,” the rate or intensity of rainfall. It is obtained from the rainfall intensity-duration-
frequency curves, shown in Fig. 6.15, and is measured in inches per hour.
Rainfall intensity is amount of average precipitation, measured in inches per hour, in a
given period of time. The determination of the intensity uses a combination of the duration
of rainfall, in minutes, along with a set of curves for the frequency of occurrence to find the
rainfall intensity. In order to enter Fig. 6.15, the return frequency of a design storm must
have been selected and also the rainfall duration (which is the sum of the overland flow time
and the time in pipe, (if applicable) will have been calculated. To use the chart, first find
the calculated duration at the bottom of the chart. Draw a vertical line up until it intersects
the curve for the selected return frequency. From this intersection, draw a horizontal line to
the left to read the rainfall rate.
With the calculation of the rate of rainfall, it is now possible to substitute the appropriate
figures into the rational formula necessary to size each design point on the site.

SYSTEM DESIGN PROCEDURE

1. The drainage structures should be located, and final coordination completed with the
site planner. Select the type of drainage structures necessary, either DI or CB.
2. Select the return period for the design storm. If there is some concern of a recurrence in a
short period of time, there is a new manner of determining the probability. The probability
of any storm exceeding the rainfall rate to be used in the design of the storm water system
for any one year, can be found by dividing the design time, in years, into 1.
Using a 25-year frequency storm as an example, what will be the probability that this
storm will be exceeded in a 1-year period of time?
P = 1 p = 004.
25
In this example, the probability that a rainfall will occur in one year which exceeds
a 25-year storm is 4 percent. Since the forces of nature is unpredictable, it is entirely
possible for a specific rainfall rate to be exceeded at any time. With the selection of a
higher average frequency of occurrence, such as 100 years, the probability of a similar
storm occurring is considerably lower. Choices on the possible recurrence of the storm
is based on economical considerations and risk trade-off.
3. A site plan showing the location of the drainage structures and contour lines shall be
used to determine the area draining into each DI and the distance from the furthest
contributing point. The type of surface of the contributing area should be available or
easily be obtained. The contour lines will allow calculation of the slope of the land into
each DI. A chart or tabulation in any convenient form should be prepared indicating the
type of surface and the square feet area of each type of surface draining into each DI.
4. Determine the imperviousness factor of the various surfaces in the watershed using
Table 6.2, keeping in mind the average slope of the land and the type of surface or surface
combinations. Use a weighed factor for areas having more than one surface type. The
weighed factor is obtained by finding the area of each type of surface and multiplying the
area by the imperviousness. This is done for each type of contributing area, adding them
together and dividing the sum by the whole area going into the DI. As an example, there
are three separate tributary areas that have been previously determined to be 0.5 acres of
grass, 0.5 acres of pavement, and 0.2 acres of roof. This would be solved as follows:
(0.5 × 0.15) + (0.5 × 090 ) + (0.20 1.00)
×
(
.
.
Weighed factor = factor = 060
1.2
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