Page 304 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SITE UTILITY SYSTEMS

6.26 CHAPTER SIX

of rainfall by the storm water drainage system. This depends on the importance of the facil-
ity, how flooding may affect access, the importance of uninterrupted service, and the value
of the equipment or material installed or stored. Since a severe thunderstorm or hurricane
may produce rainfall rates greater than anticipated, the property value may require the
selection a frequency longer than the one originally selected.
The return period selected could be based either on client preference, code requirements,
or the degree of safety desired by the engineer. Choices on the recurrence of the storm is based
on economical considerations and risk trade-off. The following criteria may he helpful as a
guide. For average sites a design storm frequency of 10 years is generally used. Certain clients
design their projects for 40 years of useful life, which may require a 50-year design storm
if flooding might cause a problem. Many municipal authorities use a 2- or 5-year design
storm for city sewers. Some use a 10-year storm. The U.S. Corps of Engineers normally uses
a 10-year storm, but for military installations a 25-year storm is used.
After the return period is selected, it is put aside for future use when the intensity is
determined.

Rainstorm Duration (Time of Concentration). The final figure in the calculation of rain-
fall rate is the determination of the length of time (duration) rain will fall during the design
rainfall, measured in minutes. The reason for the importance of duration is that the peak
flow of the rainstorm is assumed to occur the instant that a drop of water reaches the design
point from the furthest part of the contributing area, and then stops. The shorter the period
of time; the more intense the rate of rain is assumed to be.
As an example, for New York City, the heaviest total amount of rain on record for a 5-min
period is 0.75 in. Multiplied out to determine the 1-h rate at the 5-min reading gives and
hourly figure of 9 in of rain per hour. Yet the largest amount of rainfall ever recorded over
a 1-h period is 3.11 in. It can thus be seen that the rate of rainfall measured during a short
period is much more intense than the rate measured over a longer period of time.
It is recommended that a 10-min duration be used as the minimum for typical design
conditions. Less than 10 min will not usually be justifiable.
We now calculate the amount of time it takes for a drop of water to reach any design
point, commonly called the time of concentration. This is done by finding the inlet time
plus the time in pipe, if applicable.
Overland Flow Time (Inlet Time). The overland flow time, often called the inlet time, is
the number of minutes it takes a drop of water to travel from the furthest point of the tribu-
tary area to a DI. For equal distances and slope, it takes longer for a drop of water to enter a
DI from the furthest point of a lawn than from a paved parking lot. Slope of the land is also
a factor, since for any given surface, the steeper the slope, the faster water will flow.
Figure 6.16 is a direct reading chart, giving the overland flow time when the distance
from the most remote point to the DI, the average slope of grade and type of surface are
known. A special case would be on pavement where the flow of water from surfaces would
be in the form of a very shallow sheet of water covering the surface of the ground. This
phenomenon is known as sheet flow and has a high velocity.
Experience has shown that for most projects, a very short duration will result in exces-
sive runoff volume and excessive pipe sizes. Experience has shown that for most projects, a
duration of less than 10 min will result in calculating excessive volume and result in larger
pipe sizes than required.

Time in Pipe. The time in pipe is the number of minutes the theoretical drop of water takes to
flow from the outlet pipe of a DI in question to the next design point, or, from one design point
to another. This requires that the pipe be sized and the velocity of the stormwater is known.
Finding the velocity in feet per second and multiplying that figure by the actual pipe
length, in feet, gives the time in pipe, in seconds. The velocity can be found in Table 6.26.
Convert this to minutes.

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