Page 310 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SITE UTILITY SYSTEMS

6.32 CHAPTER SIX

STORM WATER RETENTION METHODS

There are circumstances where it is not possible or desirable to remove storm water from
a site as quickly as it collects. Some built-up urban areas generally have sewer systems
originally designed for low projected land development and population density, which may
result in the design outflow from the site exceeding the permitted flow into the sewer or
the environment. Therefore, many cities have adapted a storm water management program
to limit the outflow of storm water from developed sites to a flow rate equal to that before
the development was started. Similarly, if a small stream is used as an outfall, it may not
be possible to drain a large paved area that was formerly woodland into this stream without
flooding the downstream portion.
In these circumstances, some method of temporarily storing the storm water must be
provided. The methods used most often are to retain the storm water on the roof, on the
site, or both.

ROOF RETENTION

Refer to Chap. 9, Plumbing Systems, section entitled, Limited Discharge Roof Drainage
Systems, for the method used to retain water on the roof.

SITE RETENTION METHODS

Storage of water on roofs, if allowed, provides only a small portion of the retention gener-
ally required for an entire site drainage system. Therefore, additional provisions must be
made. For smaller sites where space is limited, this is best accomplished by temporarily
retaining the excess storm water in a retention basin either at or below ground. Another
popular method is to greatly oversize the discharge piping, in effect, using the pipe to store
the required volume. On-ground retention is called ponding.
The least costly method is the oversized pipe. Substituting a 72-in (DN 1800) or larger
pipe for the designed size is a possibility if the ground is deep enough to accommodate it.
If not, a retention basin should be considered.
Several factors should be considered in the selection of a retention basin. First is cost.
An owner may want a pond in an out-of-the-way corner of the site rather than the more
expensive retention basin. Second is available depth. The depth of an underground basin
may be limited by the invert of the sewer to which it would be connected, or the depth of
the stream or river which would provide the final outfall. In many cases, an underground
basin may be the only solution in the development of an urban site, possibly with a pump
to reach a higher sewer line.
The design of a retention basin begins with the concept that the total volume of storage
required will be the difference between storm water inflow and allowable outflow for a
given period of time. This requires the calculation of three variables:
1. Outflow from basin, which varies as a function of time
2. Inflow to basin, which varies as a function of time
3. Storage, which is the difference between 1 and 2 above for a specific time period
The information for retention basin design has been extracted from Technical Release
Number 55, U.S. Department of Agriculture, Soil Conservation Service. The methodology
differs from that of the Rational Method for storm water design. For example, total volume

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