Page 311 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SITE UTILITY SYSTEMS

SITE UTILITY SYSTEMS 6.33

of rainwater produced by a given storm during a particular time is used rather than rate of
rainfall in inches per hour.
When original agricultural and wooded areas are replaced with new, less impervious
surfaces such as buildings and roads, both the volume and the peak rate of discharge will
increase. The additional peak rate of discharge for the storm period is the amount of water
to be stored on site while being released at a lower predetermined flow. Following are brief
explanations of the factors to be considered in obtaining a reliable estimate of the peak rate
of discharge and total runoff volume:
1. Storm distributions. There are two storm distributions that are used for different climatic
regions: Type I and Type II. Type I storms are located in maritime climates and are found
only in limited areas west of the Rocky Mountains. Type II storms are found in regions
where high rates of runoff are usually generated from summer thunderstorms or tropical
storms.
2. Hydrologic soil group. There are groups of soils that have the same runoff potential
under similar storm conditions. Nationwide, over 8000 specific soils have been classi-
fied into four soil groups as follows:

Group A: Low runoff potential soils having a high infiltration rate even when thoroughly
wet and consisting chiefly of deep, well- to excessively-drained sand or gravel
Group B: Soils having a moderate infiltration rate when wet and consisting chiefly of
moderately deep to deep, moderately well- to well-drained soil with moderately fine to
moderately coarse texture
Group C: Soils having a slow infiltration rate when wet and consisting of soil with a
layer that impedes downward movement of water, or soil with moderately fine to fine
texture
Group D: High runoff potential soils having a very slow infiltration rate when wet and
consisting chiefly of clay soil with a high swelling potential, soil with a high permanent
water table, soil with a clay pan or clay layer at or near the surface, and shallow soil
over nearly impervious material

Caution is advised when determining the soil group for the final developed conditions,
as machine compaction or removal of upper soil layers can change the group drastically.
Advice of the Soil Conservation Service is recommended when determining the soil
group.
3. Average watershed slope. The steeper the average slope of the watershed, the greater the
peak discharge will be.
4. Land use. The most common effect of urbanization is reduced infiltration, resulting in
increased runoff volume, decreased overland travel time, and higher peak rates of flow.
The volume of runoff is governed primarily by infiltration characteristics, and is related
to soil type, type of vegetative cover, impervious surfaces, and surface retention. Travel
time is governed by slope, flow length, and surface roughness. The peak discharge is
based on the relationship of these factors.
5. Runoff curve number (CN). This is a dimensionless number selected for use in calcula-
tions that takes into account the previous land use information. The CN allows informa-
tion to be conveyed in easy tabular or graphical form.
6. Amount of rainfall. The criteria for the selection of rainfall quantity used to determine
the design parameters for the storage requirements are different than those used to design
the piping system. Total rainfall is used rather than the instantaneous rate. In fact, the
total rainfall for an entire 24-h period is used because of a phenomenon called abstrac-
tion, which is the sum of interception, depression storage, and infiltration. Interception
is rain caught by foliage, leaves, and twigs that evaporates before it reaches the ground.

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