Page 312 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SITE UTILITY SYSTEMS

6.34 CHAPTER SIX

Depression storage is rain caught in low points on grade and thus not available as runoff.
Infiltration is water absorbed into the ground. Investigations have shown that for areas
having a CN of 60 to 65, 2 in (50 mm) of rain must fall before runoff even starts. For this
reason, a 24-h period is chosen. A heavy concentration of rain falling later in a given storm
event will produce a greater peak discharge than at the beginning of the same storm.
The 24-h time period is also used to determine total volume of runoff for the same rea-
sons. As will be seen, modifications and assumptions must be made to properly estimate
runoff from the rainfall amount chosen.
The first step in retention basin design is to calculate the peak rate of discharge from
the existing site and then from the site as it will be developed. To do this, the CNs of both
the original and developed site must be found. When there is only one type of land use
and one soil group, this is easy. Otherwise, a weighted CN must be found. This is done by
determining the CN for each type of soil condition found and the number of acres for each
CN, and multiplying these two figures. The sum of all the products is divided by the total
area to find the weighted CN.
EXAMPLE A site consists of 100 acres with two types of land use. One type is 70 acres
of pasture land in good condition with soil group C, and the other type is 30 acres of grain
field with soil group B. Find the weighted CN for the site.

Solution Refer to Table 6.4 to find the runoff CN. The pasture has a CN of 88, the grain
field, a CN of 71.
70 acres × 88 = 6160
+ 30 + 2130
acres × 71 =
100 8290
8290
= 82 9 (rounded to 83)
.
100
Therefore, 83 is weighted CN for the entire site.
A similar calculation is made to find the CN for the developed site.
1. After the CN has been determined for both the undeveloped site and the developed site,
the rate of rainfall and return period should be selected. Local rainfall standards must be
used. However, if no standards exist, the maps in Figs. 6.18 (10-year, 24-h rainfall), 6.19
(25-year, 24-h rainfall), 6.20 (50-year, 24-h rainfall), and 6.21 (100-year, 24-h rainfall)
should be used. Interpolation between isobars is necessary for intermediate values. A
recommended average storm would be a 50-year, 24-h rainfall, Fig. 6.20.
2. Next, an adjustment to the 24-h rainfall figure must be made to allow for abstraction and
the other considerations that were made when the CN was determined. Table 6.5 reduces
the amount of runoff depth according to the various factors previously discussed.
3. Another item required for the determination of peak rate of discharge for the site is the
adjustment to rainfall based on the average slope of the watershed. This is the average
slope of grade to the storm water inlets, not of the piping system. Figures 6.22 (up to
2 percent slope), 6.23 (up to 7 percent slope), and 6.24 (up to 50 percent slope) give the
discharge in cfs per inch of adjusted rainfall on the site, according to the average slope
for Type II storms. Table 6.6 gives adjustment factors for intermediate slopes other than
those found in Figs. 6.22 to 6.24. Type I storm tables are not available.
With the CN calculated, it is now possible to calculate the peak rate of discharge for the
site. For example, a 300-acre site is to be developed in the northern part of New Jersey. The
present weighted CN has been determined to be 75. The average slope of the watershed is
found to be 4 percent. A 50-year, 24-h storm has been selected.

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