Page 92 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

LEED Sustainable Sites 73

Subsidiary
Concept Symbol Governing Equation Equation
Percent impervious %Imp %Imp = 100A /A —
imp T
∑ CA
C = i i
Percent imperviousness %Impness %Impness = 100C ∑ A i

TABLE 2.6.2 Summary of Percent Impervious and Percent Imperviousness Equations

The equations used to determine the two main concepts of percent impervious and
percent imperviousness are summarized in Table 2.6.2. Percent impervious is used
frequently in other calculations related to green building and sustainable development.
Percent imperviousness is used mainly as a concept for this credit, although it is based
on the rational method which is used widely in hydrological modeling.

Stormwater Management Plans and Hydrologic Models When the imperviousness of the
existing site is determined, then the new design for the proposed site must meet the
listed requirement options in the credit based on whether the existing imperviousness
is greater than 50 percent. If it is greater than 50 percent, then Case 2 applies and the
credit is obtained only if the new design reduces the existing amount of runoff from the
2-year, 24-h design storm, in total volume only, according to the verbiage in the credit.
If the existing site has an imperviousness equal to or less than 50 percent, then either of
the options in Case 1 may apply. Note that the first requirement of Case 1 applies to
both the peak rate and the total volume of runoff from both the one-year and two-year
storms.
Since the two-year event is greater than the one-year event, it might seem
reasonable to assume that calculations on the two-year storms are sufficient for Case 1.
However, this is not necessarily true. Statistically, one-year storms are more frequent
than two-year storms, and a two-year storm tends to have a higher peak rate and a
higher total volume than its equivalent duration one-year storm. However, most
current stormwater codes and regulations now have requirements on controlling the
runoff from 2-year and 10-year storms, but not usually 1-year storms, and this may
result in changes to the outflow from the 1-year storm. One of the most common
forms of stormwater management is the stormwater detention pond. The rate is
usually controlled by collecting the water from the storm in a pond and releasing it
through an outlet structure at a rate less than the peak requirement (two-year usually).
Sometimes this is referred to as release rate rules. Statistically this flow rate under
natural or preexisting conditions would occur only once every two years. However,
now with the pond and altered hydrological state on the site, there is the opportunity
for this rate to be released much more frequently. The pond can fill to similar levels
for many of the smaller storms, and the outlet control is only required to keep the
outflow less than the two-year rate. There is a chance, based on pond and outlet
structure design, that this equivalent peak two-year rate may now occur for many
smaller storms and perhaps occur on a frequency an order of magnitude greater than
preexisting. In other words, this two-year outflow rate may occur a dozen or more
times in two years. This not only makes the downstream impacts much more frequent,