Page 90 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

LEED Sustainable Sites 71

Calculations and Considerations
To understand this credit and determine potential technologies and strategies, the first
thing that needs to be understood is what the term imperviousness means. Ordinarily in
land development and watershed analyses, the features on the land may be divided
into two characteristic types of surfaces: pervious or impervious surfaces. Impervious
surfaces are typically roofed and paved areas where the roof or pavement material does
not allow any significant infiltration of water into the earth below. Pervious surfaces are
everything else. Frequently, it is required to determine the percent pervious or percent
impervious areas for a project. In that case, the percent impervious is simply the land
areas covered by characteristically impervious surfaces, divided by the total project site
area A , given in percent. If A is the total land areas with impervious surfaces and
T imp
%Imp is the percent impervious, then
%Imp = 100A /A (2.6.1)
imp T
However, many surfaces are more or less pervious than others. For instance, hard-packed
clay or a soccer field surface may be less pervious than a sandy beach. In addition, the
concern here is more with the percent of rainfall that “runs off,” not the percent that
infiltrates. There are other mechanisms by which rainfall may exit an area other than
infiltration and runoff, including evaporation and evapotranspiration. So even if an
asphalt shingle roof may be considered an impervious surface, it does not shed all the
rainfall as runoff, since some is absorbed or stored in crannies on the surface and
eventually evaporated. Therefore, to estimate runoff totals from a development, a simple
system was developed to estimate the fraction of rainfall that runs off different types of
surfaces. It is referred to as the rational method. It has been used for more than half a
century to estimate runoff rates from small sites (usually much less than 100 acres).

Rational Method, Percent Imperviousness, and Percent Impervious Many types of surfaces,
based on either the material of the surface or use of the surface and other characteristics such
as slope, have been given a typical rational method runoff coefficient C. This coefficient
i
represents the fractional percentage of a rainfall volume from a 2- to 10-year frequency storm
that is estimated to result in runoff from that particular surface. To estimate the overall runoff
from these frequency storms for the site in question, the overall runoff coefficient C is
calculated as the land-area-weighted average of the individual area coefficients C for each
i
individual land area A. This overall runoff coefficient represents the estimated fraction of
i
rainfall that will run off a site for a typical 2- to 10-year frequency storm event.
∑ CA
C = i i (2.6.2)
∑ A i

There are many state and local agencies such as the North Carolina Department of
Environment and Natural Resources that are also adopting the concept of percent
imperviousness along with the variable percent impervious. In version 2.1 of LEED-NC,
the definition of imperviousness is given as the rational method coefficient in percent, and
the adopting agencies have similarly based definitions. Therefore, percent imperviousness
(%Impness) can usually be calculated as
%Impness = 100C (2.6.3)