Page 415 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

Low-Impact Development and Stormwater Issues 375

The variables C, DF, %imp, and %impness are explained in Chap. 2 and will not be
repeated here. However, there is a need for a closer look at CN, FAR, and GFA.
CN is similar to the simpler rational runoff coefficient, but also includes other site
factors such as soil types. It is used in very well-established hydrology modeling, and
more information about its use and application can be found in most hydrology texts
and from the Natural Resources Conservation Service. How to modify the curve number
to adequately take into account LID design modifications is under development. Some
mechanisms for doing this have been introduced by Prince George’s County Department
of Environmental Resources (1999) and the DoD (2004). In addition, methods for
incorporating LID practices into other established hydrology models can be found in
the Puget Sound Technical Guidance as previously listed.
The gross floor area of a building (GFA ) can have many definitions depending on
i
the local ordinances used. Usually it includes all the floors of a building, taken from
the outer dimensions, except for unfinished attic and basement spaces. Chapter 2
discusses some modifications which can be used to also include parking garages for
the purposes of the LEED calculations. For every case, the actual areas used as such
should be summarized.
The floor area ratio (FAR) is usually defined as the gross floor area of all the buildings
on a site divided by the total site area, as in Eq. (10.1.1).

∑GFA
FAR = i for all buildings i on the site (10.1.1)
A
T
Why is it necessary to have so many variables? Well, each is useful in aiding in the
determination of different impacts of construction. Several examples are also listed in
Table 10.1.10. One simple example to explain the importance of determining many of
these variables for sustainable construction purposes can be seen if the floor area ratio
is examined. A metamodel called Sustainable Futures 2100 has been recently developed
to examine many of the complex environmental and natural resource impacts of
construction, both on-site and regional. It was used to compare a single-story house and
a two-story house of similar gross floor area, which in turn means similar FARs. It was
determined that the single-story home had less of an energy impact during construction,
but the two-story home had a higher operational energy demand. Hydrologic impacts
are different too and were not compared in that analysis, but obviously the single-story
home will use more of the land and therefore leave less for keeping the natural
hydrologic cycle of the land without additional LID measures. So, knowledge of only
the FAR is not sufficient for determining the complex impacts of construction, although
it is a good indicator of many items such as utility and regional transportation usage
related to the site.

Green Streets and Urban LID Practices
As LID practices are incorporated into the landscape, more and more references and
details for applications under various different scenarios are being developed. Some of
these are focusing on applications to linear projects such as highways and more urban
streetscapes. Special considerations may also be given to other development parameters
in conjunction with the LID practice such as vehicular flow, traffic calming, pedestrian
access, bicycle paths, landscape needs and wildlife passage, in addition to other context
sensitive solutions. Figure 10.1.7 is a photograph of a traffic circle combined with a rain