Page 96 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

LEED Sustainable Sites 77

90 Percent of Symbol for Volume
Annual Annual Rainfall Volume to Be to Be
Precipitation Estimated "Handled" per "Handled"
(in) per Event (in) Event ∗ per Event ∗
Humid >40 1 over total site WQ (0.083 ft)A
VH T
Semiarid 20–40 0.75 over total site WQ (0.063 ft)A
VS T
Arid <20 0.5 over total site WQ (0.042 ft)A
VA T
∗ Infiltrated on-site, used on-site, or treated prior to discharge.
TABLE 2.6.3 Regional Precipitation Area Definitions and Water Quality Volumes

infiltrated on-site, used on-site, or treated prior to discharge off-site can be calculated
as follows:

WQ = (0.083 ft)A (2.6.4)
VH T
WQ = (0.063 ft)A (2.6.5)
VS T
WQ = (0.042 ft)A (2.6.6)
VA T
These water quality volumes and respective rainfall definitions by area are summarized
in Table 2.6.3.
Best management practices (BMPs) are normally separated into two main categories:
nonstructural and structural. For the purposes of LEED, nonstructural BMPs include
such items as pervious pavements, swales, and disconnection of impervious areas to
promote natural infiltration of rainwater into the soils (although others might consider
swales and pervious pavements as structural controls). The structural controls include
ponds, rainwater harvesting cisterns, constructed wetlands, and treatment devices in
the stormwater collection system. Other agencies and organizations segregate the BMPs
differently. More information on this topic can be found in Chap. 10.
For any of the methods used that involve infiltration, soil types, and soil infiltration
rates must be provided to determine if the soils can handle the infiltration. This credit is
based on a set volume and not on a peak rate, so a mass balance under dynamic conditions
may not be easily shown. The LEED 2009 reference guide provides a 24-h period for
these dynamic calculations. Likewise, for the other structural controls, a period of 24 h
is used for any flow rates, discharges, and infiltration rates with respect to the water
quality volume.
In all cases, for submittal, a list must be provided of all the BMPs used including a
description of each, its pollutant removal capability, and the percent of rainfall that will
be handled by the BMP for the site. If there are special circumstances or considerations
for this credit, a narrative must be submitted describing these circumstances. Additional
information on BMPs and stormwater options can be found in Chap. 10. Figure 2.6.1
shows an example site where constructed wetlands are used for both stormwater
quantity and stormwater quality control. Figure 2.6.2 depicts signage by a stormwater
pond educating the public on its importance.