Section 2: Individual LEED Credit Reviews
LEED Credit SS-6.1:
Subsurface retention systems that allow
stormwater to infiltrate the site.
Stormwater Management
Vegetated roof systems can also be used to achieve
Rate and Quantity
the credit, although their applicability may be
limited by the attended costs (see "Summary of
First Cost Impacts" below). Also, although
Intent
technically viable, stormwater collection tanks for
on-site water reuse (primarily assumed for
Limit disruption and pollution of natural water
landscape irrigation or cooling tower make-up) are
flows by managing stormwater runoff.
considered unlikely for most GSA projects.
Requirement
Factors that may influence the viability of the
credit include the level of imperviousness of the
If existing imperviousness is less than or equal to
existing site, the lot coverage of the new building
50%, implement a stormwater management plan
footprint, the amount of hardscape and landscape
that prevents the post-development 1.5 year, 24
areas assumed for the new development, and the
hour peak discharge rate from exceeding the pre-
porosity of the existing soils. Where the credit is
development 1.5 year, 24 hour peak discharge
deemed viable, preliminary calculations may be
rate.
required to develop an overall approach, and to set
site design parameters.
OR
If existing imperviousness is greater than 50%,
Basis for Cost Assumption
implement a stormwater management plan that
results in a 25% decrease in the rate and quantity
For the purposes of this study, Credit SS 6.1 was
of stormwater runoff.
pursued only in the Courthouse model--the scope
of the Office building modernization does not
(1 point)
include site work.
For the Courthouse, two approaches were
Cost Impact = 2 or 5
estimated: 1) a scenario that relies on increased
landscaped areas to achieve the credit, and 2) a
scenario that relies on a vegetated roof system to
1
2
3
4
5
achieve the credit. These approaches are described
in the following sections. In both cases, it has been
GSA
No
Low
Moderate
High
assumed that the existing site (prior to GSA's
Mandate
premium,
premium
premium
premium
(no cost)
psble svgs
(<50K)
(50-150K)
(>150K)
purchase) consists of 90 percent buildings and
hardscape and 10 percent turf and plantings.
Practical Applications
Increased Landscape Areas
Because the majority of GSA's projects involve
As noted in credit SS-5.1, a Base Case of site
previously developed sites in urban areas, this
development features was defined for the
credit will typically involve reducing the rate and
Courthouse model based on several current GSA
quantity of stormwater runoff by 25 percent
courthouse projects. In the Base Case,
compared to existing conditions. Typical strategies
approximately 60 percent of the open site area is
to achieve these reductions include:
dedicated to paved surfaces (sidewalks, plazas,
roads, service areas), while 6 percent is dedicated to
Increased landscape areas;
turf grass. The remaining 34 percent of the site
Use of pervious paving systems, including
area consists of groundcovers, perennials,
unit pavers (for plazas), pervious asphalt or
shrubbery, and trees. The calculated
concrete (for walkways, vehicle access, and
imperviousness of this Base Case site does not
parking), or "grass pave" units (for fire lanes);
reduce runoff by 25 percent compared to the
and
GSA LEED COST STUDY
41
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