SS Credit 6.1: Stormwater Design: Quantity Control
LEED-NC Credit: YES | UNSURE | NO
Credit Intent
Limit harmful impacts to the natural water hydrology of the site by increasing ground
permeability and limiting water contamination and pollution.
Feasibility
The current state of the new GSB campus -- 2 buildings surrounded by large parking lots
-- is advantageous for this credit, which emphasizes increasing or restoring water
permeability to the site. It is assumed that Option 2 of this credit, which applies to
existing sites with existing imperviousness greater than 50%, will be followed. A simple
aerial photo of the site shows just how much of it is covered by asphalt. Even though the
project currently lacks a thorough permeability analysis by a civil engineer, we can
reasonably conclude that 50% of the site is probably impervious. Therefore to achieve
this credit, stormwater management must be improved so local hydrology is more natural.
This can probably be done at the GSB.
Figure 20: Plan view of the site. Note prevalence of asphalt.
From Google Maps.
Analysis
To achieve this credit, LEED-NC requires a 25% decrease in the amount of stormwater
runoff, based on a 2-year 24 hour design storm. The most effective way to do this is by
promoting soil permeability and stormwater infiltration, which luckily in this case means
removing the surface asphalt from the site and keeping the GSB building footprints to a
minimum; a definite plan for the site. Because the GSB design is still in the schematic
phase, it is difficult quantitatively analyze the stormwater runoff decrease. To achieve
this credit through Option 2, two values need to be determined by a project civil engineer:
1. Current discharge rate and quantity at the site.
2. Post development discharge rate and quantity for the site, consistent with
previous discharge calculation methods.
SS Credit 6.1: Stormwater Design: Quantity Control
LEED-NC Credit: YES | UNSURE | NO
It is difficult (perhaps impossible) for us to make these calculations, but at this point we
can qualitatively say that the chances for this credit are very high. In ARUP’s Pre-SD
Civil Narrative, they note the following:
“The existing site consists almost entirely of impermeable surfaces such buildings
and asphalt parking lot. The proposed development will contain a significant
area of permeable landscaping. The volume and rate of storm water entering the
University drainage system upon completion of the project is anticipated to be
less than the pre-development situation.”1
The key question for obtaining this credit becomes: How much will the amount of
stormwater entering the University drainage system be reduced by? If it is reduced by
25% or more, then the credit will be obtained. After the stormwater management system
has been designed, stormwater runoff can be predicted. During design it will be
beneficial to prescribe a system that will reduce stormwater runoff as much as possible,
but definitely more than 25%. This reduction will be greatly enhanced after the
impervious asphalt surfaces are removed. Finally, after the design is complete,
stormwater runoff should actually be measured and analyzed by a project civil engineer
to ensure the design goals were met.
1
Arup: Stanford GSB Pre-SD Civil Narrative, 30 April 2007, pg. 1 &2.