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.