The existing condition imperviousness of the project site is 62.3 percent. The goal for credit 6.1 is a 25% reduction in rate and quantity of stormwater runoff. The intent of the stormwater management credit is to limit disruption of natural water flows by minimizing stormwater runoff, increasing on-site infiltration, and reducing contaminants. We believe that we meet and exceed the intent of the credit, and request a favorable interpretation based on the following points (support calculations are available): 1. The existing project site, which is 62.3% impervious, has no stormwater management system. All runoff is collected and directed to the public storm drainage system surrounding the site, without any type of treatment for quantity or quality control. The project to replace the building and renovate the site provides extensive stormwater management features. 2. The project includes the demolition of the existing Langston Center, and construction of a new Langston Center building. The new building incorporates cisterns to capture and store all storm runoff from the building roof, for later use as irrigation water. There is, then, no runoff from the new building roof, and the area of the roof can be discounted from developed condition impervious area calculations. 3. The project includes the use of a significant amount of pervious asphalt pavement in the new parking lot. This system relies on collection and infiltration of runoff captured by the pervious pavement. The project also includes the use of a bio-retention device called a "rain garden", credited by the Commonwealth of Virginia with a 65% pollutant removal efficiency, and a "Stormceptor", credited by Arlington County with a TSS removal rate of 78%. 4. We have achieved a reduction in the peak rate of site runoff from a 10-year storm of 53%, from 11.05 cfs existing to 5.10 cfs proposed. We have achieved a reduction in impervious area, and therefore total volume of runoff, of 22.4%. Although the runoff volume reduction is not quite at the target of 25%, the runoff rate reduction is well over the target of 25%. The average combined runoff reduction is (53 + 22.4) / 2 = 37.7% 5. We therefore meet the intent of the credit by significantly reducing disruption of natural water flows by minimizing stormwater runoff (more than 25% overall), providing onsite infiltration (the porous pavement areas and use of stored roof runoff for irrigation purposes), and significantly reducing contaminants.
The calculation methodology and conclusions as described are not acceptable for achievement of SS Credit 6.1. To prove compliance with the credit requirements, use the methodology described in the LEED Reference Guide for calculating imperviousness for the baseline and design cases. Imperviousness must be reduced by at least 25% to achieve this credit. Rainwater harvesting systems that collect roof runoff for reuse can be addressed by using a corresponding runoff coefficient in the calculations, depending on collection efficiency and cistern capacity. See the Credit Interpretation Ruling (CIR) of 5/17/2002 for more detail. Hardscape surfaces can be deducted corresponding to the percent of their runoff that is held and infiltrated on site by bio-retention and infiltration systems (see CIRs from 9/24/2001 and 7/10/2001). If your calculation of 22.4% was derived from the LEED calculation method, it is reasonable to assume that a few more site improvements will help you achieve this credit. A side note in regards to your stated approach: a 1.5-year storm event is much more reliable reference than a 10-year storm event. If the existing site was less than 50% impervious, you would have had to use the 1.5-year storm data in your calculations. Applicable internationally.