EBOM-2009 SSc6: Stormwater Quantity Control 1 point

For this project, the proposed condition reduces the existing impervious area by 38% below the existing condition, therefore the project received a waiver for storm water management, quantity control. Stormwater management requirements were met by reducing the existing impervious area on site. The "2000 Maryland Stormwater Management Design Manual" requires a reduction on 20% of existing impervious to meet stormwater management requirements and, in this project, an impervious area reduction of 38% was achieved. If by reducing the existing impervious area by 38%, does our project meet the requirement of the credit?

The NRUCFC Headquarters project is located in the NRUCFC Campus, which is within the Dulles Town Center project in Loudoun County, Virginia. Dulles Town Center is a 540-acre high density mixed used development. When Dulles Town Center was originally planned for stormwater management in the early 1990s, it was done on a regional basis using regional stormwater management facilities. These facilities were designed and constructed to accommodate the future tenants and uses within Dulles Town Center, which includes the NRUCFC Campus. Due to the presence of shallow rock and the low permeability of the existing soils, infiltration measures were not feasible options as stormwater management facilities within Dulles Town Center. The NRUCFC Campus, currently an undeveloped parcel, is planned to be developed as an office park, with the NRUCFC Headquarters project being the 1st building. The NRUCFC Campus drains an on-site regional pond identified as BMP (Dry Pond) Facility at Ramp C (CPAP 1994-0081). Runoff to this facility is detained in a dry pond and discharged directly into a major floodplain downstream. This pond was designed, approved and built in accordance with County BMP standards at the time, which were based on the Occoquan Method in the document entitled, Northern Virginia BMP Handbook - A Guide to Planning and Designing Best Management Practices in Northern Virginia, 1992. LEED stormwater management criteria were not in existence at the time. Today, meeting LEED stormwater management criteria is not required within Loudoun County. However, to meet the intent of SSc6.1 (existing imperviousness of <=50%), additional BMP measures have been provided above those required by State and County regulations for the NRUCFC Headquarters project. These additional measures include enhanced extended detention dry pond, manufactured BMP device (Stormfilter) and two bioretention basins (with underdrain systems). All together, the increase in peak runoff rates for the NRUCFC Campus site is minor: -- 1-year 24-hour storm peak runoff rate (design case): 4.73% increase -- 2-year 24-hour storm peak runoff rate (design case): 3.69% increase The stormwater management plan for the existing pond assumed an impervious coverage of 72% for Dulles Town Center (including the NRUCFC Campus site). However, to minimize runoff leaving the site, the NRUCFC Campus as proposed has reduced this impervious coverage to 39% for the NRUCFC Headquarters project area and to 15% for the NRUCFC Campus area. By significantly reducing impervious area beyond that originally planned, keeping extensive existing tree stands and buffers on-site, proposing extensive landscape plantings, and utilizing existing stormwater management facility, we believe the NRUCFC Campus, and by extension, the NRUCFC Headquarters project, have met the intent of reducing post-development runoff volume (i.e. quantity) from this site. -- 1-year storm runoff volume (design case): 4.30% increase -- 1-year storm runoff volume (with NRUCFC parcel as originally designed): 7.56% increase -- 2-year storm runoff volume (design case): 3.97% increase -- 2-year storm runoff volume (with NRUCFC parcel as originally designed): 7.09% increase While we recognize the importance of reducing post-development runoff volume back to pre-development conditions, it isn\'t always feasible if existing site geology restricts the ability to infiltrate, such as is the case here. In these situations it is more practical to preserve more open space than originally planned and propose additional landscaping to achieve less runoff. Though the project does not reduce the post-development runoff volume back to pre-development conditions, the increases in the runoff rates and in the runoff volumes are minimized. Had the project honored the originally planned 72% impervious coverage, the increase in peak flows and the total runoff volumes would be much higher. Does this approach satisfy the credit intent of SSc6.1?

The existing imperviousness of the project\'s urban site is 74%, mostly due to surface asphalt parking lots. A portion of the stormwater runs offsite while the remainder is piped to drywells. The new development will have a net imperviousness of approximately 62%, most of which will be a building roof and pedestrian plaza. This represents a reduction of 17%. All of the stormwater runoff from the new development will be directed to new drywells dispersed around the site, recharging groundwater. Stormwater from the minimal remaining surface parking will be piped to a water quality swale and discharged into drywells. The intent of Credit 6 is to "Limit disruption of natural water flows by minimizing stormwater runoff, increasing on-site infiltration and reducing contaminates." Credit 6.1 addresses the first two of these intents, while Credit 6.2 addresses the third, reducing contaminates. Credit 6.1 states "if existing imperviousness is greater than 50%, implement a stormwater management plan that results in a 25% decrease in the rate or quantity of stormwater runoff." Although the net imperviousness of site surfaces will be reduced by only 17%, rather than the stated 25%, we feel this project will meet the intent of the credit because 100% of the stormwater will remain on-site and be directed to drywells, thus infiltrating and recharging ground water. Run-off leaving the site will be eliminated, impervious surface area will be significantly reduced and on-site water flows will more closely resemble the natural flows that existed before the existing surface parking lot was constructed. Do you agree that the proposed stormwater management meets the intent of Credit 6.1?

The intent of this credit is to reduce or eliminate water pollution by eliminating sources of contaminants and removing pollutants from stormwater runoff. To achieve this, a stormwater management plan must be implemented that captures and treats the stormwater runoff from 90% of the average annual rainfall (1-inch, part of a humid watershed) for the entire LEED Boundary using acceptable best management practices (BMPs) that are capable of removing 80% of the average annual post-development total suspended solids (TSS) load. The proposed site is part of a university located in an urban area and is approximately 6.75 acres with a building footprint of 5.94 acres. The project consists of a parking garage and mechanical space that are entirely underground, and four buildings at the surface. The four buildings that emerge at the surface are connected by the subsurface portion of the building and support both green and conventional roofs. Pedestrian walkways, landscaped open space and a water quality swale make up the balance of the site at the surface. The majority of the stormwater runoff from the site is treated by the large water quality swale at the southern end of the site and water quality units before being reused for irrigation on site. There is a proposed infiltration system in the southwest corner of the site, located in the small portion of the site not directly over the building footprint. Portions of the conventional roof areas (approximately 10% of the site area) cannot be directed towards the water quality swale or the infiltration system due to their location on site. Therefore these roof drains are proposed to be directly discharged to the city stormwater system north of the site. Sources of TSS are generally from automobiles, oil, grease, salt, sediment and debris. The major sources of TSS are paved areas and parking lots where the materials listed above are found. The Massachusetts Stormwater Policy from the Massachusetts DEP considers roof runoff clean, since roofs are presumed to be free of sediment and debris and therefore relatively free of pollutants or TSS. The Massachusetts DEP does not require treatment of roof runoff because it is expected to generate close to zero pounds of TSS. Thus, the 80% removal requirement is not applicable. It is understood that the USGBC requires stormwater runoff from the entire LEED Boundary to be treated, although the Massachusetts DEP allows roof runoff to be discharged directly without treatment. This project does meet the credit intent without treating roof runoff since the relatively clean roof runoff would not contribute to water pollution downstream. Previous LEED certified projects for this university have achieved a point for this credit while not treating roof runoff and stating that roof runoff is considered clean by the State of Massachusetts. We would like to confirm that this is the position that all USGBC reviewers will take when awarding this credit.

This inquiry relates to the stormwater management infrastructure on a remote campus of a local University. The project has been developed in phases. In Phase I, a 2.0 acre stormwater basin was constructed. This basin exceeds the LEED quantity and quality requirements for 4.0 acres of existing impervious area that now drains to it. The existing impervious area is not part of our LEED applicant project and was previously untreated. This stormwater basin is considered part of our LEED project site and will also treat the runoff from our LEED applicant project. We are now in Phase II, and are adding 8.29 acres of impervious surface to the LEED project site. The stormwater basin constructed during Phase I is being upgraded as part of Phase II, by adding sediment collecting forebays, to increase TSS removal from both existing and new impervious areas draining to this basin. Approximately 6.0 acres (70%) of this new impervious surface will drain into the upgraded 2.0 acre stormwater basin and will be treated to LEED quality requirements. All of the remaining 2.29 acres of new impervious surface the LEED project is adding, in addition to 1.7 acres of existing untreated impervious area, will drain into a rain garden designed to remove at least 80% of the TSS and 40% of the TP in the water. The rain garden will drain into an existing County owned regional stormwater basin, not located on the LEED project site. Because we are treating 5.7 acres of existing impervious surface that is not part of our project plus 6 acres of our LEED project\'s impervious surface, we are hoping the USGBC will allow us to claim credit equivalence for both SSc6.1 and SSc6.2, despite the fact that 2.29 acres of our LEED project\'s impervious surface will ultimately drain into an off-site regional basin. Please clarify for our team whether we are likely to qualify for credit equivalence based on the scenario described above.