ID#2502 made on
SSc6.1 - Stormwater design - quantity control
LEED BD+C: New Construction, LEED O+M: Existing Buildings, LEED BD+C: Core and Shell, LEED BD+C: Schools
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 de...
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 project team is requesting whether their efforts to reduce the post-development stormwater discharge rate and quantity from their low permeability site meets the intent of the credit, even though they are still above the pre-development levels. Based on the description provided, the project does not meet the intent of the credit. However, if the stormwater management plan was expanded to include additional on-site quantity control strategies (such as a rain catchment system), AND stream channel protection strategies, then the project could pursue the Option 1B compliance path in the NCv2.2 Reference Guide. Applicable Internationally.
Related Addenda (Corrections & Interpretations)