ID#2537 made on
SSc6.2 - Stormwater design - quality control
LEED BD+C: New Construction, LEED BD+C: Core and Shell, LEED BD+C: Schools
Short of credit acceptance, please comment on whether the following conditions are likely to be sufficient for compliance, and if not what additional measures would need to be taken. Reduce Impervious...
Short of credit acceptance, please comment on whether the following conditions are likely to be sufficient for compliance, and if not what additional measures would need to be taken. Reduce Impervious Cover: The subject site is part of Mark Center Plaza 1, which is part of a larger development, the Mark Center area of Alexandria. The Winkler Botanical Preserve Pond, treats runoff from the subject site as well as a large portion of the Mark Center area, was designed and constructed with future urban development in mind and with a purpose of treating stormwater runoff from a large, relatively impervious urban area. Prior to construction of the Winkler Botanical Preserve Pond, which treats all runoff from the subject site, the Winkler Botanical Preserve was created. This preserve was created as part of the development of Mark Center Plaza 1A & 1B (which includes the subject site) and is approximately 40 acres of undeveloped, forested land which was dedicated to the City of Alexandria as an area that cannot be developed in the future. Because such a large area of land, which is located adjacent to the subject property and helps to treat runoff from the subject site before it enters the Winkler Botanical Preserve Pond, has been dedicated for preservation, the stormwater management plan for the Mark Center area, which includes the subject site, includes significant steps to reduce impervious area in an urban setting to the greatest extent possible. Promote Infiltration: The subject site provides approximately 30% vegetated open space, which helps to promote infiltration. In addition, all runoff from the subject site is conveyed via open channels through the above referenced Winkler Botanical Preserve and into the Winkler Botanical Preserve Pond. Because all runoff from the site flows through open channel streams in an undeveloped setting that has been designed to reduce velocities to the greatest extent possible, a significant amount of runoff leaving the site will infiltrate prior to reaching the Winkler Botanical Preserve Pond. BMPs Treat Runoff Capable of Removing 80% Total Suspended Solids: As mentioned above, runoff from the subject site flows into Winkler Botanical Preserve Pond, which is a wet pond. Virginia and City of Alexandria Stormwater Management regulations are designed to promote removal of Phosphorus in lieu of Total Suspended Solids (TSS), so the design of the pond was not intended to quantify removal of TSS. However, as shown on the attached research summary, studies have shown that wet ponds remove 80% - 90% of total suspended solids. Detailed calculations will be provided to quantify TSS removal and demonstrate that the Winkler Botanical Preserve Pond does remove 80% TSS.
The applicant is requesting confirmation regarding the use of several stormwater management strategies to achieve SS credits 6.1 and 6.2 for their site, which is part of a larger development. The proposed strategies include impervious area reduction, infiltration (through open space and redirection of runoff through an existing botanical preserve), and the use of an existing detention pond to collect and treat stormwater. The applicant has explained that the project reduces impervious area because their design proposes less impervious cover than the site was originally allowed, based on the design of an existing regional stormwater management pond. However, reduction of impervious area should not be based on what may be allowed by local zoning or existing infrastructure. Rather, it should be based on what has been used within the project's development footprint (e.g., alternative pavements, green roofs) to alter the nature of proposed hardscapes. To promote infiltration, the applicant has stated that they will provide at least 30% open space on the project, as well as convey the runoff via open channels through an adjacent natural area. Recognizing that the intent of the stormwater credits are to limit disruption of natural hydrology and reduce or eliminate water pollution by removing pollutants from runoff, merely conveying runoff onto an adjacent natural area, as described, is not adequate. Treatment of stormwater runoff within the project's development footprint should be promoted to prevent degradation of any adjacent highly valued ecosystems. The existing pond could be used to meet SS credit 6.2, but the applicant must demonstrate that runoff is conveyed to the pond in a stable and non-erosive manner. The open "streams" through the botanical preserve are equally, if not more important and warrant protection too. In addition, the pond must be sized to adequately accommodate the entire site for which it serves. The project team must either demonstrate that the credit requirements for SSc6.2 have been satisfied for the area bounded by the LEED project boundary or the entire campus. Based on the narrative provided, it appears that a substantial amount of stormwater is distributed as runoff to the adjacent Winkler Botanical Preserve. This runoff must be treated within the LEED project boundary if the project team is attempting to demonstrate compliance for the LEED project boundary. The campus compliance path outlined by the AGMBC requires the aggregate data for the campus/development be used to complete the LEED Submittal Template. Additionally, please note that attachments are not included in CIR submissions.
Update April 15, 2011: Please note that all 2009 projects in multiple building situations must follow the 2010 Application Guide for Multiple Buildings and On-Campus Building Projects, located here: https://www.usgbc.org/ShowFile.aspx?DocumentID=7987. 2009 project teams should check this document for up to date guidance on all multiple building issues.
Related Addenda (Corrections & Interpretations)