While green infrastructure applications for stormwater management have become more widely accepted by institutional and governmental agencies, there has been some hesitation from private developers who are navigating the perceived cost and schedule implications of design and permitting requirements. This is compounded by the inconsistency of green infrastructure acceptance from municipality to municipality, with some encouraging their use and others reluctant to install practices they consider to be unproven.
The Gallery Automotive Dealership project – which includes a showroom and service space for BMW and MINI, along with 468 parking/inventory spaces – provides an interesting case study for how to integrate green infrastructure techniques into a private development, and how doing so can help a project get permitted and meet sustainability goals.
Nitsch Engineering worked closely with the developer, A.W. Perry, to incorporate innovative and sustainable design elements on the 11.13-acre urban infill site adjacent to wetland resources. Developing the site for all uses, while remaining cost effective and environmentally friendly, posed a challenge that was solved by the implementation of green infrastructure techniques, including 1.7 acres of porous asphalt in vehicle inventory storage areas, a small amount of wetlands fill and replication, and a green infrastructure swale and bioretention basin to collect and treat stormwater runoff generated by Rte. 3, adjacent to the site.
Integrating Porous Asphalt: Installing porous asphalt is one practice that has been around for some time but is not widely used, due to questions related to performance and durability in cold weather climates. On the Gallery project, Nitsch designed 1.7 acres of porous asphalt in two separate vehicle inventory storage areas. These areas are ideal for porous asphalt since they receive less vehicular traffic than other site areas and are not subject to pollutant loads associated with well-travelled paved areas. The porous asphalt replaced impervious parking areas, reduced typical infrastructure costs for drainage structures and piping, and allowed run-off to be treated and infiltrated within the pavement section.
Addressing Stringent Regulatory Requirements: The project straddled the line between Rockland and Hingham, making the permitting process complex as every design choice needed to be approved by separate boards/committees with very different motivators. On top of this, stormwater management regulations for Rockland are more demanding than the MassDEP Stormwater Management Standards: MassDEP required the project to control peak run-off rates, infiltrate groundwater, and provide water quality treatment for the first inch of rainfall, while Rockland required all projects to mitigate increases in the total volume of stormwater up to the 100-year storm.
To meet both sets of stormwater requirements, Nitsch focused on environmentally-sensitive site design and incorporated green infrastructure techniques. The stormwater management system included infiltrating bioretention basins with sediment forebays, subsurface infiltration systems, proprietary stormwater treatment devices, and porous asphalt. The porous asphalt replaced traditional impervious parking areas, reducing typical infrastructure costs for drainage structures and piping, and allowed run-off to be treated and infiltrated within the pavement section.
The innovative stormwater management system allowed the project to meet and exceed both the DEP and Rockland’s requirements. Although the permitting of green infrastructure practices required additional education and negotiation with the regulatory authorities, ultimately their inclusion in the project was critical in securing nine local and three state permits and allowing the project to proceed.
Measuring Success: Developing the 11.13-acre site to meet the programmatic needs while balancing topographic and environmental constraints was challenging. Providing an adequate building program, site access, and parking areas led to work within 100 feet of wetlands in both Hingham and Rockland. Additionally, the site topography and configuration of wetlands resources in Rockland necessitated filling 4,800 s/f of wetlands. It was only through the informed use of green infrastructure techniques, strategically placed porous asphalt areas, and careful site configuration that all uses were able to fit onto the site; without these techniques, the site would not have been able to accommodate the development program and the project would not have been possible.
The porous asphalt areas have been installed for one year with excellent performance. Storm events of up to four inches and the snowiest winter in Massachusetts history (114 inches in Hingham/ Rockland) produced no run-off from the porous asphalt. Even more impressive, there has been no flow observed leaving the two stormwater outfalls that convey stormwater from the site. There were no abnormal maintenance issues, and no damage to the pavement.
Scott Turner, PE, AICP, LEED AP ND, is director of planning, and Jennifer Johnson, PE, LEED AP BD+C, is senior project engineer, at Nitsch Engineering, Boston, Mass.