When is a parking lot more than a parking lot? When it is a test bed for low-impact development techniques, best management practices in stormwater control, and sustainable development.
Last month, ASCE completed a project at its Reston, Virginia, headquarters that is all that and more. Finding its entire parking lot in need of a complete overhaul in 2018, the Society turned to the ASCE Foundation for a solution that would demonstrate its commitment to the principles of sustainable design and environmental stewardship. The Foundation rose to the challenge by engaging donors, volunteers, and vendors to design and install permeable pavement, stormwater filtration systems, and other technologies that could help keep more than half the site’s potentially contaminated stormwater runoff out of the local watershed.
“The Foundation saw this as an opportunity to lead by example, using (the Society) headquarters as a guide for how to retrofit a parking lot with environmentally friendly practices and using the site as a learning laboratory to highlight potential interventions,” says Matthew J. Jones, P.E., LEED AP BD+C, M.ASCE, a senior principal of Magnusson Klemencic Associates and the parking lot project manager. The effort, he says, offered an “ideal opportunity to incorporate green stormwater infrastructure to improve the quality of runoff leaving the site as well as to slow the release of stormwater from the site.”
Reston is located in northern Virginia, within the Difficult Run watershed, which drains into the Potomac River and eventually into the Chesapeake Bay. According to U.S. Environmental Protection Agency data cited by chesapeakebay.net, 80 percent of the bay’s tidal regions are partially or fully impaired by toxic contaminants. “Difficult Run, and in particular the portion that ASCE’s headquarters lies within, is highly urbanized,” explains Mike Rolband, P.E., PWD, PWS Emeritus, M.ASCE, the chief technical officer for Wetland Studies and Solutions Inc. and a core member of the large team of experts who donated their expertise to the project. “Much of (Reston) was developed prior to the implementation of current stormwater-management requirements (that were enacted) to control water quantity and quality,” Rolband continues. So nearby streams are subject to high levels of total suspended solids, phosphorus, and nitrogen — all of which can eventually damage the Chesapeake Bay.
By renovating the parking lot with more environmentally responsible techniques, ASCE hopes to do its part to help reduce stormwater runoff and improve the quality of the water that does discharge from its surfaces. The new parking lot includes sections built with two types of permeable pavers and two types of underground stormwater filtration units. The remainder of the parking lot surface was milled and repaved with asphalt containing recycled tires.
Let it flow
Twenty-four parking spots at the front of the building were rebuilt with permeable interlocking concrete pavement supplied by the Interlocking Concrete Pavement Institute’s Foundation for Education and Research. David R. Smith, ICPI’s technical director, explains that the 80 mm thick, roughly 100 by 200 mm pavers are made of solid concrete but are spaced roughly 10 mm apart, their joints filled with a permeable aggregate that traps sediment as water flows through.
The water flows to stone base and subbase layers that serve as a reservoir. “The 300 mm thick pavement base and subbase consists of open-graded aggregate with a porosity of about 40 percent,” Smith explains, “so it can store about 100 mm of water while infiltrating it into the soil subgrade.”
The stone reservoir can store and infiltrate flows from a 100-year storm event, he says. “Because the soil subgrade is sloped, there are 30 mm thick, 200 mm high, (polyvinyl chloride), vertical check dams spaced about every 5 m to detain and infiltrate the water stored in the subbase reservoir, rather than allowing it to run downhill on the subgrade.”
Around the corner from this area, 11 parking spots were created using Stormcrete precast porous concrete panels, made by Porous Technologies LLC. The panels are “manufactured with a concrete mix that has a uniform coarse aggregate gradation and includes minimal fine aggregates,” explains Gregg Novick, the president of Porous Technologies. “These materials are used in a proprietary mix that results in a large volume of interconnected voids — 15 to 20 percent voids.” The voids allow stormwater to pass through the panels into the soils below and, from there, into a biofiltration structure called FocalPoint, made by ACF Environmental.
Rob Woodman, P.E., M.ASCE, the New England/New York regional manager for ACF, explains that the FocalPoint system uses high-flow media to treat the runoff faster and in a smaller footprint than traditional systems. “The system treats for common urban runoff pollutants, including total suspended solids, phosphorus, and nitrogen, while also providing reductions in oils, grease, and bacteria,” he says. After that treatment, the runoff flows through an outlet pipe to the existing storm drain. “A domed overflow riser holds and conveys larger storm events to this same ultimate discharge location,” Woodman says.
Another innovative technology that is being tested as part of the project is called Filterra, made by Contech Engineered Solutions LLC. Frank Birney, the firm’s senior stormwater consultant for northern Virginia and Washington, D.C., explains that this 13 by 7 ft filtering unit was “shoehorned” into a spot in the sidewalk near the building’s front steps “between finished pavement and existing utilities, behind the curb, and next to the building.”
Runoff will enter this unit not through the pavers but through an existing curb inlet, Birney explains. Stormwater will flow through a specially designed filter-media mixture “contained in a landscaped concrete container,” he says. “The filter media captures and immobilizes pollutants; those pollutants are then decomposed, volatilized, and incorporated into the biomass of the Filterra system’s micro- and macro-fauna and -flora.” The treated runoff is then discharged.
Rubber meets the road
The remainder of the parking lot — some 105,750 sq ft — was repaved with 1,241 tons of asphalt made with recycled tires, supplied by Asphalt Plus LLC. William G. Buttlar, Ph.D., P.E., M.ASCE — a professor in the Department of Civil and Environmental Engineering at the University of Missouri and holder of the Glen Barton endowed chair in flexible pavement systems — identified Asphalt Plus as the industrial partner for the asphalt paving part of the project. Buttlar also assisted with the mix design and performance testing at the Mizzou Asphalt Pavement and Innovation Laboratory. He explains that the pavement for the project included 11 lb of Elastiko brand chemically engineered crumb rubber per ton of asphalt, which equates to roughly 722 scrap tires that were recycled and kept from landfills.
Engineered crumb rubber is ideal for pavement — not just for parking lots but for standard roads and highways as well — for a number of reasons, Buttlar explains. “Rubber helps to stiffen the mix, reducing rutting potential,” he says. “It makes the pavement more strain-tolerant, which reduces the rate of cracking. And when microcracking eventually starts, the rubber crumbs help to slow or stop the movement of the cracks.” Moreover, he says, the pavement lasts longer, requires less maintenance, and uses less material than standard pavement. “In terms of life-cycle cost, crumb rubber asphalt roads are cheaper than standard hot-mix asphalt roads.”
The parking area also includes a new vegetated courtyard, which features pavers engraved with the names of members, ASCE Foundation donors, and others. Without these donations, as well as the in-kind and materials donations made toward the project, the cost would have been $700,000. With those donations, the actual outlay that the Foundation made for the services and materials was reduced to just $440,000.
The project, which incorporated ASCE’s technical guidance and engineering standards in its design construction, will serve as a demonstration of all these technologies. Special signage explaining its various components will help tell the story to visitors, including Society members and local students, once COVID-19 restrictions are lifted.
“I hope this project will demonstrate how a parking lot can be retrofitted to improve stormwater runoff leaving a site while highlighting a few of the green stormwater infrastructure elements that are available to project teams,” says Jones. “As civil engineers, we have a duty to find ways to make the built environment more environmentally friendly.”
This article first appeared in the January/February 2021 issue of Civil Engineering as "With a More Sustainable Parking Lot, ASCE Walks the Walk."