Green alleys, like this one between 4th and 5th Street, have many benefits, including reduced stormwater runoff, silt and pollutant filtration and heat reduction.
As major U.S. cities scramble to find ways to lessen the impact of stormwater on aging, overtaxed sewer infrastructure and local waterways, an unlikely hero has emerged in the form of “green” alleys — alleys retrofitted with stormwater-capturing, permeable surfaces and underground infiltration/underdrain systems. As more cities adopt them, these alleys are creating increased opportunities for innovative surface design.
The City of Richmond, VA, a recent green alley convert with five demonstration alleys and five planned for 2014, uses interlocking concrete pavers on its alley surfaces to meet three important design requirements: historical aesthetics, stormwater management and utilities access, says Michelle Virts, P.E., Deputy Director of Stormwater Utility in the city’s Department of Public Utilities (DPU).
Richmond is one of some 770 U.S. cities that funnel sanitary sewage and rainwater into a combined sewer system (CSS). Like many cities with an aging CSS, they are routinely forced to release untreated stormwater into local waterways during storm events. In order to comply with EPA pollutant and volume allocations that protect the nearby Chesapeake Bay watershed, the city must invest in costly grey infrastructure, such as larger pipes and additional underground storage tanks, and/or in less costly sustainable green infrastructure that slows the flow of stormwater into the CSS and filters pollutants naturally.
To demonstrate the benefits of green stormwater capture and filtration technologies to local stakeholders, Richmond’s Department of Public Works and DPU in 2009 kicked off a series of showcase projects, and a green alley program was of particular interest, says Virts. “We own and maintain 209 miles (336 km) of alleys so if we can get them to be a cost-effective solution [for pollution reduction], they are definitely one of our favorites, long-term,” she says. “With the alleys we really get the triple bottom-line: the environmental benefit, the economic benefit because it improves neighborhood [property values] and the social benefit of converting what was a grungy-looking alley into a nice amenity.”
Prior to installing the permeable system at 12th Street downtown, the Richmond DPU had to coordinate with all of the utilities running beneath the alley — water, gas, sanitary and storm sewer lines, power lines for city streetlights, and privately owned fiber-optic cable — resulting in a two-year delay.
Over the years, Richmond’s original Belgian block granite road and alley pavers, commonly referred to as cobblestones, have often been removed or simply paved over. But in 2009, a downtown master plan officially recognized the economic value of making the city’s early history more evident in daily life, by, for instance, removing the asphalt from streets to unveil the cobblestone underneath. So when the city and Richmond-based Timmons Group engineers looked at permeable pavement options that could mirror the look of those original stone pavers, interlocking concrete pavers were the obvious choice. “The aesthetic is very important,” says Virts. “We want to preserve that historic character when we go in and modernize the surfaces, and the pavers give us this opportunity.”
But looks aren’t everything. Most of Richmond’s utilities infrastructure runs along or underneath alleyways, and the city and designers foresaw repeated excavation for maintenance. Planners did not want inevitable utility repairs to interfere with the permeable water-capturing systems. “We wanted a surface that could easily be removed and put back, and we didn’t want utility contractors forgetting that this is a pervious surface and putting in a patchwork of standard asphalt or concrete,” Virts says. “[Interlocking concrete pavers are] a reminder and a safeguard to make sure the alley remains permeable even when repairs are needed.”
Richmond’s pilot alley design specified a strip of permeable interlocking concrete pavers flanked by heavy-duty concrete edge restraints and a stone reservoir separated from the clay subgrade with PVC liners and geotextiles to protect surrounding basements. The city’s Urban Design Committee specified gray- and granite-blend, tumbled-texture pavers in running bond patterns (see sidebar). With some of the alley sections as narrow as 8 ft. (2.4 m) wide, installation was done by hand.
The goal was to install permeable systems using stone reservoirs and perforated piping with check dams that contain volumes from two-year and 10-year storms, says Amelia Wehunt, P.E., project manager at Timmons Group. Proposed future goals specify a minimum 20 percent peak flow rate reduction; a pilot alley behind historic Monument Avenue achieved a 25 percent peak flow reduction. “We’re proud of the results,” she says. “It’s a robust standard.” The mini watershed in each permeable alley reaps instant benefits, Virts adds. “We know that some of our sewersheds only have a five-year storm capacity, so there is an immediate benefit to the capacity in our sewers.”
The pilot alley program was funded with a $211,000 grant from the National Fish and Wildlife Foundation and $213,000 in city matching funds. To take some of the burden off of the city’s general fund, Richmond’s Department of Public Works shares oversight of the official Green Alley Program with the DPU, which has a dedicated enterprise fund for stormwater management, says Virts. The five completed alleys are located in the city center, city residential areas and on the downtown campus of Virginia Commonwealth University. Five more are planned for the city’s fiscal year 2014.
The alleys chosen for the pilot program were problem alleys, with surfaces in need of repair and utilities in need of attention. The 12th Street green alley section, for instance, a 5,200-sf (4,830-m2) passage that slopes down to the flood-prone Shockoe Bottom neighborhood, is the conduit for six public and private utilities. Prior to installing the permeable system at 12th Street, the DPU coordinated upgrades to water, gas, sanitary sewer and storm sewer lines; power lines for city streetlights; and privately owned fiberoptic cable, resulting in a two-year delay, says Shawn MacIntosh, City of Richmond engineer and project manager. While alleyways with utilities are not disqualified for future green system installations, this experience prompted the DPU to choose alleys with no or limited utilities for its next set of demo projects, says MacIntosh.
That said, the permeable interlocking concrete paver system soon demonstrated its expected benefits. In the fall of 2013, about 17 months after completion, the Virginia Commonwealth University green alley section required excavation so that crews could install bollards to protect alley streetlights. The work crew stockpiled the removed concrete pavers and each layer of stone, and then cut through the PVC trench liner. Once bollard installation was complete, a patch was glued onto the cut PVC and the entire section was restored using the original materials with no surface patching required, just as planned.
Response to the pilot alleys from local residents and business owners has been very positive, says Virts. “The feedback we’ve received is that everybody loves the look of the alleys and is amazed by how the rainwater infiltrates immediately, so much so that we’ve received requests for them.” To encourage private landowners to install their own on-site stormwater management technology, the DPU offers a 50 percent credit on annual stormwater fees. Some area business owners have retrofitted their impervious surfaces with the permeable concrete paver system, she says.
“We are very happy with the success of the pilot project,” says Virts. “If we can get more of a cost efficiency, we can incorporate the alleys into our formal capital improvement program; so that as utility work is done in the alleys, the surfaces become permeable, rather than just going back to the standard surface.”