Biofiltration Street

Hardly a day passes without a headline about a drought, wildfire, or some other unsettling symptom of climate change.

With so many environmental issues in the news, it’s curious how landscape design and American culture stubbornly cling to the image of the English garden and its postcard views of Arcadian settings, clipped hedges, and watered lawns.

However, that’s not the case at the Dallas Urban Reserve.

Developed as a single-street subdivision of 50 modernist houses, the 10.5-acre project is an irregular fragment of real estate located in the highly manicured environs of north Dallas, Texas.

An extension to the DART light-rail system defines the long western edge of the property sliver, and the eastern edge is a typical 1950s subdivision of ranch homes. And like a typical subdivision, Vanguard Way ends in a cul-de-sac that also provides a fortunate link to the existing White Rock Creek linear trail system.

But that’s where convention ended and landscape architect Kevin Sloan Studio of Dallas accepted some extraordinary site conditions.

For all those years, the project site was heavily abused as an illegal landfill. Hidden behind an earlier subdivision, piles of construction litter became intermingled with mounds of discarded shingles, timber shards, and broken paving, forming a strewn field overgrown with volunteer trees and brush.

Seen purely as real estate, the site was far from the type of curb appeal one might want for an enclave of modernist homes and well-to-do patrons.

“I couldn’t wrap my mind around the idea and cost of moving one landfill to another,” says developer Diane Cheatham of Urban Edge Developers Ltd. in Dallas.

Could the debris be left in place? Or perhaps rework the site and recover the disposal fees for something more productive?

In theory, the answer to those speculations was yes, but there’s a big difference between a collage and a pile of junk. Gambling a real-estate development on the idea of a site collage was the type of problem the “High Performance Landscapes” of Kevin Sloan Studio was known for solving.

Artistic Improvisation

For this project, the solution required working less as a “designer”--formulating a concept on a drawing table and then pushing it onto the problem--and more like a “bricoleur.” By making do with the oddments at hand, the debris becomes “bricolage,” a term used by the early 20th-century writer and historian Claude Levi-Strauss to explain the artistic productions of his time that used bailing wire, newsprint, and a host of dissimilar objects as raw material for artworks.

So at the Urban Reserve, the large concrete slabs became flagstones for paving and retaining walls along the roadway excavation. Rusting steel beams and twisted parts were welded together to become an entrance sign.

The granular debris was a challenge. Once the site had been thoroughly tested and cleared of toxic materials, the cost of raking the vast amount of small material from the site was extravagant. Moreover, if the debris was left in place, mowing the lawns would fling the debris like shrapnel.

The solution was to leave the small materials in place, but visually absorb them with a tall and grassy plant palette that would naturalize. Cultivated turfgrass was allowed in the private backyards, where culling would be at a manageable scale.

The adjacent White Rock Creek Park could satisfy any recreational need for a playfield and lack of any “public turf” within the Urban Reserve.

Mitigation Strategies

However, the mitigation strategies still needed an organizing idea.

Complicated problems often give rise to complicated and expensive solutions, so the simple stroke of rethinking the street cross-section and repeating it from the entrance to the cul-de-sac transformed an otherwise conventional street into a continuous biofilter that would also serve as a backbone to link the various mitigation concepts.

In lieu of a tree-lined street, Vanguard Way slopes asymmetrically from one side to the other, directing storm waters into a chain of rain gardens separated by pockets of head-in parking.

The upslope--or what the design team termed the “dry side” of the street--is a single line of unirrigated desert willows planted in a surface of decomposed granite.

Conversely, the low or “wet side,” directs the street runoff into an array of rain gardens densely planted with bald cypress, pond cypress, and horsetail reeds.

The rain gardens are one piece of a larger biofiltration system. Each cypress and reed planted bed is backfilled with layers of soil, porous gravel, and charcoal.

Stormwater runoff enters the rain gardens through the curb slots, and after percolating through the filtration layers, is conveyed into a set of sedimentation ponds by a storm lateral. After wetland plants aid the filtration process, the water irrigates the rain gardens during a drought.

The asymmetrical street section also reinforces two different types of development lots and two sets of engineering details. Eighteen zero-lot line units line the dry-street edge, and are accessed by a continuous roll-down curb profile.

Opposing the dry side are 16 conventional 50-foot-wide lots for larger detached homes that are serviced by an alley. The remaining lots are wrapped around a cul-de-sac. The rain gardens are contained by a standard curb detail that is slotted every 12 feet to capture the runoff.

Taken together, the lush and overgrown image of the Urban Reserve is more akin to a nature preserve than to a housing development. In the same way the High Line in New York City presents an alterative landscape character to picturesque Central Park, the Urban Reserve offers an alternative to the more English-like settings of a typical housing development.

For 60 years, suburban Americans have lived like squires with houses on tidy green lawns. Designing new landscape alternatives that are as satisfying but more intelligent to their own place and to the environment characterizes one of the larger problems of our time.

This new approach has been widely celebrated. It attracted international distinction from Eco-Structure as “One of Seven Innovative Projects for 2009,” and the Texas American Society of Landscape Architects presented a “2011 Award of Technical Excellence,” the highest award conferred.

In addition to solving problems, the design of the project also benefits the surrounding neighborhood. After White Rock Creek was improved as a 14-mile linear park, the derelict condition of the site blocked access between the existing neighborhood and the new amenity.

Now that the project is complete, cyclists, joggers, parents, and children stream through Vanguard Way. And for those using the linear park, the Urban Reserve has become a pleasant side trip to admire the showcase of modernist homes.

The Urban Reserve offers many lessons for sustainable design. Although architects and landscape architects are compelled to produce the type of on-off designs that award programs celebrate, the biofiltration street is intended to be repeated in other cities and environments.

In more arid ecologies, such as the American Southwest, the rain gardens concentrate the available precipitation into satisfying ribbons of green that are self-sustaining. In temperate and northern climes that are already green, the subgrade layers filter out any de-icing salts.

The new prototype saves money for new and existing infrastructure. Considering how the asymmetrical slope eliminates the need for inlets, catch basins, or storm-water plumbing along an entire street side, tax dollars saved for new street construction are considerable. The continuous use of a laydown curb profile eliminates the cost for curb cuts that interrupt pedestrian use of the sidewalk.

The new biofiltration street is also a planning strategy to adapt existing suburban regions. Instead of forcing new and unfamiliar planning models through agencies and onto communities, existing streets and cul-de-sacs can be retrofitted with a Urban Reserve street, giving neighborhoods the opportunity to replace the water-thirsty landscapes with naturalizing ecologies.

Considering that 80 percent of the water removed from north Texas reservoirs is used to irrigate turf grass, as one example, savings to individual households are substantial.

Environmental stewardship and intelligent cost-management are likely to become more mandatory than optional issues going forward. By rethinking the typical elements of a built environment, in this case the cross section and details of a suburban street, the collective impact to the environment is heightened by multiplication.

And by allowing the landscape character of a place to become unique by doing something, a practical strategy may be at hand to move beyond the outdated cultural desire for English landscapes.

Kevin W. Sloan, ASLA, M. Arch, is the owner of Kevin Sloan Studio in Dallas, Texas. He can be reached via email at ks@kevinsloanstudio.com.

M. Diane Sloan is CEO of Kevin Sloan Studio. For more information, visit www.kevinsloanstudio.com.

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Project Design Team

Owner & Developer: Urban Edge Ltd., Diane Cheatham, Rick Fontenot

Planning & Landscape Architecture: Kevin Sloan Studio, Kevin W. Sloan, ASLA, M. Arch, AValerie Warner, AIA, ASLA

Planning & AP: DSGN, Robert McFessell, FAIA, LEED AP

Architectural Design Guidelines Civil Engineering: Brockette, Davis & Drake, Jim A. Riley, PE

Wetland Planting: Steve Moeller