Biofiltration Street

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.

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