Technology▼Solutions Using nature’s design to stem urban stormwater problems size of Ohio. From the first U.S. projects in Prince George’s County, Md., in the mid-1990s, the low-impact design movement has taken an approach to redesigning streets that uses vegetation and holding ponds to increase SE AT TLE PUBLIC UTILITIES
This year’s summer storms sent water flowing in sheets across city streets in Houston, Texas; Washington, D.C.; and elsewhere in the U.S., causing a problem that occurs in urban settings worldwide. Such runoff can overwhelm sewage systems and treatment plants and erode urban streams, with implications for further environmental impacts downstream. But an experiment in Seattle shows that storm water in urban districts doesn’t have to end up as surface flow. Building on a movement known as low-impact development, Seattle Public Utilities and researchers from the University of Washington, Seattle, have begun to redesign streets so that they function more like naturally occurring drainage systems, with meandering streams and places for water to infiltrate the ground. “The whole trick,” explains Stephen Burges, a University of Washington, Seattle, hydrologist who works with Seattle Public Utilities, is that “if you can try and hold water a bit closer to what it does naturally, where it falls, you have a much more benign effect” overall. “We’re bucking decades, and sometimes centuries, of engineering design that has to be changed,” says James Patchett of Conservation Design Forum, a company that has worked on projects in Chicago and elsewhere. “Most of the standard engineering practices [for handling water] are harmful: They cause water-quality degradation, habitat loss,” and other impacts to hydrologic systems that probably behaved very differently before being urbanized. Paved surfaces now make up a significant portion of the earth’s land surface. According to recent research, the total area of impervious surfaces in the U.S. is nearly the
Winding streets lined with shrubs provide pathways for water to seep underground.
subsurface drainage of rainfall. Some municipalities adopt other low-tech water-saving schemes, such as rain gardens or green rooftops. In Europe, where regulations on stormwater runoff tend to be older and stricter, designers have developed additional methods that include public plazas with waterworks that also serve as water storage. The end result is decreased surface flow after rains. In Seattle Public Utilities’s Street Edge Alternatives project, the city has retrofitted existing streets so that they more closely resemble the way the land drained before it was developed. A team of engineers, landscape architects, and oth-
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ers worked to reconfigure 660 ft of 2nd Avenue NW from straight and wide to narrower and gently curving, adding some complexity to the “stream” path. “We’re taking an area and making it function like it was in a predeveloped state,” says Tracy Tackett of Seattle Public Utilities, as if “it didn’t have impervious area.” The curving swales dotted with shrubs that now edge the street in place of old storm drains help to guide the movement of rainwater to unpaved areas, where it is more likely to seep through the soil into the groundwater or stay in place to evaporate later. The water can also be returned to the atmosphere by transpiration from the vegetation. Monitoring of surface flow on the street began in 1998. Since modifications were completed in 2001, measuring devices placed at the lower end of 2nd Avenue NW—where water would be expected to collect—have not detected any surface flow during storm events. That includes relatively large storms, which occur on average every 2 years. That size storm has the equivalent rainfall of ~1.68 in. (~4.27 cm) in 24 h, Tackett says. The next steps include modeling what happens to the water once it seeps underground. The city blocks included in the Street Edge Alternatives project are only one element of a larger network made of the street grid and the urbanized streams that still cut through the city, says Richard Horner, a University of Washington engineer who works on modeling for the project. Other installations on urban streams have reduced surface flow by almost half, he says, in large storm events. Horner, Burges, and their colleague Heungkook Lim have modified a sophisticated groundwater model developed for the Everglades to try to pin down the details of subsurface flow. The project has yet to examine directly the fate of pollutants normally found in surface runoff, in© 2006 American Chemical Society
cluding motor oil, automotive “road dust” that contains trace elements, and other airborne particulate matter. Horner says, “We haven’t bitten it off yet.” In the meantime, the team relies on hazardous-waste tests from other regions to extrapolate such pollutants’ behavior. In Seattle, the costs of retrofitting urban streets—~$300,000 per block—have been deemed worthwhile, because the need for treating or storing excess storm water is eliminated, and flooding and erosion in adjacent urban streams are prevented. The team members say that residents also appreciate the aesthetics of having a street treated this way. In addition to supporting the project by agreeing to plant and care for vegetation along the street’s edges, local homeowners have also allowed measurements to be taken in their yards, says Burges. Some residents have even made changes such as moving their drain spouts; this typically is prohibited by certain building codes. Retrofitting existing urban surfaces can be more expensive than planning such solutions from the beginning, says Allen Davis, an environmental engineering professor at the University of Maryland, College Park. But the approaches for new and old roads seem simple, often because they rely on time-tested methods, such as cisterns and rain barrels or native plants and soils that evolved in place, Patchett says. Other cities using these “bio-methods” at large scales include Portland, Ore., and Chicago. More complex technologies, such as high-porosity paving stones and underground retaining tanks, also encourage surface water to recharge the underlying groundwater or reenter the atmosphere, Davis points out. (The Seattle team has not used these more expensive solutions, which may be more likely to crop up in new housing or business developments, he says.) In Seattle, the team sees aquifer recharge as a bonus, because the Street Edge Alternatives project is driven mostly by the city’s need to meet storm-water requirements set by the U.S. EPA. Even so, current regulations can thwart the kinds of experiments that are necessary to get data to prove that such projects work—such as the experiment that
Seattle is conducting. “It’s kind of a catch-22,” Davis says. “People are not going to build them until the regulators let them, and the regulators aren’t going to let them until somebody does it and gets the data.” But he says he sees a slow shift in attitudes on both sides. Still, “the cost of providing water quality is exorbitant,” including the expense of water treatment plants and traditional sewage systems, points out Neil Weinstein, executive director of the Low Impact Development Center. That fact could balance the costs of streetscaping and other methods that can “integrate all these functions,” from flood control to clean water storage in aquifers. “We know we are saving money,” says Joan Nassauer, a landscape architect at the University of Michigan. Nassauer led the development of rain gardens in the country, building the first a decade ago in Maplewood, Minn., a suburb of the Twin Cities. That project saved ~10% of the costs of building new gutters and street amenities. Ten years ago, the project’s greatest expense was for native plants, now much less costly to purchase, to be planted in highly absorptive sandy soil with walls to control surface flow. Nassauer wonders, however, “As this approach proliferates, are the savings in initial capital costs being balanced by long-term upkeep costs?” Low-impact, biology-based solutions can be “much more dynamic than, say, hard-pipe infrastructure.” Even small native plantings require maintenance, which some local projects may not be calculating in the same way cities and local governments do for long-term funding for sewage pipes and other more traditional infrastructure. Nassauer also warns that “we do need to be attentive in a precautionary way to the possibility that the techniques that are so promising could have long-term effects related to accumulation of contaminants in ways we haven’t anticipated” in soils, aquifers, or even green-roof materials. Nassauer is concerned about waterborne as well as airborne contaminants, and she says “we should greet these approaches as small experiments that we want to monitor.” —NAOMI LUBICK OCTOBER 1, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 5833
