Stormwater management is off the radar for all but the most knowledgeable practitioners -- or for leaders in cities under federal consent decree to radically reduce the amount of stormwater overflowing into rivers and streams.
More municipalities fall into that category than you might think. Cities as diverse as Miami, Atlanta and Indianapolis are all under federal compliance orders to substantially upgrade the treatment of water flowing back into streams. When sewers are overwhelmed during heavy rains, untreated runoff infiltrates rivers and streams -- which is ecological bad news.
Philadelphia, however, has a plan which, if approved, may well be the booster rocket to bring green infrastructure into the mainstream. Facing the prospect of spending $8 billion to build a giant underground holding tank, Philadelphia has proposed an alternative compliance plan to the Environmental Protection Agency.
Instead of building a big tank to temporarily hold storm runoff, Philadelphia is looking to replace concrete and asphalt with turf and earth. In effect, Philadelphia proposes to systematically redesign and reinstall water-retaining surfaces over major proportions of public and private property. The city also promises to invest in restoration of stream habitats and riverfronts which will improve absorptive effect while also offering recreational amenities.
Like many other cities, Philadelphia's urban landscape is dominated by hard surfaces that accumulate contaminants that are swept into stormwater drains during rains. Much of the city is serviced by combined stormwater and sewer systems that under normal conditions treat that water before it is released. But during heavy rains, sheer volume overwhelms the treatment plants and sends raw sewage and untreated water into rivers and streams.
The conventional, "grey," approach to mitigating the damage of untreated urban stormwater runoff is to build giant underground interception tanks that hold the overflow until the sewer system can digest the excess capacity. The cost of these interceptors runs into the billions. Milwaukee, for example, spent $2.3 billion in the 1980's but still failed to fully solve its sewer overload problems.
At their best, these storage tanks offer little value beyond capturing runoff. With a "greenification" approach as proposed in Philadelphia, a city could not only comply with the Clean Water Act, but realize other environmental, social and economic benefits.
Permeable hard surfaces, green roofs, rain gardens and catchment parks are all tested applications that slow runoff. Unlike concrete, these surfaces slowly let rain in, allowing it be absorbed, evaporated, filtered and controlled as it moves into the ground and sewer systems, reducing system overloads. A "greened" acre stops 80 percent to 90 percent of the pollution compared to an impervious surface acre.
Many cities are now testing and demonstrating a wide range of adaptive interventions, but only Portland, Ore., has integrated the innovations systematically. For a dense urban center like Philadelphia to do so successfully could set important national precedent for cities such as New York, Memphis and Chicago, which are all closely watching the outcome for the Philadelphia plan.
Taken to massive scale as in the Philadelphia proposal, city officials believe they can successfully address the root causes of uncontrolled urban stormwater runoff which, in addition to watershed degradation causes massive flooding of residential and commercial property basements each year.
Key to the success of this strategy is its implementation on public-owned land, which constitutes 45 percent of impervious land in the city.
Green advocates point as well to the triple-bottom line of other elements of a green infrastructure alternative. Philadelphia's Green City, Clean Waters plan conservatively expects the city will reduce carbon dioxide emissions, improve air and water quality, and restore wetlands and other natural habitats. Coupled with the improved recreational opportunities in stream corridors and park areas are increases in property values in urban neighborhoods adjacent to improved parks and greenery.
Whether Philadelphia builds giant holding tanks or goes green, an $8 billion investment is facing the city either way. The contrasting scenarios offer compelling differences in their relative return on investment. The big interception tank would take 20 years to become operational, and offer single purpose benefit even when it is completed. Green City, Clean Waters will spend about $1 billion funded by utility increases, but spread the balance of the costs across stormwater fees paid by developers, and ongoing construction and maintenance (street repaving, public building construction, etc.) where green adaptations can become standard practice.
The EPA is examining closely the complex engineering and environmental calculations that underlie Philadelphia's plan. In the end, it must achieve the objective of reducing Philadelphia's environmental damage to streams and rivers. The city must demonstrate its commitment and capacity to execute the plan with fidelity over many years. But the prospect of tackling a problem at its source, taken to scale for true impact, is an opportunity for an unprecedented, "concrete" advance in sustainability in our urban centers.
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