Can It Be Too Big?


A brief Forbes article this week provides a quick primer on green infrastructure. This is good—the more people understand what it is, the more likely they are to approve of it in their own communities, or at least to make informed decisions when they’re asked to vote on new stormwater requirements. The article is favorable, pointing out the benefits not just for water quality, but for the economy and for cities as well—saving money for developers, in some cases benefiting investors, and creating more green space in neighborhoods.

The article, titled “Investing In Nature 101: A Triple Win for Cities, Communities, and Developers,” also looks at the role of stormwater credit trading. “Like many cities, Washington, D.C., now mandates that developers retain stormwater runoff from their properties. But D.C.’s regulation is unique,” the article says. “It allows developers to use offsite, green infrastructure solutions. Developers can meet up to half of their stormwater retention requirements by buying credits from retention projects built across the city. Right now, D.C. is the only city that allows developers to do this.”

Other credit trading programs exist, of course, allowing credits to be generated for green infrastructure as well as other types of BMPs, although each program has its differences. An article coming up in the November/December issue of Stormwater looks at credit trading in greater detail, including how to make a credit trading program flexible enough to meet a community’s needs and help with TMDL compliance.

The Stormwater article by Vaikko Allen, Derek Berg, and Jacob Dorman also looks at some potential pitfalls. Making sure that the watershed is actually protected is a key issue: “By definition, credit trading programs allow the treatment of stormwater at one location in lieu of treatment at another location, typically shifting protection away from more densely developed areas where pollution or runoff volume reduction controls are more costly toward less urban areas where they are less expensive. This can leave local watersheds unprotected.” Other things to watch out for: making sure the design of a project that generates stormwater credits is adequate, and—even more difficult—making sure that it is maintained and that it functions properly over the long haul.

A more complex issue is the question of extra capacity, which the article also addresses. While it might be tempting for a developer to oversize a stormwater BMP to create extra capacity that can then be traded, the article notes, “The simple approach of assuming the extra storage or treatment capacity is available to offset another development that would produce a design storm equal to that extra capacity is flawed. It neglects the fact that the extra capacity would be used only when the design storm is exceeded. From the perspective of pollutant load or volume reduction, one large system is less effective than multiple smaller systems with the same combined capacity because more of the small system capacity would be used more often.” The authors also cite a Natural Resources Defense Council issue brief on the topic that has this to say on the subject: “A large single SMP [stormwater management practice] sized to capture 100 gallons will capture less volume, over the course of a year, than two smaller SMPs, each sized to capture 50 gallons. Because small storms are more common than large storms, the single larger SMP’s extra capacity won’t be used very often. On the other hand, the two smaller SMPs would be more fully utilized during smaller storms, thus capturing more stormwater.”

Does your community have, or is it considering, a stormwater credit trading program? How has it dealt with some of the trickier issues involved, including long-term maintenance of BMPs and the question of sizing credit-generating BMPs?


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