Many US cities are plagued with combined sewer overflows (CSOs)–events that occur when storms overwhelm the capacity of the combined sewer system, releasing a mixture of stormwater and untreated sewage to the environment. And a growing number of those cities are under consent decrees to reduce their CSOs, although fewer of them have figured out exactly how they will do so or how to pay for it.
The article “Taking Runoff Off the Table,” in this issue, focuses on the efforts in Indianapolis, which, along with many other cities, is supplementing traditional stormwater or “gray” infrastructure–sewer lines, treatment plants–with green infrastructure to reduce the total volume of runoff. The idea is to reduce the CSOs without a massive investment in new gray infrastructure, such as adding sewer lines, separating sanitary and storm sewer systems, or building new wastewater treatment plants. Infiltrating or using the runoff is less expensive and, in many areas, mandated–a certain volume or percent of runoff must not leave the site.
An article in our May 2011 issue looked at the economics of using green infrastructure to help manage CSOs, showing how several cities–Portland, OR; Kansas City, MO; Chicago, IL; and New York City–have reduced CSOs and comparing not only installation costs, but also operation and maintenance costs of gray and green infrastructure (“Economical CSO Management”). The use of green infrastructure has continued to expand, and we’ve also covered efforts in Cincinnati (March/April 2012), Milwaukee (May 2012), Cleveland (October 2013), Atlanta (November/December 2013), Buffalo, NY (October 2014), and many others. Additional examples were highlighted at StormCon in Portland last August.
Unlike a significant capital investment in gray infrastructure, the costs of green infrastructure are spread throughout the city at hundreds, or better yet thousands, of sites: permeable pavers to replace sections of sidewalk, rain gardens in individual homeowners’ yards, bioswales in parking lots, curb cuts and stormwater planters along urban streets–even a fair number of green roofs, which can require a bit more planning and upfront investment but are extremely popular in Europe and increasingly catching on here as well. The costs of many of these installations are borne by developers, homeowners, and business owners, who often gain a stormwater credit or some other incentive for installing them.
Also unlike that of a major gray infrastructure project, the effectiveness of green infrastructure is harder to measure. Instead of knowing precisely how many gallons a day are passing through a treatment plant, we have to estimate how many gallons of runoff are never reaching that plant because they’ve been infiltrated or retained on individual sites. Maintenance of those thousands of separate, small green infrastructure features is another important question: How will they perform over time? How can we make sure people–the new owners of a home, for example–understand what the rain garden is for and how to take care of it? These are significant challenges; the July/August 2014 article “Making Rain Gardens Work” explores how two cities are quantifying green infrastructure’s effectiveness.
The use of green infrastructure -continues to spread, even in regions that were slow at first to embrace it. As many of the articles mentioned here show, we are getting better–both more creative and more efficient–at making it work.
To what extent is your city relying on green infrastructure to manage stormwater, and is CSO reduction one of the goals? Share your comments below, or write a letter to the editor at [email protected].Janice Kaspersen
Janice Kaspersen is the former editor of Erosion Control and Stormwater magazines.