Since its peak in the middle of the last century, the city of Detroit, MI, has lost more than 60% of its population, down from a high of 1,850,000 people. The decline of the auto industry, flight to the suburbs, and other factors have led to one of the most precipitous declines of any major US city. Among the other consequences, the city now has more than 31,000 empty houses and more than 90,000 vacant lots.
It’s a heartbreaking situation for the residents who remain. But if there’s an upside, however small, the city is showing how green infrastructure can be applied on a massive scale to combat combined sewer overflows.
Joan Nassauer, professor of landscape architecture from the University of Michigan, is leading a project to turn empty lots into rain gardens. Starting with four vacant lots on the city’s west side where existing houses have been torn down, her team is filling the empty spaces that once used to be basements with permeable fill, a porous engineered soil. A rain garden located between two adjacent lots directs water to these storage areas. If necessary, excess water can be released to the municipal sewer system through the basements’ old sewer drains.
That most likely won’t happen often, though. Each lot can hold up to 300,000 gallons of runoff, or half of a hundred-year storm, as she explains in this article. “We aren’t just making sure that the stormwater that falls on these properties stays there,” says Nassauer. “We’re grabbing water from half the block and putting it in these little pilot garden sites.”
Detroit has a combined sewer system and is under a mandate from EPA to reduce combined sewer overflows; it has reduced CSOs from roughly 30 billion gallons a year to about 3 billion, but it has spent billions of dollars achieving that result and is looking at perhaps another billion dollars to eliminate the problem entirely. Because the city’s soils are relatively impermeable, green infrastructure—rain gardens, bioswales, porous pavement, and the like—has not seemed like a viable option for large-scale reduction of CSOs until now.
The Detroit Land Bank Authority owns thousands of vacant houses, which it auctions off if it can and demolishes if it can’t. The pilot rain gardens are being monitored for water quantity and quality, and similar projects are planned for additional lots where houses have been or will soon be torn down.
The idea of using vacant land for stormwater storage isn’t new, of course. Sometimes existing structures are even torn down for that purpose; this article from Stormwater magazine describes how one Virginia city has purchased homes within the floodplain, demolished them, and used the land to retain runoff. This article describes similar use of vacant land in Cleveland, OH.
An added bonus for Detroit—and for the residents remaining in the areas where the rain gardens are being installed—is a potential increase in property values and a better sense of the neighborhood. Surveys have shown that neighbors feel better about the properties with rain gardens than about vacant lots maintained by the Land Bank Authority—which are mowed about once a year—and even lots that are maintained and mowed weekly.
The project is a collaboration between the Detroit Land Bank Authority and the Detroit Water and Sewerage Department. Various city departments will maintain the gardens, and Tetra Tech, which is also involved with the project, will help Nassauer’s team monitor them.
