Bioswale Ahead!

If you take a stroll through East Rock next year and stop to admire some curbside tree plantings, you may be looking at more than mere flora.You may be looking at infrastructure.

You’ll see one of the seven “bioswales” being installed as part of the Greater New Haven Water Pollution Control Authority’s ongoing Trumbull Street area sewer separation project, which aims to improve roadway drainage and reduce sewer overflow into the Mill River.

The bioswales, which will resemble ordinary sidewalk tree wells, are depressed areas slightly below street level that divert rainwater that would otherwise run into the sewer. Once in the swale, the water will seep down into the soil, reaching the water table without mixing with contaminated sewage.

Mario Ricozzi, manager of design at GNHWPCA, explained all of this at the City Plan Commission’s regular meeting at City Hall last week. The commission voted to approve the site plan.

The bulk of the sewer separation project is replacing some of New Haven’s old combined sewer pipes, which date to the mid-1800s. When those pipes are overwhelmed, Ricozzi said, the excess water flows into the river. As part of the new project, the authority has installed separate pipes for storm drainage and sanitary sewer water on Trumbull Street.

The bioswales, which will be installed over the next two years, are meant to minimize the need for further pipe replacements. An example of so-called green infrastructure, the bioswales, which rely on natural processes to divert excess rainwater, should be cheaper and more attractive than installing more sewer pipes. “It’s a value proposition,” Ricozzi said.

To test the effectiveness of the bioswales, Clark Street will feature a sort of controlled experiment in which one side of the street gets them and one side doesn’t. (Clark Street will also be a testing ground for permeable sidewalks, which let rain water through to the soil.)

“It’s one of a range of methods to deal with storm water,” said Giovanni Zinn, a project manager in the engineering department.

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posted by: Bill Saunders on November 26, 2013  6:55pm

Interesting, but seriously, how effective?
I can’t see these improving things much beyond the immeasurable.

Once the swale is saturated, water will take the path of least resistance.

Is there some sort of ‘green grant’ that is funding this?

posted by: J.R. Logan on November 27, 2013  6:21pm

Bill, from what I understand bioswales can be very effective.  I have seen some report about how much water raingardens and bioswales can take and was surprised. The way I understand it is to think of rainfall as a graph with some extreme peaks. Bioswales may not stop the water but they smooth out the peaks. Most of the time our grey infrastructure (pipes and water treatment plant) are operating at less than full capacity but need to be ready to handle the peaks. When it rains, it often rains hard and because we have so many impermeable surfaces the water sheets quickly to the sewer system. When too much water gets in it fills the pipes and then hits the overflow point which dumps sewage right into the river untreated. Even if it gets to the plant they do a less robust treatment during a heavy rain because they need to deal with all the storm water coming through the system. When a water treatment engineer looks at this problem they are likely to figure that more grey infrastructure is the solution, such as increasing the capacity of the plant or constructing underground retaining pools(which they have done). These are not bad solutions but if the problems is how to handle the water in the system at these peak events we should do everything we can to slow the water from getting into the system. Bioswales are a way to engineer a distributed solution to reduce the need for the grey infrastructure. They create a place for the water to be held temporarily during peak water flow and allow some water to peculate into the soil so it never hits the system. My understanding is that New Haven has sandy soil that can provide good drainage. So maybe it will be a particularly effective solution in New Haven. I hope this project will demonstrate what the value of these street swales can be. If they prove as effective it will justify a larger investment in green infrastructure as an alternative to grey infrastructure.  This is starting to be embraced by other cities.