In a new parking lot in Beamsville, Ont., water running off doesn’t just flow into storm sewers. Instead, the lot channels runoff into what looks like a simple patch of vegetation.
But the greenery is more sophisticated than it looks. It’s a bioretention cell, or rain garden – small trees and plants in a basin designed to catch rainwater and filter out contaminants. The vegetation takes up some water, some sinks into the soil beneath and the rest filters slowly into drainage pipes beneath, so the lot doesn’t dump runoff into municipal storm sewers during heavy rain.
Until recently, bioretention cells were more popular in U.S. cities such as Seattle, Portland, Ore., and Minneapolis, than in Canada. Now they are gaining attention here.
The Beamsville cell, along with the carpool lot’s pavement made partly with rubber from recycled tires, won the Ontario Ministry of Transportation a Recognition Award from Ontario’s environmental commissioner in January. It’s not the ministry’s only bioretention experiment. It has tested the idea for years, says Shawn Smith, a senior project engineer and adviser to the Beamsville project, and “the last couple of years, the results are starting to show a little more promise.”
Les Habitations Jeanne-Mance, an affordable housing complex in Montreal, rebuilt its parking lots in 2010. As part of that project, Éco-quartier Saint-Jacques, an environmental group set up by the borough of Ville-Marie, planted greenery along sidewalks and installed a bioretention cell to provide drainage for the largest lot.
Rather than going into the city’s storm sewers, water now filters through layers of soil, sand and rock before dispersing into the ground, says Philippe Lavallée, project manager of the Effet de terre project at Éco-quartier Saint-Jacques. There is an overflow drain in case the runoff exceeds the garden’s capacity.
Besides helping manage runoff, Mr. Lavallée says, the bioretention cell and other vegetation added to the site help reduce the heat island effect that large expanses of asphalt create, and the greenery beautifies the housing complex.
Construction may vary, but the general principles are the same. Water drains into an area with vegetation that can tolerate both dry and wet conditions. The water filters through soil and sand on top of a layer of stone or gravel.
In the Beamsville cell, 75 millimetres of mulch lie on top of a soil mix ranging from 525 to 720 millimetres, then about 100 millimetres of sand, about 300 millimetres of larger stone, and then drainage pipes perforated on top so water drains into them and is carried away, says Kyle Perdue, an engineer in training with the ministry and leader of the project.
The Montreal cell is similar but doesn’t have the drainage pipes – water disperses into the soil.
In St. Norbert, a neighbourhood on the southern edge of Winnipeg, bioretention is being applied to a slightly different problem. Lying along the flood-prone Red River, St. Norbert has dikes to keep floodwaters out of an area that includes a community centre, a school and the province’s largest farmers’ market. The trouble is dikes that keep water out can also keep it in – drainage within the dikes can be a problem.
So the St. Norbert Water Stewardship Coalition plans to build bioretention cells on the 11-hectare site. Janice Lukes, the project manager, expects the cells will not only improve drainage but help filter out pollutants such as nitrates and phosphates, which are a significant problem in the Lake Winnipeg watershed.
The St. Norbert project is still in the planning stages, with installation of the bioretention cells scheduled to start this year.
There is good scientific evidence for most of bioretention’s benefits, says Andrea Bradford, associate professor in the water resources engineering program at the University of Guelph. They certainly can control the volume of runoff, which is important to reducing erosion. They have been shown to filter out contaminants such as metals, oil and grease quite effectively. Research continues on how well bioretention removes nutrients such as phosphorus and nitrates from water, she says.
One contaminant bioretention doesn’t filter out well is road salt, Dr. Bradford says. But she adds that no other storm water management technology known today is much good at removing salt either. “That’s probably one of the biggest challenges” in storm water management, she says.
Another still-unanswered question is how much maintenance the cells will need over time – for instance, will soil eventually lose its ability to absorb contaminants and need replacing? Helping answer that question is one goal of the Beamsville project.
Though it can’t solve every storm water management issue, Dr. Bradford predicts that with awareness of the need to control storm water rising, and better guidelines available on how to build bioretention cells, we will be seeing more rain gardens in Canada in the next few years.
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