January 14, 2015

Bioswales and rain gardens reduce stormwater impact

As published in The Erin Advocate

Credit Valley Conservation is urging municipalities and landowners to use techniques that allow more rainwater to soak into the ground, instead of simply dumping it into the closest stream.

It’s part of a trend called Low Impact Development (LID) that sees the asphalt and concrete surfaces of urban areas as a threat, leading to excessive sediment and chemicals in waterways.

“We want to reconnect the natural hydrological cycling within our urban areas,” said Cassie Corrigan, a CVC Water Resource Specialist, at a workshop last fall.

Biorention planters and swales allow
more rainwater to soak into the ground.
Photo courtesy of CVC.
Some measures can be required by planning regulations, while others are optional. Not every municipality is forcing developers to incorporate LID features.

“If a developer isn’t forced to do it, they’re not going to do it,” said Corrigan.

LID practices do not just apply to new housing developments. They can be used when roads are reconstructed, when parks or commercial zones are improved and when buildings are retrofitted.

Grants are sometimes available to offset the added costs of LID and municipalities can offer incentives through their water rates to encourage property owners to make specific changes.

In practice, LID starts by dealing with some of the rain where it lands, installing permeable paving, directing residential downspouts away from driveways and sewers, harvesting rainwater for other uses (such as water gardens or flushing toilets) and installing vegetation-covered green roofs on suitable buildings.

The risk of flooding on streets and properties can be reduced with bioswales – the new name for shallow grassy ditches. Other strips of land engineered with good drainage and a variety of low-maintenance plants, shrubs and trees are called rain gardens or bioretention swales.

These slow the run-off and filter out pollutants from roads and parking areas. They improve stream habitat and ease the burden on municipal infrastructure, extending its life and reducing the investment needed to build and maintain the system.

The goal is to allow as much water as possible to infiltrate the ground or evaporate before it ends up in a storm sewer, with the benefit of adding attractive greenery to urban areas.

Even where underground servicing is needed, it may be possible to use perforated pipes that allow some of the water to return to the ground.

Flood control still requires the use of ponds, which can be either dry or partially filled between major storms.

These are not ideal, however, since the discharged water is warmer than normal and not completely filtered, and there can be a build-up of sediment. Vegetated wetlands can also be created, though these have the potential of increasing phosphorus in the discharge.

Former Town of Erin Water Superintendent Frank Smedley said in 2012 that LID would not be suitable for the planned Solmar subdivision north of Erin village, due to the high groundwater and low permeability of the soils in the area.

For case studies, guidelines and more information on Low Impact Development, go to www.bealeader.ca.