Canadian Underground Infrastructure Logo

Bioswale to Address Stormwater Concerns for City Of Cornwall

Bioswale to Address Stormwater Concerns for City Of Cornwall

More frequent storm events of ever-increasing intensity, which many attribute to climate change, have prompted municipalities across Canada to look at ways to guard against flooding to homes and businesses.

One need only look at the damage done to southern Alberta when several rivers overflowed their banks in 2013, or the Red River Flood of 1997, which inundated 2,000 square kilometres when the Red River rose 12 metres, to see the risks of failing to protect against Mother Nature.

Following the flood in Calgary, the city repaired critical sites along the riverbank that had eroded, and developed new computer models to assist with emergency response planning.

Cornwall, Ontario also undertook to review flooding mechanisms and its flood abatement measures when in 2013, a once-in-a-century probability storm caused sewer backups affecting several hundred homes. The event was the second such 1:100 probability event within a three-year period that challenged Cornwall’s infrastructure capacity to handle intense volumes of stormwater and residents’ patience in dealing with unpleasant and costly sewer backups.

The city of 47,000, located southeast of Ottawa on the St. Lawrence River, thus developed a strategy, called the “Cornwall Blueprint”. This is a comprehensive urban water awareness strategy with an initial imperative aimed at mitigating basement flooding and increasing residents’ awareness of sewer infrastructure – both on private property as well as that belonging to the municipality.

A key element of the Blueprint has been to develop a rebate strategy for homeowners encouraging them to undertake changes at their home to reduce the risk of basement flooding attributable to sewer backup. The objectives of the rebate are three-fold: reduce the risk of flooding caused by sewer backups; reduce the contribution from storm runoff and groundwater to the municipal sanitary sewer network; and encourage open dialogue between City staff and residents about flooding causes and protection. Homeowners are eligible for up to a $3,600 rebate towards costs incurred to flood-mitigate their homes. Such measures include disconnecting eavestrough downspouts and foundation drains, lot-grading improvements, and installing backwater valves and sump pumps. We quickly learned that a successful program needed a strong citizen engagement component as public knowledge of buried municipal infrastructure was limited. The program has been very successful to date with more than 200 applicants in the first year alone.

The City has also undertaken and planned comprehensive efforts to inspect and rehabilitate aging sewers and manholes, in particular in flood-prone neighbourhoods, to reduce excess stormwater contributions (so-called infiltration and inflow, or I/I). The I/I from aging infrastructure and from the homes can quickly deplete sanitary sewer capacity in an intense storm event thus contributing to sewer backups.

Bioswale an Eastern Ontario first

The Blueprint also includes some other innovative aspects such as a Low Impact Development (LID) demonstration project. LID is a keypart of a so-called Green Infrastructure design strategy for stormwater management. For this, the City is undertaking to construct a “bioswale” as part of a road re-construction and sanitary sewer separation project. The bioswale will serve as a bioretention facility essentially to temporarily capture and treat rainwater runoff thus relieving pressure on the city’s stormwater conveyance system.

Designed to remove solids and nutrients from runoff water, a bioswale is also helpful in reducing peak stormwater flows, which is its primary purpose as conceived by the City of Cornwall. 

The $500,000 project to build a bioswale – essentially a sand and compost media filled trench running along both sides of Seventh Street for about 200 metres – is a first for Cornwall and rare for Eastern Ontario, according to John St. Marseille, P.Geo., P.Eng., general manager, infrastructure and municipal works with the City of Cornwall.

St. Marseille explained the bioswale as a relatively new design philosophy of stormwater management called “low impact development,” (LID) whereby new or existing water infrastructure is blended into the natural environment in a holistic way. Contemporary stormwater management attenuates post-development runoff rates and addresses water quality treatment. However, volumes are still significantly increased. LID enables urban runoff to more closely approximate a natural environment, including water cooling and groundwater recharge.

“We look at design criteria for water quality. We look at erosion control. We look at opportunities for infiltration. We look at the whole water balance approach,” he said. “If you can try to mimic how that water would be retained and attenuated onsite without rapidly conveying it away and pushing the problem further downstream, that’s the design philosophy.” We have retained the consulting firm Aquafor Beech to provide design support.

St. Marseille compared the bioswale, which will be constructed beginning this spring, to a kind of “sponge”. 

“The sponge has the ability to absorb water. Ultimately, the sponge will be saturated and there’s going to be some excess water contribution but ideally that occurs only after the storm event has diminished. Most of that water will thus percolate into the ground rather than to go through the storm sewer pipes.”

Perforated pipes will be placed throughout the bioswale, shallow enough to allow the water level to build up before it is conveyed through the pipes into the storm water system. The perforated pipe size and depth are purposely designed to act as kind of ‘safety valve’ – thus most of the stormwater running off the road merely infiltrates into the subsurface in the bioswale. The system will be monitored after construction to verify performance. An added benefit should come in the form of downsized stormwater pipes, thus saving the municipality potential infrastructure costs in the future. The environmental benefit is the improved stormwater quality afforded by the bioswales.

Along with stormwater retention and treatment, the bioswale is also seen as an opportunity for neighbourhood beautification. Residents will be invited to choose from a selection of greenery they want to see growing in the portion of the bioswale fronting their homes. For example perennial grasses, small shrubs or trees are possibilities. “Those are pretty appealing and do a lot for neighbourhood enhancement and to the whole streetscape,” said St. Marseille. 

Sewer rehab diverts sanitary flow

Another important element of the Cornwall Blueprint is a plan to rehabilitate the sewer system in the Grand-Miron neighbourhood – an area prone to flooding. The two-year project has a budget of $5 million and has been assisted with a $2-million grant from the Ontario government.

The work involves construction of deeper sanitary sewers and storm sewer outlets in the Eleventh Street area and along parts of Grand and Miron Avenues, as well as a new dedicated storm sewer on Marlborough Street to free up sanitary sewer capacity, and replacement of an old pump station.

“The pumping station was passed its service life and needed to be replaced. The sewers as well needed to be reconnected into this new pumping station and we’re actually moving the sanitary flow into a different sewer catchment to relieve of the pressure on some of the other areas that have been prone to flooding,” St. Marseille explained. It is a challenging and comprehensive project that will benefit many in the City.

The project started last summer and is expected to be completed later this year. According to St. Marseille, it will take more than one strategy to reduce the incidents of basement flooding – hence the multifaceted approach that is behind the Cornwall Blueprint.

“If it were as simple as one single component that was contributing to the excess water into our system, we’d just repair it and that would be it,” he said, but “with aging infrastructure and more frequent and intense storms ‘flooding by a thousand drips’ has been the operative expression here so we have to deal with this one drip at a time.”

More frequent storm events of ever-increasing intensity, which many attribute to climate change, have prompted municipalities across Canada to look at ways to guard against flooding to homes and businesses.

One need only look at the damage done to southern Alberta when several rivers overflowed their banks in 2013, or the Red River Flood of 1997, which inundated 2,000 square kilometres when the Red River rose 12 metres, to see the risks of failing to protect against Mother Nature.

Following the flood in Calgary, the city repaired critical sites along the riverbank that had eroded, and developed new computer models to assist with emergency response planning.

Cornwall, Ontario also undertook to review flooding mechanisms and its flood abatement measures when in 2013, a once-in-a-century probability storm caused sewer backups affecting several hundred homes. The event was the second such 1:100 probability event within a three-year period that challenged Cornwall’s infrastructure capacity to handle intense volumes of stormwater and residents’ patience in dealing with unpleasant and costly sewer backups.

The city of 47,000, located southeast of Ottawa on the St. Lawrence River, thus developed a strategy, called the “Cornwall Blueprint”. This is a comprehensive urban water awareness strategy with an initial imperative aimed at mitigating basement flooding and increasing residents’ awareness of sewer infrastructure – both on private property as well as that belonging to the municipality.

A key element of the Blueprint has been to develop a rebate strategy for homeowners encouraging them to undertake changes at their home to reduce the risk of basement flooding attributable to sewer backup. The objectives of the rebate are three-fold: reduce the risk of flooding caused by sewer backups; reduce the contribution from storm runoff and groundwater to the municipal sanitary sewer network; and encourage open dialogue between City staff and residents about flooding causes and protection. Homeowners are eligible for up to a $3,600 rebate towards costs incurred to flood-mitigate their homes. Such measures include disconnecting eavestrough downspouts and foundation drains, lot-grading improvements, and installing backwater valves and sump pumps. We quickly learned that a successful program needed a strong citizen engagement component as public knowledge of buried municipal infrastructure was limited. The program has been very successful to date with more than 200 applicants in the first year alone.

The City has also undertaken and planned comprehensive efforts to inspect and rehabilitate aging sewers and manholes, in particular in flood-prone neighbourhoods, to reduce excess stormwater contributions (so-called infiltration and inflow, or I/I). The I/I from aging infrastructure and from the homes can quickly deplete sanitary sewer capacity in an intense storm event thus contributing to sewer backups.

Bioswale an Eastern Ontario first

The Blueprint also includes some other innovative aspects such as a Low Impact Development (LID) demonstration project. LID is a keypart of a so-called Green Infrastructure design strategy for stormwater management. For this, the City is undertaking to construct a “bioswale” as part of a road re-construction and sanitary sewer separation project. The bioswale will serve as a bioretention facility essentially to temporarily capture and treat rainwater runoff thus relieving pressure on the city’s stormwater conveyance system.

Designed to remove solids and nutrients from runoff water, a bioswale is also helpful in reducing peak stormwater flows, which is its primary purpose as conceived by the City of Cornwall. 

The $500,000 project to build a bioswale – essentially a sand and compost media filled trench running along both sides of Seventh Street for about 200 metres – is a first for Cornwall and rare for Eastern Ontario, according to John St. Marseille, P.Geo., P.Eng., general manager, infrastructure and municipal works with the City of Cornwall.

St. Marseille explained the bioswale as a relatively new design philosophy of stormwater management called “low impact development,” (LID) whereby new or existing water infrastructure is blended into the natural environment in a holistic way. Contemporary stormwater management attenuates post-development runoff rates and addresses water quality treatment. However, volumes are still significantly increased. LID enables urban runoff to more closely approximate a natural environment, including water cooling and groundwater recharge.

“We look at design criteria for water quality. We look at erosion control. We look at opportunities for infiltration. We look at the whole water balance approach,” he said. “If you can try to mimic how that water would be retained and attenuated onsite without rapidly conveying it away and pushing the problem further downstream, that’s the design philosophy.” We have retained the consulting firm Aquafor Beech to provide design support.

St. Marseille compared the bioswale, which will be constructed beginning this spring, to a kind of “sponge”. 

“The sponge has the ability to absorb water. Ultimately, the sponge will be saturated and there’s going to be some excess water contribution but ideally that occurs only after the storm event has diminished. Most of that water will thus percolate into the ground rather than to go through the storm sewer pipes.”

Perforated pipes will be placed throughout the bioswale, shallow enough to allow the water level to build up before it is conveyed through the pipes into the storm water system. The perforated pipe size and depth are purposely designed to act as kind of ‘safety valve’ – thus most of the stormwater running off the road merely infiltrates into the subsurface in the bioswale. The system will be monitored after construction to verify performance. An added benefit should come in the form of downsized stormwater pipes, thus saving the municipality potential infrastructure costs in the future. The environmental benefit is the improved stormwater quality afforded by the bioswales.

Along with stormwater retention and treatment, the bioswale is also seen as an opportunity for neighbourhood beautification. Residents will be invited to choose from a selection of greenery they want to see growing in the portion of the bioswale fronting their homes. For example perennial grasses, small shrubs or trees are possibilities. “Those are pretty appealing and do a lot for neighbourhood enhancement and to the whole streetscape,” said St. Marseille. 

Sewer rehab diverts sanitary flow

Another important element of the Cornwall Blueprint is a plan to rehabilitate the sewer system in the Grand-Miron neighbourhood – an area prone to flooding. The two-year project has a budget of $5 million and has been assisted with a $2-million grant from the Ontario government.

The work involves construction of deeper sanitary sewers and storm sewer outlets in the Eleventh Street area and along parts of Grand and Miron Avenues, as well as a new dedicated storm sewer on Marlborough Street to free up sanitary sewer capacity, and replacement of an old pump station.

“The pumping station was passed its service life and needed to be replaced. The sewers as well needed to be reconnected into this new pumping station and we’re actually moving the sanitary flow into a different sewer catchment to relieve of the pressure on some of the other areas that have been prone to flooding,” St. Marseille explained. It is a challenging and comprehensive project that will benefit many in the City.

The project started last summer and is expected to be completed later this year. According to St. Marseille, it will take more than one strategy to reduce the incidents of basement flooding – hence the multifaceted approach that is behind the Cornwall Blueprint.

“If it were as simple as one single component that was contributing to the excess water into our system, we’d just repair it and that would be it,” he said, but “with aging infrastructure and more frequent and intense storms ‘flooding by a thousand drips’ has been the operative expression here so we have to deal with this one drip at a time.”

More from Storm Water

Get our newsletter

Subscribe to our free magazine

Get Our Magazine

Paper or Digital delivered monthly to you

Subscribe or Renew Learn more

Get our newsletter

Subscribe to our free magazine

Get Our Magazine

Paper or Digital delivered monthly to you

Subscribe or Renew Learn more