What lies beneath

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With land at a premium, developers push to explore alternative storm water management systems

With its brightly coloured playground, winding paths, and mature trees, Gallanough Park, in Vaughan, may look like a regular suburban green space — but, buried beneath it, an important piece of infrastructure is hidden.

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Just below the surface of the park, there’s a new underground stormwater facility with the capacity to store 8,762 cubic metres of rainwater— or about three-and-a-half Olympic-size swimming pools’ worth.

During a major rainfall or big melt, water collects in two concrete tanks and then is slowly discharged back to the storm sewers. This gives the municipal sewers, which are at capacity, a reprieve for several hours and mitigates flooding, a cause of headaches in the area.

“You’d never know it was under there,” said Mark Bassingthwaite, founder and president of Resilient Consulting, of this type of stormwater facility. Completed late last year, the Gallanough Park facility is part of a broader flood-prevention strategy that took root in early 2023, when the City of Vaughan hired Bassingthwaite’s firm to help the municipality develop a policy framework to encourage more of these subterranean safeguards.

“The increased demand for housing along with the limited availability of land resulted in a push to explore alternative stormwater management options,” said an email statement from the City of Vaughan, which adopted its new stormwater policy this past summer.

Vaughan is not alone. A rise in serious storms in recent years, together with high land values, have developers and municipalities across the Greater Toronto Area embracing different approaches to managing flood risks, both in new residential developments and retroactively in older communities.

“Land values in certain areas of the GTA have skyrocketed, so a lot of municipalities… they’re starting to see a lot more underground solutions,” said Katya Seravalle, manager of new product development for DECAST, a manufacturer of prefabricated stormwater infrastructure.

“They can’t afford to take these large parcels of land anymore and use them for just one purpose,” said Seravalle. “It’s too much of a waste of land.”

An alternative to the space-consuming ponds you’ll see in many subdivisions, underground facilities function similarly, Bassingthwaite explained.

In a pond, there is a forebay and an aft bay, separated by a berm, which is a ridge of land. The idea is that sediment settles at the bottom of the forebay before the cleaner water spills over into the aft bay. In a system like the one in Vaughan, this process is achieved with a wall inside the tank that creates an intake area.

“Hopefully any of the heavier sediment would drop out in that area,” said Bassingthwaite, who noted sediment is vacuumed out of the tanks. Geranium is among the GTA developers who have looked below ground to prevent floods and create new greenspace for residents.

“Open-air ponds, traditional ponds — they take up space,” said Vimal Patel, vice president of land development at Geranium. “They’re nice to look at, and you could put homes backing on ponds and sell them for a premium, but what if you could make that more usable?” said Patel. On more than one project, Geranium has buried stormwater tanks under amenities.

The need for innovation to address flooding is more pressing than ever, said Andrew Binns, a professor of environmental engineering and water resources at the University of Guelph.

“These floods, based on the classical statistics — we didn’t expect those to happen so often,” he said. He points to the prevalence of so-called 100-year storms, which are storms with a one per cent chance of occurring in any given year. Since 2005, six such storms have struck the GTA.

The fact that so much land has been paved over and built upon doesn’t help. Artificial surfaces make it harder for water to be absorbed into the ground, he noted. “We’ve also changed the landscape so much, so that’s led to flood susceptibility,” Binns added.

There may be danger in not taking action, but innovative solutions aren’t without possible downsides. For one, there are higher costs. “Land value and of course other project specific conditions aside, underground systems can cost between $350-500 per cubic metre of storage while traditional ponds can be as little as $100-$300 per cubic metre,” said Ashish Shukla, associate and director at Crozier, an engineering consultancy.

Then there’s the question of maintenance, Bassingthwaite suggested. “There’s a potential ugly side,” he said. With underground stormwater tanks, for example, he said it is harder to detect issues, such as clogging, since the infrastructure is out of sight.

“It can be an innovative solution to maximize land yield in certain situations, but it’s not without risk,” he said. “And it’s not without increased inspection and maintenance costs, too, that ultimately get borne by the city,” he said.