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The front lawn of a home near Chatham, Ont., after a storm on Aug. 30, 2019.Geoff Robins/The Canadian Press

Ontario is not effectively tackling urban flooding even though the government has been aware of the need to address this hazard for at least 15 years, Auditor-General Bonnie Lysyk concluded in her annual report released Wednesday.

Ontario has never clarified the responsibilities of various provincial departments involved, the report said. And it doesn’t provide homeowners and municipalities with adequate guidance. As a result, little has been done. Ms. Lysyk urged the government to establish a framework that clearly assigns responsibilities.

“The Province could make changes to reduce the risk – by updating the Building Code and improving urban flood risk mapping tools, and better protecting the green spaces and natural spaces, such as wetlands, that provide natural protection for communities,” she said in a statement.

Urban flooding results when heavy rainfall overwhelms storm sewers and other drainage infrastructure. That can cause water to pool on streets, leading to road closings and transit-service interruptions. It can also damage transformers, utility boxes and other ground-level infrastructure. And water can pour into basements through windows, or via sewage pipes, causing extensive damage to homes and businesses.

Major urban flooding occurred in Thunder Bay and Hamilton in 2012, Toronto in 2013, Burlington in 2014, and Windsor in 2016. It’s distinct from flooding that occurs when rivers overtop their banks. While riverine flooding can be spectacular, only 3 per cent of Ontario’s population lives within river floodplains. Urban flooding is the most common form of flooding in Ontario – and the resulting insured losses are an order of magnitude greater than those from riverine flooding.

“You get a heavy rainfall event in the middle of nowhere, and nobody bats an eye,” said Glenn McGillivray, a spokesperson for the Institute on Catastrophic Loss Reduction, a research organization funded by the insurance industry.

“But you get a biblical rainfall in the middle of Toronto, like we did in July, 2013, and you have a billion-dollar problem on your hands.”

Even so, the Auditor-General’s report found that urban flooding “receives much less attention from the provincial government” than riverine flooding.

The Auditor-General found that four departments bear significant responsibilities relating to managing urban flooding: the environment, natural resources, municipal affairs and infrastructure ministries. Municipalities, meanwhile, approve local developments and maintain local stormwater infrastructure.

David Piccini, Minister of the Environment, Conservation and Parks, told reporters the government had made “historic” investments in adaptation relating to flooding and climate change, and accused previous Liberal governments of “decades of underinvestment … where we didn’t plan for growth.”

The Auditor-General’s report points to backwater valves, simple devices installed on a sewer line (typically where it exits a home) that allows wastewater to exit, but blocks it from re-entry. Ms. Lysyk said installing one during a home’s construction costs about $250. Retrofitting one into a home can cost up to $4,800, while repairing damages caused by a backed-up sewer can run $40,000 or more. Yet the Municipal Affairs Ministry’s rules are ambiguous, so the devices are rarely installed in new homes in Ontario.

Municipalities also receive conflicting guidance from provincial departments on whether they should rely on historical data to determine how to build new infrastructure, or whether they should take into account possible future increases in precipitation owing to climate change. The Auditor-General’s office surveyed chief building officials; of the 51 who responded, all of them said they relied on historical data when enforcing the Building Code.

The Auditor-General also said the province isn’t educating Ontarians about how flood risks are increasing. Her report found that development is expanding impervious surfaces such as roads and parking lots while reducing green spaces, all of which causes stormwater to run off more quickly. Meanwhile, sewers and other stormwater infrastructure are aging and climate change is increasing the frequency of intense rainfall.

A survey of 30 municipalities by the Auditor-General found that 26 of them provide no information or maps to the public concerning urban flooding. Many of those municipalities said they didn’t have sufficient data, or were concerned about legal liabilities, or reduced home prices in high-risk areas.

Mr. McGillivray said there’s little evidence that flood maps will reduce home values for extended periods of time, or that they increase legal risks.

“I’ve joked over the years that there kind of two types of municipalities,” he said. “There’s the one type that says: ‘We’d better release this information or we might get sued.’ And then there’s the other type that says: ‘We better not release this information because we might get sued.’

“I think transparency is always the better way to go.”