Canadian Underwriter

Toronto city council to consider accelerating basement flooding protection spending

December 12, 2016   by Canadian Underwriter

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A proposal to advance the 2018 cashflow for Toronto’s basement flooding protection program into 2017 will be considered by City Council Tuesday.

Toronto’s Basement Flooding Protection Program – which includes construction of surface storage ponds, an underground storm storage tank and upgrades to storm and sanitary sewers – was first approved in April, 2006.

The previous August, a thunderstorm caused flooding in the Toronto area gave rise to nearly $600 million in insured losses, Property and Casualty Insurance Compensation Corp. reported earlier. Damage from the Aug. 19, 2005 storm included the collapse of the Finch Avenue bridge over Black Creek.

City Hall - Toronto

It was that storm plus a May 12, 2000 storm (that produced rainfall of 73 mm in some areas) that prompted the city to start its basement flooding protection program.

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As part of that program, the city’s service standard required storm drainage systems to accommodate one-in-100 year storm events – up from the one-in-two to one-in-five year frequency events.

About $1.527 billion has been allocated allocated – in the Toronto Water 2017-2026 capital plan – for “projects in 67 chronic basement flooding areas across the city,” Toronto Water stated in a staff report released in October.

The allocation for basement flooding is about 13% of Toronto Water’s total 10-year capital plan of $12.02 billion.

The city’s budget committee adopted Nov. 18 a motion to ask City Council to ask city staff “to advance the 2018 cashflow for the Basement Flooding projects into 2017, should it become evident that greater than planned volume of work can be completed, and that the 2018- 2026 budgeted cash flows be adjusted accordingly as part of the 2018 Budget process.”

That proposal is on the agenda of full council’s December 13 meeting.

One project is the Fairbank Silverthorn Trunk Storm Sewer System in the Eglinton Avenue and Dufferin Street area.

“As of the end of October 2016, Basement Flooding Environmental Assessment (EA) studies for 30 basement flooding study areas have been completed,” Toronto Water stated in a recent staff report. “These EA studies investigate the causes of basement and surface flooding and recommend sewer system improvements to reduce the risk of future basement flooding during extreme storms.”

Toronto Water added that 11 studies are ongoing and one will be completed this year. Eight studies “are anticipated to be completed in 2017” while another two studies are planned for completion in 2018.

“During storm events larger than a 2 to 5 year return period, stormwater flows can exceed the carrying capacity of the underground storm sewer system and excess water will ramin on the road and flow downhill along streets, open channels, and walkways,” Toronto Water’s general manager stated in a report to city council’s public works and infrastructure committee in September, 2013.

In that report – released two months after a summer thunderstorm caused nearly $1 billion worth of insured losses in the greater Toronto area – city staff identified three main categories of issues that can lead to basement flooding.

One is when the sanitary sewer system is overloaded. This can cause storm runoff to enter the system through various points, such as maintenance hole covers, flood drains of flooded basements, surcharged storm sewers leaking through cracked pipes, illegally connected downspouts to sanitary sewers and illegally connected private catchbasins.

Another issue is if the storm sewer system becomes overloaded during extreme events and can become a source of basement flooding.

“Surcharged storm sewers can create a high water table condition around foundation walls, which under extreme storm conditions is made worse by the additional contribution of roof runoff through downspouts discharging into the storm sewer,” Toronto water said in the report. “Poor lot grading can also create ponding around foundation walls and contribute to elevated water tables. This results in water leaking into the basement through windows, doors and cracked walls and floors.”

A third source is surface flooding on streets, mainly due to poor street grading, Toronto Water noted.

“Current design standards for storm drainage developed in the mid 1970s include a design for the storm sewer system to intercept and convey the stormwater runoff from a one in 2 year to a one in 5 year return storm events,” Toronto Water said. “For larger storms, stormwater can’t be intercepted by the storm sewer remains on the road surface and flows along the streets, typically to a low point where it outlets via an overland route to the nearest watercourse. However, in older areas of the City developed prior to 1970, many of the streets do not provide a continuous flow route, are very flat or have low points with no place for the water to outlet.”

Toronto Water staff noted in the report that in some older parts of the city, sanitary and storm water runoff is collected in a single combined sewer system.





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