North York and Scarborough homes particularly hard hit

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The shape of a massive sewer and storm water makeover to prevent basement flooding in parts of North York and Scarborough is becoming a little clearer this month, as city water staff lay out their plan for mitigating the flooding for members of Toronto's executive committee on Sept. 2.

The plan is expensive - staff estimate it will cost about $195 million. And implementing it will eventually mean steep water bill increases - possibly boosted even higher as the city explores ways to mitigate flooding risks in other parts of Toronto.

In the North York and Scarborough neighbourhoods that were flooded out in August of 2005 after a deluge-like rain storm, that work will encompass some sewer reconstruction and the installation of retention tanks.

But according to Toronto Water general manager Lou Di Gironimo, it's not just the sewer pipes underground that are causing the backups - it's the lay of the land itself in the densely-constructed communities, combined with increasingly intense storms.

"What we've found is that the sanitary sewer work that we're doing is really not a lot of money in comparison to the total package, because the infrastructure out there is really not that bad from a deterioration point of view," said Di Gironimo.

"The biggest problem is storm water management over-land, and handling the flow of rain. Once the sewers fill up when we have a storm event, we then use overland flow and the grade of subdivisions to wash the excess water away. In these older communities, standards weren't in place at the time so there's no outlet to get water to the open environment. So it ponds up and gets in the groundwater of low-lying homes."

The result, said Di Gironimo, is that if those homes have a single crack in the foundation - water gets through.

And when the water is filling storm sewer pipes past capacity, it flows into sanitary sewers, pressurizes and backs up into basements.

The city is planning to embark on a number of infrastructure projects to deal with the backups: $6 million to upgrade 4.5 kilometres of sanitary sewers; $14 million to build 13 sanitary sewage storage tanks; $100 million for 54 kilometres of storm drainage system upgrades; and $33 million for 10 storm sewer storage facilities.

As well, the city will spend $12 million on basement flooding protection subsidies to homeowners, and $30 million on various diversion plans for dealing with surface storm water accumulation.

The matter will be coming before Tuesday's meeting of Toronto's executive committee.