Toronto and Mississauga Lost Rivers and Urban Flooding

Just a few GIS maps illustrating urban flooding risks in Toronto and Missisauga, showing lost rivers and historical flooding locations (Toronto May 2000 [orange symbol] and Toronto August 2005 [red symbol] storms and Mississauga July 2013 [yellow symbols]).  The lost river major overland flow paths for small catchments (less than 1000 hectare drainage area) are estimated using the rationale method with the SOLRIS Version 2 land use-derived runoff coefficients, times of concentration derived from the MNRF WRIP enhanced DEM to estimate peak rainfall intensity, and approximate uniform flow depth and spread approximations along the centreline flow path. The land use shown is the province of Ontario's SOLRIS Version 3 data, which helps show where overland flow paths are in channels, open spaces, parks and easements vs. through developed areas where original drainage features have been enclosed over time.

Toronto - East End: good slopes toward Lake Ontario, the Don River valley and Taylor Creek. Little flooding in May 2000 and August 2005 (those storms did not affect this area greatly however).



Toronto - West End: a few flood clusters along the lost river flow paths.

Toronto - North End (Newtonbrook area): significant flooding in low relief areas, clusters of flooding along other lost river flow paths. Highest concentration of flooding area in 'partially separated' sewer servicing areas where foundation drains contribute wet weather flow to the wastewater collection system and overland flow paths were not explicitly designed.

Mississauga - Malton Area: Older development, less resilient servicing standards.  Flooding locations are approximate but appear to show concentration along the downstream end of the large central 'lost river', north of the 401.

Mississauga - Cooksville Creek: A large 2000 mm diam. trunk sewer runs along the 'lost river' flow path in the centre of the map, south the the QEW.  This branches to two large trunk under the QEW and upstream.  Flooding locations are approximate.

Urbanization, Runoff, Overland Flow and Flooding - How Sprawl of Ontario Cities Drives Flood Risk and Insurance Losses in Urban Areas

Readers of this blog have seen these basic process described several times: (1) rain transforms into runoff when it hits the ground, (2) runoff accumulates and flows in rivers or municipal drainage infrastructure, (3) the capacity of the flow systems determines whether flow "backs up", "surcharges", "spills", or generally flows uncontrollably to where we don't want it to go, causing flooding.

Using Environment Canada's data and research, we have shown that rainfall intensities have not increased in southern Ontario here. In fact there are more statistically significant rain intensity decreases than increased south of 44 degrees. So the rainfall influence on runoff is not increasing. But runoff has been increasing after decades of urbanization under the today's stable or decreasing rainfall intensities.

The following maps show urban expansion in Mississauga, Oakville and Burlington Ontario from 1966 to about 2000 (data varies from 1999 to 2002). The overland flow system path based on Ontario conditioned digital elevation model is superimposed on the land use map so that the impact of urbanization and runoff into the drainage system can be considered.

 The effect of urbanization in Mississauga on runoff would be most acute in the smaller watersheds (e.g., not the Credit), where the upstream urban area has increased significantly since 1966.

Likewise in Oakville - Bronte Creek, a large watershed more slightly influenced by the city's sprawl, has not been affected to the same degree as the smaller Fourteen Mile Creek to the west, where a high relative change in land use over that smaller watershed has occurred throughout the city.

Same in Burlington - many small creek watersheds originating off the escarpment have dramatically increased urbanization over three decades. Burlington is characterized by creeks that have been realigned, straightened and encroached upon. These can be expected to be more sensitive to increased runoff rates due to expanded urbanization.

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Parts of Hamilton have been urbanized up to the watershed divide (black line) by the late 1990's / early 2000's. How does this affect runoff into the old 'core' built to pre-1960's standards?

Hamilton, wider perspective. Some wetlands remaining upstream of Dundas? :

Richmond Hill (Lake Wilcox near upper middle of map). Some urbanization around the lake flows to the Humber where flow impacts would be muted, while other areas to the south flow flow to headwater tributaries of the Don and Rouge: