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.

Lost Rivers - Newtonbrook Tributary Flooded Basements - Former North York (Toronto)

From the late 1940's to late 1950's a tributary of the Don River in the Newtonbrook neighbourhood of North York was diverted and enclosed, fully replaced by residential development by the late 1960's. The transition in this area west of Yonge Street and south of Steeles Ave. W. is shown below (source Toronto archives, compiled by Toronto Water) :

Newtonbrook development 1947 - 1969, former North York, City of Toronto

Today, watercourse corridors are preserved through residential developments - larger valley corridors are protected through Ontario's Natural Hazards policies, acts and regulations to mitigate flood risks, and smaller tributaries are now protected to preserve local hydrologic functions (e.g., water balance) and environmental features (habitat).

The Newtonbrook tributary was not preserved although it conveyed runoff from over 250 hectares of land (2.5 square kilometers). The following images show the overland flow path through the area, including multiples of the estimated 100 year flow spread (1 time, 2 times and 4 times width), and basement flooding locations reported in May 2000 (orange), August 2005 (red), and July 2013 (yellow):

Overland flow path and 100 year flow spread (1 x, 2 x and 4 x) and historic basement flooding locations.

Today stormwater management practices would control the quantity of runoff from new development so that it matches pre-development peak flows during extreme rainfall events. This practice did not become widespread until the 1980's in Ontario. The Newtonbrook drainage catchment would have been controlled by about 5 or more stormwater ponds using today's standards (about one pond every 50 hectares).

Just over 35,000 Toronto properties are centered in the 100 year flow spread limits estimated through CityFloodMap.Com's overland flow and flood risk analysis. In many cases, like in the Newtonbrook area, these flow limits correspond to areas of high basement flooding risk given that the overland flow system can overwhelm the local sewer system with significant extraneous inflows during extreme storm events. City-wide analysis of overland flow risks and basement flooding correlation is presented in the following slides:

Urban Flood Risk from Flood Plains to Floor Drains from Robert Muir

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Link to more Toronto Lost Rivers.

Lost River Walks Toronto - Yesterday's Rivers Are Tomorrow's Flooded Basements

"The objective of Lost River Walks is to encourage understanding of the city as a part of nature rather than apart from it, and to appreciate and cherish our heritage. Lost River Walks aims to create an appreciation of the city’s intimate connection to its water systems by tracing the courses of forgotten streams, by learning about our natural and built heritage and by sharing this information with others."
Source http://www.lostrivers.ca/

www.lostrivers.ca

Today's Lost Rivers are tomorrow's flooded basements.

According to Lost Rivers, Walmsley Brook was named for John Walmsley, a settler in the Leaside area or Toronto. It started as small streams near Duplex Avenue and Alexandra Boulevard which joined west of Yonge Street and flowed east to Mount Pleasant Road, then past Bayview and Eglinton, before heading south east to Laird Drive, the CPR rail line and the Don River.

Downstream reaches around McRae Drive to the outlet were largely open in 1922. The reach was subsequently filled in as shown on the images below.

Walmsley Brook, tributary of the Don River was 'filled in' and piped near McRae Drive and Laird Drive and is a part of Lost River Walks, organized by Toronto Green Community.

The catchment draining to the low reach is over 400 hectares in area. That is significant as flood hazards were typically mapped when drainage areas reached 125 hectares, or approximately half a square mile. Today it is not uncommon to map river flood hazards for drainage areas as small as 50 hectares.

Given the large drainage area and the obstructions to flow along the overland flow path, it is no surprise that this are is subject to flood risks. But it does not manifest as river flooding - instead it is a combination of urban flooding overland, and basement flooding that is aggravated by extraneous inflows to the sanitary sewer system.

Over 250 flooded basements were reported after the May 12, 2000 storm in Toronto. The flooding was concentrated in the former Walmsley Brook watershed, and extended outside of it to the south west where the sanitary sewer system crossed into the watershed.

The former Walmsey Brook alignment through Leaside is shown to the left. The same alignment is shown on the right along with reported basement flooding locations in the former watershed. Residential development west of Laird Drive and commercial development east of Laird Drive has reduced the overland flow capacity needed during extreme rainfall events.

Perspective of Walmsley Brook from outlet to Don River.

Detailed analysis using Ontario topographic data, geographic information system hydrology tools, and geo-referenced Toronto flood locations for May 12, 2000, August 19, 2095, and July 8, 2013 storms shows quantitatively that the overland flow path along lost rivers affects basement flood risk. A summary of the analysis is in a previous post.

The lesson? "You can take the river out of the neighbourhood, but you can't take the neighbourhood's runoff out of the residual flow path". Lost rivers not forgotten.

Sag in the road - lost river near current Sunnybrook Plaza is along the original watercourse alignment. Eglinton Ave East, east of Bayview Ave, 1951. Source James Victor Salmon, Toronto Public Library:
http://www.torontopubliclibrary.ca/detail.jsp?Entt=RDMDC-PICTURES-R-3298&R=DC-PICTURES-R-3298 

Walmsley Brook in Leaside has been enclosed in sewers and the overland flow system has been filled in and blocked but the residual flow path remains in place. During extreme rainfall the storm sewer capacity will be exceeded and overland (urban) flooding will occur on the original brook flow path. Basement flooding is often concentrated along the flow path after impeded surface flow enters building flood drains and overwhelms the wastewater sewer system.

Bathurst Heights reach of Yellow Creek.
Map source Lost Rivers.

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Lost Rivers describes many other lost watercourse features in Toronto. We will highlight a few of them in the weeks to come, but first here is another smaller system that demonstrated flood clusters during extreme storm events, although not as extreme as Walmsley Brook in Leaside..

According to Lost Rivers, Yellow Creek originates in the Downsview area and entered the Don River just north of The Prince Edward. The Bathurst Heights reach experienced flood clusters on May 12, 2000, and also on July 8, 2013. Note that our mapping for all July 8, 2013 flood reports is incomplete and does not show all the reported flood locations as the individual site data is not available from the city. However our May 12, 2000 mapping does reflect all reported locations and this likely reflects the broader July 8, 2013 incidents as well - as most properties are built at grades and with service connection very close to their neighbours, back-ups at one property general reflect risks and incidents at adjacent ones. Sometimes property owners do not report flooding to the city and instead work through their insurance company. In the most chronically flooded locations where no back-up insurance is available, there may be no reports.

The image below shows where Yellow Creek fits into the broader lost rivers network and shows details of flooding along the Bathurst Heights reach.

Perspective of Bathurst Heights reach of Yellow Creek, southwest of Bathurst and Lawrence, looking northward. This ArcExplorer oblique image shows exaggerated topographic relief, clearly shows the ups and downs of the former creek catchment. Historical flooding has been concentrated along the flow path upstream and downstream of Dell Park where runoff is concentrated during extreme rainfall events.

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Link to More Toronto Lost Rivers