City of Toronto Overland Flow Map - 100-Year Storm Major Drainage System Spread & TRCA Watershed Major Drainage Centreline

CityFloodMap.Com presented GIS-based, hydrologic and hydraulic overland flow analysis over the

Newtonbrook area overland flooding in urbanized areas and
historical flood reports (May 2000, August 2005, July 2013).

past couple years (the "2015" Toronto overland flow analysis and spatial analysis correlating overland flood limits to reported basement flooding in 2000,2005 and 2013, and then the "2016" southern Ontario-wide analysis that includes refined hydrologic parameters). This provides an important insight into urban flood risk management and the influence of surface flooding on sanitary/wastewater system inflows (e.g,. via doors, windows, walk-outs, depressed driveways) - the extraneous flows that particularly stress partially separated sanitary systems, causes sewer surcharge and basement sewage back-up.

Below is an interactive map of the overland flow analysis clipped to the City of Toronto including the estimated 100 year flow spread and a 2x 100-year flow spread estimate. The mapping also includes overland flow paths within Toronto (the centreline of the major drainage flow path) and outside of Toronto covering the entire TRCA watershed areas ((c) CityFloodMap.Com). The major drainage system flow spread in Toronto is essentially a buffer from this centreline considering each overland flow reach's hydrology (rational method) and hydraulics. A standard road cross section assumed to estimate the spread using each overland flow segment's longitudinal average slope (or a minimum to avoid zero's for segments across filled sinks that have no slope). More details on the analysis including the basement flooding correlation is described here:

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

Social media geotagged surface flooding pictures like the one below were found to corresponds to the mapped overland flow path in some areas:

The interactive map is below (c) CityFloodMap.Com. Note, approximate TRCA regulation boundaries were estimated from camaps.ca georeferenced image features, and TRCA shoreline/slope regulation areas have been excluded to focus more on where river flood risks exist:

View larger map

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.

***

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: