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:

Land Use Change Drives Urban Flood Risk .. Yet Hydrologists Become "Useless Appendage" To More Virile Sciences

"The unsatisfactory state of hydrology is, in the final analysis, the result of the dichotomy between the theoretical recognition of hydrology as a science in its own right and the practical impossibility of studying it as a primary discipline but only as an appendage of hydraulic engineering, geography, geology, etc." Vit KlemeŇ° in Dilettantism in hydrology: Transition or destiny?, Water Resources Research, Vol. 22, 1986.

That explains it! Nobody studies hydrology in its own right, and so this limp appendage of other scientific pursuits has been ignored. Pity - because hydrologic science and simple quantifiable land use changes can explain increased runoff and increased urban flood damages in Ontario municipalities. Why is it ignored? Because hydrology is just the foreplay to other things like dynamic hydraulic simulations and then infrastructure construction. Everybody is always stampeding to the Priestman slot, frothing over pressure head - hydraulic hussies! A veritable orgy of climate simulators, and media stimulators, rages on while hydrologists are left alone :(

So we need some "tantric hydrology" people. Slow and savoured and reflected upon. Maybe if Sting was an engineering professor instead of an english teacher he could have promoted it, and enlightened the masses to the joy of runoff coefficients. Alas, he wasn't and it is so up to CityFloodMap.com to spell it out - how increased urbanization has affected hydrologic responses in Ontario cities and magnified runoff potential, flood damages, and insurance losses.

Get ready.

Just open your peepers and gaze upon these Ontario urban area land use maps, and see how the limits of urban areas have increased over thirty years or so from 1966 to the late 1990's. It can be dramatic. For some catchments, the increase in urban land coverage can be an order of magnitude.

Sure, some of these areas developed between 1966 and 2000 have stormwater management controls up to a point. But it is commonly accepted that the on-site local stormwater controls do not completely address cumulative volumetric runoff impacts at larger scales. And the controls are finite - not controlling the rarest storms that drainage infrastructure could see in its design life. So expanded urban areas can drive downstream flood risk. Yes, every catchment is different and maybe no property is at risk downstream of some urbanized area (i.e., dedicated drainage easements and wide regulated flood plains have maintained a wide major flow path) - but in may cities the overland flow path has not been mapped managed or preserved, so increased runoff and flood risk can result.
Tantric Hydrology
Brought to you by the Flood Guru.
OK, could a movie about hydrology be any worse than The Love Guru? I guess.....

Secret is Out ! Urbanization and Runoff Explain Increased Urban Flood Risk in Southern Ontario

Ashley Madison ad: "Middle-aged
hydrologist seeking good listener,
someone, anyone, interested in science".
"Only the small secrets need to be protected. The big ones are kept secret by public incredulity."(attributed to Marshall McLuhan)

Could it be that we have ignored the obvious "secret"? That is, the effect of increased urbanization on urban flood risk in Ontario. It seems so.

Mapping of urbanization patterns in that Greater Toronto, Golden Horseshoe, and the Big K-W areas shows obvious, dramatic increases in watershed urbanization over a thirty year period from 1966 to the late 1990's. In some Toronto watersheds, urbanization increased by 986%, and that does not even consider intensification within previously developed areas. This analysis begs the question: "Why is there such a disproportionate focus on global climate change impacts to local extreme weather (now disproved) to explain flooding, compared to a focus on the obvious land use changes on hydrology, and increased runoff under historical climate conditions?"

The following map shows the increase in urbanization from 1966 according GIS mapping converted and shared by Ducks Unlimited (source is the Canada Land Inventory Land Use 1:50,000 scale mapping for Southern Ontario), and to 1999-2002 per the SOLRIS Version 1.2 land cover GIS mapping, as compiled in the Ontario Land Cover Compilation Version 2.0 and available through Land Information Ontario.
Greater Toronto Area Urban Area Growth in TRCA watersheds and Flood Risk Influence on Urban Flooding
Greater Toronto Area Urbanization and Flood Risk

Urban Growth in TRCA watersheds and Flood Risk Influence on Urban FloodingClearly, the amount of urbanization has increased dramatically in southern Ontario watersheds, including many that have experienced flooding in the past decades. In about 30 years, urbanization has increased by 986% over and above the 1966 amounts in the Rouge River Watershed, and by 696% in the Duffins Creek Watershed. In Toronto area watersheds, unlike Rouge and Duffins, the increase is also dramatic - not as much for the percentage increase, but for the relative coverage. The Highland Creek Watershed increased by an added 124% over the 1966 coverage, resulting in over 88% urbanization coverage. Similarly, Mimico Creek Watershed urban areas expanded by 135% more, also resulting in over 88% urbanization by 1999-2002. The following table shows the TRCA watershed breakdown.

How about Mississauga where there was flooding in 2013? Well, Etobicoke Creek Watershed added 341% urban coverage resulting in 64% coverage by 1999-2002. Yes it should seem greater, but we have not counted the valley areas or other large open or vegetated areas within the urban areas (e.g., hydro corridors, etc.). We have also subtracted these 'greenspaces' from the 1966 data to avoid over-reporting the 1966 coverage, as the 1:50,000 source data was not as detailed as the the SOLRIS data that classified land used down to a 15 m cell resolution. And the Cooksville Creek area in Mississauga? In 1966 urban development extended generally up to Dundas Street, while 30 years later it extended to Highway 407 as shown in the image below.

Mississauga Urbanization 1966 to 1999-2002.

Do the math. Nearly an order of magnitude increase in urban coverage in one large watershed. Doubling of urban coverage in all but one other TRCA jurisdiction watershed. And even more intensification within the old urban areas. Runoff potential has increased dramatically as a result. This explains increased flood risk in urban areas.

While stormwater management controls have been put into place for some later developments to mitigate hydrologic impacts, these are finite, perhaps up to 100 year storm standards - extreme weather events can be above or below that threshold, meaning flood damages can occur for the large events. Furthermore, many infrastructure components put in place decades ago are not sized to handle 100 year storms, and have been designed to overtop for high frequency, lower return period events (e.g., underpasses, local roadway crossings).

So why have folks jumped on the now-discredited conclusion that climate change has been driving local flood damages and losses, but ignored basic hydrology and effects of urbanization? Perhaps we can learn from Vit Klemes, perhaps our greatest hydrologist. He had reiterated his piece to the Canadian Water Resources Association (Implications of possible climate change for water management and development. Water News (CWRA), 11, 1, S2-S3) in his address to the International Interdisciplinary Conference on Predictions for Hydrology, Ecology, and Water Resources Management: Using Data and Models to Benefit Society,15–18 September 2008, Prague, Czech Republic. In this address, entitled "Political Pressures in Water Resources Management. Do They Influence Predictions?",  he states:

"[the theorists] find it easier to play trivial scenario-generating computer games while the [managers] find these games much easier to finance... And so by happy collusion of interests, an impression is created that 'something is being done for the future' while the real problems are quietly allowed to grow through neglect of the present"

Neglect obvious present hydrology. Instead, generate theoretical climate change storm impacts .. with computer "games", i.e., models, or even basic high school math probability density function manipulation. And happily fund every climate change prediction or adaptation study.

NB - the newer urban areas have higher densities and greater runoff potential than the older ones:

York Region Urban Development Intensification 1952-2002 - Impervious Surface Area Coverage Doubles Over 50 Years

And intensification within established urban areas can increase runoff potential as well. This shows changes since the early 1970's across the 7 lots beside my house in east Toronto:

Redevelopment and increased urbanization in east Toronto - no residential, single lot stormwater management controls to mitigate runoff impacts to existing drainage system.
And this shows intensification in York Region in an area developed in the early 1950's but undergoing redevelopment / intensification, expanding impervious area coverage, to this day:

Ontario Overland Flood Risk Mapping - Risk Screening Mapping to Identify Urban Flood Risk Zones Beyond Regulated Valleys

Overland flood risks often result in water damage in vulnerable urban areas. New Ontario mapping of surface drainage flow paths can identify the highest risk areas, specifically those around buildings and beyond river flood plains.

Why Is Overland Flood Risk Mapping Needed?

Analysis of historical flooding in Toronto in May 2000, August 2005 and July 2008 has revealed that basement flooding is correlated with overland flow and topographic risk factors. A building's footprint within the overland flow path is an obvious indicator of surface water damage potential - that is, water encompassing a building and entering its openings. But the proximity to the overland flow path, and its ability to negatively influence the neighbourhood wastewater system with extreme weather inflows, has also been show to be an indicator of sewer back-up risk. In this manner the overland flow spread influences flood risks on a broader spatial scale beyond the narrow overland flow path alone.

How Does Overland Flood Risk Mapping Relate to Flood Plan Maps?

Overland flood risk mapping is the natural extension of river risk mapping, up beyond the valley flood plain limits, and across 'table land' as they say in the development industry. Typically in Ontario, regulated valley areas incorporate a range of natural heritage features and hazards including flood plain, watercourse meander belt width, and unstable valley wall slopes. On table land, overland flood risk hazards run across roadways and the developed lot fabric of our cities, sometimes confined in drainage features, or sometimes not. Overland flow zones typically do not coincide with natural heritage features. like vegetated valley flood plains do.

Who Maps and Manages Overland Flood Risks?

Sometimes nobody. After all, without natural heritage features, there is less to protect under Ontario's provincial policy statement. And because the overland risks emerge on such an infrequent basis (during the most extreme rainfall events), they are not top of mind, nor are they easy to define. Progressive cities like the City of Toronto has an aggressive basement flood reduction program that assesses overland drainage systems and identifies risk management alternatives. But these overland systems are typically developed only in specific remediation areas, incorporated into InfoWorks models and characterized in Class Environment Assessment reports.


Well, in some isolated cases overland flood risks are mapped and managed in the same manner as regulated valley flood plains by Ontario conservation authorities. Typically these are areas of isolated watercourse enclosure where extreme rainfall runoff overwhelms the sewer or culvert conveyance system and flows over land. Almost exclusively, however, flood plain risk maps stop at the conveyance system outlet (i.e., headwall / outfall) and do not extend further up onto table land.

What About Insurance Industry Mapping?

Overland surface flooding flood risks, sometimes called pluvial flood areas, are mapped by companies such as JBA and used by insurance companies as input to insurance business decisions (where to insure, setting appropriate risk-based premiums) - but mapping is proprietary, and results are not used for regulation or risk management purposes. Rather, surface flood risk mapping is a business service.

Where Are Ontario City's Predominant Flood Risks?

In overland flood risk zone, not flood plains - in fact in Toronto 98% of flooding in the last three large storms was beyond river flood vulnerable areas. This is consistent with Conservation Ontario figures that identified the percentage of Ontario properties in flood plains to be in the low, low single digits.

Show Me !

Below are a couple images of overland flood risk zones derived for the Ontario South-West digital elevation model zone. The first is the chronically flooded Newtonbrook area in Toronto, and the second is the chronically flooded Brydges-Elgin area in Stratford (subject of a settled class action lawsuit).

The large map shows the topography used to identify overland flow path alignment / upstream slope for hydrologic peaking factor / contributing drainage area / overland network reach conveyance slope, and land use used to assess contributing area composite runoff coefficient.

The inset maps shows the overland flow path spread during a 100 year peak flow, and multiples of the flow path that can indicate risks to adjacent properties connected by wastewater systems. The overland flow network is defined for all drainage areas over 3 hectares in size up to 1000 hectares in size. Typically, flood plain mapping is available for the largest drainage areas and would overlap the overland flow path limits. The inset maps shows the overland flow path on an Open Street Map base, revealing where the overland flow path affects buildings and built-up areas.

Currently overland risk zones are refined for south and south western Ontario (excluding the Ottawa River basin). This represents over 800,000 overland flow segments in the major drainage network. We are evaluating distribution methods in order to share these results as well as input layers that can be used to support refinements by others. Stay tuned!

Canadian Flood Underwriter - Can Fire Underwriters Survey History Guide Urban Flood Risk Management Model?

Can the history of the Fire Underwriter Survey provide a roadmap to a future Canadian flood underwriting model?

Fire Underwriters Survey (FUS) is a national organization and was formerly the Insurers’ Advisory Organization (IAO) and Canadian Underwriters Association. As noted on their site, "FUS provides data on public fire protection for fire insurance statistical work and underwriting purposes of subscribing insurance companies. Subscribers of Fire Underwriters Survey represent approximately 85 percent of the private sector property and casualty insurers in Canada."

The history of the FUA and the organizational and technical benefits offered to its members is described in this book:

"The Underwriters, the history of the Insurers' Advisory Organization and its predecessors, the Canadian Fire Underwriters' Association and the Canadian Underwriters' Association," by Christopher L. Hives, 1985.

If "flood is the new fire" as far as Canadian insurance losses go (Fort McMurray 2016 notwithstanding), much can be learned in how the insurance industry developed the Fire Underwriters organization to organize, guide, educate, and support member insurance companies.

How did fire insurance evolve?

The first fire policy was written by the Phoenix Company in 1782 policy in Sr. John's, Newfoundland and the first Canadian company to offer fire insurance was the Nova Scotia Fire Insurance Association, founded in 1809 in Halifax. More players joined and by the mid 1800's it was apparent there was a need for standardization and organization - so in 1857 "in order to better serve the interests of the community and the participating companies, a joint body or association called the Halifax Insurance Board (was) formed".

Following a 1859 severe fire in downtown Halifax with payouts of £128,075 the board commented on causes and identified "the want of sufficient supply of water at the commencement and to a general want of management". It made recommendations for an improved city water supply for the city, more firemen, and water supply infrastructure along Barrack Street with branch lines exclusively for fires. Without these risk reduction measures, the board would increase tariffs (i.e., premiums).

The early Halifax example illustrates the partnership between government and insurance industry whereby standards for municipal services consider insurance industry needs, which it could be argued, mirror the financial needs of the economy, and the government's need to promote safety and security in the community as a whole.

Over the next century or so, a national organization was formed, evolved and grew (Canadian Underwriter's Association (CUA) founded in the late 1880's, and Insurer's Advisory Organization (IAO) in the 1970's incorporating CUA and independent members). In the late 1970s, lAO developed one of its first computer applications to determine whether a sprinkler system would function properly and if the water supply was sufficient. This replaced inefficient manual calculations.

One could draw parallels between the IAO's analysis of sprinkler fire suppression systems and the assessed benefits of flood prevention systems. In the case of large watersheds in the Toronto area, prevention features would include infrastructure such as the Lower Donlands Landform Berm, Black Creek Channel, and TRCA dams - all incorporated in JBA's flood defense layers. At the local municipal level, flood prevention features would include drainage and municipal infrastructure standards incorporated in new resilient communities, or upgrades for flood risk mitigation in communities build with lower standards (e.g., pre-1980's subdivisions with limited overland drainage design, and partially-separated wastewater systems).

Currently CGI Risk Management Services conducts  Fire Underwriters Survey (FUS) services for the Canadian insurance industry, including assessment of municipal water supply systems including adequacy of available flow rates for structures in the community, and distribution system adequacy and reliability. This information helps guide community improvements and to establish "municipal gradings for insurance purposes", according to CGI.  

The parallel in the context of urban flood risk assessment would be i) assessment of wastewater systems to safety convey extraneous flows during extreme wet weather events, without surcharging and causing floor drain sewer back-up, and ii) assessment of overland drainage systems to safely convey overland runoff ('major drainage system flows) to an adequate outlet like a channel or valley, without surrounding and entering buildings. Since the 1970's, computer assessments of wastewater system capacity has been pursued using the SWMM model, typically in the context of flood remediation or CSO reduction studies. Today, such models are more widely available, but even in jurisdictions like Toronto, they are only now being developed to cover the entire municipality and to be developed with robust standards. Over time, the risk characterization from such models could be used to establish a municipal grading for insurance purposes, likely at a block face level of detail. Overland flood risk models are only now becoming commonplace, including JBA's riverine and surface (pluvial) flood risk models based on 2D overland flow modelling. 

The difference between CGI's fire risk assessments and flood risk assessments is that no flood risk assessments are tied to 'municipal gradings'. Municipal wastewater system assessments can result in infrastructure improvements, or when not cost effective, deferred projects. But neither condition is communicated to an insurance risk body, and the insurance industry is not known to participate as a stakeholder in the municipal environmental assessment studies that guide the improvement strategies. Similarly, the overland risk assessment available to those writing individual policies are not shared with municipalities to guide macro-scale, neighbourhood-level improvements to infrastructure that could mitigate risks and premiums. IBC's Municipal Risk Assessment Tool (MRAT) estimates municipal wastewater back-up risks but these are not used to set premium rates or identify specific municipal infrastructure improvements (i.e., MRAT provides generalized, regression-based "high-low" risk mapping for one type of flood risk but does not incorporate system hydraulics to assess upgrades, nor does it assess neighbourhood overland flood risks or remediations).

Given that fire insurance and underwriting support evolved over more than 200 years, and that technical assessments of some risk factors like fire suppression sprinklers emerged only 40 years ago, flood risk underwriting likely has decades to go before an effective partnership can develop between the insurance industry and municipalities and other levels of government.

"...our industry will undergo fundamental and radical changes which will severely test the mettle of all the players in the league. 'To be sure, the 'good old days' are gone forever. Today's conditions, regarded by many as 'abnormal' are, in reality, 'but a mild rain shower compared to the storms of the future ...obviously, the future is no place for the faintheaded, the rigid traditionalists and the seekers of the status quo. The survivors will be the courageous, the alert, the flexible, the imaginative,
the planners and the managers of change. With the continued support of its Members, lAO would like to be counted among the latter."
Ted Belton's comments on the status ofthe industry at the end of 1980, following all time high 1980/1981 loss ratios.

Given current flood loss trends, Mr. Belton could have been looking out 35 years to today when investment earnings are low (again) in this low interest environment, and when extreme rainfall losses are on the rise. Imaginative change is again needed to develop a flood underwriting strategy.