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.

Nobody?

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:

http://www.fireunderwriters.ca/theunderwriters_e.asp

"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.

News Release - Ontario Canada Climate Change Partnership