Overland Flood Factors Affect Basement Flood Incidents - Correlating Insurance Perils and Mapping Risk

overland flood Toronto
Overland flow risk mapping and basement
flood history highlights risks on table land,
outside river flood plains and traditional hazard
mapping areas.
Flood is the new fire.

Blue is the new red ink, as water damage claims increase. Enough slogans though ... time for insights resulting from new overland risk mapping for the GTA!

Overland flooding had recently becoming an insured peril for residential property owners, offered by Aviva Canada.  Others are following suit.  So now is the perfect time to explore the differences in "surface flooding" types in an urban environment, to assess risk factors and even look for correlations with traditional insured perils, like sewer back-ups that cause basement flooding.  Understanding risk factors can support better decisions and help direct mitigation efforts.

First some set-up and definitions.

Media often reports the devastating flooding in Alberta and Toronto in 2013 in the same breath, but there are important distinctions to be made - High River's "riverine flooding" along the low-lying bottom of a defined valley or wide flood plain is quite different than "urban flooding" high up on the table land (i.e., many thousands of Toronto's flooded homes).  To put things in perspective, the Toronto's Don River has a drainage area of 360 sq.km contributing to valley flow, while a table land area subject to urban flooding may have as little as a 0.01 sq.km, or 10 hectare drainage area.

Riverine flooding is largely manageable - spatially fixed, quantifiable in terms of flow frequency and flood depth, and (in Ontario) regulated through the provincial policy statement on natural hazards, MNR guidelines, and conservation authority regulations.  In the past, flood plain mapping to define riverine flood risks was once the 'bread and butter' of engineering consultants, who started number-crunching U.S. Army Corps of Engineer's HEC-2 simulation models back using punch cards - yes, that long ago.  The limitation is that only a few percent of Ontarians live in flood plains according to Conservation Ontario.  And based on new analysis from cityfloodmap.com, only about 2% of May 12, 2000, August 19, 2005, and July 8, 2013 Toronto flooding occurred in river flood vulnerable areas!

Urban flooding, in contrast, affects a greater proportion of the population - 86% of Ontarians live in urban areas per the 2011 census - but this type of flooding is not as readily managed.  In fact, risk factors equivalent to flood plain maps are seldom ever mapped.  This is because urban flooding is transient and overland flow paths are not permanent physical features that are part of i) a defined valley, or ii) municipal drainage infrastructure.  Overland flow paths appear for a few minutes or hours every few decades and then subside.  So calling this 'flash flooding' is appropriate - engineering studies show severe urban flooding events are most highly correlated with high short-term rainfall intensities over minutes.  Hours later, the overland flow is gone.

To prevent urban flooding, engineers have been designing the overland drainage network though subdivision grading since the early 1980's when "dual drainage design" emerged in Ontario.  This approach recognized the limited capacity of the minor system (underground sewer system) for conveniently conveying small storm runoff, and the need to design the major system (above-ground overland system) for extreme storms.  In pre-1980 development, the major system still exists but it can run haphazardly beyond roads, through back yards, between houses and into basement windows and walkouts. And this critical, non-designed, transient overland flood risk has not ever been mapped - ta da! - until now.

New analysis from cityfloodmap.com in the slides below ties overland flood risks all together, reviewing flood incidents across regulated valley hazard areas and river flood vulnerable areas (riverine flooding), and within overland flow risk areas across Toronto table lands (urban flooding).  It provides insight into risk factors affecting overland flooding and direction on de-risk opportunities.  Most importantly, it demonstrates the correlation between traditional sewer back-up peril and the emerging overland peril, which should cause some insurance providers to evaluate portfolio risk (where should coverage be coupled or excluded in isolation, recognizing the correlations).

If flood is the new fire for the insurance industry, this new analysis, which demonstrates neighbourhood scale flood risk factors due to overland and catchment slope risks, should also prompt discussion on how flood risks are assessed, priced and prioritized for mitigation, adaptation and insurance purposes. Just as fire underwriters identify neighbourhood scale risk factors and consider fire suppression capabilities of municipalities, flood risk assessors should consider the overland risk factors, that extend well beyond the extent of individual properties. Determining risks should not only consider whether an individual property has a reverse slope driveway when broader overland flood risk factors can be readily quantified.


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