CityFloodMap.com overland flood risk analysis across the City of Toronto has been used in ESRI's ArcScene software to visualize at risk neigbourhoods and vulnerability zones where the correlation of urban overland / pluvial flooding and surcharged sanitary sewers resulting in basement flooding has been established. Flood risks exist beyond large regulated valley systems with a continuum of risk existing from river flood plain to individual building floor drain.
Additional visualization below includes identification of building structures within the estimated overland flood risk zone.
Showing posts with label overland flood. Show all posts
Showing posts with label overland flood. Show all posts
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
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:
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| Newtonbrook area overland flooding in urbanized areas and historical flood reports (May 2000, August 2005, July 2013). |
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:Social media geotagged surface flooding pictures like the one below were found to corresponds to the mapped overland flow path in some areas:
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.
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.
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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:
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.
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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:
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.
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 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!
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) :
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| Newtonbrook development 1947 - 1969, former North York, City of Toronto |
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):
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| 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:
Lost River Walks Toronto - Yesterday's Rivers Are Tomorrow's Flooded Basements
Source http://www.lostrivers.ca/
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| www.lostrivers.ca |
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.
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| 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.
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| 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. |
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| Perspective of Walmsley Brook from outlet to Don River. |
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.
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| 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 |
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| 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. |
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| Bathurst Heights reach of Yellow Creek. Map source Lost Rivers. |
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.
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| 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. |
Link to More Toronto Lost Rivers
Insurers need to keep the foot on the gas on overland flood
Insurance Business Canada reports "Insurers need to keep the foot on the gas on overland flood"
http://www.insurancebusiness.ca/expert-advice/insurers-need-to-keep-the-foot-on-the-gas-on-overland-flood-198497.aspx
Below is our comment on the article, slightly edited and with some added graphics and links.
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Not only is sewer back-up and urban overland flood difficult to explain to consumers as noted in the article, it is also difficult to separate in terms of the reality that urban overland risks drive sewer back-up risks at a neighbourhood level. This analysis shows the overlap of these perils:http://www.cityfloodmap.com/2015/08/toronto-overland-flow-factors-affect.html
Analysis is of historical flooding in Toronto in 2000, 2005 and 2013 and shows the correlation of overland risk factors (proximity to overland flow path and catchment slope) and observed basement flood reports during extreme events. From a physics, hydrology, hydraulics perspective there should be no surprise that the perils are related because the runoff accumulation that drives wastewater system extraneous wet weather inflows (and surcharges and backs up sewers into basements) is the same runoff accumulation that defines overland flow into a window well or reverse slope driveway or walkout. Raindrops do not know how they fit into insurance policy endorsements.
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| Overland flow system 1922 - former Walmsley Brook in Leaside, Toronto |
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| Overland flow system today - reduced flow capacity / encroachment. |
The Toronto back-up and overland risk correlation suggests bundling of coverage should be mandatory in some areas with high overland risk. Why? Because insurers will be covering back-up damages anyway by insuring the neighbourhood's overland risk.
Municipalities must keep a foot on the gas on infrastructure improvements that increase the underground system capacities (the sewer systems that handle the 'small' storms). But they must also look in the rear view mirror and assess the causes of flooding and revisit their overland drainage system management - that is map it, enforce drainage easements, prevent infill that obstructs flow paths and avoid the repeat of enclosures that have caused issues in the past. This approach and the example above (Leaside, Toronto, former Walmsley Brook) are shown in our presentation Urban Flood Risk from Flood Plains to Floor Drains.
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| Causes of basement flooding. Toronto Area 32 cluster area 1. No aging infrastructure is noted in the causes of flooding. |
Statements in the Insurance Business Canada article about aging infrastructure contributing to flooding are becoming tired because an old sewer doesn't lose much capacity, and even a displaced joint doesn't disrupt flow such either. Toronto has completed 30+ detailed basement flood studies and none have pointed to aging infrastructure as a key cause of flooding.
Yes, homeowners with clogged laterals have a problem but this is not an issue for most mainline municipal sewers. The discussion must be refocused on the key factors affecting risk - if an aging concrete pipe becomes slightly rougher over time due to abrasion, this in no way compares to having the overland flow path filled in and blocked and lost forever, from a system capacity point of view. Municipalities should prioritize efforts and embrace low cost programs that can reduce flood risks through regulations and policy (preserve overland flow paths) to complement more expensive, long-term capital upgrades to sewer systems. Ironically, many sewers will good remaining service life (even 50 years to go) are being replaced to increase flood capacity now - not because of aging but because pre-1980's design standards did not preserve the overland flow path and now the barely-aged sewer is being upsized to compensate.
Likewise, statements on changing climate and 'excalating extreme weather' are also becoming tired and diverting attention from real causes and solutions. Here are some facts on inaccurate statements on escalating weather: http://www.cityfloodmap.com/2015/07/storm-intensity-not-increasing-old.html
While that is convenient, it moves us from an evidence-based approach to defining and solving problems (see recent post on the how Nobel Memorial Prize in Economics winner Daniel Kahneman would explain biased thinking about weather statistics and events).
Environment Canada's data does not show increasing trends in rain intensity - its a fact - here is an update with the latest data set and it echos statements by Environment Canada in 2011: http://www.cityfloodmap.com/2015/10/bogus-statements-on-storms-in-cbcnewsca.html
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Spinach is not high in iron (sorry Popeye, it was a misplaced decimal point by a German chemist) - but a repeated story becomes fact in the mind of the public. So lets stay focused on facts and not anecdotes and continue to 'kick the tires' on assumptions when it comes to flooding... just like we would with a new Volkswagon.
Low Insurance Industry Investment Returns Push Premiums Higher
It is common (convenient?) for the insurance industry to explain increased premiums due to storm frequency. In response to our complaint on inaccurate reporting from an insurance broker's newsletter, the CBC has acknowledged "Environment Canada verified that there has been no significant change in rainfall events over several decades" in a letter to us. People are catching on that we cannot just Blame it on the Rain.
So what are the real factors pushing damages and premiums higher? CityFloodMap.Com explores many factors that increase damages and add cost pressures in our letter the the Minister of the Environment and Climate Change. But there are factors beyond the physically obvious watershed hydrology and infrastructure hydraulics.
Macro economic factors affecting insurance industry investments are the unspoken factor that puts upward pressure on premiums for property and casualty insurers in Canada. Details are in Facts of the Property and Casualty Insurance Industry in Canada 2015 is published by Insurance Bureau of Canada (IBC).
Previously, insurance company investments earned double digit returns that subsidized underwriting losses. Today, those same investments earn low single digit returns and so now underwriting has to carry its own weight - this means that premiums from underwriting have to increase to compensate for lagging investment returns. This is an excerpt from the report (page 13) to explain the impact of low interest rates on return on investment:
"Return on equity comes from two revenue streams –
underwriting and investment earnings.
In 2013, underwriting posted gains for the 11th consecutive
year. The 2013 net underwriting revenue was $648 million.
Before 2003, underwriting posted losses for 24 years in a row.
On investment, 2013 was a year of relatively low returns of 3.1%.
Return on investment moves in lockstep with the yields for
3- and 5-year Government of Canada bonds, which have fallen
for the last two decades."
So what are the real factors pushing damages and premiums higher? CityFloodMap.Com explores many factors that increase damages and add cost pressures in our letter the the Minister of the Environment and Climate Change. But there are factors beyond the physically obvious watershed hydrology and infrastructure hydraulics.
Macro economic factors affecting insurance industry investments are the unspoken factor that puts upward pressure on premiums for property and casualty insurers in Canada. Details are in Facts of the Property and Casualty Insurance Industry in Canada 2015 is published by Insurance Bureau of Canada (IBC).Previously, insurance company investments earned double digit returns that subsidized underwriting losses. Today, those same investments earn low single digit returns and so now underwriting has to carry its own weight - this means that premiums from underwriting have to increase to compensate for lagging investment returns. This is an excerpt from the report (page 13) to explain the impact of low interest rates on return on investment:
"Return on equity comes from two revenue streams –
underwriting and investment earnings.
In 2013, underwriting posted gains for the 11th consecutive
year. The 2013 net underwriting revenue was $648 million.
Before 2003, underwriting posted losses for 24 years in a row.
On investment, 2013 was a year of relatively low returns of 3.1%.
Return on investment moves in lockstep with the yields for
3- and 5-year Government of Canada bonds, which have fallen
for the last two decades."
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| Sliding investment returns means underwriting has to carry its own weight on insurance company balance sheets. |
RSA Fresh Water Flood Coverage Acknowledges "Concurrent Causation" in Flood Damage Claims
An article in candianunderwriters.ca describes the new RSA fresh water endorsement expected to be available to the large majority of RSA customers. It is encouraging to see the recognition that there has been 'concurrent causation' for flood damages:
"Some of the challenge we had with the floods in Alberta and in Toronto was this concurrent causation issue, where you had sewer backup happening at the same time as you had clean water flood coming in and you can't really say 'Part of it is brown water, part of it is clear water,'" the RSA representative said.
This represents progress in terms of characterizing the complexity of flood risks, but what is missing from the discussion (for Toronto table land type flooding as opposed to Alberta riverine flooding) is that:
Comment 1) Sewer backup water during severe storms is often brown and clear together, because during extreme storms, the extraneous inflows to the sanitary sewer system dilute the typical brown water, typically by a factor of 10 to 1. For infrastructure managers, the clear water components is often called RDII, or rainfall dependent inflow and infiltration - this clean water component can peak at up to 5 L/s/ha in a partially separated sewer system during an extreme event. Because of this, separating clear water and brown water risks is always difficult to do in practice.
Comment 2) The clear and brown water coming in typically occurs at different locations in the neighbourhood's drainage system. Low-lying areas with low exposed openings can allow clear water to enter buildings and then exits via floor drains. Those extraneous flows enter the sanitary sewer system and cause brown+clear water to enter downstream properties via floor drains. Because of this, a downstream property may be vulnerable to brown water backup, because of an upstream property's clear water risk.
RSA indicated that regarding the old sewer backup coverage:
"In the past, that endorsement was much broader and in some respects would cover flood not intentionally but did cover flood."
Comment 3) What this means is that for some insurers, the backup premiums may have been increasing to pay for clear water, overland flood damages. In my case, for example with a different insurer, the premium has increased 800% and the coverage limit has decreased for backup insurance - this could be explained by unintentional coverage of clear water, overland flooding in my city / neighbourhood. In an ideal world, RSA could decrease backup premiums by limiting coverage of uninsured perils like clear water, overland flooding, while pricing clear water flooding separately. .
Because of Comment 1 and 2, it will difficult to separate the different risk coverage
Just like cable TV, you can now pick and pay for your flood peril coverage:
"If you get Waterproof you get sewer backup and the flood, but if you don't want the Waterproof endorsement, if you don't want the flood, you can just buy limited sewer backup," RSA adds, saying that they are "putting a lot of emphasis" on training brokers.
Comment 4) This approach may meet RSA goals of limiting coverage for uninsured perils and increasing premiums for flood to help cover increasing costs during extreme events. In the end, the bottom line for the business is not dependent on having an optimal, precise vulnerability assessment for individual properties - all it needs is a sub-optimal net positive outcome on the portfolio. RSA is making an incremental adjustment toward a more sustainable, better priced risk model with the changes.
Not all properties are eligible for flood coverage though:
RSA noted that for high risk properties, it would be difficult to cover fresh water flooding "because there is almost a guarantee that something is going to happen," she added.
Comment 5) High risk properties can include those in a defined river floodplains. It is reasonable approach that coverage is not available and instead land use policies should deter such high risk developments over time. What is missing in this approach is that governments and their agencies (Conservation Authorities in Ontario) and insurance companies are not managing all risks in a complete and comprehensive way. Will RSA risk zones be shared with municipalities, or with property owners to support land use planning policies or flood proofing efforts by individuals or businesses? Not likely. Or will the sole purpose of the risk maps be to make decisions on the portfolio for the benefit of shareholders - that is likely - there is nothing wrong with that, but if so, let's take a step back and consider what the insurance industry's role really is in terms of managing society's flood risks. Are the efforts by RSA to define flood risk a benefit to society or the balance sheet? Does it make sense that individual insurers redo overland flood risk assessments independently from each other, or should a public flood underwriter make those assessments to be shared by the insurance industry and municipalities? In that manner, there would be no gaps in coverage and risk mitigation strategy.
Comment 6) What is needed to enhance the RSA sub-optimal, incremental improvement to risk pricing for flooding during extreme storms? The answer is a more robust, neighbourhood scale vulnerability model and a comprehensive approach to riverine flood risk. This could result in and mandatory coverage for highest risk overland flood properties as one outcome. Similarly, premiums for backup coverage could be increased in neighbourhoods with high overland flood risk, because clear water entering a few properties and overwhelm the sanitary sewer system in a widespread area. Our research shows how overland flood and backup (basement) flood risks are correlated in this post. Because risks are at a neighbourhood scale and policies are written at a property scale there is no way that the current insurance model can be optimal.
RSA notes in the article that "Canada doesn't have a lot of high risk zones with respect to flood."
Comment 7) In making this statement RSA should have distinguished between urban overland flash flooding and riverine flooding in a valley systems. Certainly a low percentage of properties are within vulnerable valley flood plain areas where rivers and creeks can swell. But on table land, urban flash flooding affects many neighbourhoods built before improved 1980's drainage standard improvements. These table land areas are where 'concurrent causation' really occurs.
Lastly RSA notes "That is going to be very clearly defined as what's covered and not covered"
Comment 8) Costs and revenue may be in a more sustainable balance with the RSA policy updates, but this may not necessarily be because policy coverage is more 'clear' in the future. As noted in the initial comments, there will generally always be some 'brown' mixed into the system when it comes to extreme storms.
"Some of the challenge we had with the floods in Alberta and in Toronto was this concurrent causation issue, where you had sewer backup happening at the same time as you had clean water flood coming in and you can't really say 'Part of it is brown water, part of it is clear water,'" the RSA representative said.
This represents progress in terms of characterizing the complexity of flood risks, but what is missing from the discussion (for Toronto table land type flooding as opposed to Alberta riverine flooding) is that:
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| Extraneous clear flow from groundwater infiltration and inflow points enters the 'brown' sanitary sewer. |
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| Clear water surface flooding enters doors and windows and exits low lying upstream properties via floor drain. |
RSA indicated that regarding the old sewer backup coverage:
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| Downstream sanitary sewer overwhelmed with brown and clear water creates backup, entering property via floor drain. |
Comment 3) What this means is that for some insurers, the backup premiums may have been increasing to pay for clear water, overland flood damages. In my case, for example with a different insurer, the premium has increased 800% and the coverage limit has decreased for backup insurance - this could be explained by unintentional coverage of clear water, overland flooding in my city / neighbourhood. In an ideal world, RSA could decrease backup premiums by limiting coverage of uninsured perils like clear water, overland flooding, while pricing clear water flooding separately. .
Because of Comment 1 and 2, it will difficult to separate the different risk coverage
Just like cable TV, you can now pick and pay for your flood peril coverage:
"If you get Waterproof you get sewer backup and the flood, but if you don't want the Waterproof endorsement, if you don't want the flood, you can just buy limited sewer backup," RSA adds, saying that they are "putting a lot of emphasis" on training brokers.
Comment 4) This approach may meet RSA goals of limiting coverage for uninsured perils and increasing premiums for flood to help cover increasing costs during extreme events. In the end, the bottom line for the business is not dependent on having an optimal, precise vulnerability assessment for individual properties - all it needs is a sub-optimal net positive outcome on the portfolio. RSA is making an incremental adjustment toward a more sustainable, better priced risk model with the changes.
Not all properties are eligible for flood coverage though:
RSA noted that for high risk properties, it would be difficult to cover fresh water flooding "because there is almost a guarantee that something is going to happen," she added.
Comment 5) High risk properties can include those in a defined river floodplains. It is reasonable approach that coverage is not available and instead land use policies should deter such high risk developments over time. What is missing in this approach is that governments and their agencies (Conservation Authorities in Ontario) and insurance companies are not managing all risks in a complete and comprehensive way. Will RSA risk zones be shared with municipalities, or with property owners to support land use planning policies or flood proofing efforts by individuals or businesses? Not likely. Or will the sole purpose of the risk maps be to make decisions on the portfolio for the benefit of shareholders - that is likely - there is nothing wrong with that, but if so, let's take a step back and consider what the insurance industry's role really is in terms of managing society's flood risks. Are the efforts by RSA to define flood risk a benefit to society or the balance sheet? Does it make sense that individual insurers redo overland flood risk assessments independently from each other, or should a public flood underwriter make those assessments to be shared by the insurance industry and municipalities? In that manner, there would be no gaps in coverage and risk mitigation strategy.
Comment 6) What is needed to enhance the RSA sub-optimal, incremental improvement to risk pricing for flooding during extreme storms? The answer is a more robust, neighbourhood scale vulnerability model and a comprehensive approach to riverine flood risk. This could result in and mandatory coverage for highest risk overland flood properties as one outcome. Similarly, premiums for backup coverage could be increased in neighbourhoods with high overland flood risk, because clear water entering a few properties and overwhelm the sanitary sewer system in a widespread area. Our research shows how overland flood and backup (basement) flood risks are correlated in this post. Because risks are at a neighbourhood scale and policies are written at a property scale there is no way that the current insurance model can be optimal.
RSA notes in the article that "Canada doesn't have a lot of high risk zones with respect to flood."
Comment 7) In making this statement RSA should have distinguished between urban overland flash flooding and riverine flooding in a valley systems. Certainly a low percentage of properties are within vulnerable valley flood plain areas where rivers and creeks can swell. But on table land, urban flash flooding affects many neighbourhoods built before improved 1980's drainage standard improvements. These table land areas are where 'concurrent causation' really occurs.
Lastly RSA notes "That is going to be very clearly defined as what's covered and not covered"
Comment 8) Costs and revenue may be in a more sustainable balance with the RSA policy updates, but this may not necessarily be because policy coverage is more 'clear' in the future. As noted in the initial comments, there will generally always be some 'brown' mixed into the system when it comes to extreme storms.
Overland Flood Factors Affect Basement Flood Incidents - Correlating Insurance Perils and Mapping Risk
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| Overland flow risk mapping and basement flood history highlights risks on table land, outside river flood plains and traditional hazard mapping areas. |
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.
Connecting the Dots on Climate Data - 100 year storms not increasing
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| Connecting the dots on climate change. Real dots. |
The graph above has some dots to connect as well - it is real data unlike the Environmental Commissioner of Ontario's infographic non-data, non-science, non-anything useful to anyone:
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| Connecting the dots on climate change. Fake dots .. fake like Jenny McCarthy anti-vaccine science. |
But the Environmental Commissioner of Ontario's infographic has something cyan increasing .. but there are no labels on the axes ... is it real data ... is it past trends? ... is it future predictions? ... is it useful at all to anyone? That yellow sign and exclamation point can't be good though! Actually this non-data is useless to anyone involved in urban flood mitigation, infrastructure planning and design, municipal design, floodplain mapping, insurance risk assessment, etc.
The most-reported anecdote of 100 year storms happening all the time is North York (north Toronto). This is often repeated by non-science commentators (but not practitioners). But still the plural of anecdote is not data, nor is it actionable. If North York did get eight 100 year storms in 12 years then someone should have checked the extreme value distribution when they started counting 100 year events, because pretty soon you have to shift your frequency curves up after a few storms, and stop under-reporting the underlying frequency distribution over and over and over.
Here is real North York data at Environment Canada station 615S001 (a composite data record station so its not really one place at all):
North York is a shorter record and so less reliable on trends than the longer term Toronto City station 6158355 records that had higher rainfall amounts in the 1940's to early 1960's. Yes there are some upward trends, but like a good scientist, statistician or engineer would do, look at the scale and consider the underlying variability / uncertainty in the data and you will conclude like Environment Canada has done already that there are no consistent or significant trends in rainfall frequency in Ontario or Canada.
The anecdotal North York 100 year storm epidemic is a convenient anecdote for former North York council (pre Toronto amaglamation) and their former planners and engineers, explaining the chronic flooding in a drainage system designed with no major, overland flow outlet, and that has been the topic of engineering study since the late 1980's. If this chronically flooded North York area with all those 100 year storms (i.e., Toronto's Class EA Basement Flooding Area 28) sounds familiar, is it because physically it is a lot like the south Stratford area along Brydges Street - Elgin Crescent (catchment B23), where the class action lawsuit against the City was won. That Stratford area also had back to back 100 year storms at the turn of the millennium. Here is how that Stratford area with all the storms was described in the Beacon Herald 10-years-after-stratfords-watershed-moment:
"On Elgin Crescent we've been dealing with the problem for 20 years," complained Tom Arnott, who suggested the city had shortchanged residents in the southeast end of town.
Frank McLaren said the city should be giving everyone $1,000 to clean up their basements. He told "The Beacon Herald" at the time he was facing a cost of about $35,000 for the third time in three years because of the sewer problem.
City curbside collection teams were busy working extra shifts to clean up debris. Phones were ringing at insurance company offices. And angry citizens who had warned before of the need to do something about the city's aging storm and sanitary sewers began organizing.
The initial outcome was a class action against the municipality that sought more than $200 million in compensation for alleged negligence. That later morphed into a class action spearheaded by Mike Mitchell of the Mitchell Monteith law firm (later Monteith Ritsma Phillips) on behalf of about 800 residents who had experienced losses.
Resolution did not come easily. It was almost eight years after the flood - May 4, 2010 - when an out-of-court deal was reached between the City of Stratford and representatives of the claimants.
So was it a rash of 100 year storms, happening more often, that affected Stratford and contributed to them settling the lawsuit? Or could it be that as the plaintiffs argued "the city knew about sewer problems and the potential for flooding, based on engineering reports, for three decades and did nothing about it."
Its funny how north Stratford, just on the other side of the Avon River, did not have chronic flooding when the Brydges-Elgin area did with their repeat 100 year storms. Could it be the overland drainage patterns are better north of the river and there are better slopes? Yes - as shown in the following paper (see Figures 4 and 7), the chronic flooding areas in Stratford were mostly explained by the topography, and the storms were a convenient excuse for chronic flooding issues:
Stratford drainage system review - American Water Resources Association - GIS Specialty Conference paper (Stratford Storm Master Plan, South Side Class EA)
The Environmental Commissioner of Ontario's report Connecting the Dots on Climate Data in Ontario does a great disservice to the discussion on rainfall trends and causes of flooding in Ontario. The quite worthless infographics and shunning of real actionable data on rainfall, and most disappointing, failure to explore any true causes of chronic flooding in Ontario municipalities, shows that the Commissioner's office is not up to the task of contributing to the important dialogue facing Ontario when it comes to addressing increasing flood damages and effective mitigation.
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Speaking of Straford : "Some are born great, some achieve greatness, and some have greatness thrust upon them." ... Shakespeare wrote that, but I think this could equally apply to "foolishness", i.e., some are born fools, some achieve foolishness, and others have it thrust upon them. If you find value in the Environmental Commissioner of Ontario's report Connecting the Dots on Climate Data in Ontario, then you may just fall into the last category.
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Now that Ontario has given us Connecting the Dots on Climate Data, what is next? Paint by Numbers on Health Care Reform, or Pin the Tail on The Energy Policy? Descartes gave us graphs to plot data in the 1600's - why can't the Ontario government use a few related to rainfall and present real data to build policy on?
Storm Intensity Not Increasing - Old Extremes are the New Normal - Engineering Data Review for Canada and Ontario
Storm intensity not increasing - factual review of engineering data - Canada and Ontario from Robert Muir
Storm Intensity Not Increasing. Review of Weather Event Statement in Insurance Bureau of Canada’s “Telling the Weather Story” prepared by Institute for Catastrophic Loss Reduction. Environment Canada analysis and engineering dataset review for Canada and Ontario, July, 2015. "Old extremes are the new normal".
As illustrated through these slides, Telling the Weather Story makes a statement on the increased frequency of storms and weather events, indicating that in parts of Canada, events that occurred every 40 years are occurring every 6 years, due to climate change.
The statement on increased frequency is unfounded as (based on ICLR's IPCC source and material and IBC's presentation to the Empire Club of Canada) it is based on a theoretical shift in temperature frequency from a global climate change report, and is contrary to Environment Canada’s actual analysis and data on local rainfall intensity trends.
The Telling the Weather Story statement on increased storm intensity, based on temperature theory has been i) embraced as rainfall facts and research by many organizations, ii) embellished to apply to extreme rainfall, and iii) considered in policy and economic reports. Organizations promoting the misinformation in the statement include TD Economics, The Toronto Star / thestar.com, CBC News, Canadian Chamber of Commerce, Columbia Institute Centre for Civic Governance, Civic Action, CBC Doc Zone, The Calgary Sun, CanadianUnderwriter.ca, Aviva Canada, Insurance Bureau of Canada.
Due to the unfounded Telling the Weather Story Weather Story statement, policies and efforts toward mitigating increasing urban flood damages may be misdirected to climate change mitigation, as opposed to more effective risk identification/management efforts, urban planning / stormwater management policies and infrastructure remediation / capital investment efforts that address the root causes of increased damages, not related to theoretical storm frequency shifts.
Storm Intensity Not Increasing. Review of Weather Event Statement in Insurance Bureau of Canada’s “Telling the Weather Story” prepared by Institute for Catastrophic Loss Reduction. Environment Canada analysis and engineering dataset review for Canada and Ontario, July, 2015. "Old extremes are the new normal".
As illustrated through these slides, Telling the Weather Story makes a statement on the increased frequency of storms and weather events, indicating that in parts of Canada, events that occurred every 40 years are occurring every 6 years, due to climate change.
The statement on increased frequency is unfounded as (based on ICLR's IPCC source and material and IBC's presentation to the Empire Club of Canada) it is based on a theoretical shift in temperature frequency from a global climate change report, and is contrary to Environment Canada’s actual analysis and data on local rainfall intensity trends.
The Telling the Weather Story statement on increased storm intensity, based on temperature theory has been i) embraced as rainfall facts and research by many organizations, ii) embellished to apply to extreme rainfall, and iii) considered in policy and economic reports. Organizations promoting the misinformation in the statement include TD Economics, The Toronto Star / thestar.com, CBC News, Canadian Chamber of Commerce, Columbia Institute Centre for Civic Governance, Civic Action, CBC Doc Zone, The Calgary Sun, CanadianUnderwriter.ca, Aviva Canada, Insurance Bureau of Canada.
Due to the unfounded Telling the Weather Story Weather Story statement, policies and efforts toward mitigating increasing urban flood damages may be misdirected to climate change mitigation, as opposed to more effective risk identification/management efforts, urban planning / stormwater management policies and infrastructure remediation / capital investment efforts that address the root causes of increased damages, not related to theoretical storm frequency shifts.
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| Long term Toronto rainfall intensities decreasing per Environment Canada data (5 minute to 24 hour storm durations). |
Overland Flood Risk - From Flood Plains to Foundation Drains
From foundation drain to floodplain there is a continuum of drainage features that affect flood risks and damages in urban areas - these are micro lot level factors and macro neighbourhood factors.
Yes, some property reference points could identify a few flood risk factors to address, but overland flooding risk is not apparent at the property scale - that is if we agree overland flooding is not a result of poor lot grading or obvious rain entry points. Unlike other risks like fire hazards, the structure or property does not define most of the risk when if comes to flooding. It will be interesting to see what methods Aviva has adopted for defining risk levels and overland flood endorsement premiums.
Aviva is excluding high risk properties (about 5% of properties) considering flood plain maps that identify river (and sometimes lake) flooding hazards. Micro-scale factors like the property's foundation drain and service lateral condition can affect almost any property, but issues with those do not cause overland / surface water flooding.
I'd suggest the most sensitive overland flooding occurs upstream of large floodplain-mapped valleys and up onto urbanized 'table land' where the sewer drainage system and overland drainage system are not designed to accommodate runoff from major rainfall events. Usually this would be in catchments draining a 125 hectare or smaller area in Ontario, lying beyond the regulated flood zones above where Aviva insurance would not be available. In these areas, neither the capacity of the storm sewer system, nor the adequacy of the overland drainage system (ideally on roadways and drainage easements) is apparent at the individual property scale.
The macro-scale, overland system is only apparent from runoff accumulation and concentration from 10's of hectares to even a couple hundred hectares of table land runoff that accumulates downstream. That is, a broad neighbourhood-scale macro factor representing many hundreds upstream of properties. The map of overland flow drainage areas (red text in yellow highlight) shows the continuum of drainage areas extending beyond regulated floodplains that flow into to smaller surface features that run through the urban lot fabric.
The key overland flood risk factor is whether the property is on the overland flow path where the drainage accumulates. Maybe a property is at a sag in a road where runoff could spill toward the property (i.e., it's vulnerable to overland flooding) but if there is no large upstream area contributing overland runoff toward the property, its not exposed to meaningful risk. But with a large overland drainage area from the neighbourhood flowing into the sag there is a risk to the property. And in that case, disconnected downspouts won't help reduce risk - downspouts drain a couple hundred square meters of rooftop, while large overland catchments can be a couple hundred hectares (meaning 10,000 times greater than the local roof area)
Neighbourhood scale risks factors for overland flooding can be assessed with some minor effort (relative to detailed pipe-by-pipe sewer and street-by-street hydraulic simulation models that is) by just considering topography from readily available elevations models and by applying core GIS hydrology tools.
The image to the left on 'major sags' is an example showing overland flooding risks including dwellings within poorly-drained sags. In this case it includes dwellings upstream of a railway embankment that can impede and 'back up' flows during large events.
The risk to dwellings and property within close proximity to overland flow paths can be ranked. For example a building within 3 m of a 10 hectare area runoff flow path, or 15 m from 100 hectare area flow path, would be at relatively low risk. Alternatively a dwellings within 3 m of a 100 ha flow path would be at a high risk. A building within both a flow path and in a sag would be at the highest risk. Remember, these are estimates and are qualitative. Nonetheless, experience shows that areas identified through these heuristic methods have be subject to overland flooding during extreme rainfall events.
The issue with overland flood insurance (urban, table-land type flooding) is that risks are concentrated with a small portion of downstream properties for whom risk-based premiums could be unaffordable, or for whom coverage would not be available. The majority of upstream properties would not likely ever experience overland flooding nor add coverage for it. So overland flood damages are not like wind damage (path of wind can be anywhere whereas path of water is always the lowest elevations).
At a property scale, a property with poor lot grading and a reverse slope driveway may be at high risk regardless of proximity to overland flow paths or major sags in topography. Such a property would benefit from having an overland flood endorsement in its water protection insurance. Ideally homeowners in this situation can take action to mitigate risks by improving grading, installing barriers to flow from the reverse drive, keeping grates clear of debris and perhaps installing a backflow valve on the driveway drain. Beyond these exceptions, anyone with foundation drains that could clog or a service lateral that could become root infested is at risk of flooding - but that would not be overland flooding, but rather back-up from the floor drain or seepage from the foundation wall and cracks (a different endorsement all together).
Check out new analysis of table land flood risks and insight into correlations with basement flooding / sewer back-up incidents : overland flow flood risk factors
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| River flood risks are defined by floodplain maps (called engineered floodplains) because of the detailed hydrologic analysis used to define flood flows and river and bridge surveys used to define river and valley hydraulics to define precise flood levels. Hazards in these zones are closely regulated, new development is not permitted, and redevelopment is subject to special policies if allowed. |
Yes, some property reference points could identify a few flood risk factors to address, but overland flooding risk is not apparent at the property scale - that is if we agree overland flooding is not a result of poor lot grading or obvious rain entry points. Unlike other risks like fire hazards, the structure or property does not define most of the risk when if comes to flooding. It will be interesting to see what methods Aviva has adopted for defining risk levels and overland flood endorsement premiums.
Aviva is excluding high risk properties (about 5% of properties) considering flood plain maps that identify river (and sometimes lake) flooding hazards. Micro-scale factors like the property's foundation drain and service lateral condition can affect almost any property, but issues with those do not cause overland / surface water flooding.
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| Overland flow areas in an urban setting. Areas beyond blue regulated floodplains may be several hundred hectares in size. |
The macro-scale, overland system is only apparent from runoff accumulation and concentration from 10's of hectares to even a couple hundred hectares of table land runoff that accumulates downstream. That is, a broad neighbourhood-scale macro factor representing many hundreds upstream of properties. The map of overland flow drainage areas (red text in yellow highlight) shows the continuum of drainage areas extending beyond regulated floodplains that flow into to smaller surface features that run through the urban lot fabric.
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| Engineered floodplain limits are determined through extensive engineering analysis of the valley system. More local overland flow limits may be determined through a local flood remediation study that analyzes the 'major system' of road drainage as if it is a river valley. That can be expensive. Alternatively the risk of being close to the overland flow path can be estimated by defining buffer distances from small and large runoff area flow paths. |
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| GIS Tools can identify major sags, or depressions, (called sinks) in the landscape that can indicate or amplify overland flood risk. |
Neighbourhood scale risks factors for overland flooding can be assessed with some minor effort (relative to detailed pipe-by-pipe sewer and street-by-street hydraulic simulation models that is) by just considering topography from readily available elevations models and by applying core GIS hydrology tools.
The image to the left on 'major sags' is an example showing overland flooding risks including dwellings within poorly-drained sags. In this case it includes dwellings upstream of a railway embankment that can impede and 'back up' flows during large events.
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| Ranked property risk considering proximity to overland flow path, drainage area, and major sags in topography (i.e., drainage-challenged during extreme runoff events). |
The issue with overland flood insurance (urban, table-land type flooding) is that risks are concentrated with a small portion of downstream properties for whom risk-based premiums could be unaffordable, or for whom coverage would not be available. The majority of upstream properties would not likely ever experience overland flooding nor add coverage for it. So overland flood damages are not like wind damage (path of wind can be anywhere whereas path of water is always the lowest elevations).
At a property scale, a property with poor lot grading and a reverse slope driveway may be at high risk regardless of proximity to overland flow paths or major sags in topography. Such a property would benefit from having an overland flood endorsement in its water protection insurance. Ideally homeowners in this situation can take action to mitigate risks by improving grading, installing barriers to flow from the reverse drive, keeping grates clear of debris and perhaps installing a backflow valve on the driveway drain. Beyond these exceptions, anyone with foundation drains that could clog or a service lateral that could become root infested is at risk of flooding - but that would not be overland flooding, but rather back-up from the floor drain or seepage from the foundation wall and cracks (a different endorsement all together).
Check out new analysis of table land flood risks and insight into correlations with basement flooding / sewer back-up incidents : overland flow flood risk factors
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