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 flash flood. Show all posts
Showing posts with label flash flood. Show all posts
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.
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.
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
Can Twitter Accurately Document Flooding Incidents?
Bright Planet has explored whether social media could "help in a flood", especially in the context of reviewing tweet data for insurance claims. Here's what they did in relation to the July 8, 2013 Toronto storm and flood:
"To assist with the vetting and triage of insurance claims, BrightPlanet harvested all tweets within the Greater Toronto Area. BrightPlanet then filtered and curated the tweets down to only tweets discussing specific effects of the flooding. Any tweets containing a latitude and longitude were then mapped in a heat map format in Toronto to show where the most chatter was happening about the flooding."
See the full description on their page here: BrightPlanet. The harvesed image is here:
There are a lot of tweets downtown, mostly south of Bloor Street. Matthew Dance analyzed tweets about Toronto - not about flooding, and produced this cluster map:
Of course the extents are different, but I'd suggest that we see the same pattern in BrightPlanet "flood tweets" as we do for Matthew Dance's "generic Toronto tweets". Dance explains tweet density in a few ways:
"To assist with the vetting and triage of insurance claims, BrightPlanet harvested all tweets within the Greater Toronto Area. BrightPlanet then filtered and curated the tweets down to only tweets discussing specific effects of the flooding. Any tweets containing a latitude and longitude were then mapped in a heat map format in Toronto to show where the most chatter was happening about the flooding."
See the full description on their page here: BrightPlanet. The harvesed image is here:
There are a lot of tweets downtown, mostly south of Bloor Street. Matthew Dance analyzed tweets about Toronto - not about flooding, and produced this cluster map:
Of course the extents are different, but I'd suggest that we see the same pattern in BrightPlanet "flood tweets" as we do for Matthew Dance's "generic Toronto tweets". Dance explains tweet density in a few ways:
- "The most densely Tweeted area is bounded by Bloor Street to the North and Lake Ontario to the South, connected by Young Street. There is a greater density along the Lake, away from Young to the West."
- Hot spots include the Eaton Center, Rogers Center and tourist and suburban destinations - the areas around Yonge and Bloor and Front Street, including the sports stadiums - "destinations for those interested in shopping or taking in the sights in Toronto."
- Areas that are strictly neighbourhoods (Hillsdale Avenue running east from Yonge) have low density.
- Poorer neighbourhoods like Regent Park have few tweets (between Dundas and Gerrard west of the Don Valley)
(see the site: Matthew Dance Toronto Tweets)
Where does the City of Toronto report flooding for July 8 2013? It does not match the BrightPlanet flood tweet locations very well as shown below:
Tweet-rich downtown had only 50 mm of rain and limited reported flooding, compared to 130 mm of rain in tweet-poor Etobicoke where most of the flooding was reported. Perhaps the BrightPlanet flood tweets should first be "normalized" by the density of generic non-flood tweets, to give a better representation of relative spatial flood activity?
It would also be worth checking if what people tweet more about downtown is spectacular surface flooding (e.g., a sewer geyser blowing a manhole 10 feet in the air, a Ferrari floating in an underpass lake, an amphibious GO Train) as opposed to basement flooding, which is the focus of City of Toronto flood complaints. Tourist and passersby have time to tweet about flooding, while a homeowner with sewage gurgling up their basement floor drain may have more pressing thing to do (save the family photos honey!).
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