Economics of Flood Damage Claims - Large Events Dominate Overall Losses - Should Effective Mitigation Strategies Focus On "The Big Ones"

What type of meteorologic events dominate flood damage claims? Is it from many frequent small storms or a few infrequent black-swan events? Understanding what size of events cause the most losses can help us focus on the most effective flood loss mitigation measures - this is essential for achieving high returns on investment in flood mitigation strategies. By reducing flood losses in an economically efficient manner, high benefit-cost ratios can be achieved.

This post summarizes the distribution of FEMA's flood damage claims and explores what type of storm events - big or small - govern extreme weather losses.

FEMA summarizes the number of payout claims and the total value of payouts for significant flood events, i.e., those with 1500 or more payouts. The total value of payouts, adjusted for inflation to 2018 dollars, is plotted below against the total number of claims in the events between 1978 and 2017:

FEMA Average Claim Amount for Various Event Sizes (Number of Claims) - Small Significant Flood Events with a Minimum of 1500 Claims Per Event

The median number of payouts is 4115 with a median payout amount of $29,700, adjusted to 2018 dollars. This is over a total of 116 flood events from 1978 to 2017.

Larger storm result in more extensive flood damages and numbers of payout claims as shown on the following chart that labels some of the largest tropical storm / hurricane events:
FEMA Average Claim Amount for Various Event Sizes (Number of Claims) - Small and Large Significant Flood Events with a Minimum of 1500 Claims Per Event
How do the larger events affect the flood damage and payout values? The average flood claim payout of $36,200 is above the median value reflecting the skew in catastrophic event distribution - the right tail of rare black-swan events in the probability distribution of events pulls the average above the median.

Five of the 116 event have claim counts that are over ten to forty times the median number of claims. That is, Hurricane Ike and Irene had over 40,000 claims compared to the median count of just over 4000 claims. And Hurricane Harvey had over 160,000 claims. The losses are greater for these larger events with the best-fit line showing average claim values of over $50,000 to over $120,000 for these largest significant events. What effect do these claim counts have on weighted claim amount - they increase the claim-count-weighted average loss to $60,600 - more than double the median claim amount per event that is not weighted by the number of claims in each event.

So when looking at the economic losses associated with a significant flood event, we need to consider the size of the event. And when we develop strategies and best practices for flood resilient communities and flood risk mitigation, striving for significant damage reduction and return on investment in averted flood damage losses, we must also consider what events cause the most damages. Canada's Disaster Mitigation and Adaptation Fund (DMAF), for example, requires return on investment (ROI) evaluations for eligible risk reduction projects. It would appear that to achieve meaningful flood damage reduction ROI we must target solutions toward events leading to the most damages

Looking at FEMA's significant flood events, data show that 3 of 116 events account for 54% of the total inflation-adjusted damages. Those 3 events are Hurricane Harvey, Superstorm Sandy and Hurricane Katrina. And the top 20 events, each with total event claims of over $500M, account for 81% of the total claims. So it is clear that to reduce the bulk of flood damages we have to consider how to increase resiliency in existing communities during the largest storm events. If we target flood risk reduction for the small catastrophic events, the smaller 97 events, we will be addressing only 20% of the total claim value. So the 80/20 rule, the Pareto principle, does apply to flood damage reduction.

FEMA Inflation Adjusted Significant Flood Event Payout Distribution - Pareto Distribution and the 80/20 Rule

The impact on a few recent large events on damages helps show sample-bias in catastrophic event losses as explored in a previous post. That is, up to 2004 prior to Hurricane Katrina, the distribution of losses based on the 1978-2004 sample of events did not consider the true 'population' distribution of flood events that includes very extreme, right-tail events. As Fleming demonstrated in "Yep, We're Skewed", short samples with high skew underestimate losses of the true population of events.


For statistics geeks:

Could it be that the common chorus of explaining recent floods losses as being due to climate change may in fact be explained simply by statistics and larger sample sizes overcoming short sample biases (underestimation)?

Could it be that growth in high risk areas is driving flood damages higher? AON Benfield's review of Hurricane Harvey suggests that growth in at-risk areas explains some flood impacts:

 "Given the volume of water, local infrastructure across southeast Texas was simply unable to handle such an enormous amount of rainfall in a short amount of time. This led to major water run-off that quickly accumulated across a very large area. With so much residential and commercial growth throughout this part of the state – combined with abundant concrete and poor absorbing clay soil –this only worsened the flood impact."

Solutions to flood risk mitigation therefore cannot only be local infrastructure solutions to convey enormous amounts of water but rather land use planning policies to direct development and redevelopment away from high flood risk areas. As AON Benfield notes " Hurricane Harvey’s rainfall reached the 1,000-year rainfall return period based on many time intervals during the course of a number of hours and days.", and it is not cost effective, or technically feasible, to have local infrastructure convey the runoff from events of this magnitude.


Canada Connection (for those appreciate tree-sauce, skatey-punchy, and noble antler cows):

CatIQ claim datasets have been used to evaluate flood damages across Canada according to the size of the flood event (i.e., related to the number of claims). A similar pattern of increasing damages with increasing event size and distribution is apparent in the CatIQ datasets. In contrast to the FEMA claims noted above for many hurricane events, the CatIQ data reflect basement flooding claims primarily, as overland flooding has not been insured in the past and is not widely held. What is the magnitude of these Canadian claims? Aviva Canada provided this summary of claim trends and magnitude:

"In 2014, water damage claims accounted for 44% of dollars paid out on all Aviva Canada property damage claims, compared with 39% in 2004. The average cost per residential water damage claim has increased significantly – going from $11,709 in 2004 to $16,070 in 2014, a 37% increase."

So basement flooding damages are significantly less than FEMA's large scale catastrophe claims. CatIQ data shows that for larger events (those with higher claim counts) the average claim amount does increase above the Aviva Canada values noted above. Comprehensive benefit-cost analysis used to develop ROI rankings for flood mitigation projects would apply the lower range of typical damages to frequent to moderate events and the higher damages/claim amounts to the frequent events, factored by their probabilities. The most frequent storms, typically 5 to 10 year return period events as in a recent study by Atkins for the US EPA do not generate damages.

Watershed-Scale Flood Damage Reduction Using LID BMPs - Does Green Infrastructure for New Development and Redevelopment Significantly Reduce Existing River Flood Risks

River Flood Loss Avoidance (Deferred Damages) with
Green Infrastructure / Low Impact Development BMPs
in New Development and Redevelopment in the US.
A study by Atkins for the U.S. EPA evaluates flood damage reduction across North America using Low Impact Development (LID) Stormwater Best Management Practices (BMPs). The 2015 report "Flood Loss Avoidance Benefits of Green Infrastructure (GI) for Stormwater Management" is available at this link and evaluates benefits of implementing LID BMPs in new development and redevelopment over 20 years.

Flood Losses Avoided in the Year 2040 for Various Zero Damage Thresholds
What is the value of estimated flood damage reduction from 2020 to 2040 by constructing GI / LIDs? Savings range from $63M to $136M per year in 2040, depending on whether river flood damages are assumed to start above 10 year events or more frequently above 5 year events.

Over the 20 year implementation period the deferred flood losses increase from $0 in 2020 to an average of $100 million (2011 dollars) for 5 and 10 year zero damage thresholds.

Deferred Flood Losses with LID BMP Implementation
for Stormwater Management in New Developments
After the first 10 year s of implementation the flood losses avoided averages approximately $50 million. The table to the right indicates the increase in flood reduction benefits over the green infrastructure implementation period.

Key question: is the losses avoidance significant and is there value in implementing green infrastructure to achieve river flood damage reduction?

To explore whether deferred damages are significant, lets compare the average loss reduction over the 20 year implementation with average losses in North America. Munich RE's NatCatService estimates losses in North America for meteorologic events and hydrological events. The chart at right shows that over the past 10 year from 2008 to 2017 the average losses per year we approximately $75 billion in 2017 dollars. (we could net out non-US areas like Canada to make this more apples to apples).

Munich RE Catastrophic and Relevant Event Overall Losses
Inflation Adjusted and Normalized 
The deferred flood losses over the 20 year period with LID implementation are $50 million in 2011 dollars - according to the Bureau of Labor Statistics consumer price index, the dollar experienced an average inflation rate of 1.42% per year. Therefore the annual LID loss reduction amount in 2017 is 8.8% higher than deferred losses in 2011, or $54 million (2017 USD).

Deferred annual flood losses of $54 million represents only 0.07 % out of $75 billion in overall annual losses. This suggests that near-term river flood damage reduction is not a core benefit of green infrastructure implementation.

The cost of expanded green infrastructure implementation is not assessed in the Atkins study as noted here:

"The costs of GI implementation are not included in this document. Nevertheless, new development
and redevelopment already require stormwater management expenditures, either on-site or
downstream; therefore, GI could be used to meet those requirements fully or partially for little or no
additional cost compared to overall construction costs. This study does not assume retrofitting of
existing imperviousness. Retrofitting, in addition to implementation on new development and
redevelopment, would be expected to generate more flood loss avoidance benefits but would incur
additional costs."

Given that green infrastructure stormwater controls are assumed to be included in the base cost of development or redevelopment, the micro-sized river flood loss reduction benefit comes at no addition cost, which could indicate good 'value' - however given the almost insignificant percentage of overall flood damages averted, one could question whether river flood risk reduction should be prescribed as a low impact development benefit at all. It would appear that core benefits of green infrastructure are instead the environmental ones related to water quality improvements and erosion risk reduction.

Today it is popular to promote green infrastructure citing its multi-faceted triple-bottom-line (TBL) benefits on many aspects of the environment. It would appear that riverine flood risk reduction is not one of those benefits to consider in the TBL assessment. Many groups and Canadian municipalities have noted impacts of green infrastructure on existing utilities and foundations as a dis-benefit / adverse impact of green infrastructure measures that typically infiltrate runoff into the ground. Many of these negative impacts were recently summarized in the Ontario Society of Professional Engineers' (OSPE) comments on Ontario's draft Watershed Planning Guidance which had promoted the role of green infrastructure for flood control :

The OSPE comments note:

"While green infrastructure has recognized roles in achieving watershed outcomes, including
water balance and water quality management in greenfield developments, the above statement
is inconsistent with numerous studies that discount the flood-control benefits of green
infrastructure. Numerous studies have demonstrated that green infrastructure does not provide
a flood risk reduction benefit."

To support this statement, OSPE cited numerous Master Plans, Master Drainage Plans, Class Environmental Assessment studies, Best Practice documents, local university research, and municipality and water industry comments on the Ontario Ministry of the Environment and Climate Change's draft Low Impact Development guideline.


Are LIDs Financially Sustainable in Ontario? Philadelphia Green Infrastructure Costs - 1100 Low Impact Development Projects Define Implementation Funding for Long Term CSO & Water Quality Improvement - Comparison with 24 Ontario Projects

Philadelphia Green Stormwater Infrastructure Projects Map - Over 1100
Low Impact Development Projects for CSO Control
Philadelphia has an extensive green infrastructure retrofit program with cost information - recent Ontario low impact development project costs show comparable unit cost for implementation.


The City of Philadelphia implements green infrastructure (GI), aka low impact development (LID) best practices (BMPs), to control combined sewer overflows (CSOs).  Having implemented 1100 features in a retrofit setting, Philadelphia has a clear understanding of retrofit implementation costs. The following is a summary of their green infrastructure design construction costs provided by the city program staff:

City of Philadelphia Green Infrastructure / Low Impact Development Best Management Practices - Construction, Design and Planning Budgets Per Total and Impervious Area

Construction Cost
- $175,000 per acre ($432,000 per hectare)
Philadelphia Green Infrastructure Map by SWP / LID Type 
- $270,000 per impervious acre ($667,000 per hectare)

Design Cost
- Design fees typically 20-25% of construction costs

Total Cost (Design & Construction)
Philadelphia Green Infrastructure Map - Spatial Location
of Low Impact Development Measure
- Total costs of $230,000 per acre ($568,000 per hectare)
- Total costs of $350,000 per impervious acre ($865,000 per hectare)

-  $350,000 per impervious acre ($865,000 per hectare) is the overall target/budget cost that is achieved for the program and that does not include contingencies that could be carried for individual projects within the program.
- If estimated costs exceed $400,000 per acre ($988,000 per hectare) based on design estimates and project cannot be re-scoped, it is deemed too expensive and does not go ahead.

In Ontario, green infrastructure has been promoted for stormwater management in new developments since the Ministry of Environment's 1991 Interim Guidelines. Green infrastructure measures were promoted as part of a 'source control' approach and features that promoted infiltration were called Best Management Practices (BMPs). Since then, Ontario cities have developed design targets for achieving specific water resources management goals and have implemented LID BMP measures in appropriate locations. In the City of Markham and York Region, his history was summarized in a National Water and Wastewater Benchmarking Initiative Stormwater Task Force presentation:

The presentation above summarized LID implementation costs for nine (9) recent Ontario projects including bioswales, bioretention, infiltration galleries and permeable pavement. Theses cost are receiving close attention as LID implementation targets in some regions have been increased, e.g., through the Lake Simcoe Protection Act to meet environmental protection / phosphorus reduction goals, and as generic province-wide targets are now being evaluated by the Ministry of Environment and Climate Change.

Additional Ontario LID project implementation costs have been compiled with information shared by Ontario municipalities and also the Lake Simcoe Regional Conservation Authorit. This expands/updates the project costs in slide 17 of the above presentation. These costs include construction, design, administration and in-kind staffing efforts related to implementation of LID projects in the City of Markham (2 projects), City of Brampton (1 project) Town of Whitchurch-Stouffville (1 project), City of Ottawa (2 projects), Town of Ajax (1 project), City of Mississauga (3 projects), Town of Newmarket (2 projects), City of London (7 projects), Town of East Gwillimbury (1 project), Town of Uxbridge (1 project), Town of Aurora (1 project), Town of Innisfil (1 project).

The project costs and unit costs per total catchment are are shown below:

green infrastructure construction cost Ontario low impact development implementation cost retrofit
Ontario Green Infrastructure / Low Impact Development Best Management Practice Implementation Costs (No Adjustment for Inflation to 2018 Dollars) - Normalized Unit Costs Per Catchment Area Managed
This is a link to the above compiled Ontario LID costs (let me know if you have projects to add or can suggest edits / updates): Excel - Ontario Low Impact Development BMP / Green Infrastructure Implementation Cost Summary - 24 Projects

The average cost per hectare of $575,000 for these 24 projects is very close to the City of Philadelphia budget cost of $568,000. Cost per impervious hectare treated by the LID BMP would typically be higher (i.e., catchment is less that 100% impervious). Some notes regarding the project costs:

- complete costs are not available for some projects (e.g., Markham Green Road bioswale vegetation)
- one service area has been adjusted based on different sources (e.g., East Gwillimbury area reflect municipality's project brief and not original TRIECA 2017 presentation value).
- one projects has only tender cost estimate available, not actual construction cost (e.g., Newmarket Forest Glenn Rd)
- one project from LSRCA was not included in the list as it did not proceed to construction, but nonetheless incurred design and administration costs (e.g., City of Barrie, Annadale Recreation Centre, design/administration/geotechnical/in-kind staff cost of over $78,000) - this may reflect go/no go decisions on implementation that the others also consider
- most projects are retrofits, however some are new builds (Markham Green Road, Innisfil Fire Station)
- bioswales/enhanced swales require review given the wide range in unit costs per hectare of $51,000 (Uxbridge) to nearly $1.9M (Newmarket), with obvious sensitivity to the drainage area served

Previous cost estimates cited on this blog considered unit costs of approximately $400,000 per hectare and significant concern regarding the financial viability of any widespread implementation across Ontario's 852,000 urbanized hectares. Considering the expanded project cost review and adjusting for inflation, today's Ontario green infrastructure implementation costs can be estimated to be in the order of $600,000 per hectare. This magnitude of cost is comparable to Philadelphia's budgeting cost, considering over 1100 projects. These costs support the concern related to emerging Ontario policies that have not considered implementation cost impacts or financial viability.

The Ontario Society of Professional Engineers (OSPE) has recently highlighted concerns with the implementation of green infrastructure in Ontario in comments on Ontario's Long-Term Infrastructure Plan (my bold emphasis on the recommendations)

"....OSPE recommends that the Government of Ontario:

i. Critically apply the proposed ‘risk lens’ to infrastructure investments related to extreme
weather adaptation, recognizing variations in observed and predicted trends across the

ii. Evaluate adaptation measures such as green infrastructure for stormwater management,
often cited as key mitigation measure, using the same ‘risk lens’ and consider the cost-
effectiveness of those infrastructure investments.

iii. Recognize that green infrastructure must be viewed through the same lens as
conventional infrastructure, adhering to established asset management principles and
full cost accounting—meaning it must be addressed up-front and directly, considering
system-wide costs."

OSPE has also commented on the limited role of green infrastructure for flood control and life cycle cost concerns in response to Ontario's draft Watershed Planning Guidance.


Green infrastructure LID implementation costs should be acknowledged to be potentially higher
than conventional grey infrastructure design, particularly for retrofits, and funding for additional
incremental retrofit costs should be considered in the comprehensive evaluation of alternative
management solutions beside green infrastructure and LIDs, including enhanced conventional
grey infrastructure designs with pollution prevention activities. Higher retrofit costs compared to
greenfield implementation should also be acknowledged.

Consideration for disproportionate costs should be acknowledged as a prohibitive constraint in
general and for linear development retrofits or widespread watershed implementation. A more
strategic approach to green infrastructure implementation, based on local needs and
considering local constraints (infiltration impacts and property flooding) is warranted."


The additional lifecycle cost associated with green infrastructure should be acknowledged to
support budgeting for long term operation, maintenance and depreciation.

The cost impacts of green infrastructure in existing communities should also be quantified
including costs in communities that are susceptible to infiltration stresses and sewer back-up
risks, additional treatment costs as infiltrated water is collected in foundation drains and
conveyed to treatment plants and cost of reduced service life of cast iron and ductile iron
watermains due to chloride infiltration in right-of-ways (i.e., accelerated corrosion). Such a
robust and holistic economic analysis can then support more strategic, financially sustainable
implementation policies for green infrastructure."

Let's work toward this sustainable implementation policies for all infrastructure - including green infrastructure - considering costs and strategic goals and specific performance outcomes. Low impact development implementation costs in the order of $600,000 per hectare, as shown through local and other jurisdictions, are simply not sustainable on a broad, system-wide basis.


Is Climate Change Making Flooding Worse? - Stormy Data Trumps Fake News on Extreme Weather Trends and Flooding

From the Toronto Star: "Once, when he [Environment Canada's Dave Phillips] offhandedly uttered the words “storm porn” in a pre-interview, a TV reporter built a whole segment around the phrase, because there is only one thing editors and the public crave more than a weather story or a sex story, and that is a sexy weather story." link


"Stormy Data" is in the news almost daily - the media is obsessed with stories about big rain events and flooding - but sometimes the media is full of "weather porn", i.e., sensational stories and video clips that skew the reality behind severe flooding events. Certainly flooding is a critically important issue across Canada that needs careful and sustained attention to make improvements. But the focus on changing weather as the cause is often incorrect, and the tendency to point to climate change as the cause is equally wrong... Fake News. Fake News! Confused Media!

This post talks about storm porn (sometimes used in insurance marketing), flood loss trends in Canada and the causes, and a history of flooding in the Toronto area that suggests flood events and road/bridge washouts were more frequent in the past before modern floodplain management and design practices. That is good news that Best Practices can reduce flood risk over time!

On Weather - More Extreme? No. Its Storm Porn

Almost 30 years ago we has a cable weather channel that was had simple weather forecasts on the 'tube' (The Weather Network History). Today The Weather Network gives us:

"Force of Nature - (Featured every 20 minutes on the 3's, a show-reel of significant weather making headlines around the world), and Force of Nature Extended segment where a news reporter gives an in-depth description of the footage shown."

and "Storm-Hunters - weekends at 7 and 10pm." and "Angry Planet".... - I call this "storm porn", or "weather porn".

And no fallen limb or large puddle escapes weather reporting. Reporters know to go to the underpasses because they flood in extreme weather ... just like they are designed to - but there is no capacity in weather reporting or mainstream media to even remotely consider these design facts. It is better for business to sensationalize the events. What is better for science and public policy however?

But storms are not bigger or more frequent, or more sever today than they used to be - Environment and Climate Change Canada's Engineering Climate Datasets (Version 2.3) show this, despite what the insurance industry has stated (unfortunately by mixing up predictions with observations, theory with facts, annual precipitation totals with short-duration rain bursts):

Intact Financial Weather Frequency Shift
Intact Financial video promotes disproved 40 year to 6
year weather shift (Telling the Weather Story).
The insurance industry does not properly consider storm/weather data that engineers rely on to assess flood risks and continues to state that "In Canada, weather events that used to occur every forty years are now happening every six years in some regions"  as in this video on their web site/blog. That statement about more frequent weather has been shown to be a 'made-up', theoretical bell-curve shift and not actually real data.

On Flood Causes - Many Factors

Flood incidents are caused by many factors. For example, high risk, historical land use planning:
  1. Gatineau 2017 flooding was due primarily to having 75% of buildings in the 1-in-20 year flood plain, a high risk zone that has a 5% chance of flooding every year.
  2. Toronto Island 2017 flooding was due primarily to not completing the buy-outs of the remaining high risk properties.
Or sometimes operational decisions (mistakes) result in flood incidents. The 2013 GO Train flood is a clear example of known floods risks and inadequate operational care - deeper flooding happened regularly at the stranded train site (even just 6 weeks before), and happened over the span of the line's operation, dating back to the early 1980's. But no operational procedures were in place to check water levels or stop trains from entering the floodplain. When the last train was stranded on July 8, 2013, the Don River Watershed did not receive record rain at all and the river flow was a less than a frequent 1-in-5 year flow rate, something with a 20% chance to occur every year. 

Or sometimes stuff was just built kinda small back in the day. That's right. Infrastructure is just like cars or anything else and used to be built to a lower standard of performance - cars did not have seatbelts or anti-lock brakes and guzzled gas in the 1950's. Similarly, sewer and drainage systems in the 1950's were prone to excess wet weather flow inflow and infiltration (I&I) stresses, inadequate overland flow planning/design, and no river flood hazard mapping or land use regulation.

Or the cumulative effect of urbanization and intensification over a century in urban areas aggravates the issues associated with the factors above. Same old rain results in more flooding.

How many times do we have to say it? "There has been No Collusion between storm frequency and flood frequency". OK, we meant "No Causation", but you get the message.

On Flood Damages / Losses

Damages need to be mitigated. I charted out a Best Practices approach for identifying and mitigating flood risks holistically from 'flood plain to floor drain', looking at riverine, storm and sanitary/wastewater systems in this blog post.

Insured and uninsured losses from catastrophic and relevant events in Canada are charted by Munich RE. These flood losses include categories of hydrologic events and meteorologic events (hurricanes) that are normalized by inflation and growth in GDP to provide an indication of trends over time. The chart below shows flood losses between 1980 and 2017 in Canada:
Canadian Flood Damage Trends Insurance Losses
Canadian Catastrophic and Relevant Event "Flood" Losses, Inflation and Growth Adjusted for Hydrological and Meteorological Events in USD - Prepared by Munich RE NatCatSERVICE.
Losses are creeping up. We do have to address flood risks.

On Flood Frequency - New Normal? Or Old Extremes?

We have a tendency to forget the past. Its not well documented or easy to find.  So this should help.

The Toronto and Region Conservation Authority has documented past flooding in its jurisdiction showing flooding back to 1804 in this undated document called "A History of Flooding in the Metropolitan Toronto and Region Watersheds":

Link to full report.

The report acknowledges that prior to 1850, records of flooding are limited and suggests that many have been lost -  those that survived are in letters and diaries, and do not give a complete picture of past flooding risks.

This is noted in the excerpt below:

Some nice take-aways:

We build better today: "Over the years, road bridges became higher and stronger in response to the changing type and volume of traffic that they were required to carry. Consequently, reports of bridges descruction became rarer over time."

We keep better records today: "... newspapers and other sources tended to record only the most severe events, particularly in areas which flooded almost every spring."

Seems we used to flood A LOT in the past: The number of flood events documented by watershed and tributary/site are listed below. Often small bridges were destroyed but are not listed below. Where major road bridge's were damage and had to be replaced, or where roads washed-out, those events are noted below as "Notable Events". Mill destruction was frequent but is not noted:

Etobicoke Creek Watershed
- Long Branch - 1930 to 1954 - 9 flood events (7 in the spring)
- Brampton - 1854 to 1974 - 22 flood events (13 in the spring)
- Highway 7 at East Branch - 1968 - 1 flood event
- Tributary near Dixie Road and Dundas Street - 1974 - 1 flood event

Total Number of Documented Flood Events = 33
Notable Road and Bridge Destruction Events: 1
- Brampton April 7, 2012 "severe damage to roads, bridges, buildings"

Mimico Creek
- "No flood records have been kept..."
- 2 floods are listed, in 1850 and 1954

Total Number of Documented Flood Events 2

Humber River
- Bloor Street Bridge - 1850 to 1954 - 7 flood events
- Lambton Mills - 10 flood events (5 in the spring)
- Eglinton Flats - 1804 to 1954 - 10 flood events (5 in the spring)
- Weston - 1842 to 1954 - 10 flood events (7 in the spring)
- Albion Road Bridge - 1850 to 1954 - 6 flood events (4 in the spring, Feb-May)
- Thistletown -1878 to 1954 - 4 flood events (2 in the spring)
- Gristmill, Holm, Cord - 1850 to 1893 - 4 flood events (2 in the spring)
- Humber Summit, Rowntree's Mill - 1850 to 1893 - 5 flood events (4 for mill and 1 for subsequent cottages)
- Sawmill J. Brown - 1850 to 1893 - 4 flood events
- Woodbridge - 1878 to 1961 - 19 flood events (13 in the spring)
- Mills on Main Branch - 1850 to 1925 - 6 flood events (5 in the spring)
- Bolton - 1850 to 1972 - 22 flood events (18 in the spring)
- Mills on the Upper Humber River - 1850 to 1909 - 4 flood events (3 in the spring)

Total Number of Documented Flood Events = 111
Notable Road and Bridge Destruction Events5
- Weston, October 15-16, 1954, "Lawrence Avenue Bridge destroyed"
- Eglinton Flats, June 3, 1947. "roads washed out, buildings flooded"
- Humber River, Bloor Street Bridge, Spring 1916 "completely washed out"
- Woodbridge, August 5, 1882 "approaches to bridge on main road washed out"
- Woodbridge, January 13, 1937 "roads flooded and some washed out"

Black Creek
- Floodplain near Mt. Dennis - 1878 to 1954 - 5 flood events (3 in the spring)
- Maple Leaf Drive Area - April 5, 1951 - 1 flood event (in spring)
- Tributary - March 12, 1959 - 1 flood event (in spring)

Total Number of Documented Flood Events7
Notable Road and Bridge Destruction Events2
- Near Mt. Dennis, September 13, 1878, "railroad bridge and bridge on Weston Road destroyed"
- Near Mt. Dennis, May 14, 1893, "bridges destroyed"

Don River
- Lower Don - 1804 to 1954 - 19 flood events (15 in the spring)
- Riverdale Flats - 1804 to 1970 - 19 flood events (15 in the spring)
- Mills Taylor Family - 1850 to 1902 - 5 flood events (4 in spring)
- Mills on Lower East Branch - 1850 to 1902 - 5 flood events (4 in spring)
- Sheppard Avenue Bridge - October 15-16, 1954 - 1 flood event
- Cummer Avenue Bridge - February 11, 1965 - 1 flood event (in spring)
- Gristmills - 1850 to 1881 - 4 flood events (3 in the spring)
- Thornhill - 1850 to 1975 - 11 flood events (9 in the spring) - 11 listed, report cites 12
- Mills and Small Dams, 1850 to 1881, 4 flood events (3 in the spring)
- Yonge Street at Highway 7, 1943 to 1975, 5 flood events (4 in the spring)
- Highway 7 Bridge, 1943 and 1950, 2 flood events (in the spring)
- Gristmill, Hosiel, 1835 to 1881, 5 flood events (4 in the spring)
- Bayview Avenue Bridge, 1850 to 1954, 4 flood events (2 in the spring)
- Hogg's Hollow, 1850 to 1954, 6 flood events (4 in the spring)
- Gristmill, Boyle, 1850 to 1881, 4 flood events (3 in the spring)
- Bathurst Street Bridge, October 15-16, 1954 - 1 flood event
- Mills on West Branch, 1850 to 1881, 4 flood events (3 in the spring)
- Highway 7 Bridge, 1943 to 1975, 4 flood events (3 in the spring)
- CNR Bridge Near Concord, 1878 to 1954, 3 flood events (1 in the spring)
- Small Dam, Lamer, 1850 to 1881, 4 flood events (3 in the spring)

Total Number of Documented Flood Events 111
Notable Road and Bridge Destruction Events13
- Lower Don, April 5, 1850, "Queen Street bridge destroyed, as well as Kingston Road bridge"
- Lower Don, September 13, 1878, "bridges destroyed at Gerrard Street, Queen Street, Kingston Road, as well as many smaller ones"
- Lower Don, February 28, 1902, "roads washed out"
- Sheppard Avenue Bridge (Sheppard Avenue and Leslie) , October 15-16, 1954, "Destroyed during Hurricane Hazel"
- Thornhill, April 5, 1850, "100 feet of Yonge Street washed out"
- Thornhill, spring 1943, "Yonge street washed out in several places"
- Bayview Avenue Bridge, April 5, 1850, "destroyed"
- Bayview Avenue Bridge, September 13, 1878, "destroyed"
- Bayview Avenue Bridge, October 15-16, 1954, "destroyed"
- Hogg's Hollow, April 5, 1850, "approaches to Yonge Street bridge washed out, bridge isolated"
- Hogg's Hollow, October 15-16, 1954, "Yonge Street bridge washed out"
- Bathurst Street Bridge, October 15-16, 1954, "Destroyed"
- CNR Bridge Near Concord, September 13, 1878, March 10-11, 1936, and  October 15-16, 1954, "The railroad was washed out"

Highland Creek
- Cottages and Highland Creek Drive, 1936 to 1977, 24 flood events (20 in the spring)
- Gristmill, Helliwell, 1869 to 1878, 2 flood events (1 in the spring)
- Highway 2 or Kingston Road Bridge, 14 flood events (12 in the spring)
- Sawmill, 1869 to 1878, 2 flood events (1 in the spring)
- Cottages at "The Willows", 16 flood events (14 in the spring)
- Scarborough Golf and Country Club, 1950 to 1977, 19 flood events (16 in the spring)
- Sawmill, 1869 to 1878, 2 flood events (1 in the spring)
- Military Trail Bridge, 1948 to 1977, 19 flood events (15 in the spring)
- Sawmill, 1869 to 1878, 2 flood events (1 in the spring)

Total Number of Documented Flood Events100
Notable Road and Bridge Destruction Events4
- Cottages at "The Willows", February 15, 1949, "roads washed out"
- Cottages at "The Willows", July 4, 1951, "roads washed out"
- Cottages at "The Willows", October 15-16, 1954, "roads, bridge near present Lawrence Avenue washed out"
- Military Trail Bridge,  August 27-28, 1956, "bridge destroyed"

Rouge River
- CNR Bridge, April 10, 1973, 1 flood event in spring
- Highway 2 of Kingston Road Bridge, 1878 to 1956, 5 flood events (3 in the spring)
- Caper Valley Ski Hill, February 2-3, 1978 , 1 flood event in spring
- Meadowvale Avenue Bridge, October 15-16, 1954, 1 flood event
- Mills, 1878 to 1929, 3 flood events (2 in the spring)
- CPR Bridge, October 15-16, 1954, 1 flood event
- Mills below Markham, 1878 to 1929, 3 flood events (2 in the spring)
- Markham, 1837 to 1973, 7 flood events (4 in the spring)
- Unionville, 1878 to 1973, 6 flood events (3 in the spring)
- Mills and Dams, 1878 to 1929, 3 flood events (2 in the spring)
- CNR Tracks, July 19, 1944, 1 flood event
- Mills, 1878 to 1929, 3 flood events (2 in the spring)
- Rouge Valley Inn, October 15-16, 1954, 1 flood event
- Mills on the Little Rouge, 1878 to 1929, 3 flood events (2 in the spring)
- Con.9 Markham Township, 2 flood events (2 in the spring)
- CNR Bridge, 1947 to 1954, 2 flood events
- Mills and Small Dams, 1878 to 1927, 3 flood events (1 in the spring)

Total Number of Documented Flood Events46
Notable Road and Bridge Destruction Events6
- Markham, May 16, 1937, "bridge washed-out (on present Hwy. 7)"
- Markham, October 15-16, 1954, 'town "marooned" by Hwy. 7 washouts on both east and west sides'
- Unionville, October 15-16, 1954, "Main Street washed out north of Hwy. 7"
- CNR Tracks, July 19, 1944, "The tracks were washed out"
- CNR Bridge, August 18, 1947, "Washed out"
- CNR Bridge, October 15-16, 1954, "Washed out" "passenger train partially derailed"

Duffin Creek
- Gristmill, 1878 to 1919, 3 flood events (1 in the spring)
- Pickering Village and Cottages on Riverside Drive, 27 floods (24 in the spring)
- Mills on West Branch, 1878 to 1890, 2 flood events
- Whitevale, 1878 to 1950, 5 events (3 in the spring)
- Green River, 1878 to 1954, 6 events (3 in the spring)
- Mills between Stouffville and Green River, 1878 to 1919, 3 flood events (1 in the spring)
- Stouffville, 1878 to 1972, 10 flood events, (7 in the spring)
- Mills and Small Dams, 1878 to 1919, 3 flood events (1 in the spring)
- Greenwood, 1878 to 1956, 7 flood events (4 in the spring)
- Mills on East Branch, 1878 to 1919, 3 flood events (1 in the spring)

Total Number of Documented Flood Events = 69

Grand Totals:
Total Number of Documented Flood Events = 379
Notable Road and Bridge Destruction Events31

So yes, we have always had many floods in the past and many road, bridge and rail washouts too. And urban areas have expanded considerably since 1804, meaning more places to experience high rainfall and more runoff (before we started to practice better stormwater management quantity control). While the loss of Finch Avenue during the August 19, 2005 storm was significant, we have not had any major road or bridge washouts since, only Military Trail Bridge, August 27-28, 1956 which it is noted "has not been redesigned and remains low and vulnerable to flooding". So despite high runoff stresses and more and more crossings, the loss of roads and railways has not been an issue. This suggests that today's floodplain management and hydraulic structure (i.e., bridge and culvert) practices are largely effective as well, resulting in overall resilient infrastructure.

The TRCA flood history report notes wet cellars or basements for only a couple of the nearly 400 events. That is in contrast with today when it is basement flood damages that are driving flood losses in southern Ontario, not riverine flooding.


Interesting comment on land use planning practices:
- in Hogg's Hollow, "All of the houses flooded during Hurricane Hazel remain in the floodplain, and several more have been built" ... obviously this just adds to old risk
- in The Willows, "The cottages at The Willows which survived Hurricane Hazel were removed shortly afterwards, and the valley is now parkland" ... and this is the best way to remove risk in the highest risk zones

Climate Change Impacts on Ontario Highway Infrastructure Design Shows High Resiliency - Wastewater Systems Too - Media Reports Ignore Engineering Design Practices and Intrinsic Resiliency / Design Safely Factors

There is a tendency to jump to conclusions when considering climate change impacts on drainage infrastructure. For example, if storms 80 years from now are predicted to be 30% bigger (more rain depth), do sewer pipes, culverts and bridges under our our highway's need to be 30% bigger to handle the additional runoff?

No. Effects of rain are attenuated through the system due to hydrology and hydraulics. Plus there are intrinsic design safety factors to account for uncertainty in design already.

Hydrology mitigates the impact on rainfall on runoff, so 30% more rain depth results in less than 30% higher peak runoff flowing into drainage systems. This is due to things like storage on the surface, in ditches, etc. before the rain can become runoff.

Hydraulics mitigates increases in peak flows as well due to non-linearities in how flow rates show up as flow depths in channels - there is not a 1-to-1 relationship between peak flow and depth. And sometimes hydraulics throttle how much flow can enter into systems, for example sewer grates at the surface can control how much flow gets into underground sewers.

The Ontario Ministry of Transportation completed a review of their drainage system vulnerability to climate change, showing quantitatively that the overwhelming majority of storm sewers, roadway gutters, culverts and bridges meet design standards under projected future climate conditions:

Projected rain intensity increases are 10-30%. Assuming a 20% increase, with no change in today's designs:

- 96% of storm sewers already meet design standard of flowing less than 100% full
- the average flow spread in roadway gutters increases 5-7%
- 93% of culverts meet headwater depth requirements that relate to upstream flood depths
- 96% of culverts meet exit flow velocity criteria related to erosion potential
- all bridges are assessed for regulatory (historical) storms that exceed return period events, and "These storms are generally in excess of the design storm used in determining the size of the structure
opening and erosion protection measures."

So future climate change rainfall intensities do not cause today's highway drainage systems to fail - the majority of features still meet design requirements / standards / criteria. This is in direct contrast with media reports that incorrectly assume that rainfall changes translate into infrastructure changes - in fact, recently Gord Miller stated that all culverts and sewer pipes are too small:

Specifically: “I don’t think here in Canada we understand what’s coming,” said Miller during the talk. “We have no predictability any more. One has to look from the perspective that all culverts are undersized. All sewers are undersized.”

All culverts undersized? All sewers undersized? Obviously that is incorrect based on the Ministry of Transportation's careful and comprehensive resiliency analysis.

What about wastewater systems? Are all sanitary sewers undersized too? I co-wrote a paper for the Water Environment Association of Ontario annual conference on this topic, looking at sanitary sewer system resiliency under higher potential climate change rainfall intensities. It shows that most new post-1980's subdivisions are resilient with no sewer back-up risk with potential higher future rainfall. Here is the paper with the details:

And here is the presentation:

Analysis of all sewer pipes in the City of Markham shows that very few locations are at a risk of flooding for today's or for future climate as shown in the following table from the paper:

So less than 2% of old sewers are flood prone. Just more than 1% of new sewers are flood prone. This is shown on the following map:

Blue dots show where maintenance hole surcharge to basement levels with today's climate, concentrated largely in older, partially-separated sewer service areas. With higher potential future rainfall intensities, there are more at-risk maintenance holes / locations, concentrated again in the older areas. So over 98% of sanitary sewers are not undersized with the future predicted climate.

Media reports (like the TVO article with the Gord Miller quote above generally do not rely on engineering data or comprehensive analysis to make broad statements about climate change impacts. Too bad. We deserve better or else public policy geared to climate adaptation and mitigation will be uninformed and resources to address risk will be misdirected.