Town of Oakville Class Action Lawsuit Over Wider Floodplains and Flood Damages - Is Urbanization or Climate Change the Cause?

The CBC reported on a $1B class-action claim that alleges Oakville property owners are at flood risk due to 'over-development'.  The article appeared last week: https://www.cbc.ca/news/canada/toronto/1b-class-action-claim-alleges-oakville-property-owners-at-flood-risk-due-to-over-development-1.5755264

A resident interviewed for the story said that floodplain development restrictions have grown over time, restricting development activities on private property.

The mayor of Oakville explained the change in floodplains in the story: "He said that flood plains are continuously adjusted according to developing science and that the mapping in a century-old neighborhood like South Oakville would naturally require some changes over the years."

It is true that changes in analysis methods can affect floodplain extents.  Most likely the first high-level hydraulic models, using the USACE's HEC-2 program, were coded on punch cards in a consultant's office, and models were compiled and simulated on mainframe computers off-site (I know, I saw the old punch cards in our office storage in the early 1990's).  Personal computers came into offices in the 1980's to run the same simulations.

So floodplains have been estimated for many decades but not when centuries-old neighbourhoods in South Oakville were developed. 

Documentation from the US Army Corps of Engineers speaks to the computer requirements identified in the 1982 HEC-2 manual (image at right lists mainframe computers used on the top and emerging microcomputer PC's at the bottom).  The image below it represents bridge hydraulic model parameters in the USACE's Hydrologic Engineering Centre's HEC-2 hydraulic model - that input would be used to prepare punch cards in the early 1980's.  So forty years ago modelling was pretty basic right? And there was no such modelling 100 years ago.  

Hydrology models that determine flow rates in rivers have undergone similar upgrades over the decades just like HEC-2 hydraulic models.

So again, floodplains were not mapped 100-years ago in the 1920's in South Oakville.  Floodplain limits have not been changing on their own since then, unless the upstream land uses changed resulting in more flow or unless storms are bigger now.  According to Wikipedia, Conservation Halton, who has the role of mapping floodplains and regulating hazards (i.e., under O. Reg. 162/06: HALTON REGION CONSERVATION AUTHORITY: REGULATION OF DEVELOPMENT, INTERFERENCE WITH WETLANDS AND ALTERATIONS TO SHORELINES AND WATERCOURSES under Conservation Authorities Act, R.S.O. 1990, c. C.27), has been around (in one form or another) only since the 1950's according to their web site:

"Conservation Halton was formed in 1956 as the Sixteen Mile Conservation Authority followed by the formation of the Twelve Mile Conservation Authority in 1957. In 1963 these conservation authorities amalgamated to form the Halton Region Conservation Authority which later became known as Conservation Halton."

So floodplain mapping in South Oakville has likely not been in place for more than 40 to 50 years.  The 2014 report National Floodplain Mapping Assessment - Final Report prepared for Public Safety Canada charts the ago of floodplain mapping in Canada showing mapping started in the mid 1970's - see excerpt below:

The CBC article discusses the causes of increased floodplain extents.  The key factor noted in the class action lawsuit is urbanization that can increase runoff volumes and runoff rates, thus increasing river flow rates and river flood levels.  High flood levels result in wider, more extensive floodplains.

Two reports by the Intact Centre on Climate Adaptation (TOO SMALL TO FAIL: Protecting Canadian Communities from Floods (2018), and Preventing Disaster Before It Strikes: Developing a Canadian Standard for New Flood-Resilient Residential Communities (2017)) lists other stormwater management and flood-related lawsuits in Canada.  So lawsuits related to flooding are not new.

So has there been development in Oakville and upstream of Oakville that could have increased flood risks?  First there has been development as shown in the following images.  The 1960 development limit is based on Statistics Canada dwelling age of construction in census dissemination areas (very approximate), the 1971, 1991, 2001, and 2011 development limits are from Statistics Canada as well.  The 2015 limits are according to Version 3 SOLRIS land use mapping from the Province of Ontario.

Its pretty clear that there has been development.  The urban area in Oakville in 1971 was about 3500 hectares.  In 2001 it was 8800 hectares.  In 2011 it was 9200 hectares. So that is a significant increase.

Secondly, has the development caused floodplain impacts?  Conservation Halton describes several flood mitigation measures that have been put in place decades ago to mitigate some earlier, long-standing flood risks.  These measures include (according to their web site):

Dams 

"Conservation Halton’s dams, along with many of the major dams within other conservation authorities across the GTA were built in direct response to the devastation associated with Hurricane Hazel (October 1954). Most of these facilities were constructed in the 1960’s and 1970’s, however none have been built since then as a more passive approach to hazard management, including land acquisition and regulation, were adopted instead of costly engineered structures."

• Scotch Block Reservoir
• Hilton Falls
• Kelso
• Mountsberg

Flood Control Channels

"Conservation Halton built three flood channels between the late 1960’s and 1970’s to safely move water through our communities and into Lake Ontario as quickly as possible. The three channels are Hager-Rambo in Burlington, Milton and Morrison-Wedgewood in Oakville. The channels are designed to move large flood flows which may result from rapid rainfall or a longer rain event away from historically developed flood sensitive / prone areas."

So works are in place to address earlier-noted flood risks, say up to the 1960's and 1970's.  More recent development has been supported by robust planning and risk mitigation measures, including effective stormwater management.  There is a risk that development that has occurred between the 1970's and the early 2000's could have increased flood risks - after that time more robust mitigation are generally in place to account for cumulative watershed effects, e.g., due to higher runoff volume.  Intensification within existing development areas can also increase runoff and contribute to higher flood risks.

The CBC story discusses the role of different factors saying "At its core, the claim blames increased flood risk in South Oakville on urban development. But there are other factors that can affect an area's risk for flooding, and the most important of those may be climate change."

Is climate change the most important factor? Have observed rainfall volumes increased during storms or have design intensities for rare, extreme rainfall events increased?

To answer those questions one can review the published Engineering Climate Datasets from Environment Canada to evaluate how annual maximum rainfall amounts and design intensities have changed over the years.  The data on observed maximum annual rainfall, measured over various durations of 5 minutes to 24 hours, show no increase at long-term climate stations surrounding Oakville.  The Pearson Airport climate station to the east of Oakville shows no increases in observed annual maxima going back to the 1950's (see Environment Canada chart below).

 

When observed rainfall extremes decrease as noted above, so do the derived design rainfall intensities.  The next table shows how design rainfall intensities over a 5-minutes duration have decreased since 1990.

There are decreases for 2-year intensities, for which there are a lot of observations, and decreases for rare 100-year intensities too (note: the intensities inched up temporarily after the July 8, 2013 storm but have trended back down now).

The Town of Oakville actually uses the downtown Toronto rainfall gauge for their design guidelines.  A recent study for the Town confirmed that the Toronto gauge data can be used to design in the future as well.  Town consultant Wood assessed future rainfall and Town’s existing design intensities (Review of Future Rainfall Scenarios, December 2018), and asked and answered this question:

"1. Should the Town of Oakville maintain its rainfall standard based on the Toronto City Environment

and Climate Change Canada station or move to a database within the boundaries of the Town?

Recommendation: Maintain the Toronto City ECCC station as the basis for the Town’s design IDF

relationship."

The IDF relationship is the Intensity-Duration-Frequency characteristics used to design drainage systems).  The Town's consultant recommended using the Environment Canada data that is showing decreasing annual maximum rainfall. 

Specifically what is happening at the Toronto station used for Oakville drainage design? Annual maximum measured rainfall is generally declining for all durations - the 12-hour duration rainfall even has a statistically significant decrease (bottom middle chart below).

These observed decreases result in engineering design intensities that decrease as well. Over a 5 minute duration, these design intensities have been decreasing since the 1990 IDF updates for the Toronto rainfall gauge.  The rare 50 and 100 year rainfall intensities are decreasing the most a shown in the table below.
 

To the west of Oakville, in Hamilton, the annual maximum rainfall observations at the Royal Botanical Gardens show decreases or no change in rainfall since the 1960's:

The Hamilton Airport observed trends are also lower for short durations (see chart below). Trends for long durations are flat since the early 1970's.

 

Looking wider beyond those four stations above, a review of Southern Ontario trends shows in a previous post shows the trends at 21 long-term climate stations: https://www.cityfloodmap.com/2020/05/southern-ontario-extreme-rainfall.html. This is a summary figure and table that show decreases in frequent storm intensities and virtually no change in extreme infrequent storm intensities:

Southern Ontario IDF Rainfall Intensity Trend Chart by Duration - Environment and Climate Change Canada's Engineering Climate Datasets, Pre-Version 1.00 (up to 1990) to Version 3.10 (up to  2017)

 

So.

Development has increased significantly since the 1960's, and has doubled since mitigation works were constructed in the early 1970's to 2001 after which stormwater management measures have become more robust.  So development seems to be an important factor.

Rainfall extremes have not changed since the 1950's and 1960's at surrounding climate stations, or in southern Ontario in general. So rain does not appear to be a factor resulting in higher and wider floodplains - while Milli Vanilli can Blame it on the Rain (see below), CBC could do some fundamental fact checking on the topics in the story.

The CBC story suggests "it's difficult in general to "decouple" the effects that climate change and urbanization have on flood risk" and "determining that one played more of a role than the other is challenging" - perhaps in general it is difficult, and perhaps it is challenging.  But the difficult work has been done in this case already.  Statistics Canada has mapped urbanization growth in Oakville, and Environment and Climate Change Canada has charted and analyzed extreme rainfall trends in the region as well.   

Given the specific data here, CBC does not appear to offer any support for this statement "At its core, the claim blames increased flood risk in South Oakville on urban development. But there are other factors that can affect an area's risk for flooding, and the most important of those may be climate change."

***

Here is a higher resolution video showing the land use progression in Oakville (you can enlarge it once it starts to play):

Can We Use Daily Rainfall Models To Predict Short Duration Trends? Not Always - Observed Daily and Short Duration Trends Can Diverge

One can assess trends in rainfall intensities over various durations and return periods using Environment Canada's Engineering Climate Datasets.  National trends based on updating 226 station IDF curves were shown in an earlier post.

What are the trends in regions of Canada that have experienced significant flooding in the past?  And do the trends projected by models for long durations (1 day precipitation) match observed data trends?  No - some 24-hour trends are decreasing despite models estimating they will go up (or have gone up because of increasing temperatures).

Also, what is happening with observed short duration intensities, the ones responsible for flooding in urban areas, compared to the observed 1-day trends?

The data show short duration and long duration trends diverge. Therefore relying on models of 1 day precipitation to estimate what is happening with short duration, sudden, extreme rainfall should be done with caution.

A couple charts help illustrate these observed data trends and show what is wrong with relying directly on models to project local extreme rainfall.

This is the trend in observed rainfall for southern Ontario climate stations, using median changes in IDF statistics:

Southern Ontario Extreme Rainfall Trends

Long duration intensities are decreasing and short duration intensities are decreasing even more.  The extreme intensities (red dots = 100 year, orange dots = 50 year) decrease more than the small frequent storm intensities (green dots = 2 year).  Observed data diverges from Environment Canada models that suggest intensities are going up due to a warmer climate (see recent CBC article).

These are the trends for Alberta observed rainfall when new data are added and are reflected in the most current v3.10 datasets:

Alberta Extreme Rainfall Trends

In Alberta, long duration intensities decrease significantly (100 year is down by 4% on average).  Meanwhile the short duration intensities increase.  The long duration decrease is contrary to Environment Canada's simulation models that estimate 1 day rainfall at a sub-continental scale.

In northern Ontario, trends are different than in southern Ontario as shown below:

Northern Ontario Extreme Rainfall Trends

In northern Ontario the long duration intensities have increased but short duration intensities have decreased on average.  So we see short and long duration rainfall trends are diverging when we consider new data.

Climate modellers may suggest that simulated 1 day precipitation can guide what happens during short durations too.  Observed data suggest otherwise.  Trends actually diverge.

In brief, for this sample of regions shown above, we see these trends:

Location                 Short Duration Trend         Long Duration Trend

Southern Ontario      Larger Decrease                        Decrease
Northern Ontario            Decrease                              Increase
Alberta                            Increase                               Decrease

Remember "All models are wrong, some are useful".  Climate models do not accurately project changes in extreme rainfall in Canada based on observed data.  Furthermore, simulated 1 day precipitation trends from models cannot be used to assume short duration trends related to flooding in urban areas - short and long duration rainfall trends are observed to change in opposite directions in sample regions across Canada.

When using 1 day rainfall trends to estimate short duration trends, given the actual observed data trends above, it may be appropriate to conduct sensitivity analysis on potential shorter duration trends, especially if those shorter durations influence system behaviour (e.g., 'flashy' urban drainage systems).  Those short duration trends trends may be in an opposite direction or magnitude than the 1 day trends. For example, in Northern Ontario the 1 day 100-year intensities have increased 2% as a result of the most recent IDF data updates, however the intensities for durations of 2 hours or less have mostly decreased.

The following chart compares the 30 minute, 1 hour and 2 hour 100-year intensity trends with the 24 hour 100-year trends at 226 climate stations across Canada.

The correlation of short duration trends with 24 hour trends is weak with R-squared value of 0.12 for 2 hour trends, 0.06 for 1 hour trends and 0.006 for 30 minute trends.  This suggests that short duration trends are not correlated with 24 hour trends.   

***

Given recent flooding in British Columbia, it is worthwhile looking at trends in design rainfall intensities in BC. This chart shows that extreme rainfall intensities (red dots with 100-year return period, orange dots with 50-year return period) have not increased for most durations - the 12 hour duration intensities are up slightly on average (less than 0.5% increase), while other duration intensities have decreased by more than 2 % on average (5-minute 100-year intensities).

For the longest duration of 24 hours, intensities have decreased on average - typical 2-year intensities are unchanged on average while the moderate and extreme intensities (5-years, 10-year, 25-year, 50-year and 100-year) have decreased on average.

The following tables shows trends in observed annual maximum rainfall over various durations at BC climate stations with long-term records. These are called the Annual Maximum Series (AMS). Trends in derived design rainfall intensities above (e.g., 2-year to 100-year rainfall rates) follow these trends in AMS.

Past, Present, or Future? CBC Ombudsman finds "It would have been wrong to state categorically that Canada has already seen an increase in extreme rainfall events" - So What is Causing Flooding?

Fort McMurray Historical Ice Jam Events

Flooding caused by extreme weather has unfortunately been a significant threat to Canadians for decades.

Recent flooding in Fort McMurray, Alberta highlights how devastating flooding can be, causing widespread damage and even loss of life.  Often flood risks are long-standing and challenging to address.  For example, Fort McMurray flooding has been affected by ice jams since 1875, based on Review of flood stage frequency estimates for the City of Fort McMurray: Final report by
Alberta Environmental Protection, Technical Services and Monitoring Division (https://era.library.ualberta.ca/items/f6f26d4e-005d-462b-8d7c-0428db8f7d27).

In order to effectively manage flood risks, it is important to understand the causes.  Understanding if flood risks are increasing and if current management practices are working to prevent or mitigate risks is important.

Often journalists focus on changes in weather and climate as the primary cause of flooding, or increased flood damages.  In fact, historical land use practices and development that began in high risk areas a century before modern flood hazard mapping explains baseline flooding in many parts of Canada.  Redevelopment and intensification in these hazard areas can lead to increasing damages over time. Stay tuned for a future post with some examples.

Recently the CBC Ombudsman reviewed how CBC journalists report on past, present and future changes in extreme weather related to flooding in its In Our Backyard series, initiated last summer.  The full review is here: https://cbc.radio-canada.ca/en/ombudsman/reviews/Past-Present-or-Future.

What did the Ombudsman find?  Well, that "It would have been wrong to state categorically that Canada has already seen an increase in extreme rainfall events", recognizing that has already been reviewed in detail by CBC Radio-Canada Ombudsman Guy Gendron: https://site-cbc.radio-canada.ca/documents/ombuds/reviews/Review%20Robert%20Muir.pdf

The Ombudsman also found that "journalists could have been clearer with their choice of tenses". He
pointed to CBC reporting showing that changes in extreme precipitation are predicted in the future, but have not occurred already from past to present:

"The key stories make no direct claims, for instance, that more severe storms have been observed in Canada. What I saw was often a real effort made to lay out the issues with broad strokes, and avoid getting bogged down in details. Take this excerpt, for example, from the Harrison column:

According to the federal government's recent assessment, Canada's Changing Climate Report, there is "high confidence" that:

• Canada is warming at twice the global rate, and our north is warming at three times that rate.
• We can expect more extreme heat, warmer winters, earlier springs and rising sea levels.
• Precipitation will increase in much of the country.
• Weather extremes will intensify.

The last two bullet points are careful to use the future tense. If, as it appears, Mr. Harrison was the architect of this series, there’s no sense of an attempt to mislead, change facts or distort reality. There are appropriate distinctions made between observed phenomena and predicted phenomena."

So this is positive that CBC recognized the difference between observed phenomena and predicted ones.

While the Ombudsman got it right - more severe storms have not been observed - recent "In Our Backyard" reporting at CBC has already mixed up observations and predictions on this topic.  The Flooding tab in this report https://www.cbc.ca/news2/interactives/inourbackyard/ presents the following:

CBC states that extreme rainfall that had a 50 year recurrence time is now happening every 35 years., implying an observed phenomena. What report is CBC referring to? It is Canada's Changing Climate Report: https://changingclimate.ca/CCCR2019/chapter/4-0/

Specifically, Section 4.3.2.2 Projected changes and uncertainties includes a chart Figure 4.20 b) with simulation model projections.  Here it is:

The red line representing extreme 50-year storm events has model simulations from past to present to future, showing a predicted decreasing recurrence time (often called a "return period", the inverse of the storm's annual  probability of being exceeded).

Obviously the CBC can do better in terms of getting some basic details right - it may even be worthwhile getting "bogged down" in important details like the difference between observed and predicted changes in the factors affecting flooding.  As the Ombudsman wrote, there is a need to avoid 'shortcuts' that create 'ambiguity':

"I am not prepared to conclude that this was a violation of policy, but rather as a reminder that there cannot be shortcuts in language if they create ambiguity. This is a particular challenge in broadcast, where being concise is so critical, but editors and reporters should not leave out any word if it is necessary to sharpen the clarity of the reporting. When CBC is referring to the future, it would be better to say so. That way viewers won’t be left guessing."

Mixing up past, present and future continues to create ambiguity, leaving CBC viewers misinformed about actual extreme storm trends.  Unfortunately, this can divert attention from the other factors that affect flooding and that should be given our attention when managing long-standing risks.

Are More 100 Year Storm Happening? Yes and No. A Proliferation of Rain Gauges Can Now Record More 100 Year Storms, But Fixed Locations Show No Increase

There are many sensational media stories about ghost storms and ninja storms hitting urban areas, and a steady claim that we are experiencing more extreme rainfall, that is, higher intensities for a given probability (called return period), or greater frequency of given design intensities. Often it is stated that we are experiencing more 100 year storms today and that is a "new normal" brought on by a changing climate.

How does the number of climate stations, or rain gauges, that are in operation affect the number of observed extreme events. Well, let's look at Toronto for example.  Several past extreme events were reported in the Staff Report on Impact of July 8, 2013 storm on the City's Sewer and Stormwater Systems dated September 6, 2016: (https://www.toronto.ca/legdocs/mmis/2013/pw/bgrd/backgroundfile-61363.pdf)

During the May 12, 2000 extreme rainfall event, Toronto operated 16 rain gauges as shown on the staff report map below.

Fifteen years later, during the August 19, 2005 storm, the City operated 31 rain gauges as shown below, so almost double the number of rain gauges.  Look at the higher density of gauges in north Toronto where many higher August 19, 2005 rainfall depths were observed.

Then 8 years later, during the July 8, 2013 storm the city operated even more rain gauges, i.e., 35 in total.

And then a few years later, on August 7, 2018, the city operated 43 rain gauges - even more than 2013. I don't have a map but here is a super-cool graph summarizing Toronto Open Data rainfall totals at those gauges over a period of 5 minutes to 24 hours.

And now today as of July 17, 2019, Toronto has 45 active rain gauges as shown in the following map presented to the Ministry of Environment Conservation and Parks' stormwater stakeholder group participating in development of minimum standards for ECA pre-approval.

So let us summarize the trend in the number of rain gauges in the chart below.

Astute blog readers will notice that the number of rain gauges has increased almost 300% since the year 2000. Yes, almost three times the number of rain gauges now. Obviously, more extreme events can be observed and recorded when the number of rain gauges increases dramatically.

The following table shows that in the year 2000, there was a rain gauge every 39.4 square kilometres (16 gauges per 630.2 square kilometres). By 2019, there is a gauge ever 14 square kilometres.

***

So what is happening at fixed locations where rain intensities are measured? In Toronto and Mississauga, many trends are downward according to the Engineering Climate Datasets:

 

As a result, design intensities for short durations have been decreasing since 1990:

***

To recap, many more rain gauges today mean we 'see' more storms - these are typically needed to support basement flooding Municipal Class EA studies (rainfall needed to calibrate hydrologic and hydraulic simulation models), to guide operational activities too.  Many municipalities have installed rain gauges to support inflow and infiltration management programs.

We have a "finer mesh net" to catch these events and add them to our records - we have almost 3 times more rain gauges in Toronto since 2000.

But no. Storm are not becoming more intense. If we see more of them, it is because we are looking harder for them with more extensive monitoring efforts. Given this expanding intensive network of rain gauges today, it is not uncommon, statistically speaking, to observe many 100-year storms over a short time period.  This earlier post explores those statistics in the GTA - https://www.cityfloodmap.com/2019/03/are-six-100-year-storms-across-gta-rare.html.

Is Wild Weather and a New Normal for Severe Rainfall Responsible for Urban Flooding, or Urbanization and Hydrologic Stresses? Case Law Points to Urbanization Driving Runoff and Flood Effects.

Everyone has an opinion on the weather and media is saturated with stories linking extreme weather with flooding. It makes sense. Flooding happens during severe storms. The bigger the storm the bigger the flood damages in fact.

But media and groups including the insurance industry and some researchers have suggested that flooding and flood losses have increased due to changes in weather patterns characterized by increased intensity or frequency of rainfall events.

That is not true. And there is no data to support that explanation.

Why?

Because rainfall intensities have not changed according to official Engineering Climate Datasets that review and analyze trends in extreme rainfall to inform engineering design across Canada.

Some media are correcting this false explanation that new wild weather, or a new normal, is causing flooding, like the CBC.

The CBC Ombudsman has ruled that CBC News reporting violated standards of journalistic practice in reporting more 100 Year storms linked to urban flooding - see the scathing report. It begins:

"Review by the Office of the Ombudsman, French Services, CBC/RadioCanada of two complaints asserting that the articles by journalist Marc Montgomery entitled How to mitigate the effects of flood damage from climate change and Response to a climate change story, posted on September 19 and November 19, 2018, respectively by Radio Canada international (RCI), failed to comply with the CBC/Radio-Canada Journalistic Standards and Practices regarding accuracy and impartiality."

and regarding this claim in the article on changing storm patterns:

“We are experiencing storms of greater magnitude, more volume of rain coming down over short periods of time these days due to climate change. That is causing massive flooding.”

the CBC Ombudsman concludes that (my bold):

"One only had to examine the official Environment Canada data for Ontario as well as for the

entire country to acknowledge that the claim made in the article was inaccurate. Such

acknowledgement would at the same time have addressed the complainant’s criticism regarding

the lack of data to corroborate Dr. Feltmate’s claim about the increased frequency of extreme

rainfall events in Canada."

While Environment and Climate Change Canada have refuted insurance industry claims on storm frequency shifts in the past (see Canadian Underwriter correction on the IBC/ICLR Telling the Weather Story theoretical shifts mistakenly reported as real data).

Yet the insurance industry has continued to promote the 'causation', with opinion pieces (not any peer-reviewed paper or analysis) saying climate-change effects on rainfall drive flood losses. See Financial Post piece. 

If not rainfall, what causes more flooding, more flood damages?

Canadian courts have pointed to urbanization as a driver, as in the landmark case of Scarborough Golf Country Club Ltd v City of Scarborough et al.. The decision indicates that urbanization markedly increases runoff stresses that cause runoff, erosion and flooding. Some highlights:

i) "Expert evidence confirmed the effect of the city's rapid urbanization and water control plans on the creek." 

ii) "It is important to note that the case is not presented primarily as a complaint against flooding but rather that the markedly increased flows and increased velocity of flow have caused and continue to cause damage to the creek bed and the adjacent tableland.", and 

iii) "There can be no doubt that the storm sewer facilities and urbanization of the lands to the north of the Club are the cause of the effects just described and that the difference in flow and velocity of flow is very substantial." 

So urbanization markedly increases runoff, flows and velocities, while there are no observed changes in extreme rainfall. Mapping clearly shows the significant expansion of urban areas in southern Ontario municipalities - see post and images below:

The IPCC has reviewed the size and frequency of floods at larger regional scales in their extreme events report and noted limited to medium information to assess changes, also noting the effects of changes in land use and engineering (see page 8):

"There is limited to medium evidence available to assess climate-driven observed changes in the magnitude and frequency of floods at regional scales because the available instrumental records of floods at gauge stations are limited in space and time, and because of confounding effects of changes in land use and engineering. Furthermore, there is low agreement in this evidence, and thus overall low confidence at the global scale regarding even the sign of these changes."

IPCC notes low confidence in the sign of changes at a global scale, meaning flood magnitudes could be going up or down.

Other factors driving losses? Research shows for some severe weather event types like hurricanes the driver is GDP growth, e.g., "research is robust in concluding that, for many decades into the future, the primary driver behind increasing economic losses related to hurricanes is expected to be societal growth"  

More factors? Maintenance of infrastructure affects its performance and flood risks. For example, TRCA described that flooding of the Keating Channel and lower Don River, which affects Toronto's Don Valley Parkway was due to a lack of maintenance:

"Since its construction between 1914 and 1922, the Keating Channel has been subject to heavy sediment loads, requiring regular dredging to maintain sufficient depths to allow for and maintain shipping activities at the mouth of the Don River. Between 1950 and 1970, widespread development throughout the Don Watershed and the construction of the Don Valley Parkway increased sedimentation rates by up to four times that of the pre-was era. After 1970, decreases in the number of new watershed disturbances and improved sediment control structures likely contributed to the decline in sedimentation in the Keating Channel to levels similar to the pre-war era. A reduction in shipping activities within the Keating Channel, combined with restrictions on the open water disposal of dredgate imposed by the International Joint Commission (IJC) in 1974, resulted in a cessation of dredging in the Keating Channel. In the following five to six years, the Keating channel filled with sediment and debris to the point where it became visible under all but high lake levels, resulting in increased flood risk along the lower Don."

So flood risks increase due to fluviogeomorphology (the transport and deposition of sediments in a watercourse) and hydraulics - when dredging stops, sediment builds up, hydraulic capacity is reduced and flooding is increased along the river. 

Yet despite flooding dating back to the 1800's, as reported in the Inquiry for Premier Davis, and despite impacts on rail lines in the Don River floodplain over decades, flooding has been attributed to climate change effects. Even by the Environmental Commissioner of Ontario. The fact is there is no new normal with "wild weather", but the same old issues and extremes:

Hydraulics affect sewer system capacity and flood risks as well. Modifications to store sewage and prevent discharge to the environment can constrain capacity and contribute to higher back-up risks, as documented in approved Class Environmental Assessment Studies in Ontario. Call this "The Law of Conservation of Poop" - holding back sewage in the collection system to prevent overflows causes surcharge levels to rise, sometimes closer to basements, increasing basement flooding risks. The excerpt below from the Toronto Area 32 Municipal Class EA describes "Causes of Flooding" related to operation of the tanks installed to protect Lake Ontario and beach water quality:

And while stormwater runoff and sewage level are rising in storm and wastewater collection systems due to urbanization and hydraulic constraints, risks are being increased by lowering basements, exposing higher value finishing and contents to flood damages - in Toronto, the rate of basement lowering, tracked through Toronto Open Data building permits for foundation underpinning, has increased significantly as shown in this post. The chart below shows the data trends:

A new report "Canada’s Changing Climate Report" lead by Environment and Climate Change Canada confirms that there is no change in extreme rainfall in Canada based on observations (see Chapter 4) saying "There do not appear to be detectable trends ...":

This certainly contradicts claims made by an insurance industry-funded research group that have indicated there is 'a lot of data to show it' when it comes to bigger storms. A February, 8, 2018 presentation to the Standing Senate Committee on Energy, the Environment and Natural Resources included this statement:

"So when you see in the news and the media people talk about storms seem bigger and more intense and so forth, those perceptions are correct. And there's a lot of data to show it."

But a review in a recent presentation to the National Research Council's 2018 workshop on flooding that showed there is no data to support the statement. Concerns with insurance industry statements on frequency shifts were also expressed by Environment and Climate Change Canada staff in relation to the Telling the Weather Story 40 year to 6 year weather shift. Staff had concerns with statements that could confuse theory and actual changes. Here is an excerpt from communications regarding the Telling the Weather Story normal bell curve theory shift:

"The presentation looks to be a simple conceptual model for communicating the underlying idea – if one assumes a standard normal, then a shift in the mean implies an attendant change in extremes – which is fine as far as it goes. If this is used as the basis for statements about actual changes in extreme rainfall in Canada, then I would have concerns."

Here was the specific question posed:

Here is a graphic showing the theoretical shift in question, an arbitrary 1 standard deviation shift in a standard normal 'bell curve' (probability density function):

The Environment and Climate Change Canada report also speaks to theoretical shifts in probability density functions, like the Weather Story bell curve shift. This is the example showing a shift right in the distribution of extreme events Figure 4.2.1:

The reality is that in some regions when it comes to extreme rain intensities there is not a shift to the right but a shift to the left, meaning less extreme events, as shown in this annotated curve that reflects southern Ontario rain intensity shifts:

The 'green' shift to the left reflects an overall decrease of 0.4% in rainfall design intensities at 21 long term climate stations since 1990, considering durations related to urban flooding, i.e., 5 minutes to 24 hours. That analysis of the new Version 3.0 Engineering Climate Datasets was presented in this post.

There is often a statement that changes in means will lead to changes in extremes in a distribution of probabilities - this makes sense. This concept is reflected in IPCC reports as well:

But data shows that the means, the 2 Year storm rain intensities, the events that we have the most observations of and the most confidence in assessing trends are decreasing the most. The Version 3.0 datsets review for southern Ontario shows on average a drop of -0.8% in those rain intensities, as shown on this table in the first column:

In this region, the extremes can be expected to decrease along with the means - on average that is happening too for the 100 Year rain intensities.

The Environment and Climate Change Canada report notes 'medium confidence' in increases in annual precipitation across the country and "low confidence in quantifying regional or national total amounts of precipitation" - so medium confidence in it going up but low confidence in saying how much, especially at more local spatial scales, or regions.

Since little or no infrastructure is designed to address annual precipitation, the reports limitations on the annual precipitation statistic are irrelevant to cities facing challenges like urban flooding during extreme, short duration events. Based on CatIQ datasets, a higher number of flood claims and a higher value of claim is associated with rare storm volumes falling over duration of minutes and hours and not annual totals.

The key take-away is that extreme rainfall has not been observed to change, whether for higher frequency events like 2 Year storms, or for low frequency, rare events, like 100 Year storms.

It is easy for the media to confuse annual precipitation with rain extremes, and in the case of Canada’s Changing Climate Report, CBC News reported that urban flooding related to intense rain will increase too - CBC has since corrected that article noting the report did not find increased short-duration rainfall linked to basement flooding:

The Environment and Climate Change Canada report cites research that points to land use change having a "key role" in affecting flooding, for example for the southeast Prairies flood in 2014. Here is the excerpt on attribution of flooding to rainfall or other factors, saying "Anthropogenic influence may have influenced rainfall, but landscape modification played a key role in increased runoff":

This is consistent with reporting by the American Society of Civil Engineers who in their Adapting Infrastructure and Civil Engineering Practice to a Changing Climate document state: "It is important to point out that land-use changes (e.g., urbanization) can result in substantial flooding impacts, independent of climatic forcing functions." - see page 12.

Regarding attribution, it is also consistent with a recent report on extreme rainfall event attribution that also identifies a lack of association of extreme convective storms, those responsible for much urban flooding, with anthropogenic climate change effects. For example the National Academies of Sciences, Engineering, and Medicine. 2016 report Attribution of Extreme Weather Events in the Context of Climate Change states (see page 97):

"Studies of trends in the United States find different results depending on the time period and spatial region chosen, but there is no broad agreement on the detection of long-term trends in overall severe
convective storm activity such as might be related to anthropogenic climate change."

Regarding land use influence on runoff and flood risk, this is also consistent with analysis by the University of Guelph's Engineering Department on changes in urban 'runoff coefficients' (the fraction of rain that runs off and can contribute to flood stresses) due to urbanization like in the Don River watershed:

That analysis was intended to 'disentagle' the impacts of climate change and land use change. Green bars are pre-urbanization coefficients showing we had a small fraction of rain becoming runoff, while blue bars show significant increase in runoff potential after 50% urbanaization. Note there is uncertainty in flow monitoring too, just like in precipitation monitoring, but we see a 10 times, 1000% increase in runoff potential in summer months, when we have the highest rain intensities, due to urbanization. The urbanization effects are MASSIVE - the Scarborough Golf court case reiterated this fact over and over referring to "markedly increased flows".

Compared to urbanization effects on flows, meteorologic effects are a big "nothing burger", with no observed changes and just a lot of theory and speculation. We should design for uncertainty in the future, and incorporate cost-effective adaptation considerations or flexibility for future adaptation (ASCE's Observational Method for climate adaptation) however we should not mischaractierize past trends and risk factors driving today's infrastructure performance limitations.

The University of Guelph analysis also indicates that spring peak flow rates will decrease with climate change effects that reduce winter snowpacks and spring melt flood potential. The follow chart shows the decrease in spring peaks in the rural Moira River watershed:

The Environment and Climate Change Canada report recognizes the impacts of temperature on snow patterns in Chapter 4: "As temperatures increase, there will continue to be a shift from snow to rain in the spring and fall seasons.". The report also cites research that "The reduction in spring snow pack and the ensuing reduction in summer streamflow in British Columbia have been attributed to anthropogenic climate change". Other cited research notes "Such a change in the form of precipitation, from snow to rain, has profound impacts in other components of the physical environment, such as river flow, with the spring freshet becoming significantly earlier." - the University of Guelph research shows that the winter period flows increase from November to early March in the Moira River example, and the peaks decrease significantly from late March and April. This decrease in peaks will result in a decrease in spring flood risks in watershed affected by such events.

So there is no new wild weather, or new normal driving flood damages. Case law in Ontario defining the effects of hydrology, or urbanization, findings of inquiries into Don River flooding for Premier Davis, Municipal Class Environmental Assessment studies investigating basement flooding causes and solutions, and Environment and Climate Change Canada's Engineering Climate Datasets that examine trends in observed rainfall intensities show us that hydrology, hydraulics, fluviogeomorphology explain today's flood risks, and there is has been no shift in rainfall intensities, despite median and insurance industry 'weather stories' and claims.