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:

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):


"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

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: 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)


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.   

Super Models vs Dowdy Data - How Climate Models Diverge From Observations On Extreme Weather

A recent special article in the Financial Post noted the difference between models and observations on extreme rainfall: link

Recent reporting by CBC and Radio Canada International (RCI) have reported shifts in extreme rainfall frequency, stating that there is confirmation that a warmer climate is now making extreme rainfall more frequent and intense.  The confirmation, however, was from models analyzed by Environment Canada, and not actual measured rainfall.

As pointed out in the Financial Post article, both CBC and RCI confused models with actual observed data in stating broad confirmations.  They overlooked limitations in the models to represent local events and extreme events, omitted data that showed all the models were wrong in some regions (projected increasing rainfall when data showed decreasing rainfall), and failed to mention that other climate effects like less snow in a warmer climate can decrease flood risk, mitigating precipitation increases.

Fundamentally, observed rainfall frequencies and model frequencies are not consistent, despite RCI and CBC reporting.  The following tables show the clear difference between what models project could happen and what actual data show has happened.

This first table relates to the recent CBC and RCI reporting on a North American climate model.  The model predicts that 100 year storms become 20 year storms (i.e., for a given intensity), meaning more frequent.  Alternatively, the model says that intensities of a given frequency are higher.  In contrast, the observed data for Canada show a slight decrease in 100 year intensities at 226 climate stations, meaning storms of a given intensity are are not more frequent, but rather slightly less frequent when recent data are factored in.

Extreme Rainfall in Canada - Trends in Modelled vs Observed Data for 100 Year Storm

The second table below is for the 50 year return period storm - it shows projected model return period shifts of 50 to 35 years from model.  The results are averaged across Canada.  In comparison, 226 climate stations across Canada have observed that results in a slight decrease in 50 year storm intensities.  Like the 100 year storm above, that means actual storm frequencies are lower now.  Old 50 year return periods are now longer than 50 years now.  

Extreme Rainfall in Canada - Trends in Modelled vs Observed Data for 50 Year Storm
The CBC reported the above 50 to 35 year model shift as actually having already occurred in its In Our Backyard interactive: (see flooding tab)
The CBC claimed that intensities in Toronto are greater today, resulting in more flooding.

While it is challenging to draw conclusions from trends at individual climate stations, shifts at a couple of  Toronto climate stations are shown in the 100 year and 50 year tables as well to check the CBC reporting.  The Toronto Pearson International Airport and Toronto City (aka Bloor Street) gauges have very long records to compare old and new intensities.

The old Pearson 100 year 24-hour storm intensity (top table) is now a  417 year storm, meaning it occurs much less frequently now.  Alternatively, the magnitude of the 100 year storm intensity has dropped from past to present, meaning such storms are less severe.  This decline occurred despite that climate station recording the large July 8, 2013 storm.  The 50 year storm is now a 108 year storm, again less frequent than before.

Clearly local data at Pearson Airport, just outside of Toronto is not changing the same way that the Canadian model projections are.  Observed frequencies are longer, while the model estimated them to be shorter.

The Toronto City climate station shows only small changes in 24-hour storm frequency.  The 100 year frequency is slightly shorter at 97 year. Meanwhile the 50 year frequency is slightly longer at 52 year.  These changes are nominal and represent no significant overall change.  They are consistent with the average changes at 226 stations across Canada that also showed no appreciable change when 10 additional years of data were analyzed.  Across Canada, 100 year and 50 year rainfall intensities decreased slightly overall - the 100 year intensities decreased 0.5% and the 50 year intensities decreased 0.6%.

Clearly local data at Toronto City, essentially downtown Toronto, shows no change in extreme storm frequency or intensity, contrary to the CBS's reported model estimates.

To not rely on just a couple Toronto stations, one can look at at changes in intensities at all long term southern Ontario climate stations that have recent data updates.  Comparing the Engineering Climate Datasets v2.00 with data up to 2007 and v3.10 with data up to 2017 one can see a slight decrease in 50 year and 100 year 24-hour intensities, on average.  The stations and their lengths of record are shown below:

Southern Ontario Long Term Climate Stations with Recent IDF Updates (v2.00 to v3.10) - Environment Canada Engineering Climate Datasets
Overall, there are 978 station-years of data to analyze trends.

In southern Ontario the 100 year 24-hour intensities decreased by 1.0% while the 50 year intensities decreased by 0.9%, when additional data was added.  This suggests that the regional trends in Toronto per the Toronto City climate station, showing no overall change, are consistent with other stations in the region.  The southern Ontario data does not support the North American or Canadian model estimates reported by CBC and RCI that expect shorter return periods and higher intensities.

So beware of media reports that mix up models with actual observed data.


The following image expand on the tables above, showing where CBC and RCI made reference to the climate model results, and the text used to describe 'confirmation' of changes in rainfall.  Links to comparison charts (some that were in earlier posts) and tables are also included, showing the actual observed data trends and indicating Environment Canada source material.

Click to enlarge:

Comparison of 100 Year Return Period Rainfall Trends in Canada - Climate Models vs Observed Data, CBC and RCI Reporting

Comparison of 50 Year Return Period Rainfall Trends in Canada - Climate Models vs Observed Data, CBC Reporting