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.   

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) https://www.cbc.ca/news2/interactives/inourbackyard/
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 


How Have Rainfall Intensities Changed in Canada Over the Past 10 Year? Not Much. Extreme 100-Year Rainfall and Short Duration Intensities Causing Flooding Are Lower

Environment and Climate Change Canada's Engineering Climate Datasets including rainfall intensity duration frequency (IDF) statistics are regularly updated as observation records become longer, and more and more stations have sufficient data to analyze.

What do the recent updates show? There is no new normal in design rainfall intensities.  Over the past 10 years, the severity of extreme rainfall has decreased on average.

Short duration sudden rainfall rates responsible for flooding in urban areas have also decreased overall - only the frequent, low intensities show an overall increase, which can be expected given additional precipitation in Canada. Of course some regions may have different trends (a previous post has shown that the southern Ontario frequent intensities (i.e., 2-year return period) have decreased).

Where do design intensities, the statistics in IDF curves and tables, come from?

Annual maximum series (AMS) of recorded rain intensity are collected for duration intervals of 5 minutes to 24 hours.  These series are used to derive probability density functions to describe the frequency distribution of rainfall, and that can be used to determine specific 'return period' design intensities.  The return period is the inverse of the probability of a rainfall intensity (or volume) over a certain duration occurring during a given year.  So a 100-year intensity has a 1/100 or 1% chance of being exceeded each year, while a 2-year intensity has a 1/2 = 50% chance per year. Storm sewers are designed to convey 2, 5 to 10-year return period rain intensities - 5-year is most common.  Flooding, especially extreme flooding, occurs at higher return periods becoming more severe above the 25-year return period and increasing for 50 and 100-year intensities.

The recent version 3.10 update to IDF statistics analyzes rainfall data up to 2017.  These intensities can be compared to the version 2.00 datasets that included data up to 2007.  A total of 226 stations were analyzed to check for changes in intensity - this total includes about 72 stations that have been relocated, but by not more than 5 km from their previous location.  The same trends are apparent for all the exact match stations (92 stations) and stations with new IDs but unchanged coordinates (154 stations).

The following chart shows the ratio of new intensities to old intensities for these 226 stations, so 1.0 means no change in design intensities.

Extreme Rainfall Trends in Canada - Design Intensities by Duration and Return Period

What are the take-aways?

1) rainfall design intensities are generally unchanged over the past 10 years, considering 3313 station-years of additional data,

2) extreme rainfall intensities, the 100-year rates (red markers in the chart), have decreased - the shortest duration intensity governing urban flood risk has dropped the most,

3) short duration intensities that govern sewver design, 5-year return period intensities (purple markers) over 5-minute to 2 hour durations are unchanged on average,

4) 2-year intensities (green markers), the low intensity rainfall that is exceeded in 50% of years, has increased slightly - these intensities do not govern infrastructure design and are unrelated to urban flash flooding or flood damages.

Popular media has focused on theoretical changes in rainfall intensity, sometimes confusing those projections with actual changes in rainfall intensity that have been measured or observed.  See this review of recent CBC coverage in the Financial Post.  Increasing damage amounts are erroneously linked to changes in rainfall due to a changing climate.

If popular media were to focus on observed data, and actual trends in extreme rainfall statistics, like the trends reviewed above, it would have to temper claims of a new normal in extreme weather.  Data do not show increases the critical rainfall intensities - in fact, on average, extreme intensities have decreased.

Changes in v2.00 to v3.10 dataset intensities are shown in the tables below.

Rainfall Trends in Canada

The analysis above is based on assessing the effect of adding additional data to the v2.00 IDF data intensities.  It is also possible to assess the effects of new data by splitting the series into old and new halves to compare IDF intensities and look for trends.  The following charts show the change in two long-period climate stations in the Toronto area.   Rainfall volumes are shown for a 24 hour period - intensities would be simply the volumes divided by 24 hours.

Toronto Pearson International Airport Climate Station - Changes in 24 Hour Rainfall Frequencies

For the Toronto Pearson International Airport climate station, the return periods of the old period volumes (blue line) have shifted right in the new data set, meaning longer return periods for a given volume, i.e., lower frequency.

The chart also compares how a climate model has predicted return periods have changed from 1961 to 2010, covering approximately a similar period.  Those model frequency shifts were reported by the CBC (link: https://www.cbc.ca/news/technology/extreme-rainfall-climate-change-1.5595396) and considered a 1 degree warming scenario. The climate model predicts lower return periods for a given volume, meaning that volume occurs more frequently - that is not consistent with observed local data at this station that has shown significantly longer return periods in the new period.

Toronto City Climate Station - Changes in 24 Hour Rainfall Frequencies

For the Toronto City (downtown) climate station, the return periods of the old period volumes (blue line) have shifted slightly right in the new data set, meaning slightly longer return periods for a given volume, i.e., slightly lower frequency.

The chart again compares climate model return periods for 1961 to 2010.  Again, the model, which represent a large area, and not necessarily the specifics of the Toronto area predicts lower return periods for a given volume, meaning that volume occurs more frequently - that is not consistent with observed local data at this station that has shown no significant change.

It is possible to look at the change in intensity as opposed to the change in frequency.  The following chart for Toronto Pearson International Airport climate station presents the same data but expresses the changes in terms of intensity, as opposed to frequency.

Toronto Pearson International Airport Climate Station - Changes in 24 Hour Rainfall Volumes
Often you can read in media reports that both the frequency and intensity increased over time - this is a peculiar way to express changes as that data can be used to show a change in one or the other but realistically not both at the same time.  To show the change in frequency and the change in intensity would mean allocating the change in some proportion to the two.

***

Do we have enough weather stations to analyze trends in observations - yes! - we are getting more and more stations and data over time - see previous post regarding additional Environment Canada stations since 1990.

In addition, municipalities are adding 100's of stations to support local studies as described in another post. More rain intensity data than ever before.

Although the data shows less extreme rainfall in Canada, some confuse models that predict future conditions and measured data.  The CBC misinterpreted a model predicting that 50 year storms would happen every 35 years in a time period out to 2015, and reported that this projections has already happened - read more about that here.

Do We have Enough Climate Stations in Canada To Track Trends in Extreme Rainfall?

Some have suggested that we have lost so many climate stations due to cut backs in the 1990's that we can't accurately detect trends in extreme rainfall.  But many are confusing manual climate stations with the stations that collect rainfall intensity data, often automatically.  The number of stations measuring extreme rainfall has been increasing since 1990.

Declining number of stations was noted in the ECO's report 2018 GREENHOUSE GAS PROGRESS REPORT CLIMATE ACTION IN ONTARIO: WHAT'S NEXT? - (see Appendix D)
https://docs.assets.eco.on.ca/reports/climate-change/2018/Climate-Action-in-Ontario.pdf

CBC New has also referred to this concept in responding to a complaint to the CBC Ombudsman regarding accuracy in reporting on extreme weather trends.  What has been cited as evidence of that decline is the chart in Appendix D in the ECO report above. CBC's Director of Journalistic Standards Paul Hambleton wrote:

"The report suggests several possible reasons for this inconsistency, including issues with data collection: There simply are not enough rain gauges. Rainfall data is collected using rain gauge buckets that can record both amount and intensity of rainfall. After a series of federal budget cuts in the 1990s, there are fewer rain gauge stations across the country than there were 60 years ago."

Fewer rain gauge stations? Or fewer "manual" rain gauge stations?  Yes there is a difference.

What does that chart show?  It summarizes declining manual stations in Canada and is a excerpt from the paper in Atmosphere-ocean An Overview of Surface-Based Precipitation Observations at Environment and Climate Change Canada (Mekis et al., 2018) - https://www.researchgate.net/publication/324041502_An_Overview_of_Surface-Based_Precipitation_Observations_at_Environment_and_Climate_Change_Canada

The chart of manual station count in Canada is Figure 2a in the paper on the left below.
Number of Manual Climate Stations in Canada

This chart has been referred to in discussions on extreme rainfall trends.  For example, in the ECO report this chart has been related to intensity-duration-frequency of isolated localized storms as in the excerpt at right:

Readers of this blog will have seen extensive analysis of the trends in extreme rainfall across Canada, including annual maximum series and intensity-duration-frequency (IDF) trends.  The data used is that of Environment and Climate Change Canada, distributed in the Engineering Climate Datasets.

What do Engineering Climate Datasets show us in terms of number of stations that collect and analyze extreme rainfall and IDF trends - they have been increasing!  And the number of station-years of data has been increasing - that means more long-term data to support more reliable statistical analysis.  Good news. The following table summarizes the trends:

Rainfall Intensity Data in Canada
Number of Climate Stations in Canada With Rainfall Intensity Analysis

The newer datasets include more stations, a 22% increase in station count since 1990. And the number of station-years has increased by 48% since 1990 - that's almost 50% more data to analyze and derive IDF design curves since I graduated and started working in this field.

How have the number of stations with extreme rainfall analysis, increasing since 1990, compared to the number of manual stations decreasing since 1990? See chart below:

Climate stations in Canada - trends, count, type
Number of Climate Stations in Canada - Manual and Intensity-Duration-Frequency Stations.  Manual stations decreasing while IDF stations and number of station-years of data increasing. (note: v2.00 (557 stations) and v3.00 (596 stations) not shown on chart)

The Mekis et al. figure is shown in blue and the IDF station trends in orange. Obviously the decline in manual stations does not relate at all to the trends in IDF stations.  As noted in other blog posts, municipal IDF stations have also proliferated over past decades, complementing the IDF stations charted above.

So when CBC's Paul Hambleton writes: "After a series of federal budget cuts in the 1990s, there are fewer rain gauge stations across the country than there were 60 years ago" he missed an important detail - yes manual stations that are expensive to operate have declined, as we expect.  It makes sense that we have fewer manual climate stations since 1990. 
Technology changes.  A good summary of the changes in equipment is described by Mekis et al. - image above are from the website https://www.wikiwand.com/en/Rain_gauge that describes the history of rain gauges and their evolution.

But what about automated weather stations? And what about the number of stations used to collect extreme rainfall information and rainfall intensities? Has the number of stations that define extreme rainfall decreased since 1990? No.

IDF stations have increased from 532 to 651 stations since 1990, many with longer periods of record - we have more extreme weather data to rely on today!  The CBC and others should clearly be more careful when interpreting data on climate station and extreme rainfall  monitoring.  

Yes, we're getting more extreme rainfall, and it's due to climate change, study confirms .. well not so fast

CBC News has a new report "Yes, we're getting more extreme rainfall, and it's due to climate change, study confirms" https://www.cbc.ca/news/technology/extreme-rainfall-climate-change-1.5595396

The byline is "Federal scientists predict more frequent and severe rainfall in future", referring to this research paper Human influence has intensified extreme precipitation in North America by Megan C. Kirchmeier-Young and Xuebin Zhang
https://www.pnas.org/content/early/2020/05/26/1921628117

The research paper refers to "heavy rainfall", i.e., Kirchmeier-Young the lead author and research scientist at Environment and Climate Change Canada stated "We're finding that in North America, we have seen an increase in the frequency and severity of heavy rainfall events."

Kirchmeier-Young also refers to "extreme rainfall" and makes a connection to urban flooding in the CBC article:

"And as we continue to see warming, we will continue to see increases in the frequency and severity of extreme rainfall," Kirchmeier-Young said. "And heavy rainfall is one of the major factors in flash flooding, particularly in urban areas."

The CBC relates extreme weather to rising insured flood damage trends in Canada since the early 1980's.

Let's review:

1) What 'heavy rainfall' events were reviewed in Kirchmeier-Young's research paper?

2) Is 'heavy rainfall' for a climate researcher the same as 'extreme rainfall' for an engineer?

3) Do 'heavy rainfall' and precipitation trends follow 'extreme rainfall' trends used in engineering design?

4) Do 'heavy rainfall' events studied in the research paper cause damaging flood events, and flash flooding 'particularly in urban areas?

5) What are the trends in 'extreme rainfall' in Environment and Climate Change Canada's Engineering Climate Datasets, the data used by engineers to analyze and design infrastructure to manage flash flooding risks in urban areas?

6) What does Kirchmeier-Young's research paper reveal about previous extreme rainfall and flooding events in Canada - has climate change increased runoff that could aggravate flood damages?

1) What 'heavy rainfall' events were reviewed in Kirchmeier-Young's research paper?

The research paper abstract indicates "Here, we address the question of whether observed changes in annual maximum 1- and 5-d precipitation can be attributed to human influence on the climate."

What does "1- and 5-d precipitation" mean?  This is the amount of rainfall over one to five days, so 24 to 120 hours. While precipitation can include snowfall too, the focus is on rain.

Note, the research paper actually refers to 'heavy precipitation' and not 'heavy rainfall'.

The authors have confirmed that short-duration rainfall was not reviewed, only annual maximum daily rain.

2) Is 'heavy rainfall' for a climate researcher the same as 'extreme rainfall' for an engineer?

No.

The research paper states:

"We focus on the annual maxima of 1-d (Rx1day) and 5-d (Rx5day) rainfall. Rx1day is important for flash floods as well as infrastructure design. Rx5day is relevant to large-scale river flooding."

A training session on the use of rainfall intensity design curves from a climate scientist (link: http://projects.upei.ca/climate/files/2012/07/IDFtraining-Auld-final.pdf) indicates that shorter times influence flooding (underline and all-caps emphasis are in the original material, not added here):

 "An urban centre could experience flooding from heavy rains falling over a SHORT period of time, such as A 5 TO 30 MINUTE PERIOD."

"• A rural highway with deep ditches on its shoulders would not likely be impacted by an intense rainfall lasting only 5 to 15 minutes, although the paved road itself would see ponding of water.
• A heavy rainfall event lasting 1 to 6 hours might be more significant for filling the ditches and overflowing the roadway."

So short durations are important for flooding.

From the insurance industry perspective, an Institute for Catastrophic Loss Reduction paper in Journal of Flood Risk Management notes the importance of short-duration rainfall (link: https://onlinelibrary.wiley.com/doi/pdfdirect/10.1111/jfr3.12168) states

"Subdivisions built before the 1970s are less likely to be serviced by major systems (Watt et al., 2003), and are thus more vulnerable to overland flooding from extreme short-duration rainfall events."

A civil engineer will tell you that rarely that the a 1-day rainfall is not 'important for flash floods'.  Why? Because urban flooding is caused by short-duration rainfall.  Designers of storage facilities such as stormwater manage ponds may consider design rainfall events up to 24 hours.

In the Canadian Water Resources Journal, authors of Flood processes in Canada: Regional and special aspects (link: https://www.usask.ca/hydrology/papers/Buttle_et_al_2016.pdf) representing six universities across Canada, INRS-ete, and Environment Canada review "key processes that generate floods in Canada":

"Similarly, floods can be generated across most of the country by rainstorms with large depths and/
or intensities (Figure 1). Thus, convective and frontal systems can generate large short-duration rainfall intensities (Alila 2000) which can occur in all regions (Table 1). Nevertheless, the significance of such storms to flood generation varies across the country, with the greatest
depths and intensities for short-duration events in southern parts of Canada and the smallest in the Arctic. These short-duration events are often responsible for flood generation in relatively small drainage basins, given the greater chance of high-intensity rainfall occurring over
the entire basin (Watt et al. 1989)."

"Short-duration events are often responsible for flood generation".

"Small drainage basins" is equivalent to urban drainage systems. In the municipality where I was Manager, Stormwater our storm sewer drainage systems averaged just over 50 hectares in size.  Urban drainage systems that are 'flashy', responding quickly to rainfall running off hard surfaces, are characterized in engineering design by a 'time of concentration' that is the response time of the drainage area, and which is used to determine the extreme rainfall durations relevant to infrastructure design.  It is never 24 hours or one day.  Typical times of concentration are measured in minutes and up to hours.

The Ontario Ministry of Transportation describes the design rain storms that may be used to analyze rural and urban areas, including the duration of the storm (link:   http://www.mto.gov.on.ca/english/publications/drainage/stormwater/section10.shtml):



Storms of duration up to 24 hours are applicable to rural land uses.  Storms of up to 4 hours (including flashy Chicago hyetograph temporal distributions) are applicable to urban areas.  The SWMM Knowledge Base, a discussion forum for the standard U.S.EPA Stormwater Management Model and other modelling platforms, provides insight into what storm durations practicing civil engineering / urban system modelling professionals use.  In the discussion thread "Design storm duration" (link: https://www.openswmm.org/Topic/3967/design-storm-duration) a duration of 24 hours is deemed by one practitioner to be 'ultra conservative' ("ultra conservative choice of a 24-hour storm but it hardly can be justified when no detention storage is involved"), another states that in small urban systems the 5-minute rainfall governs peak flows ("a small (25 acre) urban, very impervious, drainage area was that the peaks were almost the same no matter the duration, and that they were driven by the peak 5-minute rainfall"), and Ben Urbonas, Ben Urbonas,
President of Urban Watersheds Research Institute and Owner, Urban Watersheds, LLC (LinkedIn: https://www.linkedin.com/in/ben-urbonas-2a319338/)  notes the use of durations of 2-6 hours ("All of our design storms are front loaded intensity types and range from 2-hour to 6-hour durations depending on watershed area.").

Marsalek and Watt's paper Design storms for urban drainage design in the Canadian Journal of Civil Engineering shows design storm durations of often 1 hour duration, sometimes up to 6 hours (US Soil Conservation Service (SCS) for rural areas, as highlighted in their Figure 1.

Marsalek and Watt tablulate design storms with duration and categorize the use of the storms for different hydrological studies, including urban/sewer design and other applications, such as the study of large rural basins. Table 2 from their urban drainage review shows durations of up to 1- 12 hours for Canada's Atmospheric Environment Service's (AES) storm, and 1, 3 and 4 hour storms for sewer sizing in other jurisdictions (see highlights below).

Practitioners in Ontario, Canada will know that longer duration storms are considered for large regional wastewater systems that have a slow response to long-high volume storm events.  These govern large trunk sewer system performance, but not local sewer system performance that is dominated by short duration rainfall.  Even small wastewater system trunks may be governed by short duration rainfall intensities where there are direct inflows, which is common for many flood prone systems.  Analysis of trunk system response in the Kitchener-Waterloo Region showed wastewater trunk peaks flows for most-highly correlated to the 5-minutes rainfall intensities in on Master Plan study (i.e., more than longer durations).


Rivard's paper in the Journal of Water Management Modelling entitled Design Storm Events for Urban Drainage Based on Historical Rainfall Data: a Conceptual Framework for a Logical Approach (link: https://www.chijournal.org/Journals/PDF/R191-12) summarize early work on characterizing storms in Canada and in the highlighted excerpt notes that 1- 12 hour durations represented convective (thunderstorm) and synoptic scale events. See the highlight to the right.

Rivard also summarized what storm durations are of interest for urban design graphically as follows:


So in small to medium basin, up to a 3 hour duration is critical, and for a very large urban basin, up to 6 hours.  Twenty four hour durations and longer are critical to large rural basins.

The statement "Rx1day is important for flash floods as well as infrastructure design." is therefore inconsistent with professional engineering practice in Canada.

Environment and Climate Change Canada publishes Engineering Climate Datasets including Intensity-Duration-Frequency statistics describing rainfall, both common, moderate and extreme, used from infrastructure design.  The durations analyzed are from 5-minutes to 24-hours.

So again, no, 'heavy rainfall' in a climate research paper is not the same as 'extreme rainfall' an engineer uses for infrastructure analysis and design. Rainfall over 1-5 days periods is not the same as extreme rainfall over minutes to hours used to design conveyance systems in urban areas - those 'flashy' systems with short 'time of concentration' characteristics.

The statement in the research paper "Rx1day is important for flash floods as well as infrastructure design." is questionable.  One-day rainfall is way at the fringe of influence on flash flooding.

3) Do 'heavy rainfall' and precipitation trends follow 'extreme rainfall' trends used in engineering design?

Kirchmeier-Young's research found that 1-day duration simulated precipitation from various models has increased over past decades, and this trend follows observations from HadEX2 (a global gridded dataset).

We can compare the HadEX2 trends across North America, and subregions shown in the research paper, with extreme rainfall trends based on Canadian climate station observations.  Let's start with the 1-day, 24-hour annual maximum rainfall trends across Canada.

The chart below shows how annual maximum rainfall has changed according to Environment and Climate Change Canada's version 3.10 Engineering Climate Datasets for all storm durations from 5-minutes to 24-hours.


For 24-hour durations, 4.9% of all stations have a significant increase, 91.2% have no significant change, 2.3% have significant decreases and 1.5% of stations had no data.

Comparing to earlier datasets:

                                                     Version 2.30           Version 3.00            Version 3.10

No significant 24-hour trend            91.5%                    91.1%                         91.2%

Significant 24-hour increase              5.3%                      5.4%                           4.9%           

So the percentage of data that has no significant trend is relatively steady, and represents over 90% of the data. The percentage of data that has a significant increase in 24-hour rainfall is decreasing relative to the earlier datasets.

Canadian Engineering Climate Dataset trend data does not show increases consistent with the research paper.

4) Do 'heavy rainfall' events studied in the research paper cause damaging flood events, and flash flooding 'particularly in urban areas?

No.  Flash flooding is due to short duration, high-intensity rainfall.

The severe thunderstorms that are responsible for urban flooding and that occur over minutes to hours are different than the storms that occur over hours to days as indicated in the RSI IDF training presentation noted above:


For this reason, those interested in turban flooding drivers should look at short duration rainfall extremes - see below.

5) What are the trends in 'extreme rainfall' in Environment and Climate Change Canada's Engineering Climate Datasets, the data used by engineers to analyze and design infrastructure to manage flash flooding risks in urban areas?

Short duration rainfall is responsible for urban flash flooding.  Environment and Climate Change Canada's Engineering Climate Datasets indicate the following on annual maximum rainfall trends across Canada:


The short durations from minutes to a couple hours have low percentages of significant increase, just like the 24-hour data noted above.  The amount of significant increases expected due to chance is 2.5% increasing and 2.5% decreasing.

In a review of an earlier dataset by Environment Canada's Shephard et. al in 2014 (link: https://www.tandfonline.com/doi/pdf/10.1080/07055900.2014.969677) these amounts of changes were deemed not significant:

"Based on this IDF single station analysis, and the more general single station climate results from the 1965–2005 period presented in Section 4a, we conclude that the annual maximum short duration rainfall values across Canada typically do not show a significant trend."

And more recently in Canada’s Changing Climate Report, such changes in short duration extreme precipitation were explained by chance (link: https://changingclimate.ca/CCCR2019/):

"There do not appear to be detectable trends in short-duration extreme precipitation in Canada for the country as a whole based on available station data. More stations have experienced an increase than a decrease in the highest amount of one-day rainfall each year, but the direction of trends is rather random over space. Some stations show significant trends, but the number of sites that had significant trends is not more than what one would expect from chance (Shephard et al., 2014; Mekis et al., 2015; Vincent et al., 2018)."

The short duration intensities used for infrastructure design, derived based on annual maximum series, have not increased in many regions based on compiled studies (see previous post: https://www.cityfloodmap.com/2018/03/extreme-rainfall-and-climate-change-in.html).  A review of design intensities in southern Ontario shows overall increases in short duration values (see previous post: https://www.cityfloodmap.com/2020/05/southern-ontario-extreme-rainfall.html).

So no change in how infrastructure is designed based on short-duration design intensities (that is, not including checks or 'stress tests' for future changes).

6) What does Kirchmeier-Young's research paper reveal about previous extreme rainfall and flooding events in Canada - has climate change increased runoff that could aggravate flood damages?

Nothing.  The storms that lead to widespread urban flooding are not addressed in the research paper.  The processes driving 1-5 day rainfall are different than those driving short-duration rainfall.  There are no significant increases in the short-duration rainfall that causes flooding based on Engineering Climate Datasets as shown above.

Why then have damages increased over decades? Possible reasons are:

a) growth in net written premiums: more insured properties = more losses


b) urbanization: more pavement means more runoff and impacts

The landmark case Scarborough Golf Country Club Ltd v City of Scarborough et al. (Ontario Court of Appeal, 1988, http://members.storm.ca/~river/letters/Scarboro%20Golf%20Club%20v%20City%20of%20Scarboro%20OCA%201988.pdf) decision indicates that Toronto-area urbanization markedly increased runoff stresses that caused runoff, erosion and flooding:

“Expert evidence confirmed the effect of the city's rapid urbanization and water control plans on the creek.”

“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

“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.”

Cities are growing and there is more runoff as shown here in some regions:

Urbanization and Flood Risks

c) inconsistent data: the data source for losses cited by CBC changed from 2008 onward

Changing data methods can lead to different results (see previous post on this: https://www.cityfloodmap.com/2018/06/catastrophic-losses-in-canada-have.html)



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The research paper makes a reference to an attribution study for the 2013 Alberta flood.  It states:

"Additionally, event attribution studies have identified an increased probability of some individual extreme precipitation events in this region due to anthropogenic influence (4, 5)"

Reference 4 is:

B. Teufel et al., Investigation of the 2013 Alberta flood from weather and climate
perspectives. Clim. Dynam. 48, 2881–2899 (2017). (link: https://link.springer.com/article/10.1007/s00382-016-3239-8)

So we have one Canadian storm assessed. Findings are:

"Event attribution analysis suggests that greenhouse gas increases may have increased 1-day and 3-day return levels of May–June precipitation with respect to pre-industrial climate conditions. However, no anthropogenic influence can be detected for 1-day and 3-day surface runoff, as increases in extreme precipitation in the present-day climate are offset by decreased snow cover and lower frozen water content in soils during the May–June transition months, compared to pre-industrial climate."

So greenhouse gases may have increased precipitation, but that is offset by less snow, resulting in no change in runoff, compared to pre-industrial climate.

So with no change in runoff, can there be a change in flood damages attributed to the precipitation change?  The net effect is no increase in risk.

***

To wrap it up, CBC has relied on a research paper that looks at rainfall events (1-5 day precipitation) that are not related to urban flash flooding and are not related to the events that lead to significant damages (convective thunderstorms with peak intensities over minutes to hours).  The research does not review short-duration rainfall that is relevant to infrastructure design governed by short 'times of concentration' - i.e., they are 'flashy'.  The research does not appear to be consistent with trends in 24-hour annual maximum rainfall observed at Canadian climate stations and as published in Environment and Climate Change Canada's Engineering Climate Datasets - data show few statistically significant increases and the percentage of significant increases is decreasing slightly for 24-hour rainfall across Canada.  The short-duration rainfall intensities responsible for urban flooding show no consistent changes, and any significant changes are explained by chance, according to Environment Canada.

Many factors go into increasing flood damages.  Changes in rainfall does not appear to be one of those factors.  Media should take the time to dive deeper into the technical details they reference to improve the accuracy of reporting, so that the public is better informed about complex issues.
Urban flooding is a complex issue, and an important challenge to address that requires significant funding and attention.  A better understanding of the causes of flooding, and any changes in design rainfall, is required to mitigate flooding in the most objective, cost-effective manner.  CBC has relied more on model predictions than on actual data in the past, even confusing the two (see previous post: https://www.cityfloodmap.com/2020/05/what-covid-19-taught-us-about-observed.html).  In this recent report it has not met its JSP principle for accuracy by confusing longer-term precipitation and short-duration extreme rainfall.

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BONUS - Trends in short-duration rainfall, based on annual maximum observations, from Environment Canada's version 3.10 Engineering Climate Datasets are summarized below (link: https://climate.weather.gc.ca/prods_servs/engineering_e.html).  These tables consider stations with a long period of record and recently updated data for regions across Canada.