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."

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