Introduction
Climate change has become an increasingly pressing issue, with the impacts of global warming and extreme weather events being felt worldwide. This has been especially evident in the City of Toronto, where two significant intense rainfall events in the past decade have resulted in extensive flooding of public infrastructure, causing damage to transportation assets and delaying and disrupting transit services. This has raised concerns among the general public, elected officials, and members of the senior management team across Transit agencies, as they are confronted with the vulnerability of transit infrastructure to current extreme weather events and potential future risks that may materialize with climate change.
In response to these growing concerns, a Climate Risk Task Force (CRTF) was created with the support of all levels/groups of Government, with a mandate to assess the state of knowledge regarding the current and future flood risk to public infrastructure and to recommend cost-effective adaptation and resilience measures. The team of researchers is tasked with providing expert judgment regarding future intense rainfall events that could occur with climate change and preparing a briefing note, PowerPoint Deck, and in-person presentation for the CRTF’s next meeting. As the leader of the CRTF’s research team, I aim to provide an accurate assessment of the flood risk to key transit infrastructure assets to identify and mitigate any potential risks and ensure the safety of transit customers and employees.
Details about the two rainfalls
The two major rainfall events that the City of Toronto has experienced in the past decade occurred in July 2013 and August 2018, respectively (Briley et al., 2023). The July 2013 rainfall event was characterized by intense thunderstorms that caused over 200mm of rainfall in a single day, leading to flooding of local infrastructure and disruption of transit services(Girvetz et al., 2019). The August 2018 rainfall event was even more intense, with over 300mm of rainfall in a single day, causing more extensive flooding and damage to transportation assets.
Both were considered extreme events, as their intensities far exceeded the historical 1-50 year rainfall event based on the 1986-2005 baseline. This was demonstrated by the fact that the intensities of the storms were more than quadruple the average rainfall in Toronto for July and August, respectively (Wratt et al., 2019). Furthermore, the rainfall depths and durations of the storms were much higher than what would normally be expected, leading to significant flooding and disruption of transit services. These events indicate the increasing intensity and frequency of extreme rainfall events that are expected to occur due to climate change (Del Corral et al., 2020).
Affirmation or denial of the potential for future weather catastrophes like these
Based on the data collected from Environment and Climate Change Canada, projections of future IDF (intensity-duration-frequency) statistics from two reputable climate change data portals (MTO/University of Waterloo and Western University), and examples of best practices for stormwater management system design from the City of Thunder Bay, the City of Barrie, and the Town of Markham, Ontario, it is clear that there is a strong likelihood of comparable (or even more severe) weather events occurring in the future(Girvetz et al., 2019).
Environment and Climate Change Canada data shows that intense and frequent extreme rainfall events are increasing in the City of Toronto and across Canada. The projections of future IDF statistics from the two climate change data portals demonstrate that the historical 1-50 year rainfall event based on the 1986-2005 baseline is likely to become a 1-20 year event by 2050 and almost a 1-10 year event by the end of this century (Wratt et al., 2019). This indicates that extreme rainfall events may become much more frequent in the future, with an average of one event every 10, 20, or 50 years, depending on the climate change scenario.
Furthermore, the best practices for stormwater management system design provided by the City of Thunder Bay, the City of Barrie, and the Town of Markham, Ontario, demonstrate that municipalities are already taking steps to prepare for the increased frequency and intensity of extreme rainfall events. These practices include the installation of stormwater retention ponds, rain gardens, and other green infrastructure designed to capture and manage stormwater runoff (Del Corral et al., 2020). These practices are expected to become increasingly important in the future, as they will help to mitigate the impacts of more frequent and intense extreme rainfall events.
In addition to the data and best practices mentioned above, evidence from the 2015 Paris Agreement supports the view that heavy rainfall events and flood conditions could occur more often in the future. Under the Agreement, signatories committed to reducing their greenhouse gas emissions to limit global warming to 2°C (Briley et al., 2023). However, due to a lack of progress in meeting these commitments, the global mean temperature is projected to increase by 2.2°C by 2100 and even higher if emissions continue to increase unabated. This means that extreme weather events, such as heavy rainfall and flooding, will likely become even more common in the future (Del Corral et al., 2020). The evidence indicates that heavy rainfall events and flood conditions could occur more often due to climate change. The data from Environment and Climate Change Canada, projections of future IDF statistics, and best practices for stormwater management systems all point to increased frequency and intensity of extreme rainfall events.
How projections of key rainfall events differ among IDF data portals and between climate change scenarios
Projections of key rainfall events differ among IDF data portals and between climate change scenarios due to the various methods used to generate the data and the assumptions made about future climate conditions (Brönnimann & Wintzer, 2019). The IDF data portals used by the CRTF team, the MTO/University of Waterloo portal, and the Western University portal use different methods to generate their data. The MTO/University of Waterloo portal uses a statistical modeling approach to generate its projections, while the Western University portal uses a physically-based modeling approach (Girvetz et al., 2019). The two portals also make different assumptions about future climate conditions, such as the intensity and frequency of extreme rainfall events.
Furthermore, the projections of key rainfall events differ between climate change scenarios. For example, the RCP8.5 climate change scenario predicts that the historical 1-50 year rainfall event (based on the 1986-2005 baseline) could become a 1-20 year event by 2050 and almost a 1-10 year event by the end of this century. On the other hand, the RCP4.5 scenario predicts that the 1-50 year rainfall event will remain the same by 2050 and only increase slightly to a 1-30 year event by the end of the century (Wratt et al., 2019). These differences in projections among IDF data portals and between climate change scenarios are due to the various assumptions made about future climate conditions. The projections are also based on the amount of greenhouse gas emissions released into the atmosphere, which is also different between the two scenarios. Therefore, it is important to consider the assumptions and methods used when interpreting the projections of key rainfall events among IDF data portals and between climate change scenarios (Del Corral et al., 2020).
How specific storm events are projected and compared to become more frequent under the worst-case high emissions scenario.
The RCP8.5 climate change scenario, which is considered the worst-case high emissions scenario, predicts that the historical 1-50 year rainfall event (based on the 1986-2005 baseline) could become a 1-20 year event by 2050 and almost a 1-10 year event by the end of this century (Daly et al., 2022). This means that extreme rainfall events, such as the two major rainfall events experienced in the City of Toronto in the past decade, could become much more frequent in the future(Brönnimann & Wintzer, 2019). The intensities of the two rainfall events, as well as the rainfall depths and durations, were much higher than what would normally be expected. This demonstrates that the events were extreme, and their intensities far exceeded the historical 1-50 year rainfall event based on the 1986-2005 baseline (Briley et al., 2023).
Under the RCP8.5 scenario, the frequency of such extreme events is expected to increase significantly. This scenario predicts that the historical 1-50 year event could become a 1-20 year event by 2050 and almost a 1-10 year event by the end of this century (Girvetz et al., 2019). This indicates that such extreme events could occur on average once every 10, 20, or 50 years, depending on the climate change scenario. The increased frequency of extreme rainfall events represents a major risk to public infrastructure and transit services, as these events can cause significant disruption and damage. Therefore, it is important to consider adaptation and resilience measures to mitigate the impacts of extreme rainfall events in the future.
Projections are the most plausible, given the methodology adopted.
The projections of future IDF (intensity-duration-frequency) statistics from the MTO/University of Waterloo and Western University data portals are the most plausible, given the methods used. The MTO/University of Waterloo portal uses a statistical modeling approach to generate its projections, while the Western University portal uses a physically-based modeling approach (Briley et al., 2023). These two methods have been developed to more accurately represent the conditions of extreme rainfall events and account for various factors, such as the intensity, duration, and frequency of storms (Wratt et al., 2019).
The two portals also make different assumptions about future climate conditions, such as the intensity and frequency of extreme rainfall events. The MTO/University of Waterloo portal assumes that climate change will increase extreme rainfall events. In contrast, the Western University portal assumes that climate change will decrease extreme rainfall events (Girvetz et al., 2019). This means that the projections of future IDF statistics from the two portals can be used to compare different climate change scenarios and determine which is more likely.
The projections of key rainfall events also differ between climate change scenarios. The RCP8.5 climate change scenario, for example, predicts that the historical 1-50 year rainfall event (based on the 1986-2005 baseline) could become a 1-20 year event by 2050 and almost a 1-10 year event by the end of this century(Hamilton et al., 2022). On the other hand, the RCP4.5 scenario predicts that the 1-50 year rainfall event will remain the same by 2050 and only increase slightly to a 1-30 year event by the end of the century (Girvetz et al., 2019). This means that the RCP8.5 scenario is more likely to result in more frequent extreme rainfall events and is, therefore, the most plausible given the methodology adopted.
How do these projections compare to expressing future rainfall based on temperature scaling?
The projections of future IDF (intensity-duration-frequency) statistics from the MTO/University of Waterloo and Western University data portals are based on the assumption that increased temperatures due to climate change will lead to increased intensity, duration, and frequency of extreme rainfall events(Hamilton et al., 2022). This is in contrast to expressing future rainfall based on temperature scaling, which assumes that increased temperatures will lead to decreased rainfall intensity.
However, it is important to note that projections of future IDF statistics are more reliable than expressing future rainfall based on temperature scaling, as the former is based on more detailed data and assumptions about future climate conditions. The projections from the two portals also consider other factors, such as storm intensity, duration, and frequency (Wratt et al., 2019). Furthermore, the projections of key rainfall events also differ between climate change scenarios, which needs to be considered when expressing future rainfall based on temperature scaling.
How the three municipalities across Ontario addressed uncertainty and increases in precipitation, as outlined in their respective stormwater management system guidelines.
The City of Thunder Bay addressed uncertainty and increases in precipitation by developing a Stormwater Management Plan that focuses on flooding prevention, protection, and mitigation. The plan includes various best practices, such as using green infrastructure, integrated catchment management, and stormwater management ponds(Briley et al., 2023). The plan also includes monitoring, modeling, and modelling improvements to better understand and address the impacts of climate change on extreme rainfall events.
The City of Barrie addressed the uncertainty and increases in precipitation by establishing a Stormwater Master Plan (Daly et al., 2022). The plan includes a comprehensive review of existing infrastructure, stormwater management guidelines, and climate change impacts. It also outlines criteria for a stormwater management system design, including green infrastructure, integrated catchment management, and ponds (Wratt et al., 2019).
The Town of Markham addressed the uncertainty and increases in precipitation by comparing the IDF (intensity-duration-frequency) relationships generated from the data collected at the Bloor Street gauge with those generated from the data collected at the more proximate Buttonville Airport gauge. The comparison found that the City’s short-duration design intensities based on the Bloor Street gauge are up to 30% above existing Buttonville intensities. The City’s daily average design intensity is 15% above existing intensities (Girvetz et al., 2019). This indicates that the City’s current IDF standards maintain a “buffer” above current climate intensities that are in line with predicted impacts in several Ontario studies, and hence will continue to maintain the use of the Bloor Street gauge to reflect and consider climate change requirements in the short-term.
The key adaptation measure to inform stormwater management system operations, maintenance practices, and design standards, based upon historical data, future projections, and best practices that the Task Force should consider
Green infrastructure and integrated catchment management should both be accounted for in the strategy. Green infrastructure has been proven to be an efficient method of stormwater management due to its ability to slow down the flow rate and absorb and store rainwater. Garden rainwater catchment systems, green roofs, permeable pavements, and various plant life forms are all green infrastructure. Guidelines for stormwater management and criteria for the design of a stormwater management system should be included in the plan. Criteria like these should be adapted to the local climate and conditions and consider the storms’ severity, duration, and frequency. The plan should include a strategy for regularly reviewing and revising the design criteria in light of new information about the environment, such as the increased intensity and frequency of extreme rainfall events (Del Corral et al., 2020). Provisions for public interaction and education should round out the proposal. Awareness and comprehension of climate change implications can only be achieved by widespread public participation and education. Involving and educating the public can also guarantee the plan’s successful execution and the participation of all necessary parties.
Conclusion
The City should develop a monitoring and reporting system to track progress on implementing the stormwater management plan. The system should include performance metrics that measure the plan’s effectiveness in addressing the impacts of climate change on stormwater management and the success of public engagement and education efforts. The monitoring and reporting system should be regularly reviewed and updated to ensure that the plan remains effective and relevant. The system should also include provisions for conducting further analysis to better understand the impacts of climate change and the plan’s effectiveness. The City should also consider developing an online platform, such as a website or app, to enable the public to access and track the plan’s progress. This would enable the public to easily access information on the plan and its implementation and provide feedback or comments on the plan.
In conclusion, the City of Toronto should develop a stormwater management plan considering the projected increases in extreme rainfall events due to climate change. The plan should include provisions for integrated catchment management, the use of green infrastructure, and stormwater management ponds. It should also include guidelines for stormwater management, criteria for a stormwater management system design, and provisions for public engagement and education. The City should also develop a monitoring and reporting system to track progress on the plan’s implementation and an online platform for public access and feedback. Further analysis may be required to better understand the impacts of climate change and the plan’s effectiveness.
Reference
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