Update of Intensity-Duration-Frequency (IDF) Curves for the City of Waterloo and the City of Kitchener Prepared By: Donald H. Burn, Ph.D., P.Eng. Department of Civil and Environmental Engineering University of Waterloo, August 2012.
The executive summary states there is no significant change and some decreasing trends for short duration rain intensities - that is, the design parameters that affect urban infrastructure flood risks:
"Annual maximum rainfall data for durations ranging from five minutes to 24 hours were analyzed for trends using the Mann-Kendall non-parametric trend test. Although no statistically significant trends were identified, there were noticeable patterns in the magnitude and direction of the trends in the rainfall data, as a function of the rainfall duration. Based on the available rainfall data for the period 1971 to 2007, new intensity-duration-frequency (IDF) curves were developed for the Waterloo Wellington A climate station; the curves can be used as IDF curves for the City of Waterloo and the City of Kitchener. The Pearson Type III (PE3) distribution was identified as the preferred distribution function for the data and formed the basis for estimating the quantiles required to form the IDF curves. The rainfall intensity values for the new IDF curves tend to be lower than the corresponding values for the existing curves for rainfall durations of up to one to two hours and generally slightly higher than the rainfall intensity values for the existing curves for the longer rainfall durations. The results indicate that the existing IDF curves for the City of Waterloo and the City of Kitchener are likely somewhat conservative for rainfall durations less than two hours, although the impacts of climate change could result in more severe events in the future."
The University of Waterloo findings are consistent with the 'general lack of detectable trend signal' in past rainfall observations as reported by Environment and Climate Change Canada (ECCC). ECCC reported even some regional decreasing trends (St. Lawrence region of southern Quebec and the Atlantic Provinces) for the short duration intensities affecting urban drainage systems:
Trends in Canadian Short‐Duration Extreme Rainfall: Including an Intensity–Duration–Frequency Perspective Mark W. Shephard, Eva Mekis, Robert J. Morris, Yang Feng, Xuebin Zhang, Karen Kilcup & show all Pages 398-417, Published online: 19 Nov 2014 (Atmosphere-Ocean).
What do the University of Waterloo findings look like? These intensity-duration curves for 5-year, 25-year and 100-year rain frequencies show that for duration less than 2 hours, the storm severity has been decreasing:
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| IDF update to assess climate change impacts for sewer design shows decreasing 5-year rainfall intensities for durations less than 120 minutes (2 hours) - small flashy urban drainage systems now have lower extreme rainfall risk than with previously higher rainfall. Therefore earlier designs using older, higher rainfall design intensities have a safety factors against extreme weather risks for frequent events, and climate change has not adversely affected Kitchener-Waterloo erosion risks or erosion damage potential (erosion risks are often governed by frequent storm stresses). |
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| IDF update to assess climate change impacts for sewer design shows decreasing 25-year rainfall intensities for durations less than 90 minutes (one and a half hours) - small flashy urban drainage systems subject to nuisance flooding now have lower extreme rainfall risks. Therefore earlier storm drainage designs using older, higher rainfall design intensities, have a safety factors against extreme weather risks, and climate change has not adversely affected Kitchener-Waterloo flood risks or damage potential. Wastewater systems that have extraneous flow stresses (i.e., inflow and infiltration) and that respond to short duration rainfall appear to have lower capacity stresses with climate change IDF update. A Class Environmental Assessment Master Plan study in Kitchener has shown that peak wastewater flows in the trunk systems (e.g., Ottawa, Manchester, Montgomery trunks) are most highly correlated to short duration rainfall intensities. A Class Environmental Assessment study of the Sandrock Greenway trunk has also shown that smaller local wastewater systems respond to short duration rainfall intensities, even without high inflow potential in catchments with fully and partially-separated foundation service catchments). |
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| IDF update to assess climate change impacts for sewer design shows decreasing 100-year rainfall intensities for durations less than 90 minutes (one and a half hours) - small flashy urban drainage systems now have lower extreme rainfall risk today compared to previously higher intensities. Therefore earlier designs using older, higher rainfall design intensities have a safety factors against extreme weather risks for rare storm events, and climate change has not adversely affected flood risks or flood damage potential. |
Why do derived IDF values decrease? Because the observed maximum rainfall amounts in the Annual Maximum Series (AMS) have been decreasing. The following two graphs show trends in annual maximum rainfall recordings from 1971 to 2007. The 5-minute duration maximum rainfall has been decreasing and the 2-hour maximum rainfall have been flat - it is over this range that IDF intensities have been shown to be decreasing n the earlier graphs.
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Comparing University of Waterloo analysis with politician statements:
Prime Minister Justin Trudeau recently stated "We are a government grounded in science". If so, why has he stated that 100 year storms may occur every 10 years or sooner following 2017 flooding in Gatineau? There is no data to support the storm frequency statement made by the Prime Minister.
Comparing University of Waterloo analysis with insurance industry statements:
The insurance industry, interested in flood damages and the effect on business activities, has stated that storms that happened every 40 years are now occurring every 6 years. Environment and Climate Change Canada has clearly refuted this frequency shift, most recently in Canadian Underwriter saying its studies do not support the insurance industry statement. As noted in the recent article:
"Associate Editor’s Note: In the 2012 report Telling the Weather Story, commissioned to the Institute for Catastrophic Loss Reduction by the Insurance Bureau of Canada, Professor Gordon McBean writes: “Weather events that used to happen once every 40 years are now happening once every six years in some regions in the country.” A footnote cites “Environment Canada: Intensity-Duration-Frequency Tables and Graphs.” However, a spokesperson for Environment and Climate Change Canada told Canadian Underwriter that ECCC’s studies “have not shown evidence to support” this statement."
The following detailed review of insurance industry statement and comparison with Environment and Climate Change Canada's Engineering Climate Datasets clearly shows how the insurance industry has confused projected rainfall intensity shifts with trends from past observations:
