A comparison of the OSG and the 2007 IDF curves revealed that for short durations the intensities from the 2007 IDF Curves are less than the OSG IDF Curves on average by 5% and 10% for the 5 year and 100 year curves respectively. However, for the longer durations the intensities for the 2007 IDF Curves are actually greater than the OSG IDF Curves on average by 11% and 17% for the 5 year and 100 year curves respectively. The intensities at the shorter durations would influence storm sewer sizing while the longer durations will influence stormwater pond sizing. The lower intensities at the short durations will tend to result in smaller storm sewer sizes while the larger intensities at higher durations will tend to increase stormwater management pond requirements.
Given that the percentage differences in intensities between the IDF curves is within the margin of error associated with data collection and hydrologic assessments, it was ISD’s opinion not to update the OSD IDF curves. As part of the Stormwater Levels of Service review, the need to revisit the IDF curve selection was identified."
Because stormwater ponds are not designed with IDF curve statistics alone, i.e,. they are designed by simulating temporal patters of storms in hydrologic/hydraulic models, it is questionable if an IDF shift in long durations alone would alter the design of a stormwater management pond. That is the design storm (or hyetograph) pattern would influence the pond performance more than the input IDF used to adjust storm volume. Also, some municipalities assess pond performance assuming the outlet is completely blocked, as an operational worst case scenario, such that all collected runoff is routed through the emergency spillway. In this extreme operating condition, a small change in rainfall volume may not affect overall performance (i.e., spillways operate efficiently as weirs, able to accommodate additional flow release with limited increase in operating level (pond water surface elevation).
ANOTHER BONUS - the City of Hamilton has also reviewed IDF curves in 2015 and found no change that warranted updates in their current standards (R.Muir personal communication with Hamilton engineering staff May, 2018).
As part of the assessment, the City of Markham updated local IDF curves for the long-term Toronto City climate station that its design standards are based on, as well as the Toronto International Airport (Pearson) and Markham Buttonville Airport stations IDF curves. The findings related to the wastewater system resiliency assessment, which are also relevant to storm drainage infrastructure, were as follows:
■ “The Pearson station 100-year data showed no change since the ECCC 2013 dataset, and a decrease since the 1990 dataset (average decrease of 3.2%). The Buttonville station 100-year data showed an average increase of only 1.1%. Therefore 100-year short-duration intensities are considered to be stationary for the purpose of the existing system capacity assessment under today’s climate - past rainfall intensities (IDF data) maybe used to assess current wastewater system wet weather performance.”
• the 1963 IDF curves is conservative relative to the estimates made in the 2000 IDF curves.
Thus, adoption of the 2000 curves would effect a relaxation of planning standards for many
types of infrastructure.
• the 1963 curves were conservative relative to the current (2000) estimates and even relative
to the projected (2020 and 2050) values for many duration/return interval combinations. In
those instances, it is reasonable to retain the 1963 intensities."
So even looking into the future, the city's design rainfall intensities from the 1960's are more conservative. So of course if there has been flooding in Welland it's due to other design considerations (like return period level of service for sewer design, etc.) and not because rain is now, or will be, higher than design intensities.
YET ANOTHER BONUS - the City of Niagara Falls has also reviewed its design IDF curves and found lower rainfall intensities. The following is an excerpt from the MOECC (now MECP) LID Guidance Manual (second draft, page 129):
"Many Ontario municipalities have conducted climate change and/or IDF analysis studies to provide direction for municipal infrastructure planners in light of climate change risks. Of note is the City of Niagara Falls which conducted an IDF curve update and climate change analysis as part of their 2015 Master Drainage Plan Update Study. Updated IDFs for four of the five climate stations within the City were found to generate rainfall volumes and intensities that were slightly lower than those generated by the previous IDF curves (Hatch Mott MacDonald, 2015). Additional analysis conducted for Niagara Falls found that the “average annual rainfall volumes for the past 15 years (2000 to 2014) were actually 5.5% lower than the long term average, and significantly lower (by 12.6%) than the average annual rainfalls in the 1970’s, 80’s and 90’s; and the frequency of the larger rainfall events (> 25 mm) that cause most of the stormwater management and combined sewer overflows problems were all significantly lower than the long term average (by 15-25%)” (Hatch Mott MacDonald, 2015)."
YET ANOTHER BONUS - The Windsor/Essex Region Stormwater Management Standards Manual reviewed City of Windsor Airport IDF trends - see December 2018 report: https://essexregionconservation.ca/wp-content/uploads/2018/12/WE-Region-SWM-Standards-Manual.pdf
"Table A-3.9.1b showing Windsor Airport extreme rainfall trends from 1995 to 2015 continues to illustrate a decreasing trend for short-duration events from 5min to 30min duration for nearly all return periods. The trends illustrate an increasing trend in 1 hour, 2 hour, 12 hour and to a lesser extent the 24 hour durations."
The review includes data up to 2015 - Table A-3.9.1b is shown below:
YET ANOTHER BONUS - The Ontario Ministry of Transportation (MTO) completed a comprehensive study entitled “The Resilience of Ontario Highway Drainage Infrastructure to Climate Change” in 2015 that indicates consistently decreasing intensities predicted for short durations affecting urban flooding:
“The IDF predictions in the 2014 UR study (2) also give rainfall predictions with significant variability with location, storm duration and return period (frequency) which can be compared to the 2007 MTO IDF curves. Predicted storms with durations less than 6 hours are less intense than those observed in 2007, for all return periods. Longer duration storms do not always hold to this pattern, with the 6 and 24 hour storms often predicted to become more intense, particularly in Northwestern Ontario.”
The study noted ranges in predicted increases and decreases for bias-corrected climate models:
“In some areas rainfall intensity increased from 0% to just above 30% where in other areas there were rainfall intensity reductions in the from 2 to 10%.”
The Ontario Ministry of Transportation noted in the report that modern drainage infrastructure including sewers, culverts and bridges are resilient to increases in design flows that may occur due to climate change:
“An overwhelming percentage of the storms sewer networks tested appeared to have sufficient excess capacity to hand the increases in design flow rates up to 30%. Similarly, the sample of highway culverts analysed showed adequate capacities, for a large percentage of the culvert, to handle the rage of low rate increases investigated without the need to be replaced. The bridges tested also appeared to suffer no risk to structures as a result of the flow increases.”
In response to this low risk, the Ministry’s Highway Standards Branch has developed a policy for assessing risks based on future climate that incorporates flexibility in design. The 2016 memorandum entitled Implementation of the Ministry’s Climate Change Consideration in the Design of Highway Drainage Infrastructure states:
“Designers are to exercise engineering judgement to determine whether the infrastructure will meet current and future design criteria through appropriate sizing of the infrastructure or through providing allowances for future adaptation measures.”
The Ministry’s approach of allowing for future adaptation as opposed to increasing infrastructure capacity as a result of future criteria (i.e., higher rainfall intensities) is similar to the ASCE approach that advocates the Observational Method that promotes “Design for low regret, adaptability, and robustness, and revisit designs when new information is available.”
A separate post explores IDF trends in the Version 3.0 Engineering Climate Datasets for long term southern Ontario climate stations - previous post link. Here are the results showing an overall decrease of 0.4% in all design intensities, and decreases for all return periods (bottom row) - smaller storms with more observations decreasing the most - and decreasing for most durations (right column) - especially the shorter durations of less than 1 hour:
Update: The Globe and Mail responded by 'working their Google' and citing examples of flood damages increasing, diverting from the question on extreme weather frequency. We have explained that flood damages can increase for many reasons and that they have fallen prey to 'attribute substitution'. Check out this recently published paper on heuristic biases and challenges in framing and solving problems related to extreme weather and flooding:
The CBC Ombudsman has recently reviewed data provided on this topic, cited Environment and Climate Change Canada statements and agreed as shown in this post https://www.cityfloodmap.com/2019/01/cbc-ombudsman-decision-finds-lack-of.html
The Minister of Environment and Climate Change Catherine McKenna has also weighed in on this topic indicating that there is no evidence of changes in extreme precipitation (i.e., short duration rainfall affecting flooding).
The letter (at right) was to clarify comments made by Prime Minister Trudeau following Gatineau 2017 flooding that "The frequency of extreme weather events is increasing, and that's related to climate change".
The Minister's letter, drawing from the recent Canada's Changing Climate Report, does not support the Prime Minister's statement on extreme weather frequency.