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.
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