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Robert Muir Extreme Rainfall Trends - NRC Workshop on urban rural storm flooding February 27 2018 Ottawa

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The National Research Council's Workshop on adaptation to climate change impact on Urban / rural storm flooding February 27, 2018. Presentation on: National, regional, local IDF trend
analysis & hyetograph selection to
define risks and system stresses
Robert J. Muir, M.A.Sc., P.Eng.
Manager, Stormwater, City of Markham

Published in: Engineering
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Robert Muir Extreme Rainfall Trends - NRC Workshop on urban rural storm flooding February 27 2018 Ottawa

  1. 1. Woodbine 1 Workshop on adaptation to climate change impact on Urban / rural storm flooding February 27, 2018 National, regional, local IDF trend analysis & hyetograph selection to define risks and system stresses Robert J. Muir, M.A.Sc., P.Eng. Manager, Stormwater, City of Markham
  2. 2. Woodbine Outline 2 • Environment and Climate Change Canada (ECCC) Engineering Climate Datasets (version 2.3) – Annual Maximum Series Trends and Significance – Regional Trends In Observed Extremes • Local IDF Trends Since 1990 • Future IDF Uncertainty • Hyetograph Selection as Critical Risk Factor (how IDF data is applied in practice)
  3. 3. National Annual Maximum Rainfall Trends Data • ECCC added trend analysis in the v2.3 Engineering Climate Dataset, showing trend direction and statistical significance at each station: http://climate.weather.gc.ca/prods_servs /engineering_e.html • Ottawa Airport  Decrease 5-min to 6-hrs.  Increase 12 to 24 hrs.  Statistically significant decrease over 10-min, 15-min, and 1-hr. 3
  4. 4. Very Few Significant Trends in Canada (random) • 93% of trends are not significant or ‘no data’. • 5-min rainfall maxima have significant increases at 2.7% of stations. • Decreasing regional trends in southern QC and Atlantic. Increasing on coasts (SW BC and NFLD). 4 Significantly Down 3% Significantly Up 4%2.7% http://www.cityfloodmap.com/2015/12/trends-in-canadian-shortduration.html
  5. 5. 5http://www.cityfloodmap.com/2015/12/canadian-extreme-rainfall-map-climate.htmlhttp://www.cityfloodmap.com/2015/12/canadian-extreme-rainfall-summary-by.html
  6. 6. National Data Trends Contradict Media & Marketing • Insurance ‘facts’ & infographics refuted by ECCC as inadequate for decision making on risks: 6 • Data facts from ECCC: – “No Detectable Trend Signal” (Atmosphere- Ocean, 2014) – “No significant change in rainfall events over several decades” (CBC letter, 2015) – Advertising Standards Canada complaint resolutions 2015-2016 – “ECCC studies have not shown evidence to support statement” (Cdn Underwriter, 2016) – “If this is used as the basis for statements about actual changes in extreme rainfall in Canada, then I would have concerns.” (personal communication ECCC, 2018) http://https://www.slideshare.net/RobertMuir3/storm-intensity-not-increasing-factual-review-of-engineering-datasets
  7. 7. Mixing-up Annual Precip. Data and Extreme Rainfall Risks “Indeed in Canada, in southern Canada we are getting about 18% more rainfall on an annual basis than was the case just over 100 years ago. So when you see in the news and the media people talk about storms seem bigger and more intense and so forth, those perceptions are correct. And there's a lot of data to show it. I'm just giving one quick illustration here.” Blair Feltmate, ICCA, February, 8, 2018 Standing Senate Committee on Energy, the Environment and Natural Resources 7 http://http://senparlvu.parl.gc.ca/XRender/en/PowerBrowser/PowerBrowserV2?fk=474513&globalStreamId=3&useragent=Mo zilla/5.0%20(Windows%20NT%206.1;%20WOW64;%20rv:52.0)%20Gecko/20100101%20Firefox/52.0http://senparlvu.parl.gc.ca/XRen der/en/PowerBrowser/PowerBrowserV2?fk=474513&globalStreamId=3&useragent=Mozilla/5.0%20(Windows%20NT%206.1;%2 0WOW64;%20rv:52.0)%20Gecko/20100101%20Firefox/52.0
  8. 8. Less Extreme Short Duration Rain - Southern Ontario • 97 % of trends not statistically significant (mild trends up & down). • 2.3% statistically significant decreases and only 1.0 % significant increases in intensity. No short duration significant increases. 8 97% http://www.cityfloodmap.com/2016/02/ontario-climate-change-trends-going.html
  9. 9. No Change in Annual Maxima = No Change in Derived IDF • Southern Ontario 21 stations with 30+ years of record have median IDF changes since 1990: – 2-Yr 5-Min -0.5 % – 5-Yr 10-Min 0.0 % 9 http://www.cityfloodmap.com/2018/01/short-duration-frequent-rainfall-show.html
  10. 10. No Change in Annual Maxima = No Change in Derived IDF • Southern Ontario 21 stations with 30+ years of record have median IDF changes since 1990: – 2-Yr 5-Min -0.5 % – 5-Yr 10-Min 0.0 % – 5-Yr 1-Hr -1.2 % – 10-Yr 2-Hr -0.6 % 10 http://www.cityfloodmap.com/2018/01/short-duration-frequent-rainfall-show.html
  11. 11. Local Trend - Lower 5-Minute IDF For 2 to 100 Year • Toronto Downtown and Mississauga Airport 5-15 min. intensities decreasing for all return periods (like 5 minute data in this table). • 24 hour trends are downward as well despite July 8, 2013 storm. 11
  12. 12. Southern Ontario Extreme Rain Trends Decreasing http://www.cityfloodmap.com/2017/11/less-extreme-short-duration-rainfall-in.html http://www.cityfloodmap.com/2017/09/less-extreme-ontario-rainfall.html http://guelph.ca/wp-content/uploads/SMMP-Appendix_E_Combined_IDF_Report.pdf “new rainfall depths are lower than current by 10 to 15 %”, EarthTech https://drive.google.com/open?id=1Gxmg8gtkzZuv-ZqiqYpc3tQ9r-Ie1v1p 12https://drive.google.com/open?id=1AngUYFFlm-RqQlmSC0gZqxy8nV61BW8J
  13. 13. 100-Year 15-Minute Future IDF Uncertainty • Various options: – Tool / Method – Station / Cell – RCP/ Conf. Bands – Timeframe • Wide diversity in future volume across options. • No consistency in governing option across durations. IDF CC Tool 2050-2100 CCDP 2065-2095 RCPs RCPs RCPs RCPs 10-90% 10-90% 10-90% Local Climate Stations Local Grid Cells 14
  14. 14. 100-Year 15-Minute Future IDF Uncertainty • Various options: – Tool / Method – Station / Cell – RCP/ Conf. Bands – Timeframe • Wide diversity in future volume across options. • No consistency in governing option across durations. 15 H L
  15. 15. 100-Year 2-Hour Future IDF Uncertainty • Various options: – Tool / Method – Station / Cell – RCP/ Conf. Bands – Timeframe • Wide diversity in future volume across options. • No consistency in governing option across durations. 16 H L
  16. 16. 100-Year 24-Hour Future IDF Uncertainty • Various options: – Tool / Method – Station / Cell – RCP/ Conf. Bands – Timeframe • Wide diversity in future volume across options. • No consistency in governing option across durations. 17 H L
  17. 17. Is IDF data critical to design? Does it account for real system stresses? 18
  18. 18. Is IDF data critical to design? Does it account for real system stresses? 19
  19. 19. IDF data is just the “tail on the dog” ... don’t let “the tail wag the dog” when defining risk 20
  20. 20. IDF Data Use (To Create Hyetograph) is Most Critical to Risk • Regulatory floodplain hyetographs can underestimate 100-year rain by 90%. 21 0 50 100 150 200 250 300 0 100 200 300 400 500 RainfallIntensity(mm/hr) Time (minutes) Markham Storm (3 Hr AES) 100 Year TRCA Storm (AES 12 Hr) 100 Year Markham 100-Yr IDF 100-Yr TRCA 100-Yr Markham 100-Yr TRCA 100-Yr Risk Gap
  21. 21. Hyetograph Selection Effects Major Runoff Peaks (Storm) • Various 100-year storms to assess peak flows in Toronto InfoWorks models. • “Chicago” distribution gives higher overland, major system design flows: • 108% higher in small catchment • 60% higher in medium catchment • 36% higher in large catchment 22 Description Minor Peak (m3/s) Major Peak (m3/s) Chicago AES Chicago AES Area 15 – Small Catchment 0.16 0.15 0.25 0.12 Area 15 – Large Catchment 3.74 3.66 0.45 0.33 Area 17 – Small Catchment 0.12 0.10 0.34 0.22 Area 17 – Medium Catchment 2.79 2.72 0.32 0.20 Area 17 – Medium Catchment 1.28 1.25 1.49 1.01 Area 17 – Large Catchment 49.09 48.37 0.07 0.05
  22. 22. Hyetograph Selection Effects Surcharge (Sanitary) • Several wastewater design storms were used to assess surcharged manholes representing basement flooding risks in Markham’s all-pipe InfoWorks model. • “Chicago” storm has highest surcharge/flood potential (conservative): – 20 times more flood risk compared to historically-based design storms. – 4.3 times more flood risks compared to Region operational / trunk storm. – Higher risk for Today’s IDF than other storms with Future IDF (stress tests for Climate Change resiliency, demonstrates safety factors in design) 23 Design Storm # MH Flooded (< 2m freeboard) % MH Flooded (< 2m freeboard) York Region 25-Year 'Historical' 60 0.42 % Toronto May 2000 'Historical' (25-50 Year) 51 0.35 % Ottawa 100-Year 'Historical / Scaled' 13 0.09 % Markham 100-Year Chicago 259 1.80 %
  23. 23. Hyetographs & Hydrology have safety factors to account for complex reality • Chicago storm: “the synthetic storm pattern would undoubtedly have a greater return period than the IDF curve from which it was derived” (Marsalek and Watt, 1984) • “the science of hydrology has evolved into a practice that relies heavily on empirical relation- ships or concepts, that describe the major hydrologic process in a relatively gross way…The present best state of the art in this field still relies heavily on the use of judgement of the practitioner to balance the complex reality with the need for useful simplifications. “ (Ryerson University) 24
  24. 24. Conclusions • Newest ECCC Engineering Climate Datasets Version 2.3 trends and significance for each station/duration define extreme rain intensity changes: – Short duration intensities define urban flood risk (not annual precip.) – Local station and even regional trends may be up or down. – Local IDF changes follow observed intensity trends (e.g., lower IDF values in southern per Ontario ECCC datasets & countless studies). • Media / marketing focus on rainfall trends diverts focus from key flood risk drivers and the most effective mitigation measures (see next presentation). • Future IDF predictions high variable (but not critical to define cost effective mitigation measures due to certainty in existing system limitations) • Hyetograph selection more critical to defining risk (i.e., design storm to set level of service for mitigation) than IDF refinements. 25

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