Environment Canada Denies Change in Rainfall Events Over Past Decades

While there are many reasons to address climate change, increasing rainfall and resulting flood damages is not one of them.  Environment Canada, federal the agency responsible for collecting and analyzing weather data in Canada, agrees.  In response to a complaint on rainfall frequency reporting at CBC, Environment Canada has verified that there has been no significant change in rainfall events over several decades:

Excerpt. Environment Canada verifies no significant change
 in rainfall events over several decades.
Extreme rainfall reporting often goes like this, leading many to believe rainfall is increasing and leading to inaccurate reporting:

1) A convective summer thunderstorm drops 100 to 150 mm of rainfall over an urban areas that has been intensifying over decades (i.e., more runoff), and that can have drainage infrastructure constraints due to operational or structural changes (i.e., less flow capacity).

2) Flooding happens. 

3) Weather personalities share a characterization like "wow, we had a month of rain this afternoon folks", forgetting to mention that it doesn't rain much in the summer anyway and that 100 to 150 mm of rainfall is not an unusual event from an extreme value statistical point of view.

4) Many photos and videos are shared showing flooding in road underpasses, sometimes with expensive cars and stories about the owner - but the intrinsic built-in design vulnerability of underpasses in drainage design is not acknowledged.  (See 'subway' or 'depressed roadway' in MTO design criteria documents.  Or 'underpass' in Toronto design criteria.)

5) The insurance industry settles many flood claims at a great cost because it cannot distinguish if the event met policy restrictions, technically cannot characterize the neighbourhood scale factors that could exclude coverage, and perhaps legally cannot take the chance that unclear policy clauses on sewage back-up will not stand up in court if claims are denied.

6) There is no follow up discussion on the importance of runoff and how development intensification increases it, or that runoff mitigation measures are often sized to handle "smaller than extreme" events. (Public appreciation for hydrologic principles is very low, with focus group surveys suggesting many believe a 'watershed' is an building on the back lot for storing water.)

7) There is no follow up discussion on flow capacity and how it is constrained due to operational or structural changes to drainage and wastewater infrastructure. (This is a liability issue for the operator if the impacts of the changes were not carefully addressed.)  Also no follow up on the intrinsic vulnerability of underpasses which are sized by design, based on economic and technical constraints to handle only moderate storms and flows. (My experience is that capacity to handle a 100 year storm in an underpass, with 100 % redundancy in terms of back-up pumps, and stand-by diesel power, is rare - the depressed 400 series highway in Windsor may be the only one with this high level of service in the Ontario (i.e., Herb Gray /Windsor Essex Parkway).

8) Climate change modellers predict more extreme rainfall under some scenarios, despite having no models to simulate convective thunderstorms, and despite not using predictive models with a spatial or temporal resolution to analyze short term rainfall events (i.e., minutes and hours durations).  (The Ministry of the Environment in Ontario's expert panel indicated "Climate change science and modelling currently is not at a level of detail suitable for stormwater management where knowledge of the intensity, duration, frequency of storms and their locations and timing is required." - as per the report at this link)

9) People want answers and increased rainfall frequency or severity seems plausible, and besides, runoff discussions and flow capacity discussions are complex and not addressed.

10) Its 2015, and climate change temperature trends over years and decades are mistakenly considered to be the same as the short term extreme rainfall trends.

This is where we are today in the midst of an important discussion on climate change mitigation and carbon pricing.  Many climate change reports conflate temperature and rain.  Insurance industry conveniently points to rain as the reason insurance premiums are increasing, not saying how low investment returns by property and casualty insurers are forcing those companies to no longer subsidize underwriting activities as in past decades - that is increasing premiums.

Here are some resources and facts on the issues with today's rainfall frequency reporting:

1) Rainfall extremes happen.  Environment Canada publishes extreme value statistics as part of their Engineering Climate Datasets and exceeding "100 year storm" rainfall is not unusual.
  • All the data is here: http://climate.weather.gc.ca/prods_servs/engineering_e.html
  • It is a challenge to navigate but the intensity duration frequency files have information to show at each weather station when and how "100 year storms" were exceeded.  For example in Toronto's 61 year record at the Bloor Street gauge, we exceeded 100 year extremes in 1954, 1962, 1977, twice in 1986 (NB - neither August 19, 2005, not July 8, 2013 exceeded 100 years in downtown Toronto).
Excerpt from Table 1 Annual Maxium in file: idf_v2-3_2014_12_21_615_ON_6158355_TORONTO_CITY.txt
Version 2.3 data set for Ontario file: IDF_v2.30_2014_12_21_ON.zip

  • Rainfall intensity for duration of interest that cause flooding in urban areas is not increasing as Environment Canada has verified in response to our complaint on inaccurate reporting in this story :
Excerpt. CBC confirms Environment Canada has verified no change in rainfall over decades..
  • Here is our summary of Canada-wide rainfall trends in the Engineering Climate Dataset which shows for 565 weather gauges :
    • there is no statistically significant change in rainfall intensity for 86% of data points
    • there is a statistically significant decrease for 2 to 3% of data
    • there is a statistically significant increase for 3 to 5% of data
    • there is incomplete data for trend analysis 7% of the time
    • for shortest duration rainfall that affects urban flooding, just less than 3% of data shows increases in intensity (5 minute intensity increases at 2.7% of gauges)
     
    Climate Change Canada
    Real climate data from Environment Canada shows no change in historical rainfall intensities.
    Trend data file used: idf_v2-3_2014_12_21_trends.txt
    From Environment Canada's file: IDF_Additional_Additionnel_v2.30.zip

2) Flooding happens when rainfall extremes exceed the design capacity of our drainage systems and because those systems are becoming more and more constrained.
  • Underpasses are designed for 10-year to 25 year storms in some cases (check the Toronto criteria here in the design standards).
  • Over a many years, underpasses have a high risk of flooding, for example, sized for a 10-year storm flood frequency there is a 41.0% chance of flooding in 5 years, or sized for a 25-year storm there is a 33.5% chance over 10 years.  This is a nice summary table of risks: 
Source : Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Protection,
NCHRP Report 761, 2013.

  • The Don River floodplain that floods the Don Valley Parkway in Toronto and that flooded the GO Train rail line on July 8, 2013 floods because of an intrinsically low design capacity.  Or put another way, there is inadequate "freeboard", or buffer, between the elevation of the travelled lanes and rail bed and the moderate and extreme water levels of the river.
  • The 1981 inquiry into Toronto's Don River flooding sheds light on past railway flooding, including the area of the infamous GO Train flooding on July 8, 2013.  The 81 page report is available at the Toronto Reference Library:
    http://www.torontopubliclibrary.ca/detail.jsp?Entt=RDM209381&R=209381
  • The inquiry report provides a chronology of 40 Don River flood events during both ice-jam and ice-free conditions dating back to spring 1801 when aboriginals could not spear fish for salmon due to the post-flood high turbidity levels.  Railway line damage and flooding during ice-free conditions (i.e., no ice blockage of bridges) was reported in the "Great Flood" on September 13, 1878, the spring of 1914, and during two storms in 1980 (March and April).  Limited flooding was also reported on May 11, 1981 including the area of the Bayview Extension and the Toronto Brick Yards - the stranded GO Train incident occurred was just south-west of the Brick Yards (now the Evergreen Brick Works).  The report indicates that train operation has halted, or trains were detoured during floods, including December 25, 1979, January 11, 1980, March 21, 1980, April 14, 1980, February 11, 1981 and May 11, 1981.

3) After extreme events weather personalities / presenters compare weather extremes to climate "normals", or averages, which exaggerates the severity of the storm event.
  • Environment Canada publishes normals including average monthly rainfall amounts are low in the summer.  For example, in Toronto the average rainfall for July is 63.9 mm, as shown below or at this link

  • The rainfall statistics for Toronto show that even a moderate frequency "10 year storm", with 67.6 mm of rain over a day, exceeds the monthly average by 6%.  So why would we be surprised that some more extreme storms exceed the monthly total - as noted above, there is 41% chance we will have a 10 year storm every five years?
    Table 2a Return Period Rainfall Amounts (mm) in file: idf_v2-3_2014_12_21_615_ON_6158355_TORONTO_CITY.txt
    Version 2.3 data set for Ontario file: IDF_v2.30_2014_12_21_ON.zip
Nobel Memorial Prize in Economics winner Daniel Kahneman explains why humans struggle to think statistically. We substitute heuristics for methodical thought.  So an evaluation of infrastructure vulnerabilities is not within our capabilities in many cases - please see our next post for an exploration of heuristic biases that can influence reporting on extreme rainfall, flooding and climate change.

***

Extreme rainfall trends in Canada (Environment Canada Engineering Climate Datasets) are documented in the following posts:

Static Maps: http://www.cityfloodmap.com/2015/12/severe-storm-trends-canada-rainfall.html

Interactive Map: http://www.cityfloodmap.com/2015/12/canadian-extreme-rainfall-map-climate.html

Table Summaries: http://www.cityfloodmap.com/2015/12/canadian-extreme-rainfall-summary-by.html

Chart and Table: http://www.cityfloodmap.com/2015/12/top-weather-story-in-canada-2015-less.html

Long-term Station Table: http://www.cityfloodmap.com/2015/12/long-term-climate-change-short-term.html

Environment Canada Denies Changes: http://www.cityfloodmap.com/2015/10/bogus-statements-on-storms-in-cbcnewsca.html

Contradicting Insurance Industry Claims: http://www.cityfloodmap.com/2015/12/trends-in-canadian-shortduration.html

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