Review of historical Intensity-Duration-Frequency (IDF) data in Markham, Ontario, Canada as well as IDF trends in southern Ontario and across Canada. Presented to the Southern Ontario Municipal Stormwater Discussion Group. Comparison with Insurance Industry and media reporting on climate change effects (e.g., Telling the Weather Story). Review of design hyetographs and design standards updates to improve resiliency.

1. Municipal Stormwater Management
Discussion Group
City of Markham
IDF & Design
Hyetograph Review
Town of Whitchurch-Stouffville
February 25, 2016
1

2. Outline
• IDF History and Review
• IDF Trends
–Local to National Data
–Climate Change
–Insurance Industry „Data‟
• Design Hyetograph Review
• Design Standards Update
2

3. IDF History
• Intensity Duration Frequency data has
been used for drainage design since
Markham was a township (pre 1972).
• IDF data appears based on Toronto
Bloor Street data. Local Buttonville
Airport gauge records begin in 1986.
3

4. IDF Review
• City reviewed design IDF volumes in recent Stormwater Guidelines
update using Buttonville Airport records up to 2007.
4
• Current IDF data are conservative since decades-old Toronto Bloor
Street gauge volumes are higher than today‟s local values
• On average 16 % higher volumes in Markham design IDF.

5. IDF Trends – Local
• Environment and Climate Change Canada have added trend
analysis in the version 2.3 Engineering Climate Dataset, showing
direction of trend and statistical significance at each station:
http://climate.weather.gc.ca/prods_servs/engineering_e.html
• Trends are on annual maximum observed volumes for each duration
and suggest direction of IDF trends (extreme values).
• Local Buttonville observed trends are mostly downward and not
statistically significant.
• „Reference‟ Toronto Bloor Street observed trends are also
downward, some statistically significant.
5

6. IDF Trends – Local
• Environment and Climate Change Canada have added trend
analysis in the version 2.3 Engineering Climate Dataset, showing
direction of trend and statistical significance at each station:
http://climate.weather.gc.ca/prods_servs/engineering_e.html
• Local Buttonville trends are downward:
6

7. IDF Trends – Local
• Environment and Climate Change Canada have added trend
analysis in the version 2.3 Engineering Climate Dataset, showing
direction of trend and statistical significance at each station:
http://climate.weather.gc.ca/prods_servs/engineering_e.html
• Local Buttonville trends are downward:
7

8. IDF Trends – Local
• Environment and Climate Change Canada have added trend
analysis in the version 2.3 Engineering Climate Dataset, showing
direction of trend and statistical significance at each station:
http://climate.weather.gc.ca/prods_servs/engineering_e.html
• Local Buttonville trends are downward:
8

9. IDF Trends – Local
• As annual
maximum
values trend
lower, extreme
IDF intensities
decrease as
well.
• “Toronto City”
Bloor Street
trends are
lower for all
durations and
for all return
periods.
• Pearson Airport
has mixed
trends.
9
Source
Environment Canada Engineering Climate Dataset
ftp://ftp.tor.ec.gc.ca/Pub/Engineering_Climate_Dataset/IDF/
Up to 2007 per Dataset v2.3, to 2003 per Dataset v1, to 1990 per hardcopy records
by www.CityFloodMap.Com, 2016

10. IDF Trends – Southern Ontario
• 97 % of data trends
are not statistically
significant (mild
trends up or down).
• More statistically
significant
decreases than
increases in
intensity.
10
Not
significant
at the 5%
level
97%

11. IDF Trends – Ontario Long Term Stations
• Stations with
45+ years
record and
recent data
only (since
2005).
• More
decreases
than
increases in
intensity.
• More
statistically
significant
decreases.
11

12. IDF Trends – Ontario Long Term Stations
• Stations with
45+ years record
and recent data
only (since
2005).
• More statistically
significant
decreases than
increases in
intensity.
12

13. IDF Trends – National
• Environment Canada paper in Atmosphere-Ocean “Trends in Canadian
Short‐Duration Extreme Rainfall: Including an Intensity–Duration–Frequency
Perspective”, by Mark W. Shephard, Eva Mekis, Robert J. Morris, Yang
Feng, Xuebin Zhang, Karen Kilcup & Rick Fleetwood (November 2014).
• Show increases and decreases in extreme rainfall frequency across
Canada, only a few percentage of statistically significant increases or
decreases, e.g. :
– (Abstract): “The single station analysis shows a general lack of a detectable
trend signal, at the 5% significance level … 30-minute to 24-hour durations
show that, on average, 4% of the total number of stations have statistically
significant increasing amounts of rainfall, whereas 1.6% of the cases have
significantly decreasing amounts.
– “For the shortest durations of 5–15 minutes, the general overall regional trends in
the extreme amounts are more variable, with increasing and decreasing
trends occurring with similar frequency…”
– “This indicates that at most locations across Canada the traditional single station
IDF assumption that historical extreme rainfall observations are stationary
(in terms of the mean) over the period of record for an individual station is
not violated.”
http://www.tandfonline.com/doi/abs/10.1080/07055900.2014.969677#.Vrc6d7I4HGI
13
Not
Increasing
due
to
climate
change,
etc.

14. IDF Trends – National (Few Significant Trends)
14
94% of 5410
statistics show
no significant
trend

15. IDF Trends – National
15
http://www.cityfloodmap.com/2015/12/canadian-extreme-rainfall-map-climate.html

16. Sample Bias 101
16
Sample mean x
Population mean µ
µ
x biased < µ

17. IDF Trends – „Sample Bias‟ vs. Climate Change
• Skewed rain distributions have sample
biases that underestimates true
„population‟ mean in small samples
(short rain records).
• Bias increases with higher skew.
• Increase in rain intensity can reflect
greater chance of right tail events, not
necessarily climate change impacts.
17
Above graphs per Kirk G. Fleming's article
"Yep, We’re Skewed", VOLUME 2/ISSUE 2
CASUALTY ACTUARIAL SOCIETY
µ
x

18. IDF Trends – Insurance Industry „Data‟
18

19. IDF Trends – Insurance Industry „Data‟
19

20. IDF Trends – Insurance Industry „Data‟
20
http://www.slideshare.net/RobertMuir3/storm-intensity-not-increasing-factual-review-of-engineering-datasets

21. IDF Trends – Insurance Industry „Data‟

22. IBC / ICLR Weather Story Statement
• Institute of Catastrophic Loss Reduction (ICLR) report for Insurance Bureau of
Canada (IBC) stated an increasing frequency of weather events, including storms,
due to climate change.
• Shift in “return period” from 40 years to 6 years noted (event with a 2.5% chance
per year (1/40) now has 17% chance per year (1/6))
• General data used by Environment Canada noted in the reference, but no actual
analysis is provided.

23. Weather Story Review – Frequency Shift Theory
• The Telling the Weather Story presentation describes shifts in average and extreme
climate data using a theoretical “bell curve” with no actual climate data.
• Review shows a whole standard deviation shift in the average assumed to make
extremes more frequent (40 year event becomes 6 year event in a standard bell curve).

24. Weather Story Review – Frequency Shift Error
• After presenting data on observed temperature shifts of many degrees, an error
appears to be made associating “rainfall rate of so many millimetres per hour” with
a 40 year to 6 year frequency shift if “you just shift the mean by one degree’.
• The association of rainfall event frequency is inconsistent with IPCC discussion on
temperature frequency, and the stated shift of ‘one degree’ is inconsistent with a
one standard deviation shift in bell curve average required for a 40 to 6 year shift.

25. Weather Story Review – Frequency Shift Data
• A 40 year to 6 year return period shift is based on a one standard deviation shift in
the mean of a standard normal distribution, or bell curve. Cumulative probability
tables show this theoretical shift from z = 1.96 to z = 0.96 (i.e., mean shift of 1.0).
Exceedance probability of
P = 17 % with
z= 1.96 - 1.00 = 0.96
Return period of
1/P = 6 years
Exceedance probability of
P = 2.5 % with
z= 1.96
Return period of
1/P = 40 years

26. Weather Story Review – IPCC Source Review
• The Telling the Weather Story release presentation references an Intergovernmental
Panel on Climate Change (IPCC) report that discusses general aspects of climate data
variations (shifts in statistical average, variability and skew), but does not analyze
actual data and references only temperature, and not rainfall.

27. Weather Story Review – Comparison to Data
• The Telling the Weather Story’s one standard deviation, bell curve shift in climate
data average contradicts Environment Canada data trends for rain intensity.
“Weather Story” increase
from 16.5 mm to 23.1 mm
over 15 minute duration
(one standard deviation
increase of 6.6 mm)
Actual decreasing trend
“Weather Story” increase
from 29.7 mm to 40.4 mm
over 2 hour duration (one
standard deviation
increase of 10.7 mm)
Actual decreasing trend
“Weather Story” increase
from 48.3 mm to 63.1 mm
over 24 hour duration
(one standard deviation
increase of 14.8 mm)
Actual decreasing trend

28. Progression of “Weather Story”
• Started with a theoretical
discussion from IPCC on the
changes in the distribution of
temperature
• Presented a theoretical shift
in climate data using a bell
curve and a one standard
deviation shift in averages
• Associated theoretical shift in
event frequency to weather
events, implying storms,
happening in parts of Canada
• Referenced Environment
Canada data but did not use
Environment Canada rain
data or analysis that does not
support such intensity trends
• Organizations promoted
theory as fact, associate
weather with severe rain
events, explaining increasing
insurance premiums & tie to
risk mitigation policy.

29. IDF Trends – Insurance Industry „Data‟
• Many organizations promote the Weather Story statement as fact,
describe a this statement based on theory as „reality‟ based on
„research‟, credit the statement to Environment Canada as the source,
and use it support economic and other policies.
• Due to the Weather Story statement inaccuracies, policies and efforts
toward mitigating increasing urban flood damages may be misdirected to
climate change mitigation, as opposed to more effective risk
identification/management efforts, urban planning/stormwater
management policies and infrastructure remediation/capital investment
efforts that address the root causes of increased damages, not related to
theoretical storm frequency shifts.
• More scientific rigour is needed to ensure that real data are used in
assessing important design parameters and setting infrastructure
management policy.
• Several insurance companies have recently corrected/updated websites.

30. Design Hyetograph Review
• Not all 100
year storms
are created
equal.
• „Watershed‟
storms applied
in moderately
sized urban
catchments
can
significantly
underestimate
design flows.
• Hyetographs
are as
important as
IDF data.
30

31. “100 Year” storm can have < 2 Year intensities !
31
TRCA Rouge River “100
year” storm 1-hour intensity
of 40.71 mm/hr is only a
14.5 year frequency
Markham “100 year” storm
intensity is greater than 68.66
mm/hr over peak hour (exceeds
100 year frequency)
IDF Data from Environment
Canada’s Engineering Climate
Dataset Version 2.3 for Toronto
(Bloor Street climate station)
TRCA
Rouge
River “100
year”
storm 15
minute
intensity
of 40.71
mm/hr is
only 66%
of 2 year
frequency
Markham
“100 year”
storm
intensity is
averages
178
mm/hr
over peak
15 min
(exceeds
100 year
frequency)

32. Design Hyetograph Review
• IDF data
show
some
watershed
storms do
not reach
short
duration
design
intensities.
• Markham
3-hour
AES*
storm is
conser-
vative.
32

33. Design Standard Update
• Maintain IDF values despite decreasing trends in intensities at Toronto
reference station and lower local intensities.
• Increase in Rational Method C-values to reflect higher % impervious
(increase single family value from 0.50 to 0.70, i.e., + 40%)
• Add conservatism with „return period factors” above 10 year storm:
+10% for 25 year, + 20% for 50 year, + 25% for 100 year.
• Cumulative increase for single family 100 year peak flow is +75%.
33
1952 32 % 1971 45 % 1981 56 % 2002 70 %

34. Conclusions
• Markham IDF review shows design values are conservative relative
to local rainfall characteristics.
• Rainfall intensity trends observed at local & reference Toronto
climate stations are mostly decreasing (some statistically significant).
• Decreasing observed rainfall intensities are reflected in extreme
value trends (IDF return period data) – City design criteria are not
being relaxed.
• Decreasing or „flat‟ rainfall intensities are consistent with southern
Ontario stations and long-term Ontario station trends.
• Increasing rainfall trends may be explained by statistical bias in short
records with skewed distributions and insurance industry errors.
• Design storms vary significantly in short duration intensities (some
below IDF values) and can underestimate runoff in small urban areas
– attention to hyetographs required.
• Runoff coefficients and returns period factors have been adopted to
reflect intensification and to add conservatism for extreme events.
34

35. 35
Contact
Robert J. Muir, M.A.Sc., P.Eng.
Manager, Stormwater
City of Markham
rmuir@markham.ca