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BGC ENGINEERING INC.
Mountain Creek Hazards and Risks in the Canmore AreaDr. Matthias Jakob, PGeo. (BC), P.Geol. (AB)
April 28, 2014
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Outline
• The Storm of June 19-21, 2013
• Notes on the hydroclimate
• Debris flows and debris floods
• Cougar Creek debris flood
• Hazard and Risk Assessment
• Outlook for Alberta standards
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The evolution of an ugly storm
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Low 1(Cerberus)
Low 2(Hades)
Low 3(Poseidon)
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Und then came the rain…
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325 mm
200 mm
50 mm
Canmore
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Pictures of a catastrophe
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Kananaskis Station Rainfall
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Changes in Heavy Precipitation
YEARS
Ret
urn
Per
iod
(yea
rs)
Tot
al P
reci
pita
tion
per
Eve
nt (
mm
)
200 mm
100 mm
1990 2000 201019901970 198020101950
3 years
4 years
5 years
6 years
Source: Stull et al. 2014
Heavy precipitation has become more frequent during the past 15 years
The return period of heavy precipitation has decreased (now: 1 in 3 chance)
= BanffX = Bow Valley= Kananaskis
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Bow River Flows (1909-2011)
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Bow River Flows (1879-2013)
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transportation zone
deposition zone
Elements at risk
The Debris Flow System
Artwork: BGC Engineering
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The Classic FanThe Classic Fan
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DEBRIS FLOODSProblems: debris aggradation, avulsion & bank erosion
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Cougar Creek Fan
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Cougar Creek Fanprobably one of Canada’s most densely developed fans
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Floodplain
Canmore
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Towards a systematic Towards a systematic hazard and risk assessmenthazard and risk assessment
Hazard Recognition
Frequency-Magnitude Analysis
Hazard Intensity Mapping
Consequence Determination
Risk Calculations
Risk Evaluation
Development Decision
Risk Reduction
Numerical Runout Modeling
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Historical Air Photographs
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Evidence of Previous Events
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Test Trenching Program
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Test Trenching
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Bridge River Tephra (~2500 yrs)
Paleosols (old soils)
Deciphering the “deep past”
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Dendrochronolgy
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Frequency-Volume Analysis(How often, how big?)
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Return Period (Years)
Deb
ris F
lood
Vol
ume
(m3 )
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Debris Flood Modeling: Scenario Analysis
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IDT
(Jahre)
Volumen
(m3)
2 1:30 to 1:100 40,000
3a3b
1:100 to 1:300 60,000
4 1:300 to 1:1000 160,000
51:1000 to
1:3000260,000
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1:400(June 2013 Simulation)
90,000
2 3a 3b
4 5 6
2 3A 3B
4 5 6
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Combined Hazard Map
• Hazard is expressed as intensities (flow depth times velocity square)
• The problem: neither the exact probability nor the risk is known for specific lots.
• Risk maps are more intuitive in that they show the real risk
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Note that this hazard map is from a study in the US and shows debris-flow hazards for a 500-year return period
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Quantitative Risk Analysis (QRA))
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“…estimation of the likelihood that a debris flood scenario will occur, impact something, and lead to undesirable consequences.”
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Risk Map (example)
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• PDI >1:10,000 (Red)
• PDI of 1:10,000 to1:100,000 (Orange)
• Yellow means that the risk is < 1:100,000.
PDI: Probability of Death of an Individual
Note that this map is not an actual risk map but serves as an example of how risk maps could look like
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Group Risk (example)
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• Group risk is unacceptable
• Needs to be lowered to the ALARP zone
• Goal of mitigation is to bring group risk into the ALARP zone
As low as reasonably possible
Unacceptable Risk
Acceptable Risk
Potential Fatalities
An
nu
al
Pro
ba
bil
ity
of
Cu
mu
lati
ve
Fa
tali
tie
s
Uncertainty Range
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Building Damage, Safety Risk
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Vulnerability Criteria(Buildings)
Assessed Building Values
Model Results Building Damage Level
Building Damage Cost Probability of Life Loss
Elements at Risk
+
Vulnerability Criteria (Safety)
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Building Loss Potential
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5
• $600 k annualized direct building damage costs on fan• Total costs likely more than a factor of 2 higher
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Conclusions
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• Cougar Creek and many other creeks in the Alberta Rocky Mountains are very hazardous landforms with high risk potential
• Climate observations, theory and modeling all point towards a high likelihood that extreme precipitation events will increase in frequency and magnitude. This emerging trend should be reconciled with landuse planning and structural mitigations
• Given that many fans are heavily developed, risk exists that must be quantified systematically and transparently
• Risk should be expressed as in loss of life risk and economic risk• The new Alberta Guidelines will aim to attain those goals• Mitigation measures should strive to reduce risk to tolerable levels
and optimize costs and benefits.
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Thank you!