Guidelines for Snow Avalanche Risk Determination and Mapping. David McClung University of British Columbia

Transcription

1 Guidelines for Snow Avalanche Risk Determination and Mapping David McClung University of British Columbia

2 Why do we need guidelines? Costs: 14 fatalities/year, $0.5 M/year property damage, $10 M/year avalanche control, $2 M/year static defenses (Jamieson & Stethem, 2002; McClung & Schaerer, 1993). Mapping practice & policy mostly developed in response to accidents a proactive approach is preferred Mapping practices varied depending on the industry or region affected by accidents; consultants often defined mapping standards and acceptable risk Standards/guidelines should result in more uniform practices and thus reduce loss of life & property from avalanches Risk based methods allow comparison and combination with other hazards Photo: Kevin Fogolin

3 Canada: 702 fatalities since 1782 BC: 61% AB: 13% QB: 11% NFD: 10% Terr: 5% Every railway in BC is threatened; 1370 paths threaten highways in BC; Fatalities:>90% self-triggered in backcountry travel Not recognized as a natural hazard by the Govt. of Canada; Highest fatalities of any natural hazard in Canada

4 April, 2008: Alaska: Transmission line: $35M Photo: Scott Willis

5 Land Managers Guide to Snow Avalanche Hazards in Canada Recognition of potential avalanche problems Methods used for avalanche hazard mapping Elements of a hazard / risk map and report Selecting avalanche expertise Typical mitigation and mapping for land uses Avalanche protection

6 Guidelines for Snow Avalanche Risk Determination and Mapping in Canada Limitations of mapping Types of snow avalanche mapping Definition of risk and avalanche terms Risk guidelines for various applications (risk matrices) Typical methods used for risk determination

7 Recognition of potential avalanche problems Photo: BC Ministry of transportation Photo: Bruce Jamieson Photo: BC Ministry of Forests The obvious avalanche problem Can this area produce destructive avalanches?

8 Risk determination for avalanches Avalanche risk in guidelines is defined in terms of: Photo: Alan Jones Frequency (or return period) Consequences expressed in terms of: 1) Destructive potential (Canadian 5-class scale); and/or 2) Predicted impact pressures Probable exposure is also required in some applications such as transportation routes Photo: Alan Jones

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10 The 5 part destructive Canadian size system has been adopted in: 1. USA 2. New Zealand 3. Iceland 4. Recently by the forecasting community in Europe (Volume instead of mass) Any technician or forecaster with a small amount of training can register the avalanche size

11 Size 1: avalanche..sluff

12 Size 2: Slab avalanche released by snow loading: natural

13 Size 2

14 Size 3 avalanche: Whistler Bowl, B.C.

15 Four main types of avalanche maps Locator map Avalanche atlas Zone map From CAA, 2002 From Schaerer, 1989 Linear risk / hazard map

16 Applications addressed by the guidelines Transportation routes Occupied structures Forest harvest areas Photo: Tim Jensen Utility or energy corridors/structures Photos: Alan Jones Work sites Recreation operations

17 Example applications of guidelines

18 Photo: Alan Jones Transportation Routes Mica Dam Access Road, Monashee Range

19 Transportation Routes

20 Avalanche risk zones for occupied structures Kicking Horse Mountain Resort, Purcell Range Photo: KHMR/Steve Parsons

21 Avalanche risk zones for occupied structures Critical values of 30 years, 300 years and 30 kpa, and colour scheme are similar to those used in Switzerland New construction of permanently occupied structures only in White zone, some temporarily occupied structures may be permitted in Blue zone

22 Avalanche risk zones for occupied structures Kicking Horse Mountain Resort, Purcell Range Red Zone Blue Zone White Zone

23 Forest Harvesting Table VI: Risk ratings for expected avalanche size and expected avalanche frequency for forest harvest resulting in damage to forest cover Risk is rated qualitatively as low (L), moderate (M) and high (H) Frequency range (events/a) Average frequency (events/a) Qualitative risk for avalanche size 2 3 >3 >1 1:3 1:1 M H H 1:3 1:30 1:10 L M H 1:30 1:300 1:100 L L H Table VII: Risk ratings for expected avalanche size and frequency for forest harvest when down-slope transportation corridors, facilities or essential resources may be affected. Risk is rated qualitatively as: low (L), moderate (M) and high (H). Frequency range (events/a) Average frequency (events/a) Qualitative risk for avalanche size 2 3 >3 >1 1:10 1:3 M H H 1:10 1:100 1:30 L M H <1:100 1:300 L L H

24 Forest Harvesting Nagle Creek, BC Coquihalla Highway Photo: Alan Jones NOT ACCEPTABLE ACCEPTABLE? Photo: Jim Bay (From CAA, 2002)

25 Frequency / Return Period: 1.Records of avalanche occurrences 2. Vegetative damage to forest 3. Estimate far into the runout zone using extreme value statistics and known return period for high frequency of events (McClung, 2000, 2005) x*: dis tan ce T T x X b exp( */ ) 0 0 b..scale parameter of extreme value fit T : Return period X : length

26 x X >45 paths..fit to Gumbel distribution 6 b

27 Runout Ratio: x/ X b: scale parameter: slope of the line

28 Notes on avalanche dynamics: 1.Cannot be used to determine runout..use statistical methods and probability or records 2. Classical Voellmy boundary conditions: not correct: avalanches don t behave with both turbulent and terms

29 Example: Explosive control

30 Size 3 avalanche: Whistler Bowl, B.C.

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33 Conclusion: Initial avalanche motion is a transition in states: 1. Sliding block 2. Fluidization into a dense core with high volume fraction of solids 3. Therefore, use scaling methods and start dynamics calculations at maximum speed in the middle of the path

34 N Ratio pdf 1% 5% 10% K-S A-D C-S 29 s l-l (0.19) 0.17(1.37) 0.42(3.21) U / S m 106 beta (0.10) 0.42(1.37) 2.31(8.55) / S Um 29 s l-l (0.19) 0.22(1.37) 0.24(4.64) Um 118 beta (0.10) 0.25(1.37) 2.33(8.55) / H Um / H s l-l: shifted log-logistic distribution beta: beta distribution

35 Maximum Speed (m/s) New approach- calibrate model with data on measured maximum speed Vertical Fall (m 1/2 )

36 Land-use Planning Context Definition Speed calculation of zones depends from on destructive dynamics potential model (impact pressure) as a function of speed speed Empirical determination of frontal stopping position for the design event

37 Example: Aulta avalanche path in Switzerland Calculated speed Slope angle 25 degrees Measured speed

38 Blue zone..does it exist? From dynamics: deceleration is about 2 4 m/ s Blue zone pressures are < 30 kpa Assume speed 20m/s and density kg / m Total time to stop is 5 seconds which gives Blue Zone width of 50 m for constant friction 0.4 g cos

39 More realistic assumption is that friction increases as the avalanche slows If the speed has exponential decay with time with average deceleration 2 as 4 m/ s t V 20e cos t The time taken is still 5 seconds but the Blue Zone width is about 15m.

40 Conclusion : Blue Zone width is smaller than the uncertainty in dynamics calculations and avalanche dynamics will disappear from zoning applications. When friction is shared by: 2,kv terms (Voellmy assumption): friction drops as speed slows to allow wider Blue Zone but avalanches don t behave this way.

41 Conclusions For most applications, expected size and frequency are combined to assess acceptable risk For Occupied Structures, it is expected that dynamics will disappear from zoning schemes to be replaced by empirical runout methods Canada is a leader in risk-based planning for avalanche applications