12

SNOW AVALANCHE SIZE CLASSIFICATION

David M. McClung and Peter A. SchaererNational Research Council of Canada, Vancouver, B.C.

Introduction

When avalanche events are reported, it isnecessary to in61ude an estimate or measure of their SiE.If a simple, consistent method of sizing avalanches can befound, obvious benefits can be derived. For example, datafrom different storm cycles, years, or areas can becompared. People such as avalanche forecasters, observersor consultants, as well as skiers, can clearly profit frumthe data once a good method is established.

Unfortunately, in the great majority of casesthere is no visual record of the avalanche as it falls. Theobserver is left with only a few measurable factors afterthe event. Some of these are: mass, runout distance, pathdimension, depth, and spatial extent and water content ofdebris. In addition, there are other variables which may beestimated such as area swept out by the flowing ·avalanche,degree of path confinement, and damage to structures orvegetation. In this paper it is argued that a meaningfulestimate of size should take into account all observables.Other measures of size proposed such as volume of snowmoved, avalanche mass, and estimated kinetic or potentialenergy seem inadequate to describe avalanche size when takensingly. This is due to the inherent complexity of thephenomenon of avalanches in motion. In the present papersize classification systems with emphasis on the system inuse in Canada are discused. Experience with the Canadiansystem is described by data collected at Rogers Pass, B.C.

Size Systems in Use or Proposed

1. Recommendations by the Working Group on AvalancheClassification of the International Commission on Snow andIce (I.C.S.I.)

In a comprehensive paper on avalanche class­ification (de Quervain, et ale 1973), the I.C.S.I.recommended that a set of dimensional measurements berecorded when reporting events. Those measurementsrelevant to avalanche size are: width and thickness offracture, length and width of avalanche path, and dimensionsand volume of the deposit. .

13

The data recommended by the I.C.S.I. arepotentially very useful in determining avalanche size.However, some essential elements such as density of thedeposit are not required so that avalanche mass, forexample, cannot be calculated. In addition, no scheme ispresented to combine the measurements into a size factor.Thus, the I.C.S.I. recommendations do not constitute a sizeclassification system although the format prescribed is usedinternationally for reporting destructive avalanches.

2. U.S. Reporting System

In the U.S.A. avalanches are classified using fivesizes. The sizes are based upon an estimate of the volumeof snow transported down the avalanche path. The five sizes(Perla and Martinelli, 1976) are:

1. Slough or snowslide less than 50 m (150 ft.) of slopedistance [approximately 25 m (75 ft.) vertical] regard­less of snow volume.

2. Small, relative to path.

3. Medium, relative to path.

4. Large, relative to path.

5. Major or Maximum, relative to path.

The important feature of the system is that Sizes2-5 are related to the path in question. This restrictionproduces a measure which is not compatible with the conceptof size. By relating avalanche size relative to the path,the implication is that size depends on location.Specifically, the same avalanche may have a wide variationin size according to the path it falls on. In addition, thesystem does not make use of all the observable factors; forexample, mass of the avalanche or water content of debris.Furthermore, the system is subjective and yields data whichare meaningless to anyone not familiar with the paths inquestion.

3. Systems Proposed in Japan

Several systems have been proposed in theoreticalpapers by Japanese researchers. None of these, however,appears to be used systematically for reporting avalancheevents.

14

Shoda (1965) proposed a logarithmic scale basedupon avalanche potential energy, 10910MgH, where H isthe vertical drop, M is avalanche mass, and g isacceleration due to gravity.

There is some merit in Shoda's system because Mand H are potentially measurable. Furthermore, the conoeptis valid even for the processes in a falling avalanche whichcontain non-conservative frictional forces. However, thesystem does not make use of all the observable variables;for example, volume of snow transported or depth of deposit.In addition, there is no implied velocity dependence so thata slow, wet avalanche might be classed nearly the same as amore destructive, fast moving, dry one.

Shimizu (1967) proposed three size systems. Oneof-these is identical to Shoda's. The other two are:10910M and a scale based logarithmically on destructivekinetic energy, 10910tMVT2 , where VT is theavalanche terminal velocity.

The system based on mass alone suffers from thesame disadvantages as Shoda's system but to a greater

.degree. The system based upon kinetic energy has some meritbut Shimizu's actual discussion of it is phrased in terms ofunknown factors and he admits it is not a usable form(Shimizu, 1967).

4. Systems Used in Switzerland

Researchers in Switzerland have proposed a varietyof specific systems in various research papers. None ofthese, however, is used systematically in practice.

de Quervain (1975) proposed a two-class systembased upon the objects that avalanches affect. Thoseaffecting people were designated "tourist" while thoseaffecting villages were designated "catastrophic". FOhn(1975) used a similar system in which he established thefollowing classes: slough, slope avalanche, and valleyavalanche with the latter two designations used as synonvmsfor tourist and catastrophic.

FOhn et ale (1977) related size to the areacovered by the avalanche in a three-class system: small(sloughs), medium, and large (50000 m2 ).

,15

None of these systems makes use of all importantmeasurable variables. Also, it would seem that there arenot enough size classes to discriminate adequately in mostcases.

5. Canadian Reporting System

The system used in Canada is based upon estimatedpotential destructive effects. It is an extension of thatoriginally introduced by M. Atwater in the U.S. (U.S.D.A.,1961, rev. 1968). R. Perla introduced the system intoCanada and it was adopted by the Canadian AvalancheCommittee in 1977.

The basic idea of the system is to estimate thepotential destructive effects of the avalanche at terminalspeed in approximately the Aiddle of the path. The fivesizes are listed in Table I, along with suggested values fortypical associated factors. In Table I the size of theobjects affected increases with size classification so thereis a strong implied relation between size, avalanche mass,and area swept out by the falling avalanche.

The Canadian system attempts to integrate allobservable variables associated with the events into asimple estimate of size. The disadvantage of such a systemis that there is some subjective judgment involved. This isthe primary reason that only five classes are used. Onlyapproximate estimates are necessary in order to classifyevents in the system. Field experience shows that observerswith a reasonable amount of experience will generally agreeon the size of a given avalanche.

The principal objective of the system is toprovide an effective vehicle for communication between theobservers themselves and others (not necessarily observers)who may wish to use approximate data describing recordedevents.

Factors for the Canadian Size Classification System

The factors listed in Table I are determined fromdata and theoretical analysis and only approximate valuesfor each size are given.

16

1. Suggested Typical Mass

Figures 1-4 show mass/size data for 744 avalanchesfor the winters 1978-79 and 1979-80. These data are allfrom Rogers Pass, British Columbia and were collected by thestaff of the National Research Council. Figure 1 shows thatonly about 6% of the avalanches were estimated as Size 4 orlarger. Figure 2 shows that most2avalanches estimated asSize 2 had masses in the range 10 t-l0 3t. Howeve2, thedistribution is skewed toward smaller sizes so 10 twastaken as the typical Size 2 mass. Figures 3-4 show similarresults for Sizes 3 and 4.

Figure 5 depicts a mass/frequency distribution for6534 avalanches at Rogers Pass observed by the NationalResearch Council from 1966 to 1980. When compared to Figure1, it can be seen that the sizing is heavily dependent onmass, as is expected. Clear differences are evident,however, and therefore, mass is not the only importantvariable.

It should be emphasized that the data in Figures 1and 5 will reflect the character of the avalanches at RogersPass where many of the paths are frequently controlled byartillery fire which tends to reduce the avalanche masses.Also, the observations are made at the highway so that thenumber of Size 1 and Size 2 avalanches recorded will not beaccurate. This is because many of these stop higher up onthe mountain where they cannot be seen either due to weatherconditions or terrain. For these reasons, Size 1 avalanchesare not generally recorded by the National Research Councilstaff.

2. Path Factors

Destructive effects of avalanches are related tovelocity as well as mass. The only dynamic model whichprovides velocity predictions in terms of path variables isthe two parameter model of Perla, Cheng and McClung (1980).Their model implies that path length is the fundamentalvariable where path length is the distance along the inclinefrom starting zone to the avalanche stop position. Themodel describes the motion of centre of mass of theavalanche and relates motion to friction along the path by adifferential equation. Higher terminal velocities areobtained on longer paths because the driving force isapplied over a longer distance (time). The Appendix of this

17

paper gives a short derivation and an approximate estimationof the dependence of maximum terminal speed in path length.Vertical drop may be of greater practical interest due togreater ease of estimation. It is obviously related to pathlength for typical mountain terrain.

3. Typical Impact Pressures

The impact pressures listed in Table I wereobtained from the product of expected flow density andVT 2 for typical path lengths. This calculation isdescribed in the Appendix. In addition, estimates of impactpressures from measurements made at Rogers Pass, B.C., aswell as other estimates (e.g. Salm, 1966; Perla andMartinelli, 1976) were used. It is not intended in theclassification system to estimate impact pressures beforeascribing a size to an avalanche. Rather, the suggestedtypical values are expected to follow from the observabledata associated with the events.

4. Water Content of Deposit

Water content of the avalanche deposit is usuallyestimated to be dry, moist, wet, or very wet as explained inthe International Classification for Snow (InternationalCommission on Snow and Ice, 1954). It may cause the size toshift downward as terminal velocity usually decreases aswetness increases. This effect is expected in spite of thefact that density increases with wetness. Fieldobservations show that increasing wetness greatly reducesthe area swept out by the flowing avalanche.

The effect of water content in the Canadian systemis integrated into the size estimate by the observer. Thus,it does not appear explicitly in Table I.

Conclusion

Avalanche size classification is a potentiallyimportant variable in reporting events. Each proposedsystem has obvious advantages and disadvantages. Due to thecomplexity of the problem and the fact that in most casesfew observable data are available, a dilemma results: oneis faced with a somewhat subjective measure if unmeasureddata are estimated or an unsatisfactory designation ifmeasurable data are strictly adhered to.

18

References

Acknowledgements

C. 1977.and conventionalJ. Glaciology,

Statistische Aspekte beiInterpraevent 1975, pp. 293-304.

F~hn, P.M.B. 1975.Lawinenereignissen.

F~hn, P., Good, W., Bois, P., and Obled,Evaluation and comparison of statisticalmethods of forecasting avalanche hazard.Vol. 19, No. 81, pp. 375-387.

Perla, R. and Martinelli, M. 1976. Avalanche Handbook.U.S.D.A. Forest Service, Agriculture Handbook 489. u.S.Govt. Printing Office, Washington, D.C~, 238 p.

Perla, R. 1980. Avalanche release, motion and impact. InDynamics of Snow and Ice Masses, S.C. Colbeck, Ed. AcademicPress, New York, pp. 397-462.

International Commission on Snow and Ice. 1954. The Inter­national Classification for Snow. Tech. Memo. No. 31.Assoc. Committee on Geotechnical Research, National ResearchCouncil of Canada, Ottawa, 15 p.

The authors wish to thank P.A. Anhorn for hisassistance with data analysis and collection. This paper isa contribution from the Division of Building Research,National Research Council of Canada and is published withthe approval of the Director of the Division.

The Canadian system, which is described in thispaper, provides a reasonable compromise between thealternatives. The disadvantage of the subjectivityinvolved is more than outweighed by the advantage of thecommunicable estimate of size that results. Experience withthe system thus far shows that it is useful and practical.The qualitative size estimates yield data which are usefulto avalanche specialists and non-specialists alike as wellas for applied purposes. The observers who produce the natamust be experienced and they must be checked forconsistency.

Bakkeh¢i, S., Cheng, T., Domaas, U., Lied, K., Perla, R.,and Schieldrop, B. 1981. On the computation of parametersthat model snow avalanche motion. Canadian GeotechnicalJournal, Vol. 18, No.1, pp. 121-130.

19

Perla, R., Cheng, T., and McClung, D. 1980.parameter model of snow avalanche motion. J.Vol. 26, No. 94, pp. 197-208.

A twoGlaciology,

Quervain, M. de, Crecy, L. de, LaChappelle, E.R., Losev, K.,and Shoda, M. 1973. Avalanche Classification.Hydrological Sciences Bulletin, XVIII, 4, pp. 391-402.

Quervain, M. de. 1975. Avalanche Formation. In AvalancheProtection in Switzerland. U.S.D.A. Forest Service GeneralTechnical Report RM-9 (Trans. of Lawinenschutz in derSchweiz, Buendnerwald, Beiheft Nr. 9, 1972) pp. 6-18.

Salm, B. 1966. Contribution to avalanche dynamics.International Symposium on Scientific Aspects of Snow andIce Avalanches. 5-10 April, 1965. LA.S.H. Publication No.69, pp . 19 9- 21 4 .

Schaerer, P.A. 1975. Friction coefficients and speed offlowing avalanches. In Snow Mechanics. Proc. of theGrindelwald Symposium-,-1-4 April, 1974. IAHS-AISH Publ. No.114, pp • 425- 4 3 2 .

Shimizu, H. 1967. Magnitude of avalanche. In Physics ofSnow and Ice. Proc. International Conf. on Low Temp.Science, Sapporo, Japan. H. Oura, Ed., August 14-19, 1966,Vol. 1, Part 2, pp. 1269-1276.

Shoda, M. 1965. An experimental study on dynamics of snowavalanches. International Symposium on Scientific Aspectsof Snow and Ice Avalanches, 5-10 April, 1965, Davos,Switzerland. A.I.H.S. Publ. No. 69, pp. 215-229.

D.S.D.A. Forest Service. 1961. Snow Avalanches. U.S.D.A.Handbook No. 194, January 1961, rev. October 1968, U.S.Govt. Printing Office, Washington, D.C., 84 p.

Canadian Snow Avalanche Size Classification System andTypical Factors

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TABLE I

.TypicalSize Description Mass

1 Relatively <lOtharmless topeople

2 Could bury, 102tinjure orkill a person

3 Could bury a 103tcar, destroya small bldg.or break a fewtrees

4 Could destroy 104 ta railway car,large truck,several bldgs.or a forestwith an areaup to 4 hectares(40000 m2 )

5 Largest snow lOStavalanchesknown; coulddestroy avillage or aforest of40 hectares

TypicalPathLength

10 m

100 m

1000 m

2000 m

3000 m

Typical ImpactPressures

1 kPa

10 kPa

100 kPa

500 kPa

1000 kPa

21

60 ----..,.---...,----r------r-----,

50

t;'2. 40

VllLJ

IU 30 0Z I..LJ« 0-I a::::«

0>« u

20 I..LJ

a::::

~

0z

101 EV ENT

'\0

1 2 3 4 5

S I Z E

FI GURE 1

FREQUENCY VS SIZE FOR 744 EVENTSFROM ROGER'S PASS, B.C.

.B R 6 104 - 1

22

70 ~--~~~~r----""'T"""---'---"

60

50

ef2-

-40Vl

I..LJ

IUZ«-.l

30«>«

20

10

10-102

102

-103

10 3-10 4 104-10 5 105

MA SS,

FIGURE 2

FREQUENCY VS MASS FOR 330 EVENTSESTIMATED AS SIZE 2 FROM ROGER'SPASS, B.C.

23

60 .....--~---,......--....,....---r----...

50

40ef2.

(/)

u.J

:::I:U 30z<C-J

<C><C

20

10

10-102

102

-10 3 10 3-10 4 10 4'-10 5 105

MASS, t

FIGURE 3

FREQUENCY VS MASS FOR 368 EVENTSESTIMATED AS SIZE 3 FROM ROGER'SPASS, B.C.

BR 6104-3

24

10-10 2 10 2-10 3 10 3-10 4 104-10 5 105

MASS, t

FIGURE 4

FREQUENCY VS MASS FOR 45 EVENTSESTIMATED AS SIZE 4 FROM ROGER'SB.C.

O....--........-----.........~........~~--- .....---

70 .....---,...--.......,~--~--~---.,

60

20

10

50

i:f2-

V) 40u...J

IUZ<t:---J

<t: 30><t:

25

60 -------,.....----,----r----...

50

;:f2.40

-VIw:c:u 30z«---J

«>«

20

101 EV ENT

)

10-102

102

-103 10 3-10 4 10 4-10 5 10 5

MASS, t

FIGURE 5

. FREQUENCY VS MASS FOR 6534 EVENTSFROM ROGER'S· PASS, B. C.

BR 6104-5

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APPENDIX

Estimates of Maximum Terminal Velocity

The centre of mass model (Perla, Cheng, andMcClung, 1980) contains essentially the same boundarycondition physics as the widely used Voellmy model. Thisresults in a two-parameter representation with dragexpressed as the sum of a constant term (coefficient ofsliding friction), ~, and of a term depending on V2 (where Vis the instantaneous velocity) which is written MID. MID isthe ratio of mass to V2 drag and is the sum of several termsfor some of which D may depend on M.

The differential equation for the motion of the centof mass along an increment of path ds with inclination e is:

~ §~2 = g(sin6 _ ~cos6) _ ~ V2 (1 )

with ~ and MID constant over the entire path, the generalsolution to (1) is (with V = 0 at S = 0):

( 3 )

( 2 )

( 4 )

( 5 )

SI !

d s I] 2

V '" rg .; 2g (s lne t - ~ cose t

-2DS S 2DV = [e -r;r f 2g (sin6 - ~cose )e """""'H"

o

Now if M/D»S, we find sensitivity to MID is lost and

S 1

V '" [f 2g (s i n6 - ~ cos6 )d s I] "2o

In reality, sensitivity of VT to variations inMID is nearly lost when MID is close to So, total pathlength (e.g. Bakkeh¢i et ale in press), where path lengthmeans distance along the slope from the starting zone to theavalanche stop position. Thus, MID has a practical upperlimit near So.

If terminal speed is reached on about half the total pathlength, 80/2, then an appropriate upper limit for VT is:

. Assume the path is broken into segments ofconstant averaged slope angle, i.e. the track (t) and therunout zone, as in Perla (1980). For constant averageincline in the track, 6t, (3) yields:

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Typically, expected numerical values for thesecond square root term in brackets are in the range 1-2 (mksunits) and therefore we see that maximum VT scalesapproximately as ISo.

This analysis along with expected flow densitiesexplains the path length and impact values in Table I. Itmay he noted that even though (5) is simplistic it is inaccord with estimated speed data at Rogers Pass, B.C. (e.g.Schaerer, 1975). Measured speed data there are actuallyless than 10 m/sec and 50 m/sec for path lengths 100 m and2500 m. Apparently the ideal maximum values predicted by(5) are rarely achieved, if at all.

The specification of impact pressures (densitytimes Vt

2 ) in the classification system is also consistentwith (5). Flow densities of interest are in the range 1-400Kg/m3 (2 orders of magnitude) while path lengths of interestare in the range 10 m to an absolute maximum of 104 m (3orders of magnitude). However, the extremes will not bepresent together so that the impact pressure range is lessthan 5 orders of magnitude.

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Discussion

Daffern:

One of my reasons for coming to these kind ofworkshops, as well as furthering my own knowledge, is totake some of the material back and try and apply it to theteaching of back country skiers. I think this is one pieceof information that we are going to have to be very carefulwith. The thing that bothers me is the word "harmless" inyour Size I avalanche classification. As far as the backcountry skier is concerned, there are only two sizes ofavalanches--one that can kill him and one that can't. To mymind a slough can be just as deadly by causing a person inthe mountains to lose his footing as can a large amount ofsnow that can come down and bury him. I just urge people touse caution in using this classification with back countryeducation classes.

McClung:

That is an excellent point. As I mentioned, thissystem is not for use by people without experience.

Fesler:

I think it is a good idea to recognize that we areonly able to establish the mass and volume within an orderbf magnitude because often field measurements aren't takenunder the best of conditions for obtaining accuratemeasurements. One further comment I have is on relating theparallel categories of mass and pressure potential. It hasbeen my experience that some of the very largest avalanches,in terms of the mass, aren't necessarily the ones that wouldhave produced the highest pressures because they may nothave had the highest velocities associated with them. Iwould say that we don't necessarily have a correspondencebetween the mass and the pressure potential.

McClung:

I did not mean to emphasize mass so much as otherfactors are definitely important. The two graphs that Ishowed--percentage of events versus mass and percentage ofevents vesus size--don't look the same in the case wherethere are other factors affecting the system. Mass is oneimportant factor; others are: how far the avalanches run,whether it is wet or dry.

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Stethem:

I am interested in the use of the system in termsof field forecasting because avalanche occurrences are oneof the most important inputs a forecaster has. One of theproblems I have encountered in correlation of data after thefact in improving a system is that if you have twelve ClasslIs and you try and establish an index for the day'sactivity you can add them up and call it 12. If you havefour Class 3's, you add them up and call that 12. The twothings are very different. What do you think of the idea ofsome form of a multiplier for each of 1, 2, 3, 4, 5, forexample, .1 for 1, .5 for 2, 1 for 3, 2 for 4, and 5 for 5,so that this index added daily means more to the forecasterwhen he is analyzing his data.

McClung:

I see what you mean. That is directed at aspecific research-oriented application. I don't really haveany guidelines for you on that. .

Stethem:

The problem is that, in Canada, this system iswhat we are teaching in the schools and it is what everybodyis using. If people come to the point, like I have, wherethey have to try and correlate the data by simply adding upthose numbers, they are going to have trouble. Maybesomebody should think about introducing some multiplier.

McClung:

Well, that is a possibility. Or perhaps you couldgive an explanation of the complexity of the problem andtell him it is not that simple.

Cronmiller:

In the guidelines that were sent out, you mentionthat we have to look at the aeposit after the avalance in~rder to fill out an occurrence report. Well, I go down~nto the deposit and I measure and I kind of have to equateIt with the objects I am looking at in the debris •. I takemy measurements that way and then mail in the occurrencereports accordingly. ..

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McClung:

I would submit that you are not taking yourmeasurements correctly or you are not classifying your sizescorrectly if you are doing that. What you have to do isvisualize if you can the point on the path where thegreatest damage would occur; that is, at terminal velocity.It is just not the mass of the deposit. You have to look atthe avalanche path, the runout--whether it is wet or dry,whether the path is confined or unconfined--and anydestructive effects you can see which are rare.

Stevens:

Size classification is a very subjective thing.It is going to be important for the forecaster to spend afair amount of time with his people in the field so that thesystem establishes some credibility.

Stoneman:

One thing that has bothered me for years is thatthere are no guidelines here. It has been very difficult tohave patrolmen reporting their avalanches since everybodyhas a different idea of size. It seems that this is the onemajor thing we have to work on and get some guidelines.

McClung:

That is what we are trying to do. If you noticed,Ed LaChapelle referred to Size 4 avalanches. That was theUSFS system which I didn't discuss but those Size 4avalanches meant nothing to anybody in Canada that did notknow the system. I submit that if you didn't know thespecific avalanche paths that he was talking about, itdidn't mean anything even to those people from the U.S.A.,so I agree that development of some sort of standard isimportant and we are trying to work toward that goal.