SNOW STABILITY EVALUATION By Janet Kellam, including adapted material from NAS 2005, Jill Fredston

CourseObjectives:Bytheendofthislecture,participantsshouldbeableto

• Identifybull’seyeinformationaboutsnowinstability• Understandhowtoseekcluesandhowtointegratespecificdatafrom

observationsandstabilitytests• Understandhowtodevelopanevaluationroutinethatobjectivelyexamines

dataandavoidsbiaseddecision‐making• Beabletoidentifyinstabilitytrends

LectureDescription:Snowstabilityevaluationiswhenweintegrateallourinformationaboutsnowpack,weatherandterraininordertoanswerthequestion“willitslide?”Snowstabilityisrelativelyeasytodeterminewhentheconditionsarevery,verybadorvery,verygood.Evaluationbecomesmorechallengingwhenstabilityconditionsliebetweenthetwoextremes,whichcanbemuchofthetime.Thelecturewillcoverthebasicprocessofevaluatingsnowstabilitybyrecognizingbull’seyecluesandidentifyingsnowandweatherfactorscontributingtoinstability.Equallyimportant,itwillshowhowtoprioritize,organizeandsystematicallyintegratealloftheinformationfromweatherdata,observationsandstabilitytests.WorkshopDescription:Inthisdecision‐makingworkshop,participantswilldivideintosmallgroupsandbepresentedwithanavalanchehazardevaluationproblemthatputsthembetween“arockandahardplace.”Participantswillneedtocommunicatewhattheyaregoingtodo,howtheyaregoingtodoit,andwhatdatatheyarebasingtheirdecisionupon.Inordertodefinetheseanswers,participantsmustfirstbeabletodeterminewhatthestabilityofthesnowis.Thisworkshopexaminesanuncontrolled,backcountrysnowpack.Decisionsmustbebasedonconcreteinformationandacceptablepractice.Afterpresentingthebasicoutdoorsettinginpicturesandwords,facilitatorswillopenthefloortoquestionswiththeparticipantsposingspecific,datagatheringquestions.“Isitsafe?”isnotavalidquestion.Thepurposeofthisworkshopistopracticeseekingandintegratingbull’seyeinformationaboutterrain,weather,snowpackandhumanvariablestodeterminewhetherornotapotentialavalanchehazardexists.

SNOWSTABILITYEVALUATIONLECTURESnowstabilityevaluationisanongoingprocess.Itbeginswiththefirstsnowfallanddoesnotenduntileverysnowcrystalhasmelted.Itisimportanttonotethatsnowonaslopeisadynamicsystem.Stabilityevaluationshouldnotonlyaddressthesnowpackatasinglepointintime,itmustalsoanticipatechangingstabilityasthemanyfactorsinfluencingthesnowpackchange.Synopsis:Avalanchesdonothappenbyaccident,theyhappenforparticularreasons.Snowpack,WeatherandTerrainandtheinteractionsbetweenallthreeofthesecriticalfactorsarekeywhenevaluatingthestabilityofaparticularslopeaswellasalargerregion.

• StabilityEvaluationisbasedonrealdataandmultipleobservations.Bewareofmakingassumptionsorbeinginfluencedbypersonalbiasesandoutsidepressure.

• Anorganizedsystemiscriticalforstabilityevaluation

• Keepoperationalrecords

• Collaborateandshareinformation

• Review“missed”forecastsandstabilityevaluations(nooneisright100%)

Avalanchesoccurwhenstressonthesnowpacksystemisequaltoorgreaterthanthestrengthofthesnowpacksystem.First,thesnowpackmustfailandinitiateacrack.Secondlyitmustpropagatethecrack(fracture)toproduceanavalancheonaslope.RECIPEFORANAVALANCHE ASLOPE 1)INITIATIONASLAB 2)PROPAGATIONAWEAKLAYERATRIGGER

THEPROCESSOFSTABILITYEVALUATION: Priortogoinginthefield:

• Reviewyourrecordsofsnowpackhistory,conditionsandassessments• Accessavalancheadvisoriesforcurrentconditionsandsnowpackhistory• Identifywhattypeofavalancheproblemsyouanticipate

o Newsnow,loosesnow,windslabs,wetsnow,warmingtemperatureso Persistentweaklayersanddeepslabconditions

• Identifywhereyouanticipatethedifferenttypesofavalancheproblemso Overallaspect,elevation,whatregionsorlargeareaso Drainagesorsmallerareaswithinaregiono Specificslopes

• Utilizeweatherproductsforpast,presentandfutureweather• Utilizelocalknowledge‐(professionalandrecreationalcontacts)

o PastconditionsandcurrentobservationsInthefield:

• Makeobservationsfromthesecondyoustepoutthedoor• DataCollection‐Makeongoingtargetedobservationsasyoutraveloversnow

o Observeandtestthesnow,useavarietyoftechniquesinavarietyoflocations.Visualclues,informaltests,columntests,blocktests

• Lookformultiplecluesandredundancyfromavarietyofsources• Lookforinstabilitynotstability• Adjustandupdateyourevaluationthroughouttheday• Don’tmakeassumptions• Mothernature’swarningsignstakeprecedence

o #1‐activesignofinstability‐naturalortriggeredslideso #2activesignofinstability‐shootingcracks,whumphing

• Additionalconditionsthatmayleadtoavalancheproblemso Significantnewsnow,wind,warmingtemperatures,persistentweak

layers• Remember,humantriggeredavalanchesoftenoccurbeforenaturalslidesor

evenifnaturalslidesdonothappen• Anticipatefutureproblems

o Askyourself“SoWhat”and“Whatif”o Considerthresholdsthatmayrepresentchangingstability

• KeepFieldNotesImportantfactorsforstabilityevaluations:

• Terrain‐updateyourevaluationsforvaryingterrain• Recognizepatternsandtrends

o Currento Historico Whenunprecedentedeventsbecomepossible

• Rateofchangeappliedtothesnowpacksystem• SpatialVariability• Uncertainty

DATACOLLECTIONNOTES:Dataisspecificormeasuredinformation,observationsandtestresults.Thekeytoevaluatingsnowstabilityishavingaccesstoreliableinformationaboutthestress/strengthrelationshipofthesnowpackinquestion.Bychoosinginformationthatprovidesahighdegreeofcertaintyinitsmessageandbyseekingavarietyofcluesandtestresultsthateitherreinforceorrefutethemessage,youcanquicklymakeaneducatedevaluationbaseduponmeaningfulinformation.Thekeytothisprocessisgoingforthe“bull’seye”data.Thisgetstotheheartoftheproblem.Payattentiontoitsmessage.Noonecluetellsthewholestory;informationmustbeintegratedfromavarietyofsources.Stabilityevaluationisanongoingprocess;opinionsmustbeconstantlyupdatedandnewopinionsmayariseandbedevelopedbaseduponnewinformation. Terrain:Istheterraincapableofproducinganavalanche?SlopeAngle:Concept‐thegreatertheslopeanglethegreaterthestressonalltheslabboundaries(i.e.,thecrownface,flanks,stauchwall,andbedsurface/slabinterface)duetogravity.Verifymeasurementswithaninclinometer;don’tguess.

• Slabavalanchesincoldsnowarepossibleonlywithinacertainrangeofslopeangles,generallybetween25and60degreesplus.Primetimestartingzonesrangebetweenapproximately35and45degrees.Theseshouldnotbetakenasabsolutenumbers,butasanapproximaterangethatvarieswithsnowclimateandspecificavalancheconditions.

• Onenotableexceptiontothisrangeofanglesareslushflowavalanches.

(Slabreleasesofwatersaturatedsnow.)Theseoccurinhighlatitudemountainrangesandhavebeenreportedinitiatingincreekbedswithslopeanglesoflessthan15degrees.Slushflowsarenotwetsnowreleases,whichcommonlyoccuratlowerlatitudesinresponsetomid‐winterthawsorspringtimesolarradiationandwarmtemperatures.

• Humantriggeredavalanchescanbeinitiatedfromaslopeofanyangle

undercertainconditions,eventheflats,aslongassomeportionoftheslopeisconnectedtoavalancheterrainandinstabilityexists.

• Differenttypesofinstabilitiestendtofailatdifferentslopeangles.Pay

attentiontowhichanglesandaspectsarefailingwhenacyclestarts,orslidesaretriggered.

SlopeAspect:(inrelationtosun/wind)Leeward,shadowedorextremelysunnyslopesarepotentiallymoreunstable.

• Leeward:becauseofincreasedstressduetorapidloadingandlayeringofwindtransportedsnow.

• Shadowed:becauseofthegreaterpropensityforthedevelopmentof

facetedcrystalsandthereducedrateofstrengtheningthroughsettlement.(Duetotheabsenceofbeneficialwarmingbythesunandgenerallycoldertemperatures;thisalsocreatesapotentialforlargertemperaturegradients).

• Extremelysunnyexposures:becauseoftherapidweakeningofthebonds

betweensnowgrainsfromsolarheatandthepercolationofmeltwaterintoandbetweensnowpacklayers.Alsothelikelydevelopmentofcrustlayersthatnewersnowmayormaynotbondto.

SlopeConfiguration(roughness/shape):Concept‐anchorpointsnotonlyprovideholdingabilityforthesnowpack,buttheycanactaspointsofdifferentialstressconcentration(i.e.immovableobjectssurroundedbyanotherwiseflowingsnowpack).Othervariationsalongaslopecanalsocreateareasofadditionalstress.

• Slabsfrequentlyfracturejustbelowconvexitiesandbetweenanchorpointsbecausethesearetheareasofgreatesttensilestress.

• Roughirregularsurfaces,suchasboulderfieldsorheavyvegetation,

generallyprovidebetteranchoringforasnowpackthansmoothsurfacessuchasgrassyortundracoveredslopesorsmoothrockslabs.Oncetheirregularitiesarecoveredup,theupperlayersgainnoanchoringbenefit.

• Assnowdepthincreases,theeffectivenessofanchorsdecreases.

• Steeprockyterrainisoftenpronetohavingshallow,weakfacetedsnow

formaroundburiedorvisiblerockoutcroppings.Ifapersistentburiedweaklayeralsoexistsacrosstheslope,theseshallowspotsmaybecometriggerpoints.

• Avalanchescanoccuronallsteepsnow‐coveredslopes,giventheproper

conditionsofinstabilityregardlessofslopeshapeorroughness.

Weather:Hastheweatherbeencontributingtotheinstability?Precipitation(type,rate,amount):Concept‐Increasedweightfromsnoworraincausesincreasedstress.Agivensnowpackcanonlyadjusttoacertainamountofstress(i.e.load)andonlyatacertainrateofspeed.Howmuchnewstressisthesnowpackcapableofhandling?Howwellisthenewsnowlayerbondingtotheoldsnowsurface?Don’tmakeassumptionsorguess,checkitout.

• Generally,warmermoreconsolidatedsnowlayersontopofcolderlesscohesivelayerstendtoreducestability.Conversely,warmsnowfallingoverwarmsnowtendstobondwellandstrengthenthroughsettlementifotherfactorsarenotinvolvedlikewind,rainorwarmingtemperatures.

• Unlikesnow,rainprovidesnoadditionalstrength,onlyweightand

lubrication.Rainfallingonnewsnowleadstounstableconditionsveryrapidly,evenbeforemuchweightorlubricationisadded.Afullscientificexplanationhasnotbeenfoundasofyet,butthisconditionwarrantsaconservativestabilityevaluation.Rainonoldsnowisslowertoreact.

• Asanexampleofrapidloading,astormproducing12”snow

accumulationduringonedayismorelikelytoresultinincreasedinstabilitythanonewhichdeposits12”over3days.Recordmeasurementsduringstormsofstormsnowdepthandinchesofwaterversustimeinordertomeasuretherateofprecipitation.

Wind (wind speed, direction, duration, amount of snow transport): Concept‐Depositionofwindtransportedsnowincreasesthestressinleeward(windloaded)areas,enhancesslabformation,andbuildscornices.Windspeed/directionalongwiththetypeandamountofsnowavailablefortransportdeterminesthedepthanddistributionofnewlydepositedwindslabs.

• Snowtransportedfromwindwardslopescanrapidlyincreaseinstabilityonleewardslopesduetotheinabilityofthesnowpacktoadjusttothenewweight.Aweaklayerorapoorbondbetweenlayerswithlittleinternalstrengthmaynotbestrongenoughtoholdtheweightofthenewlydepositedload.

• Eachsnowtypehasawindthresholdspeed(thespeedatwhichsnow

startstomove).Lowdensityunconsolidatedsnowismoresusceptibletowindactionthanolder,hardersnow.Howmuchsnowiscreatingloading(andwhere),andhowmuchisbeingsublimated?Fieldobservationsofskiandfootpenetrationintothesurfacesnowareausefulindicationofhowmuchsnowisavailablefortransport.Measureactualwindloadinganddeposition.Patternscanvarywitheachwindeventonthesameslope.

• Twoimportantobservationsneedtobemadewhenconsideringtriggeringavalanchesinwindslabs.Big,fat,freshwindpillowscanbetriggerpointsduetotherecentloadordepositionandthusthegreaterstress.Oftenthecaseduringorimmediatelyfollowingastorm;thisconceptisutilizedforskicuttingnewsnowinskipatrolsnowsafetymeasures.Inthecaseofolderorharderwinddepositionandburiedweaklayers,susceptibletriggerpointsaremorelikelyfoundnearthethinneredgesofwindslabsoratthinspotsintheslabwhereunderlyingweaklayersareclosertothesurface.(Weaklayersarecommonlyfoundaroundorontopofrockoutcroppings.)Skicuttingisnotutilizedforoldorhardwindslabs,asthistypeofslabislesspredictableandtendstobreakabovetheperson.Thebottomline‐bewaryinanywindaffectedterrain.

Temperature(max/min,trend,andfreezingline):Concept‐Thewarmerthesnowpacktemperature,thefasterinternalchangestakeplace.Whateffectarecurrentandprojectedtemperaturechangesgoingtohaveonthesnowpack?Howwillthesechangesaffectthesnowstability?

• Assnowpacktemperatureswarm,therateofdeformationincreasesandbothcreepandglidedownhillmaybeaccelerated.Whathavethetemperaturesbeen?Keeprecords!

• Warmersnowtemperaturesalsomeanfasterratesofmetamorphism,

regardlessofthetypeofchange.RemembertheBettyCrockerPrinciple.Itisthepresenceorabsenceofatemperaturegradientthatdeterminesthetypeofmetamorphism.Itistheambienttemperatureofalayerandthetemperaturegradientthatdeterminestherateofmetamorphism.

• Rapidorprolongedwarmingofthesnowpack(near0degreesC)or

intensesolarradiationoftenresultsingreaterinstability.Conversely,coolingofawarmsnowpackusuallyresultsinamorestablecondition,allotherfactorsbeingequal.

• Persistentburiedweaklayersinacolddrysnowpackmaypersistforlong

periodsoftime.Persistentgraintypessuchasburiedsurfacehoarorlargesizefacetedsnowmayremainintactforweeks,months,sometimesmostofaseason.Whentheseconditionsexist,thepossibilityoffalse‐stabletestsanddelayedactionavalanchesmustbeconsidered.

Snowpack:Willtheweaklayerfailandtheslabpropagateacrack?

SlabConfiguration(depth,distribution,andstructure):Question#1:Doyouhaveaslab?Ifso,whatisitsdepthanddistributionacrosstheslope?Whatareitsboundariesifitweretoslide?

• Examinetheslopeinrelationtowinddirectiontohelpdetermineslabdistribution.Useyourprobetodeterminedepthandboundariesandrelativehardnessorweaklayers.

• Caution:Don’texposeyourselftopotentialharmbyworkingbeneathaslopethatissuspect.Picksafespotstodigapit,utilizesmallertestslopes.Ifconditionsarefairlyunstable,saferlowangleslopesmayprovidealltheinformationyouneed.

• Remembertheconceptofspatialvariabilityacrossaslope.Youare

lookingforrepresentativepatterns,notjustoneobservationinonelocation.

BondingAbility:Question#2:Howwellistheslabbondedtothelayerbeneath?Aretherepersistentburiedweaklayers?Whatisthenatureandpossibledistributionofweakspots,triggerpointsorshallowsnowareas?Windexposed,rockyslopesareoftensuspectforshallow,weakareas.

• Confirmthenatureoftheweaklayersbyprobing,diggingquickarmpits,skitrackshears,snowmobilebankhits,performothersheartestsandstabilitytests.Lookforbull’seyecluessuchascracking,collapsingandwhumphingbytravelingoffthebeatentrack.Ifpreviousavalanchesexist,examinefracturelinesandflankprofilestodeterminethecauseoffailures.

• Poorlybondedlayerscomeinallforms.Digdowntoanysuspectedweak

layerandtestit.Insomecasesofnewsnowinstabilityitmaybeveryhardtodiscernaslabandaweaklayerorapoorbonduntilyouperformsometests.

• Beawareofthefactthatyoumaynotbeabletodetectinstabilityusing

testsalone.Sometriggerpointsortenderspotsarehardtofindanddangeroustoapproach.Useextremecautioninyourchoiceoftestsites.

SensitivitytoForce:Question#3:Whatkindofforcewillittakeandwhereplacedtomaketheweaklayerfail?Cantheslabpropagateacrack?Aretherepersistentburiedweaklayers?Whattypeofinstabilityorcharacterofavalancheareyoudealingwith?Integrateresultsfromallofyourtestsandobservations.Lookforredundancyandmultipleclues.Beobjective‐avoidassumptionsanddon’tletpersonaldesiresbiasyourinterpretations.

Althoughinsomecasesofdeepslabinstabilityitmaynotbelikelytotriggeranavalanche,theresultscanbecatastrophicandonalargescaleduetothedepthandextentoftheslab.Forexample,deeppersistentinstabilityoftenrequiresgreaterforceandoftenjusttherightplacementofforce,orjusttherighttimingsuchaswhenadditionalstressesarebeingaddedtothesnowpack(snowfall,significantsolarradiation,dramaticwarming,windtransport).Deepslabinstabilitycanproducefalsestableresultsinstabilitytests.Thebottomlineisyou(oranyoneelse)reallydon’twanttobeonorbeneathadeepslabwhenitgoes.

DATAINTERPRETATION:Organizingandincorporatingsnowpackfieldtestswithstabilityevaluations

1) Strength2) Structure3) Energy

1)Strength‐oftheweaklayersinrelationtotheslab Dotestsindicateconditionsconduciveforinitiation? Columntests‐testforfailureofweaklayers Compressiontest,Stuffblocktest Non‐standardizedsnowtests‐shoveltilt,loadedcolumn,handshears2)Structure‐representingstructuralinstability 4of5Lemonscanindicateinstability WeakLayerDepth:<1meter WeakLayerThickness:<10cm GrainType:Persistent(facets,depthhoar,surfacehoar) GrainSizeDifference:>1mm(atweaklayerboundary) HardnessTransitionsbetweenlayers:>1stepinhandhardness3)Energy­therateofreleaseinthesnowpack Dotestsindicateconditionsconduciveforpropagation? Qualityofshear‐Q1,Q2,Q3 Fracturecharacter‐SP,SC,RP,PC,BRK BlockTests‐testsindicatethepotentialforpropagation ExtendedColumnTest,PropagationSawTest Rutschblockfullrelease

MAJORCLUESTOINSTABILITY:Generally,whenanunstablesnowconditionexistsmultiplecluesandapparenttestresultswillalsoexist.Thekeyistopiecetogetherpatternsoflocations,slopeaspects,elevations,therangeofslopeanglesandspecificconditions.Whenforecastingintothefuture,itiscriticaltoevaluatestabilitytrends.

BULL’SEYECLUES

THECLUE THEMESSAGERecentavalancheactivity The#1Bull’sEyeClue

Whenrecentactivityhasoccurred,slopesofsimilaraspectandelevationaresuspect.

“WhumphingNoises” Thesuddencollapseofanunderlyingweaklayer.

Indicatesextremeinstabilityandavalanchesareverypossibleorlikelyonsteepenoughslopes.

ShootingCracks Theweaklayerfailingandtheslabpropagatinga

crackorfracturing.Typically,thelongerordeeperthecrackthemoreserioustheinstability.Avalanchesareverypossibleorlikelyonsteepenoughslopes.

REDFLAGS

HollowSounds Hollow,drum‐likesoundswhenaslabissteppeduponindicatethepresenceofalessconsolidatedandpotentiallyweaklayerbeneathaharder,moreconsolidatedlayer.Unstablewindslabsoftenexhibitthischaracteristic.Rememberwindslabsmaybetriggeredfromthethinneredgesoftheslaborfromweakerspotswithintheslabboundaries.

Windtransport,snowplumes Visiblealongexposedridgelineswhenthewind

istransportingsnowfromwindwardtoleewardslopes.Conditionscanbecomeunstableveryquickly.Ifthewindtransportpersists,avalanchecyclesmayrepeatasslopesarereloadedwithwindblownsnow.Closerinspectionrevealswindtexturedsurfacesandoftenhollowsounds.Crossloadedslopescanresultatmidtolowerelevationseventhoughplumingmaynothavebeenobserved.

StormActivity Newsnowcanmakeconditionsmoreunstable,especiallyifrapidratesofprecipitationandsnowaccumulationoccur.Iftheonlyproblemisnewsnowinstabilityandnotdeepslaborburiedweaklayers,warmerclimateswilltendtostabilizefasterthancoldertemperatures.

Shear,StabilityandPropagationtests Easyshearsandveryeasyshearsfromavariety

ofstabilityandpropagationtestsindicateinstability.Lookforpatternsofinstability.Negativeornoshearresultsdonotwarrantastableevaluationuntilallotherfactorsareconsideredandmultipletestsareconductedinavarietyoflocations.

ExplosivesResults Ifinstabilityissuspected,lackofresultsmaybea

resultofpoortimingorplacement.Utilizedatafromavarietyofsourcestomakeyourstabilityevaluation,notjustexplosiveorartilleryresults.

SPECIALNOTESONPERSISTENTWEAKLAYERS:Inthecaseofburiedweaklayersordeepslabinstability,thecluesmaynotbeasapparentastheyarefornewsnowproblemsandtestresultsmaybemixed.Becauseofprolongedinstabilityandgreateruncertaintyassociatedwithpersistentweaklayersitisimportanttobeconservativeandwatchforanychangesinweatherthatmaybeincreasingthestressonthesnowpack.Inthecaseofpersistentburiedweaklayers(facetedsnoworburiedsurfacehoar),acumulativethresholdmaybereachedbygradualloadingovertimeandalargefailurecanresult.Sometimesthisthresholdfordeepslabreleasecanoccurwithseeminglysmallchangesinstressandstrength.Theinstabilitymayappeardormantuntilahumantriggercollapsesathinner,weakspotontheslope.Ifthesnowpackisunstable,adeepslabavalancheonceinitiated,canpropagatelongdistancesthroughaseeminglyimperviousslab.Itisimportanttoconsiderunseenfactorsthatmaybeinvolvedinadeep,largeslabonaslopewithaburiedweaklayer.Gravityinducesdownhillcreepoftheslabandstresscanconcentrateattheslab/weaklayerinterfacewhilethisgradualloadingisoccurring.Intensesolarradiationmayalsoplayaroleinacceleratingdownhillcreep,thusaddingstress.(Lookatrooftopsandroofavalanches.)Shallowsnowpackareasinsteeprockyterrainarenotoriousforbecomingtriggerpointswiththeseconditions.Persistentweaklayers,onceburied,shouldalwaysbeconsideredaredflagandfurtherinvestigationisessential.Itisimperativetoweighallthefactorsandmakeconservativedecisionswhenassessingstabilityofslopeswithpersistentburiedweaklayers.Carefulterrainselectionwillbeyourbestdefenseduringtimesofprolongeddeepslabinstability.

AVALANCHES Observed and potential

- Character

- Type

- Trigger type

- Trigger size

- Size – destructive

- Size – relative

- Number & distribution

- Water (moisture)

- Runout distance

SNOWPACK - Spatial/Temporal variability

- Weak layer/interface presence

- Weak layer type (storm/persistent)

- Weak layer depth

- Weak layer strength/stress

- Bed surface

- Slab thickness

- Slab stiffness

- Load on weak layer(s) – H20

- Snow surface condition

- Snow available for transport

- Snowpack temperatures

- Moisture content

- Propagation potential

-Compaction

WEATHER Scale – space/time

- Current and future

- Precip (type, intensity, duration)

- Wind (speed, direction, duration)

- Temp (magnitude, trend)

- Energy balance (solar radiation)

- Humidity

- Sky condition

TERRAIN Spatial Scale

- Incline

- Aspect

- Shape

- Forest cover

- Ground roughness

- Terrain size

- Elevation (vegetation band)

- Traps (terrain complexity)

- Overlapping terrain

- Glaciation

Avalanche hazard factors (examples of commonly used factors)

The strength and weight given to these factors is a judgemental assessment with no hierarchy of data type

Evidence and Data

The Beacon, Winter 2005 ! Volume 9, Number 22

“Deep-slab instability stilllurks…beware, these slabs mayinspire false confidence luringbackcountry travelers furtherout onto slopes before releasinglarge areas….”

As a user of the CAIC daily avalanche bulletins, you nodoubt have read or heard us mention deep-slab insta-bility and the potential for big avalanches. But just what

is deep-slab instability? Why is it so difficult to assess? Why isit so deadly? And what can you do about it?

To answer these questions we put our heads together tocompare experiences with different winters, weather patterns,and snowpacks; we reviewed past accidents for commonthemes; and we checked a few avalanche books, but not toosurprisingly found there is very little information about deep-slab instability. In this short article we will summarize the factsof deep-slab instabilities, what you can look for, and what youcan do about it.

Deep instability simply refers to weaknesses in old snowlayers, and deep-slab avalanches are those that break into oldsnow layers. We know from work done primarily by the SwissFederal Institute of Snow and Avalanches in Davos, that thedeeper the weak layer, the less stress a person exerts on thelayer. In fact, a person’s weight has little additional effect onweak layers buried deeper than about one meter. Knowingthis, we decided to look at Colorado avalanche accidentsinvolving deep releases. We defined a deep release as any slabavalanche with a fracture line greater than three feet.Conversely any release equal to three feet or less is a shallowrelease. We ended up with a sample of 239 human-triggeredslab avalanches where we know the type of avalanche and thefracture-line (crown face) depth and width. On average, 25percent of avalanche accidents involve deep releases inColorado and other western states. Let’s take a look at thefacts of deep instabilities and deep-slab avalanches.

Fact 1: Deep insta-bility often involveshard-slab avalanches.

Hard slab by definition isa cohesive slab layer havinga density greater than 300kg/m3. Skis barely cut intoit. On the hand-hardnesstest, it is usually pencil hard.It is usually formed whenstrong winds redepositsnow. (See Figure 1)

Hard-slab avalanches arenasty and unpredictablebeasts, and in Colorado ourhigh elevations and strongwinds mean widespreadhard slab conditions. In factin Colorado 59% of ava-lanches deeper than threefeet involved hard slabs,whereas only 11 of 175(6%) avalanches three feetor less were hard slabs.

Fact 2: The snowpack appears to be very strongand able to withstand lots of weight.

There is no doubt that hard slab is strong snow-because ofits high density that results from the close packing of very smallgrains. Generally speaking, the thicker the slab, the stiffer andstronger the slab. Thick hard slabs often conceal their trapfrom the unwary backcountry traveler with an illusion of sta-bility, and it is stability, not strength, that is the important char-acteristic. These slabs can be very strong but are only as stableas the weak layer below. Failure and fracture start first in theweak layer, not the slab. In Colorado deep slab avalanchescatch an average of two people per avalanche compared to1.7 people for less deep avalanches. Why? See Fact 4.

Fact 3: The weak layeris days, weeks, oreven months old.

Persistent weak layers arethe usual problem layer indeep-slab avalanches. Theselayers are composed of rela-tively large and cohesionlessgrains that are slow tochange shape or gainstrength. These snow grainsinclude depth hoar andfacets, along with surfacehoar crystals. These weaklayers often form during aprolonged period of fair andmild weather when a strongtemperature gradient domi-nates the snowpack. It is

230

220

210

200

190

180

170

160

150

140

130

120

110

100

90

80

70

60

50

40

30

20

10

K2000 1500

P1000

1F500

4F F2N/m

Snow SurfaceSN

OW

DEP

TH (cm

)

Hard SlabLayer

Weak FacetLayer

HAND-HARDNESS

Figure 1: Cumberland Pass,Colorado, fatal avalanche fracture

line profile, February 6, 1999

Figure 2: Buried surface hoarlayer near Wolf Creek Pass

(Photo: Tom McKelvy)

Deep Slab Instability

by Dale Atkins and Knox Williams

From the Winter 2005, Volume 9,Number 2 issue. Posted with permission.

often during these same periods when the three C’s—clear,calm, and cold—create weak surface hoar on the snowsurface. Additional snows eventually bury these weak layers(See Figure 2), and it is not uncommon for a persistent weaklayer to produce avalanches a month or two later.

Persistent weak layers are especially troublesome becauseeven though they gain strength over time and sometimes cansupport considerable loads, when fractures do occur they canpropagate long distances. A persistent weak layer is analogousto a line of closely spaced dominoes standing on end. Both arestrong in compression and can support much weight; however,both are weak in shear. When one domino topples and fallsinto its neighbor it can trigger a cascade of toppling dominoes.Once a fracture starts in the weak layer, it may propagate longdistances through the layer.

Fact 4: Deep slabs release above you. Usually the stronger the slab, the farther fractures propa-

gate, and therefore the bigger the avalanche. There is evenmore bad news: these avalanches fracture higher above orfarther away from the trigger point—your position. (See Figure3) The result is that deep-slab releases are harder to escapefrom. Soft and shallow slabs more often break at or near yourfeet, giving you a slightly better chance to escape (but anyavalanche once set in motion is difficult to escape). InColorado the width of the average fracture line for a shallowrelease is about 260 feet; however, the width for a deeprelease is about double that at 540 feet across.

Fact 5: Deepslabs aretriggeredwhere theslab is thin.

This fact is keyto understandinghuman-triggereddeep-slab ava-lanches. Thoughour records tellof human-trig-gered avalanchesup to 12 feet deep, it is essential to recognize that the fractureline did not occur underfoot. The victim triggered the ava-lanche from a thinner spot where their body weight couldaffect the weak layer. (Figures 4 & 5) Once fracture occurs,cracks are driven by high energy and quickly propagate into

the deep slab areas. A personexerts nearly two times theforce on a layer one-halfmeter (50 cm) deep com-pared to the same layerburied one meter down, andnearly four times the forcecompared to the same layerburied two meters. It is notuncommon to see a skier orsnowboarder cut across alarge pillow of wind-driftedsnow only to see the fracturesshoot out as the rider reachesthe edge or bottom of thedrift. The same happens but

The Beacon, Winter 2005 ! Volume 9, Number 2 3

Figure 4: A large slab avalanche near Red Mountain Pass triggered bybackcountry skiers while standing on the ridge far to the right of the

avalanche. The skiers felt the shallow snow collapse beneath them andwatched cracks shoot out and release the avalanche. The fracture line

or crown face ranges from 2 to 8 feet deep. November 30, 2004. (Photo: Messmore Kendall)

Figure 3: Skier triggering a deepslab when he hit an area of

shallow, weak snow near a rockband. The final fracture was far

above the trigger point. Thedashed border represents the area

shown in first photo. (Photos:Tom Fankhanel, February 1984)

continued on page four

Figure 5. The additional stress of a skier canaffect a weak layer where the slab is shallow.

WeakLayer

Skier

The Beacon, Winter 2005 ! Volume 9, Number 24

with all too often tragic results when a skier crosses the thinspot of a large slab and sets an entire slope into motion. Theproblem for all of us in the backcountry is, of course, that wedo not know where the slab may be thin.

Fact 6: Snow pits and stability tests like theRutschblock and Compression Test can be mis-leading.

Snowpits are a great way to learn about snow and ava-lanches, but snowpits can be deceptive because they onlyreveal information from one point in the snow cover.Snowpits can also be misleading when diggers interpret thewrong layers. Many backcountry travelers tend to focus on the strong snow layers instead of weak layers. Anothercommon mistake is to dig in areas of deep deposition wherethe weak layer is far below the snow surface. When the weaklayer is buried more than a meter down it is difficult to assessthe weakness.

Stability tests can also be misleading when assessing deep-slab instability. Because the weak layer is deep and often old,stability tests often score in the moderate to high (stable)range for two reasons. First, because the weak layer is deep,the thick slab attenuates the applied stress, reducing its affecton the weak layers. Second, it is easy to unknowingly dig inthe wrong spot, where the weak layer is deep or is strong.With time, the super-weak zones in the weak layer will getsmaller and slopes become less sensitive to triggers. Thisexplains why we tend to see more frequent but smaller ava-lanches during a storm. But as the days pass we see fewer butlarger avalanches. As the super-weak zones shrink you arealso less likely to dig in the right spot to encounter theweakest snow, so stability scores may be high. This can givefalse confidence and a perception the snow is more stablethan it really is.

Fact 7: Deep-slab avalanches produce largeforces and crushing weight.

The middle of a roiling and crushing avalanche is a badplace to be, and this is especially true of deep-slab avalanches.In nature when it comes to gravity-enhanced events, biggerand heavier is almost always faster and more powerful andthis is especially true with deep-slab avalanches. Certainly adeep-slab avalanche implies more snow, but also the densityof that slab will likely be greater, resulting in an even moremassive avalanche. The power of a deep slab avalanche isbest viewed from afar.

What Should You Be Alert To?Deep-slab avalanches are ornery and unpredictable. These

avalanches are not only deeper but also wider than shallowreleases, and thus more dangerous. Even worse, while the firstskier or snowmobiler might trigger it, it is more likely that the5th, 10th, or even 20th traveler will hit the weakest spot andcause the avalanche. During times of deep-slab instability it iseasy to be lulled onto steep slopes because they seem stable.

To avoid this trap, here are some points of which to be aware:

• Avoid hard slabs on steep slopes. Hard slabs are notoriouslyunpredictable and best left alone. Hollow drum-like soundsare the most obvious clue to unstable conditions.

• Dig snow pits in spots of shallower snow and/or where theslab is thin. This makes it easier to test the weak layerbecause the applied force in a Rutschblock or compressiontest will easily reach the weak layer. Stability tests per-formed at the top of slopes are less reliable than tests donealong the edge and lower on the slope. Of course this cancreate the interesting dilemma of how much risk you’rewilling to take to get good data. If you are concerned aboutthe snow, but unwilling to take risks to collect that infor-mation, we suggest you find a different, less-steep route.

• In snowpits, look for and test persistent weak layers, suchas a thick layer of depth hoar or a thin layer of buriedsurface hoar. A dangerous combination is a persistent weaklayer sitting on top of a crust (e.g., melt-freeze, sun, or raincrust).

• On steep slopes, be leery of the edges of pillows on wind-drifted slopes. At the top of a pillow or drift the snow willbe deep, and you will find lots of strong snow between youand the weak layer. Where the drift tapers lower on theslope, less slab separates you from the weak layer, and youcould more easily trigger an avalanche.

• Generally winters with low snowfall or winters with periodsof prolonged dry spells-and occasional periods of strongwind-are best known for creating persistent weak layersand deep-slab instabilities.

What Should You Do About It?Uncertainty is the operative word when dealing with deep-

slab instabilities, and the backcountry traveler who assumesstable conditions may be in for a rude and painful surprise.Your first task is to learn as much as you can about currentconditions. On a broad scale, the CAIC hotlines and emailswill relay information that we have received on hard slabs ordeep-slab instability. On a small scale-like the slope you wantto be on-it is up to you to get data through observations,snow pits, and stability tests. Use the tips listed above in“What Should You Be Alert To?”

Your second task is to apply the habits of safe backcountrytravel to the max-habits such as one at a time, don’t bunchup, travel the edge not the center of slopes, don’t crossbeneath steep slopes, etc. You know the drill.

These tasks are your “due diligence,” and they will lessenyour uncertainty. But there is an unavoidable bottom line:When deep-slab instabilities lurk the only way to stay safe isto avoid steep slopes. This answer might sound like a “cover-your-you-know-what”-type response, especially when youwatch people rip turns on steep slopes, or listen to yourfriends tell of their latest powder adventure. However, if youtackle steep slopes with deep instabilities your safety does notrely on skill, technique, or equipment; it relies on luck. Luck issomething we would prefer to rely on in the casinos and notin the mountains. !

Deep Slab Instabilitycontinued from page three

INTEGRATING STRENGTH, ENERGY, AND STRUCTURE INTO STABILITY DECISIONS

“So you dig a pit and then what?”

By Ian McCammon and Don Sharaf

from The Avalanche Review, 23/3

“You get in zee pit, you get zee information, you get out” – Peter Schaerer told one of us during a training course. Along with his advice that you should never stop observing changes within the snowpack was the stern admonition that snow pits should be focused, efficient, and QUICK.

For years, avalanche educators have taught students to dig snow pits to look at stratigraphy, identify snow crystals, and perform stability tests. But correlating snow profiles with stability has never been an exact science, and many students came away unsure of exactly how they were supposed to use snow stratigraphy in their stability decisions. Moreover, many of them lacked clear objectives for their pit analysis and ended up wasting time collecting tedious and semi-relevant information. The result fell far short of Peter Schaerer’s sage advice. Rather than training people to make quick, informed decisions, we seemed to be creating an army of winter recreationists who could spend half an hour or more in one snow pit, but couldn’t tell you how, why, or when they would use the information they found.

Two years ago, we began teaching a simple approach to interpreting snowpit results on upper level avalanche courses and professional training seminars. Many students said the approach produced an “a-ha!” moment for them and they were excited to learn a simple way to focus their efforts in their snow pits. In this article, we’ll describe our approach and how we teach it, in hopes that others may find it useful in helping their students to make quick and informed stability decisions.

A three-part model

The discipline of fracture mechanics tells us that three things need to happen in order to produce the large-scale shear fracture that initiates a slab avalanche: 1) the fracture must begin at some point under the slab where the shear strength of the weak layer is overcome by applied stresses, 2) the shear fracture must liberate enough energy from the snowpack to sustain its own propagation, and 3) there must be a “path of least resistance” in the snowpack structure along which the shear fracture can propagate.

These three components of strength, energy, and structure are a simplification of the complex interrelationships that produce slab avalanches. But from a teaching standpoint, they provide an effective way of summarizing important stability information that students can gather from a few simple field procedures.

Figure 2. A fracture mechanical model of slope stability.

Strength

The backbone of most Level 1 stability classes is the standard stress-strength model. This model says that a weak layer will fail when you apply enough stress to it. An important implication of this model is that when stress and strength are very nearly balanced, unstable conditions will exist and slabs may be easily triggered by the weight of a skier or snowmachine. In this model, the role of stability tests is to assess whether stress and strength are in a critical balance. If your stability test scores are low, the weak layer is in a critical state of balance. But if your stability scores are consistently high, the weak layer is less likely to be triggered. Many students come away from avalanche courses with a simple rule of thumb: low scores are bad, high scores are good. Thus it’s no surprise that some of them come to base their go/no go decisions almost entirely on cursory avalanche observations and test scores from one or two snow pits.

Research has shown that the stress-strength model works pretty well most of the time: high test scores generally do correlate with stable conditions. But not always. A disturbing number of accidents occur during “false stable” conditions, where tests indicated stability but avalanches were still triggered by a skier or rider. In these cases, practitioners in the know say “Well, that’s spatial variability for you” and shake their heads in recognition of the tough job they have chosen. But for decision makers who rely on stability tests the message is deeply troubling – the stress-strength model is not the whole picture.

Energy

In 1998 , Ron Johnson and Karl Birkeland described a formal rating system for a phenomenon that field practitioners had noticed over the years: when stability tests fractured with a clean and fast shear, triggered avalanches were more likely. In 2001, Schweizer and Weisinger described a similar system used with rutschblock tests in Switzerland and in 2002, van Herwijnen and Jamieson described a system of fracture character used in Canada Exactly what these schemes are measuring remains unclear, but one trend stands out: fast and clean shears release their fracture energy quickly, and are more frequently associated with unstable conditions. See Karl Birkeland’s article on stability, shear quality, and fracture character in the previous issue of The Avalanche Review for more details.

Fracture mechanics tells us that the higher the fracture energy release rate, the greater potential the fracture has for propagation. Shear quality and fracture character may only provide a very rough estimate of the fracture energy release rate, but when used in conjunction with stability tests, shear quality seems to provide valuable information regarding the likelihood of avalanche triggering.

Structure

Imagine two snow profiles. In one, an interface between light-density storm layers produces a moderate and clean shear 30 cm from the surface. In the second, a layer of facets beneath a hard wind slab produces the same shear

quality and score at the same depth. Even though the two weak layers have the same strength and release their energy at the same rate, few practitioners would treat the two snow packs the same.

Figure 3: Two profiles with similar strength and energy, but different structural properties

In an effort to characterize some of the “red flags” that professionals use in comparing such profiles, McCammon and Schweizer (2002) described five stratigraphic features of weak layers that statistically correlate with skier-triggered avalanches.

Weak Layer Depth ≤ 1 m Weak Layer Thickness ≤ 10 cm Hardness Difference ≥ 1 step Weak Layer Grain Type - Persistent (SH, DH, FC) Grain Size Difference ≥ 1 mm

Table 1: Five Structural “Lemons”

. These features, referred to here as “lemons”, appear to be rough indicators of how well a snowpack might concentrate shear stresses in a weak layer. The more lemons in a weak layer, the more structurally weak the snowpack.. Schweizer et al. (2004) extended the initial concept, and refinement of the system continues.

Taken alone, strength, energy, and structure only do a fair job of predicting skier triggered avalanching. But fracture mechanics tells us that when all three factors are present, shear fractures are more easily initiated and are more likely to propagate large distances. Bruce Tremper uses the analogy of a combination lock; when all three tumblers of strength, energy and structure fall into place on a particular slope, conditions are primed for avalanching. False stable conditions exist when the tumblers of energy and structure are present, but strength tests indicate stability. Under these conditions, wandering onto an isolated weak spot can cause localized fracture that can propagate into an avalanche.

CT14 Q2

4 2.0

0

0 3 0.5

j 0.3

2 0.5

3 0.5

1 0.5

m 1.5

m 1.5

Teaching

The standard way that we approach teaching mechanics and stability assessment using strength, energy, and structure typically goes like this:

1) Review the stress-strength model and its limitations

2) Describe shear quality as a way of quantifying elastic energy release

3) Introduce the lemons as a method of analyzing snow structure

4) Incorporate the lemons into a quiz for strength, energy, and structure

The Test + Pit

In Snow Sense, Jill Fredston and Doug Fesler describe three questions to ask when digging a test pit:

1) What is the weakest (significant) layer?

2) How much force does it take to make that weak layer fail?

3) What is the depth and distribution of that weak layer?

These are very good questions to help focus snow pit observations and keep the observer from analyzing layers that are unimportant for an immediate go or no go decision. A test + pit takes the three questions and then asks you to get more structural information about the weak layer. To answer the five lemon categories, you need this additional information about the weak layer only. Obtaining this information shouldn’t take more than a few minutes .

1) What is the hardness of the weak layer and the layer immediately above it?

2) What is the grain size of the weak layer and the layer immediately above it?

3) What is the grain type of the weak layer? Persistent?

4) What is the weak layer depth (answered before with the previous questions)

5) What is the weak layer thickness?

Helpful hint: When teaching about the lemons we strongly encourage the students to write down the lemons into the back of their field books (along with the three objectives of a standard test pit).

5) Introduce the concept of the ‘Test +’ pit as an efficient way to analyze snow strength, energy and structure in a snow pit

As always, information from a snowpit shouldn’t override class 1 information (natural avalanches, recent loading, shooting cracks). However, we have found that when students look at snowpits within a larger mechanical framework, their efforts are more focused, efficient, and productive. So get in those snowpits and then… get out!

REFERENCES:

Birkeland, K., 2004, Comments on using shear quality and fracture character to improve stability test interpretation. The Avalanche Review, Volume 23 #2,

Johnson, R. and Birkeland, K. 1998. Effectively using and interpreting stability tests, Proc. ISSW, Sun River, OR.

McCammon, I. and Schweizer, J. 2002. A field method for identifying structural weaknesses in the snowpack, Proc. ISSW, Penticton, BC., pp.477 - 481

Schweizer, J., Fierz, C. and Jamieson, B. 2004. Assessing the probability of skier triggering from snow layer properties, Proc. ISSW, Jackson, WY.

Schweizer, J. and Weisinger, T. 2001. Snow profile interpretation for stability evaluation, Cold Regions Science and Technology, 33 (2-3): 179–188.

van Herwijnen, A. and Jamieson, B. 2002. Interpreting fracture character is stability tests, Proc. ISSW, Penticton, BC., pp. 514 - 520

Putting it all together A real-world quiz

strong low

weak low

weak high

strong high RB6 Q1 L4

RB2 Q1 L5

RB3 Q3 L2

RB7 Q3 L2

Test result strength energy structure

Strength Energy

Structure

strong

strong

weak

weak

AVALANCHE HAZARD EVALUATION FIELD CHECKLIST Doug Fesler & Jill Fredston Alaska Mountain Safety Center, Inc.

An “accident’ is a happening that is not expected, foreseen, or intended, sometimes resulting from negligence, often from ignorance, and typically resulting in injury, loss, or damage. Avalanche “accidents” are not a matter of mischance; they happen for particular reasons. The interrelationship of four critical variables—terrain, weather, snowpack and man—determines whether or not a potential avalanche hazard exists. Although these important variables are frequently changing, these changes are often detectable. Not only can critical information be observed, it can be measured, tested, evaluated and acted upon. The bottom line is that our route selection and hazard evaluation decisions are only as good as the data we seek—the primary causes of avalanche accidents are attitude and ignorance. Our attitude “filters” the data and warps it to our needs or desires. Our ignorance keeps us from seeking the answers beforehand. Warning: Using this checklist may save your life. Follow these simple steps: 1) Seek out critical data; 2) Evaluate the potential level of hazard (red, green, yellow); 3) Add a level of caution for the “unknown” and 4) Continually re-evaluate your situation without letting your attitude persuade you away from the facts. CRITICAL DATA HAZARD LEVEL* ACTION PARAMETERS: KEY INFORMATION G Y R Go No Go TERRAIN: Is the terrain capable of producing an avalanche? Slope Angle (how steep? exposed?) [] [] [] Slope Aspect (leeward, shadowed, or extremely sunny?) [] [] [] Slope Configuration (smoothness, anchoring, shape effect?) [] [] [] Overall Effect: [] [] [] [] [] WEATHER: Has the weather been contributing to the instability? Precipitation (added weight, stress?) [] [] [] Wind (significant snow transport and deposition?) [] [] [] Temperature (rapid/prolonged warming, weakening?) [] [] [] Overall Effect: [] [] [] [] [] SNOWPACK: Could the snow fail? Slab Configuration (depth, distribution, and structure?) [] [] [] Bonding Ability (nature and distribution of “tender” spots” [] [] [] Sensitivity to Force (easy shears, clues to instability?) [] [] [] Overall Effect: [] [] [] [] [] HUMAN: Could you be a trigger or a victim, and are you prepared for the consequences? Attitude toward life, risk, goals, data?) [] [] [] Technical Skill Level (high/low, so what?) [] [] [] Physical and Mental Ability (tired, weak, strong?) [] [] [] Appropriate Equipment (prepared for the worst?) [] [] [] Overall Effect: [] [] [] [] [] DECISION/ACTION: Do better alternatives exist? Go/No Go: Why? (What assumptions are you making?) [] [] [] [] [] * HAZARD LEVEL SYMBOLS: Think of data as being either red, green, or yellow lights. G=green light (go, OK) Y= yellow light (caution, potentially dangerous), R= red light (Stop, Dangerous). © December 1989, Alaska Mountain Safety Center, Inc. 9140 Brewster’s Drive, Anchorage, Alaska, 99516

i

Acknowledgements: A very special thank you and acknowledgement goes to Jill Fredston for all of her previous work on this topic. A significant portion of the materials in this notebook are hers, including much of the workshop. Her guidance, mentoring and insights have been invaluable over the years. In addition, I have included concepts and references to stability evaluation methods and field records done by Ian McCammon. His perspectives, research and innovative teaching have helped to further how we look at information, comprehend basic concepts and make decisions in avalanche terrain. The material in this lecture, just like snow stability evaluation, is an integration of many pieces of information. It is a compilation of the work of a large number of avalanche professionals over the decades. I thank them all. References: Books- Tremper, B. Staying Alive in Avalanche Terrain, 2nd edition, The Mountaineers, Seattle 2008. Chapter 6 Stability Fredston, J. and D. Fesler. Snow Sense: A Guide to Evaluating Snow Avalanche Hazard,

Anchorage AK. Alaska Mountain Safety Center, Inc, 1994 McClung, D. and P. Schaerer The Avalanche Handbook 3rd Edition, The Mountaineers,

Seattle 2006. Chapters 7 & 8, Appendix 6A-Common Biases or Decision Traps in Avalanche Forecasting

McCammon, I. Snow and Avalanche Field Notebook, SnowPit Technologies, PO Box 9038 Salt lake City, UT 84109

Articles-All available on websites Website: University of Calgary Applied Snow and Avalanche Research (ASAR)

http://www.schulich.ucalgary.ca/Civil/Avalanche/papers_all.htm Jamieson, Bruce. 1999 slightly revised 2000, 2004. Skier triggering of slab avalanches:

Concepts and research.The Avalanche Review 18(2), 8-10. Jamieson, B., T. Geldsetzer and C. Stethem. 2001. Case study of a deep slab instability

and associated dry slab avalanches. Proceedings of the International Snow Science Workshop in Big Sky Montana, October 2000. American Avalanche Association, PO Box 1032, Bozeman, Montana, 59771, USA, 101-108. Reprinted in Avalanche News 60, 29-36.

Jamieson, B. and J. Schweizer. 2005. Using a checklist to assess manual snow profiles. (yellow flags) Avalanche News 72, Canadian Avalanche Association, Revelstoke, BC., 57-61.

Schweizer, J.,C. Fierz and B. Jamieson. 2005. Assessing the probability of skier triggering from snow layer properties. Proceedings of the International Snow Science Workshop in Jackson Hole, Wyoming. USDA Forest Service, Fort Collins, CO, 192-198.

Schweizer, J., K. Kronholm, B. Jamieson and K. Birkeland. 2006. Spatial variability – so what? In: J.A. Gleason (editor), Proceedings of the 2006 International Snow Science Workshop in Telluride, CO., 365-377.

ii

Website: Forest Service National Avalanche Center-NAC tech pages

http://www.avalanche.org/%7Enac/NAC/techPages/techPap.html#CA Birkeland, K.W. and D. Chabot. 2006. Minimizing "false stable" stability test results:

Why digging more snowpits is a good idea. Proceedings of the 2006 International Snow Science Workshop, Telluride, Colorado.

Website: SnowPit Technologies

http://www.snowpit.com/articles/articles.htm McCammon, I. and J Schweizer. 2002. A field method for identifying structural

weaknesses in the snowpack, Proceedings of the 2002 International Snow Science Workshop, Penticton, BC 477-481

Website: American Avalanche Association –The Avalanche Review archives

http://www.americanavalancheassociation.org/pub_archives.html Note: back issues of The Avalanche Review are archived on the internet the summer after publication. 2009- A special series on fracture propagation was published. The Avalanche Review 27 (4) pg 23-28, April 2009 McCammon, Ian. 2006. A User’s guide to the obvious clues method. The Avalanche

Review 25 (2) pg 8-9, December, 2006. Additional articles from ISSW 2004, 2006, 2008 (archived on the internet through avalanche.org, ISSW and the American Avalanche Association.) 2010 to be archived and available soon. Atkins, Roger. 2004. An avalanche character checklist for backcountry travel decisions.

Proceedings of the International Snow Science Workshop in Jackson Hole, Wyoming. USDA Forest Service, Fort Collins, CO, 462-468.

Birkeland, K. and R. Simenhois, 2008. The Extended Column Test: Test effectiveness, spatial variability, and comparison with the propagation saw test, Proceedings of the International Snow Science Workshop Whistler, BC. Birkeland, K., Simenhois, R., Heierli, J., 2010 The effect of changing slope angle on extended column test results: Can we dig pits in safer locations? Proceedings of the International Snow Science Workshop in Squaw Valley, California. 55-60. Jamieson, B.,Schweizer, J.,Statham, G., Haegli P., 2010 Which obs for which avalanche type? Proceedings of the International Snow Science Workshop in Squaw Valley, California. 155-161. Statham, Grant, 2008. Avalanche Hazard, Danger and Risk-A Practical Explanation Proceedings of the International Snow Science Workshop Whistler, BC.

iii

Statham, G., Haegli P., Birkeland, K., Greene, E., Israelson, C., Tremper, B., Stethem, C., McMahon, B., White, B., Kelly, J. 2010 A conceptual model of avalanche hazard.

Proceedings of the International Snow Science Workshop in Squaw Valley, California. 686-692.

Sascha B., Jamieson, B., Schweizer, J., 2010 When to dig? Thoughts on estimating slope stability. Proceedings of the International Snow Science Workshop in Squaw Valley, California. 424-430. Schweizer, J., and Jamieson, B. 2010 On surface warming and snow instability. Proceedings of the International Snow Science Workshop in Squaw Valley, California. 619-622.