Cold Regions Science and Technology

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1 Cold Regions Science nd Technology 97 () 97 Contents lists ville t ScienceDirect Cold Regions Science nd Technology journl homepge: Mesurements of snow sl displcement in Extended Column Tests nd comprison with Propgtion Sw Tests A. vn Herwijnen,,K.W.Birkelnd WSL Institute for Snow nd Avlnche Reserch SLF, Dvos, Switzerlnd USDA Forest Service Ntionl Avlnche Center, Bozemn, MT, USA rticle info strct Article history: Received 7 Decemer Received in revised form July Accepted July Keywords: Snow frcture Snow stility tests Avlnche forecsting Prticle trcking The Extended Column Test (ECT) hs ecome incresingly populr for ssessing snowpck stility. Wht hppens to the snow sl nd the underlying wek lyer during the test remins lrgely unknown. Such work hs een done for the Propgtion Sw Test (PST), ut not for the ECT. We therefore nlyzed high-speed videos of ECTs nd djcent PSTs using prticle trcking to etter understnd the mechnics of the ECT. Our results show tht in n ECT, tpping on one end of the column hd no oservle effect on the opposite end, nd tht frcture initites in n re of the wek lyer directly under the shovel t the free edge of the column. We oserved no signs of progressive dmge ccumultion in the wek lyer during tpping, ut rther single rpid collpse when frcture initited. In contrst, in PSTs, we oserved sl ending prior to wek lyer frcture. During frcture, wek lyers in ECTs compct on the order of severl mm, similr to mesurements otined from PSTs. Mesured propgtion speeds, on the order of to m s, were lso similr to those from PSTs. The similrities etween ECT results nd those with PSTs give us confidence tht the frcture mechnics re similr regrdless of the triggering mechnism. From prcticl perspective, our results suggest tht the ECT is indeed mesuring the propensity of crck to propgte t the smll scle of the ECT lock, thus providing informtion on criticlly importnt property of snow stility in our tests. Elsevier B.V. All rights reserved.. Introduction Dry snow sl vlnches re preceded y frcture of wek snowpck lyer elow cohesive sl of snow. An initil wek lyer crck must first rech criticl size efore it expnds rpidly, relesing sl vlnche (e.g. Schweizer et l., ). Wek lyer crcks cn form nturlly (e.g., due to snow loding) or rtificilly (e.g., due to skier loding or explosive triggers). Avlnche workers nd reserchers utilize snow stility tests to chrcterize snow strtigrphy nd estimte whether prticulr slope is stle or unstle. Idelly, results from these tests should provide unmiguous informtion on the ese of crck initition nd the propensity for crck propgtion, oth of which re required for snow sl vlnche relese (e.g. vn Herwijnen nd Jmieson, 7). However, given the inherent limittions of the vrious tests in replicting the vlnche relese process nd the sptilly vrile nture of the mountin snowpck, snow stility test results re not unmiguous nd require expert interprettion (Schweizer nd Jmieson, ). Nevertheless, numer of different studies demonstrte the usefulness of snow stility tests for ssessing vlnche risk (e.g. Föhn, 97; Simenhois nd Birkelnd, 9; vn Herwijnen nd Jmieson, 7). Snow stility tests consist of isolting lock of snow of pre-defined dimensions nd incrementlly loding the lock up to mximum lod. Corresponding uthor. Tel.: + 7. E-mil ddress: vnherwijnen@slf.ch (A. vn Herwijnen). While one gol of these tests is to identify potentil wek lyers, the primry gol is to evlute the lod required to frcture them, i.e. to crete n initil crck tht will propgte. Trditionlly, the most commonly used tests were the Rutschlock test (Föhn, 97), consisting of. y m lock of snow loded y skier, nd the compression test, consisting of y cm lock of snow loded y tpping on snow shovel (Jmieson, 999). While the loding step t wek lyer frcture minly reltes to crck initition, dditionl informtion on crck propgtion is otined through qulittive frcture oservtions (e.g. Schweizer nd Jmieson, ). In recent yers, reserchers developed two new field tests to investigte frcture initition nd frcture propgtion long the wek lyer, nmely the Extended Column Test (ECT) nd the Propgtion Sw Test (PST) (Guthier nd Jmieson,, ; Sigrist nd Schweizer, 7; Simenhois nd Birkelnd, 9; vn Herwijnen nd Jmieson, ). The ECT involves isolting lock 9 cm cross-slope y cm upslope nd then progressively loding one side of the lock y tpping on snow shovel, similr to compression test (Simenhois nd Birkelnd,, 9). Avlnche workers hve rpidly dopted the ECT since its introduction in. It hs ecome the most populr stility test in the pst two yers y SnowPilot users (Chot et l., ), eing conducted in lmost % of the pits (Birkelnd nd Chot, ). The usefulness of the test s forecsting tool for predicting vlnche conditions hs een studied in severl different snow climtes nd countries (e.g. Ross nd Jmieson, ; Simenhois nd Birkelnd, -X/$ see front mtter Elsevier B.V. All rights reserved.

2 9 A. vn Herwijnen, K.W. Birkelnd / Cold Regions Science nd Technology 97 () 97 Tle Overview of the snow properties t ech field site. Site N ECT N PST θ (degrees) F H (m) ρ sl (kg m ) A. DH 9 9 B. SH 7 9 C. FC D. SH E. SH 7 F. SH 9 G. SH The numer of ECTs (N ECT ), the numer of nd PSTs (N PST ), slope ngle (θ), wek lyer grin form (F), sl depth (H) nd men sl density (ρ sl ) re shown for ech site. The oserved wek lyer grin forms were: depth hor (DH), surfce hor (SH) nd fceted grins (FC). 9; Winkler nd Schweizer, 9). However, the processes leding to wek lyer frcture in the ECT hve thus fr not een investigted. The PST involves isolting lock tht is cm cross-slope y cm upslope (or the wek lyer depth, whichever is greter) nd then cutting the wek lyer with snow sw until wek lyer frcture (Guthier nd Jmieson, ). The populrity of the PST hs risen stedily since its introduction. It ws the third most common field test mongst SnowPilot users for the winter of / (Birkelnd nd Chot, ). In contrst to the ECT, the PST hs een the suject of severl detiled studies focusing on the mechnics of the test to etter understnd some of the fundmentl processes involved in snow frcture (e.g. Bir et l., ; vn Herwijnen nd Heierli, 9; vn Herwijnen et l., ). Much of this work utilizes prticle trcking, where video recording of the test is mde with high speed cmer nd mrkers re inserted into the snowpck to nlyze snow displcement using prticle trcking softwre. In this study we pply the sme prticle trcking technique used in PST studies to the ECT. The gol is to etter understnd the mechnics of the ECT, such s whether or not the whole lock is ffected y the tpping, where the frcture initites, nd whether frcture speeds in the ECT re similr to those mesured with the PST. An improved mechnicl understnding of the test will help to etter interpret test results nd evlute potentil limittions.. Methods.. Dt collection We collected dt t seven sites during the / winter in southwest Montn, USA (Tle ). At ech field site, we did mnul snow profile to otin informtion on snow strtigrphy nd performed one or severl ECTs nd PSTs ccording to the procedure outlined in Greene et l. (). In some cses, our ECTs hd wider columns, llowing us to etter evlute wek lyer frcture t distnces frther from the tpping re (Fig. ). Our PSTs lso hd longer columns, nd we modified the columns to hve slope norml, rther thn verticl, column ends, s suggested y other reserch (Bir et l., ; Heierli, ; vn Herwijnen nd Heierli, ). After excvting test columns from the surrounding snow cover, we inserted lck plstic mrkers into the snow wll ove the wek lyer. Using cmer on tripod pointing directly t the pit wll, we mde video recordings of the tests t frmes per second. We stood off to the side during the ECTs while tpping so s not to ostruct the cmer view of the mrkers (Fig. ). A chllenge encountered during ECTs ws tht snow often fell in front of the mrkers directly elow the snow shovel during tpping, nd mrkers fell out of the pit wll s the lock ws compressed (Fig. ). This mde it difficult, or in some cses impossile, to ccurtely trck the displcement of mrkers immeditely elow the shovel. This prolem did not ffect mrkers further wy from the shovel... Dt nlysis We utilized prticle trcking velocimetry (PTV) lgorithm to nlyze the displcement of the mrkers in the imges of the video recording (Crocker nd Grier, 99). Coordintes re ssigned to the centroid of ech mrker y using sptil nd pss filter to serch for locl rightness minim. In this wy, the position of the mrkers in ech video frme cn e determined with men ccurcy of. mm. The ccurcy of the prticle trcking softwre depends on the size nd the qulity of the imges, i.e. signl-to-noise rtio. We used coordinte system ligned with the slope: the x-xis pointing in down-slope direction prllel to the snow surfce, the z-xis pointing towrds the ground norml to the snow surfce. We defined the displcement of mrker in the slope prllel direction u x nd in the slope norml direction u z s the displcement reltive to the initil position, tht is, the verge position of the mrker p prior to movement. In the following, solute displcements U ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u x þ u z re lso reported. For propgting crcks, there is dely etween u z of susequent mrkers. The time dely etween the onset of movement etween mrkers is proportionl to the distnce etween the mrkers nd ws used to clculte the propgtion speed c of frcture, s outlined in vn Herwijnen nd Jmieson (). We estimted uncertinties in c using Monte Crlo method to ccount for the PTV mesurement uncertinty nd uncertinties ssocited with the scling distnce mesurement (±%). By mking rndom drws from the proility distriutions for the input quntities (ssumed norml), we evluted the resulting distriution. The vlues of c reported here re the men of the respective distriutions nd the reported uncertinty is twice the stndrd devition of the respective distriutions, which gives level of confidence of pproximtely 9%. Fig.. () The experimentl setup consisted of recording ECTs with lck mkers inserted into the pit wll on high speed videos. () During tpping, flling snow prtilly oscured mrkers directly under the shovel, mking trcking those mrkers difficult or impossile.

3 A. vn Herwijnen, K.W. Birkelnd / Cold Regions Science nd Technology 97 () Results We nlyzed the displcement of the sl in 9 ECTs nd PSTs t seven different sites with slope ngles (θ) rnging from to (Tle ). The mjority of wek lyers consisted of uried surfce hor (SH), while one wek lyer consisted of depth hor (DH) nd one of fceted grins (FC). We tested vriety of sls (Fig. ), including soft new snow (site D), soft fceted snow (sites A, F nd G), rounded snow (sites B nd E), nd wind deposited snow (site C). Sl thickness, mesured verticlly, rnged from. to.7 m nd men sl density rnged from to kg m... Snow displcement in ECTs Tpping on the lock during ECTs minly compcted the snow directly elow the snow shovel nd hd little influence deeper in the snow cover or further wy from the shovel. For exmple, the snow column in test ECT_E ws sustntilly compressed directly elow the shovel prior to wek lyer frcture (Fig. nd ). In this experiment, the wek lyer consisted of uried surfce hor which frctured fter 7 tps. Frcture rrested efore crossing the entire column nd crck through the sl occurred pproximtely hlfwy through the column (Fig. ). The solute displcement of mrkers in the upper hlf of the sl directly elow the shovel exhiited the lrgest displcements (drk nd light lue curves in Fig. c). On the other hnd, prior to wek lyer frcture, we did not mesure ny displcement of mrkers closer to the wek lyer nd further wy from the shovel (green, ornge nd red curves in Fig. c). Note tht for these ltter mrkers the displcements oserved fter ech tp, indicted with the dshed lines in Fig. c, were rtifcts resulting from snow flling in front of the mrkers. Prior to wek lyer frcture, we did not mesure ny displcement of the mrkers closest to the wek lyer in ECTNs, i.e. ECTs in which the frcture rrested efore crossing the entire column. In ECT_F, the initil wek lyer frcture ws not oserved in the field. Susequent tps then resulted in dditionl wek lyer compction (Fig. ). In ll ECTPs, i.e. ECTs in which the frcture crossed the entire column, we lso did not oserve ny displcement for mrkers closest to the wek lyer prior to wek lyer frcture. This is clerly seen for row of mrkers directly ove the wek lyer in test ECT_B in Fig.. These results lso show tht sl displcement due to wek lyer frcture strts t the free Fig.. Results from experiment ECT_E (ECTN 7). () Imge tken t the strt of the test. The colored dots show the mrkers for which the displcements re shown. () Imge tken t the end of the test showing considerle crushing of the column, prtil wek lyer frcture nd sl frcture. (c) Totl displcement with time. The colors of the displcement curves correspond to the mrkers indicted with the colored dots in (). The dshed lines correspond to tps. end of the column efore propgting to the other side. This ehvior ws oserved in ll ECTPs. In ll our ECTs, u z rnged from. to 9. mm in the re where the wek lyer frctured (Tle ). In ECTPs, u z fter wek lyer frcture either decresed (ECT_D, ECT_D, ECT_D nd ECT_F) or incresed (ECT_B nd ECT_B) throughout the column (Fig. ). In ECTNs, on the other hnd, u z decresed to zero typiclly within. to. m from the free edge of the column (Fig. ). Note tht in some experiments we could not determine the totl slope norml displcement due to sliding of the lock fter wek lyer frcture... Snow displcement in PSTs Progressively swing the wek lyer during PSTs resulted in ending of the sl prior to wek lyer frcture, s previously reported (Bir et l., 7 Site A 7 Site B 7 Site C 7 Site D 7 Site E 7 Site F 7 Site G FF F P FF F P FF F P FF F P FF F P FF F P FF F P Fig.. Overview of the snow strtigrphy t ech site. The hnd hrdness is shown for the lyers ove the wek lyer (lue) s well s for the wek lyer (red). The depth is shown reltive to the top of the wek lyer.

4 A. vn Herwijnen, K.W. Birkelnd / Cold Regions Science nd Technology 97 () 97 ; vn Herwijnen nd Heierli, ). While the slope norml displcement ssocited with sl ending ws generlly smll, on the order of. mm, it ws clerly oserved in the u z curves of mrkers closest to the free end of the column, independent of susequent frcture distnces (rrows in Fig. ). Like in ECTs, sl displcement cused y weklyerfrcturestrtedtthefreeendofthecolumneforepropgting to the other end of the column (Fig. ). Note tht in experiment PST_F the initil wek lyer frcture ws not oserved in the field. Susequent swing then resulted in dditionl nd rupt wek lyer collpse (Fig. ). After wek lyer frcture, u z rnged from. to. mm in the PSTs (Tle ). In PSTs where the wek lyer frctured through the entire column, u z fter wek lyer frcture generlly decresed (PST_B, PST_C, PST_C nd PST_G) long the column, except for experiment PST_B (Fig. 7). In PSTs where the wek lyer did not frcture through the entire column, u z decresed to zero within. to. m from the free edge of the column (Fig. 7)... Propgtion speed Propgtion speeds in ECTPs rnged from to m s (Tle ), while in PSTs c rnged from 9 to m s (Tle ). Despite our reltively short columns, smll vritions in c with distnce were oserved (not shown), lthough no consistent trend ws present. Propgtion speeds in ECTs either incresed (ECT_A, ECT_A, ECT_B, ECT_B nd ECT_D), were reltively constnt (ECT_C nd ECT_D), or decresed (ECT_A, ECT_D nd ECT_F). Likewise, propgtion speeds in PSTs either incresed (PST_B), were reltively constnt (PST_A, PST_C, PST_C nd, PST_E), or decresed (PST_B nd PST_G). We mesured the propgtion speed for oth ECTs nd PSTs t three sites (sites A, B nd C). Vlues from oth tests were in resonle greement, except for site C, where c mesured in the ECT ws lrger thn in oth djcent PSTs (compre Tles nd ). Despite the lrge rnge of c vlues, they re consistent with previous mesurements (Johnson Tle Overview of ECT results nd PTV mesurements. Test Test result W (m) (mm) c (m s ) ECT_A ECTP.. ± ECT_A ECTN.. ECT_A ECTN.... ECT_A ECTP.. ± 9 ECT_A ECTP.9. 7 ± ECT_B ECTP....7 ± ECT_B ECTP ± ECT_C ECTP 7.. ± 9 ECT_D ECTN ECT_D ECTN....7 ECT_D ECTP ± ECT_D ECTP.... ± 7 ECT_D ECTP ± ECT_E ECTN ECT_F ECTP.... ± ECT_F ECTN.... ECT_F ECTN.... ECT_G ECTN....7 ECT_G ECTN ECT results re shown ccording to Greene et l. (): ECTP## frcture initites nd propgtes cross the entire column on tp ##; ECTN## frcture initites without full propgtion on tp ##. Width of the column (W), PTV mesurement ccurcy (), the rnge of slope norml displcement in the re where the wek lyer frctured (u z )nd frcture speed with mesurement uncertinty (c) re lso shown. The column slid down-slope fter frcture. Frcture propgtion eyond the re elow the shovel ut not cross the entire column. et l., ; vn Herwijnen nd Jmieson, ; vn Herwijnen et l., ) (Fig. ). Comining our dt with dt from previous studies results in sttisticlly significnt reltionship etween propgtion speed nd men sl density (Person r =., p =. ; Fig. ). Furthermore, c lso correlted significntly with u z (Person r =., Fig.. Slope norml displcement with time for the row of mrker closest to the wek lyer in tests () ECT_F (ECTN ) nd () ECT_B (ECTP ). The colors of the lines correspond to the distnce of the mrkers to the free edge of the column in m, s indicted in the legend. The dshed lines correspond to tps.

5 A. vn Herwijnen, K.W. Birkelnd / Cold Regions Science nd Technology 97 () ECT_B ECT_B ECT_D ECT_D ECT_D ECT_F ECT_A ECT_D ECT_D ECT_E ECT_F ECT_F ECT_G ECT_G.. Horizontl distnce (m). Horizontl distnce (m) Fig.. Men totl slope norml displcement fter wek lyer frcture with horizontl distnce long the column for () ECTPs nd () ECTNs. The colors of the lines correspond to the experiments indicted in the legend. p =. ; Fig. ), confirming results previously reported y vn Herwijnen nd Jmieson ().. Discussion While the PST hs een the suject of severl detiled studies focusing on frcture initition, frcture propgtion nd crck fce friction (Bir et l., ; vn Herwijnen nd Heierli, 9, ; vn Herwijnen nd Jmieson, ), thus fr the processes leding to wek lyer frcture nd frcture propgtion in the ECT re mostly unknown. The results presented here therefore provide vlule insight into the mechnics of the ECT nd llow for detiled comprison with the PST. Note tht in some cses we used non-stndrd test columns in our experiments. Some cution is therefore wrrnted when pplying our results to stndrd tests, s very little is known out the influence of test geometry on results. Overll, the mesurements suggest tht there is no grdul dmge ccumultion in the wek lyer during tpping in n ECT. Indeed, no noticele displcement ws oserved in mrkers directly ove the wek lyer prior to frcture (Figs. nd ). Furthermore, the mesurements lso show tht tpping on the column minly compresses the snow directly elow the shovel nd does not ffect the fr end of the column (Fig. ). We therefore hypothesize tht wek lyer frcture in n ECT cn only occur fter the sl hs een compressed enough to trnsmit sufficient stress to the wek lyer for frcture. At the criticl loding step, the wek lyer ruptly frctures over n re roughly equl to the width of the shovel, since in most ECTNs u z decresed to zero within. to. m from the free edge of the column (Fig. ). Thus, in our mostly soft sls, criticl crck lengths were roughly. to. m. Hrder sls my differ in this respect. In contrst, in PST n unstle crck is creted y grdully cutting the wek lyer with snow sw until crck propgtion egins. Even though criticl crck lengths in PSTs re typiclly round. to. m, longer criticl crck lengths hve lso een documented (e.g. Ross nd Jmieson, ). Progressively cutting the wek lyer with snow sw results in ending of the overlying snow sl (Fig. ), providing the energy necessry for crck propgtion (Bir et l., ; Heierli et l., ; Sigrist nd Schweizer, 7; vn Herwijnen nd Heierli, ). Thus, when performing PST, sl properties re not ltered s they re during n ECT c Fig.. Slope norml displcement with time for the row of mrkers closest to the wek lyer in tests () PST_D, () PST_F nd (c) PST_B. The colors of the lines correspond to the distnce of the mrkers to the free edge of the column in m, s indicted in the legend. The rrows indicte the mrker displcement ssocited with sl ending prior to wek lyer frcture.

6 A. vn Herwijnen, K.W. Birkelnd / Cold Regions Science nd Technology 97 () 97 PST_B PST_B PST_C PST_C PST_G... Horizontl distnce (m) 9 7 PST_D PST_F.. Horizontl distnce (m) Fig. 7. Men totl slope norml displcement fter wek lyer frcture with horizontl distnce long the column for () PSTs with complete wek lyer frcture nd () PSTs with prtil wek lyer frcture. The colors of the lines correspond to the experiments indicted in the legend. Though the processes involved in reching the criticl crck length prior to propgtion differ for ECTs nd PSTs, our oservtions on wek lyer frcture re consistent for oth tests. First, the totl slope norml displcement fter wek lyer frcture ws similr (compre Figs. nd 7). This result lso confirms tht tpping on the snow column in ECTs does not result in incresed wek lyer collpse, s lredy oserved for compression tests y vn Herwijnen nd Jmieson (). Second, similr fluctutions were oserved in the totl slope norml displcement fter wek lyer collpse in oth tests. These likely relte to smll scle chnges in microstructurl properties of the wek lyer (e.g. crystl size or lyer thickness) nd hve lso een oserved in much longer PSTs (vn Herwijnen et l., ). Third, in oth tests, sl displcement due to wek lyer frcture strts t the free end of the column nd propgtes to the other end (compre Figs. ndc). Fourth, if crck propgtion rrests efore crossing the entire column, susequent tpping or swing results in further compction of the wek lyer (compre Figs. nd).fifth,oservedcrckpropgtion speeds were similr for oth tests (Fig. ), nd no consistent trends in c were oserved. This is in line with results presented y vn Herwijnen et l. (), who found no consistent trend in propgtion speeds in much longer PSTs. All in ll, our oservtions on crck propgtion in ECTs re consistent with results otined from PSTs. Tle Overview of PST results nd PTV mesurements. Test r c (m) L (m) (mm) c (m s ) PST_A.7,.. PST_A... 7 ± PST_B ± PST_B..... ± PST_C ± PST_C..... ± PST_D..... PST_E.,.. PST_E.7.. ± PST_F..... PST_G ± Criticl cut length (r c ), column length (L), PTV mesurement ccurcy (), the rnge of slope norml displcement in the re where the wek lyer frctured (u z )ndfrcture speed with mesurement uncertinty (c) re shown. The column slid down-slope fter frcture. Wek lyer frcture rrested efore reching the end of the column. From prcticl point of view, our mesurements show tht ECT nd PST results relte to frcture initition nd propgtion, oth of which re necessry for vlnche relese. Both these snow stility tests therefore provide vlule informtion with regrd to snow slope stility evlution, s lredy shown in vrious studies (e.g. Ross nd Jmieson, ; Simenhois nd Birkelnd, 9; Winkler nd Schweizer, 9). However, mny questions remin s to how results from smll scle stility tests relte to lrge scle processes involved in vlnche relese, such s the influence of the free edges on test results, the significnce of prtil crck propgtion nd the role of snow sl compction in ECTs.. Conclusions Using high-speed video recordings in comintion with prticle trcking velocimetry, we nlyzed the displcement of the snow sl in 9 ECTs nd PSTs t seven different sites. Despite the fct tht the ECT is widely used snowpck stility test, little ws known out wht hppens to the sl nd wek lyer during the test. Our results prtilly fill this gp, providing etter understnding of the mechnics of the ECT. Our results led to some importnt conclusions:. Tpping only ffects the snow immeditely elow the shovel since we did not oserve ny mrker displcement t the fr end of the column prior to wek lyer frcture. This suggests tht the ECT is indexing not only crck initition, ut lso crck propgtion.. Tpping minly compresses the sl. We did not oserve ny progressive dmge in the wek lyer prior to frcture, when the wek lyer collpses.. Propgtion speeds mesured in ECTs were consistent with mesurements in djcent PSTs nd previously pulished results. During frcture, wek lyers in ECTs compct on the order of severl mm, similr to mesurements otined from PSTs. Mesured propgtion speeds (on the order of to ms ) re lso similr to those from PSTs. The similrities etween our results nd those with PSTs give us confidence tht the frcture mechnics re similr regrdless of the triggering mechnism. However, importnt differences do exist etween the PST nd ECT. In PST, n unstle crck is creted y cutting the wek lyer with snow sw until crck propgtion occurs. The crck length is thus grdully incresed resulting in ending of the overlying sl. On the other hnd, with the ECT the frcture is initited y tpping on the top of

7 A. vn Herwijnen, K.W. Birkelnd / Cold Regions Science nd Technology 97 () 97 ECT PST vn Herwijnen et l. () vn Herwijnen nd Jmieson () Johnson et l. () c (m s ) c (m s ) density (kg m ) u y (cm) Fig.. () Propgtion speed with men sl density. () Propgtion speed with men totl slope norml displcement fter wek lyer frcture. Note the logrithmic scle. The rs indicte the mesurement uncertinty. the lock. No sign of progressive dmge in the wek lyer ws oserved, nd it ppers tht t the criticl loding step, i.e. the finl tp, the wek lyer frctures over n re directly under the shovel. Depending on snow conditions, this crck will either e lrge enough to propgte or frcture will rrest. Building on oservtionl studies which show tht ECT results correlte with stility (e.g. Simenhois nd Birkelnd, 9), this study shows tht ECT results relte to frcture initition nd propgtion, oth of which re required for vlnche relese. Though this increses our confidence in ECT results for ssessing vlnche dnger, we continue to cution people tht stility tests re only one prt of vlnche dnger ssessments. In the end, holistic pproch utilizing wether, snowpck, nd vlnche oservtions, in ddition to stility test results, is necessry for good decision-mking in vlnche terrin. Acknowledgments We thnk Ron Simenhois for reviewing this pper for us. Thnks lso go to the Glltin Ntionl Forest Avlnche Center for providing informtion nd logisticl support for this work, nd to Brd Crpenter, Wes Frnsworth, Eric Knoff, Alex Mrienthl, nd Royn Wooldridge for helping out in the field. References Bir, E., Simenhois, R., Birkelnd, K., Dozier, J.,. A field study on filure of storm snow sl vlnches. Cold Regions Science nd Technology 79,. Birkelnd, K.W., Chot, D.,. Chnges in stility test usge y SnowPilot users. Proc. Int. Snow Sci. W., Anchorge, AK, USA, pp.. Chot, D., Khrl, M., Birkelnd, K., Anker, C.,. SnowPilot: new school tool for collecting, grphing, nd dtsing snowpit nd vlnche occurrence dt with PDA. Proc. Int. Snow Sci. W., Jckson Hole, WY, USA, p. 7. Crocker, J., Grier, D., 99. Methods of digitl video microscopy for colloidl studies. Journl of Colloid nd Interfce Science 79, 9. Föhn, P., 97. The Rutschlock s prcticl tool for slope stility evlution. IAHS-AISH Pul., pp.. Guthier, D., Jmieson, B.,. Evluting prototype field test for wek lyer frcture nd filure propgtion. Proc. Int. Snow Sci. W., Telluride, CO, USA, pp. 7. Guthier, D., Jmieson, B.,. Evlution of prototype field test for frcture nd filure propgtion propensity in wek snowpcklyers. ColdRegions Sciencend Technology, Greene, E., Atkins, D., Birkelnd, K., Elder, K., Lndry, C., Lzr, B., McCmmon, I., Moore, M., Shrf, D., Sterenz, C., Tremper, B., Willims, K.,. Snow, Wether nd Avlnches: Oservtion Guidelines for Avlnche Progrms in the United Sttes. Americn Avlnche Assocition, Pgos Springs, CO, USA. Heierli, J.,. Anticrck Model for Sl Avlnche Relese. Ph.D. thesis Universität Krlsruhe (TH). Heierli, J., Gumsch, P., Ziser, M.,. Anticrck nucletion s triggering mechnism for sl vlnches. Science,. Jmieson, J., 999. The compression test fter yers. The Avlnche Review,. Johnson, B.C., Jmieson, J.B., Stewrt, R.R.,. Seismic mesurements of frcture speed in wek snowpck lyer. Cold Regions Science nd Technology,. Ross, C., Jmieson, B.,. Compring frcture propgtion tests nd relting test results to snowpck chrcteristics. Proc. Int. Snow Sci. W., Whistler, BC, Cnd, pp. 7. Ross, C., Jmieson, B.,. The Propgtion Sw Test: slope scle vlidtion nd lterntive test methods. Journl of Glciology, 7. Schweizer, J., Jmieson, B.,. Snowpck tests for ssessing snow-slope stility. Annls of Glciology, 7 9. Schweizer, J., Jmieson, J., Schneeeli, M.,. Snow vlnche formtion. Reviews of Geophysics,. Sigrist, C., Schweizer, J., 7. Criticl energy relese rtes of wek snowpck lyers determined in field experiments. 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Snowpck properties ssocited with frcture initition nd propgtion resulting in skier-triggered dry snow sl vlnches. Cold Regions Science nd Technology,. vn Herwijnen, A., Schweizer, J., Heierli, J.,. Mesurement of the deformtion field ssocited with frcture propgtion in wek snowpck lyers. Journl of Geophysicl Reserch Erth, F. Winkler, K., Schweizer, J., 9. Comprison of snow stility tests: extended column test, Rutschlock test nd compression test. Cold Regions Science nd Technology 9, 7.