Ninth International Conference on Permafrost ... - IARC Research

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The Role of Permafrost in the 2002 Ten Mile Creek Debris Torrent, Yukon, CanadaPanya LipovskyYukon Geological Survey, Whitehorse, CanadaCrystal HuscroftThompson Rivers University, Kamloops, CanadaAntoni LewkowiczUniversity of Ottawa, Ottawa, CanadaBernd EtzelmüllerUniversity of Oslo, Oslo, NorwayIntroductionIn June 2002, a catastrophic debris torrent initiated from amoderate north-facing slope in the headwaters of Ten MileCreek, in central Yukon, Canada. Field evidence indicatesthat permafrost was a major contributing factor that causedan initial landslide which then triggered the debris torrent.The mechanism of failure in the initial landslide appearsto be unique in comparison to other permafrost-relatedlandslides (i.e., retrogressive thaw failures and active layerdetachments) documented in the region (Lipovsky et al.2006, Lipovsky & Huscroft 2007, Lyle 2006).SettingThe landslide source zone is located at 1084 m elevation,15 km southeast of the town of Carmacks in central Yukon,Canada (61°58′45.4ʺN, 136°08′20.7ʺW). Based on fieldobservations and aerial photograph analysis of the terrainimmediately surrounding the source zone, the pre-failureslope is estimated to have been moderately steep (up to 27°)with a typical boreal forest cover consisting of low shrubs,mosses and mature spruce trees.The landslide left a bowl-shaped scar up to 160 m wideand 100 m long with a steep headwall 12–31 m high (Fig.1). Springs flow from the base of a secondary slump scarin the floor of this bowl. Following the initial failure, anensuing debris torrent descended 500 m in elevation andtraveled 4.7 km down the narrow v-shaped valley of TenMile Creek. The main debris lobe came to rest after crossingthe South Klondike Highway, clogging its culvert and fillingthe adjacent ditch with debris. Superelevation measurementstaken on a channel bend in the runout zone indicate that thetorrent traveled at a maximum velocity of 11 m/s (40 km/hr)with a peak discharge of 1300 m 3 /s.Along much of the debris torrent path, a swath of treesaveraging 35 m wide was cleared through mature forestadjacent to the former stream channel. Silty loam diamicton(containing 35–43% coarse fragments) was deposited upto 1.3 m thick along most of the debris torrent path, exceptwithin a 600 m long canyon segment confined by steeprock walls. Ongoing remobilization of these deposits bysubsequent stream flow has caused sedimentation of localsalmon habitat at the mouth of the creek where it flows intoNordenskiold River.Surficial geological materials exposed in the landslideheadwall consist of a stony till blanket up to 12.5 m thickoverlying at least 22 m of glaciofluvial sand and gravelexhibiting prominent bedding structures. At numerouslocations up to 3.7 km downstream from the source zone,discrete blocks of sandy material deposited by the debristorrent and originating from this lower glaciofluvial unit showintact primary bedding structures. In order to preserve thesefeatures over such a long transport distance, the sedimentmust have been frozen both before and during transport.This implies that in the landslide source zone permafrostmust have extended into the glaciofluvial unit found at least12.5 m below the ground surface.Local Permafrost ConditionsTen Mile Creek is located within Yukon’s extensivediscontinuous permafrost zone. Permafrost is commonlyfound on north-facing slopes in this region, particularlyFigure 1. Aerial view of landslide source zone showing debristorrent channel exiting at lower left corner. Maximum height of theheadwall is 31 m and the bowl is approximately 160 m wide.189
Ni n t h In t e r n at i o n a l Co n f e r e n c e o n Pe r m a f r o s tbeneath thick organic mats associated with boreal forestcover.Surface probing behind the landslide headwall in latesummer 2004 indicated an active layer thickness of 42–86cm where the organic mat was at least 40 cm thick. Wherethe organic mat was thinner, the permafrost table was notencountered within 1.05 m of the ground surface, howeverit was assumed to be present at greater depth. Thermistormeasurements and lateral probing into the lower headwalldid not detect permafrost in the glaciofluvial unit at this time(ground temperatures 1 m into the face were ~1°C).DC resistivity surveys conducted in 2006 suggest thatpermafrost is up to 5 m thick on the gentle north-facing slopebehind the landslide headwall, and up to 15 m thick on thesteeper north-facing slope adjacent to the landslide sourcezone (Fig. 1). The resistivity surveys also confirmed theabsence of permafrost in the landslide headwall, implyingthat rapid lateral thaw has occurred since the failure occurredin 2002.Landslide Failure MechanismUnfrozen glaciofluvial sand and gravel exposed in thelower unit of the landslide headwall are highly permeable andporous, with a capacity to store and transmit a large amountof groundwater. This is confirmed by the presence of springsflowing from the floor of the landslide source zone scar.Based on aerial photograph interpretation, these sedimentsare part of a larger geological unit consisting of beddedglaciofluvial deltaic materials that extend laterally furtherupslope and capture groundwater from the upper drainagebasin. As a result of this geomorphic configuration, the siteof the landslide source area represents the location of greatestgroundwater convergence within that basin. We hypothesize,therefore, that the base of the impermeable permafrostlayer within these sediments confined groundwater flowand allowed high pore pressures to accumulate below thepermafrost during the spring snowmelt period in June 2002.During the same time period, no abnormal precipitationwas recorded and no earthquakes occurred in the vicinity.We suggest that pore pressures increased until a thresholdwas reached that allowed rupture or “blow-off” (Cavers2003) of the unfrozen materials and detachment of the entirepermafrost layer above.We infer that subsequent rapid drainage of storedgroundwater supplied a large volume of water whichfacilitated the catastrophic debris torrent. Similar failuremechanisms are common in non-permafrost areas throughoutwestern Canada where groundwater pressures are insteadconfined by surficial material stratigraphy rather than byfrozen ground (Cavers 2003). Alternatively, the initialmovement could have partially blocked a small streamchannel to the northeast allowing water to accumulate in apond. However, we were unable to locate any evidence ofsuch ponding in the field.Local and regional climatic data show that at least fourdecades of climate warming have occurred in central Yukonsince 1930. Any corresponding permafrost warming and/or thinning may have weakened the permafrost layer andlowered the pore pressure threshold required to initiate afailure and cause detachment. Dataloggers were installedin 2005 to monitor the long-term air and ground surfacetemperatures above the landslide source zone.ImplicationsThe results of this study have important implicationsfor future development and land use planning in the area,as the geomorphic setting of the landslide source zone iswidespread throughout much of central Yukon. This casestudy illustrates that significant hazards are associated withthis type of landslide. It also highlights the need to performdetailed evaluations of basin characteristics, permafrostconditions, and surficial material stratigraphy severalkilometers upslope of any area targeted for human land use.In particular, stream crossing designs and development onfans in permafrost regions should carefully consider the riskof debris torrents triggered by groundwater blow-off failuresbeneath frozen ground.AcknowledgmentsFunding was provided by NSERC and the Universityof Ottawa to A. Lewkowicz; by the University of Oslo toB. Etzelmüller; and by the Yukon Geological Survey to P.Lipovsky and C. Huscroft.ReferencesCavers, D.S. 2003. Groundwater blow-off and piping debrisflow failures. Proceedings, 3 rd Canadian <strong>Conference</strong>on Geotechnique and Natural Hazards, Edmonton,Alberta, June 9–10, 2003: 151-158.Lipovsky, P. & Huscroft, C. 2007. A reconnaissanceinventory of permafrost-related landslides in the PellyRiver watershed, central Yukon. In: D.S. Emond, L.L.Lewis & L.H. Weston (eds.), Yukon Exploration andGeology 2006. Yukon Geological Survey, 181-195.Lipovsky, P.S., Coates, J., Lewkowicz, A.G. & Trochim,E. 2006. Active-layer detachments following thesummer 2004 forest fires near Dawson City, Yukon.In: D.S. Emond, G.D. Bradshaw, L.L. Lewis & L.H.Weston (eds.), Yukon Exploration and Geology 2005.Yukon Geological Survey, 175-194.Lyle, R.R. 2006. Landslide susceptibility mapping indiscontinuous permafrost, Little Salmon Lake, CentralYukon. M.Sc.E. Thesis. Queen’s University, 351 pp.190
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ContentsPreface ...................
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Mapping and Modeling the Distributi
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Satellite Observations of Frozen Gr
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xiiIce Wedge Thermal Regime in Nort
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xivPermafrost Response to Dynamics
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NICOP SponsorsUniversitiesUniversit
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Deep Permafrost Studies at the Lupi
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Effect of Fire on Pond Dynamics in
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Cryological Status of Russian Soils
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Acoustical Surveys of Methane Plume
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Permafrost Delineation Near Fairban
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Preparatory Work for a Permanent Ge
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A Provisional Soil Map of the Trans
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Martian Permafrost Depths from Orbi
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Time Series Analyses of Active Micr
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Impact of Permafrost Degradation on
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DC Resistivity Soundings Across a P
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Modeling Thermal and Moisture Regim
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A Provisional Permafrost Map of the
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Alpine Permafrost Distribution at M
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Cryogenic Formations of the Caucasu
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Modeling Potential Climatic Change
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A Hypothesis: A Condition of Growth
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Modeled Continual Surface Water Sto
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Freeze/Thaw Properties of Tundra So
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Discontinuous Permafrost Distributi
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Thermal Regime Within an Arctic Was
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Seasonal and Interannual Variabilit
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Twelve-Year Thaw Progression Data f
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Continued Permafrost Warming in Nor
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Landsliding Following Forest Fire o
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A Permafrost Model Incorporating Dy
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Seasonal Sources of Soil Respiratio
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Greenland Permafrost Temperature Si
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The Importance of Snow Cover Evolut
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The Account of Long-Term Air Temper
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The Combined Isotopic Analysis of L
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Adaptating and Managing Nunavik’s
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Human Experience of Cryospheric Cha
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HiRISE Observations of Fractured Mo
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A Soil Freeze-Thaw Model Through th
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Mapping and Modeling the Distributi
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First Results of Ground Surface Tem
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Historical Changes in the Seasonall
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Rock Glaciers in the Kåfjord Area,
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Snowpack Evolution on Permafrost, N
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Climate Change in Permafrost Region
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Maximizing Construction Season in a
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