Ninth International Conference on Permafrost ... - IARC Research

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Landsliding Following Forest Fire on Permafrost Slopes, Klondike Area,Yukon, CanadaJim Coates, Antoni G. LewkowiczDepartment of Geography, University of Ottawa, Ottawa, OntarioIntroductionIn the boreal forest, fire is often followed by widespreadactive layer detachment sliding (McRoberts & Morgenstern1974a, 1974b, Lewkowicz & Harris 2005a, 2005b). Forestfire, with a recurrence interval of 25–300 years, kills adulttrees, destroys much of the insulating mossy organic layer,and blackens the ground surface (Dyrness et al. 1986).Seasonal thaw depths (active layer thickness) generallyincrease in the years following forest fire (Yoshikawa et al.2003), although this can vary according to the slope aspectand state of vegetation (Swanson 1996, Lewkowicz & Harris2005b). When the heat reaches the permafrost, it may thawthe ice-rich transient layer, which lies just below the averagemaximum depth of thaw (Shur et al. 2005). Water released bythis process may raise soil porewater pressures sufficiently todestabilize slopes and cause active layer detachment sliding(McRoberts & Morgenstern 1974a).Active layer detachment failures occur when all or aportion of the active layer separates from the permafrostbeneath and moves as a semi-competent, unsaturated massdownhill over the lubricated slip surface of the thaw plane.Failures occur within the active layer or the transient layerand are triggered by high porewater pressures over frozenground (Lewkowicz & Harris 2005b). The depth of the initialfailure plane is limited by the position of the permafrost table(Harris & Lewkowicz 2000).Klondike Detachment FailuresNumerous forest fires occurred during the summerof 2004 in the Klondike Goldfields region of the YukonTerritory, an area of extensive discontinuous permafrost.Significant detachment failure landslide activity developedin subsequent weeks in Steele Creek, a small drainage basinlocated about 60 km south of Dawson City at approximately63°35′N and 138°59′W.Preliminary observations of the failures and near-surfacethermal regime were made through freeze-up of 2004 andcontinued in the summers of 2005 and 2006. Detachmentfailures were mapped, and individual sites were surveyed.Table 1. Failure characteristics (n = 37).Mean Max MinFailure Angle 23 32.0 12.0Length (m) 32 105.5 5.0Width (m) 7 23.0 1.6Depth (cm) 48 160.0 17.0Scar (m) 23 88.5 2.0Length/Width ratio 4 10.6 1.2Two-dimensional DC resistivity transects were used toexamine subsurface conditions in the area.Thirty-five detachment failures occurred in 2004 along a3.7 km section of the main Steele Creek Valley and on slopeswithin its tributaries. Five new failures developed by mid-August 2005, and several failures from 2004 reactivated. Nomore failures developed in the summer of 2006.FormThe failures in the Steele Creek Valley varied in lengthfrom 5–105 m, in width from 7–23 m, and in depth from17–160 cm (Table 1). Only elongate detachment failureswere observed (length-to-width ratio >1; e.g., Lewkowicz& Harris 2005a). The majority of these took place in coarsegrainedsoils with high pore-water pressures at the time offailure. Headscarps were coincident or proximal to convexbreaks-of-slope. At the headscarps of nearly all the failures,tension cracks were observed with roots stretched acrossthem. These tension cracks were more common on convexbreaks-of-slope which are concentrations of stress. Nearmany of the failures, the organic mat was thinner near theheadscarp but thicker downslope as a result of burning orpre-fire vegetation conditions.Failure surfaces were generally higher than the inferredfrost plane and dipped towards the centre of the failure scar.Displaced soil and organic material in most debris pileswas highly disturbed. Trees were left standing in debrispiles, indicating that the organic layer moved withoutoverturning until it lost momentum or reached material thatwould not detach. It then piled up with liquefied mineral soilsandwiched between folded layers of the original surfaceorganics. At some of the larger failures, the moving massacquired sufficient weight and momentum to scour down tothe permafrost table. Failure angles were all moderate (Fig.1), with none below 10°.MechanismForest fire contributed to detachment failure activity onpermafrost slopes by destroying the surface organic mat,causing burned surface temperatures to rise, thawing activelayers by up to 20 cm (+30%) deeper than adjacent unburnedslopes and weakening the surface root structures. Deeperthaw melted transient layer ground ice, raising soil porewaterpressures.These landslides appeared to have behaved as flows withinunfrozen soils. The permafrost affected the failures byproviding an aquiclude, which raised pore-water pressures,and by supplying water released from the transient layer dueto thermal disequilibrium caused by the forest fire.49
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 tFrequency0.500.450.400.350.300.250.200.150.100.050.000-4.99 5-9.99 10-14.99 15-19.99 20-24.99 25-29.99 30-34.99Angle (º)Figure 1. Histogram of landslide failure angles (n = 37).Failures in Steele Creek occurred where the subsurfacemineral soil consisted of completely saturated coarse-grainedmaterial with low cohesion, the organic mat was weak on thebreak of slope but strong below, and there was significantdownslope stress. The saturated soil mass liquefied andbegan to flow below the organic mat, which was elevatedabove, and detached from, the mineral soil. Tree rootsthat were still anchored in the mineral soil supported thesuspended organic mat.SignificanceThe failures are similar in planar dimensions to thosedescribed previously in other permafrost regions (e.g.,Lewkowicz & Harris 2005a, 2005b, McRoberts &Morgenstern 1974b). However, they are towards the upperend of the range of slope angles observed, and no lowangledforms were present. Moreover, in contrast to mostother shallow failures over permafrost, the frost table did notact as a failure plane, and soil materials were cohesionlessand coarse-grained. The critical role of permafrost in SteeleCreek was to release water from upslope as the transientlayer thawed, and by acting as an aquiclude, to generate highpore-water pressures near the base of the slope.Fossil failures in the Steele Creek Valley and otherYukon locations (e.g., Lipovsky et al. 2006) indicate thatdetachment failure is an episodic process controlled byfire. In unglaciated areas such as Steele Creek, given a firerecurrence interval in the order of 100 years, this process haslikely occurred thousands of times during the Pleistocene andmay be responsible for elements of the form of the region’sslopes. These include permafrost slopes having a generallygentler gradient and more defined mid-slope break of slopethan nonpermafrost slopes.The potential for climate change-induced thaw ofpermafrost as well as larger and hotter forest fires raise thepossibility of greater active layer detachment failure activityin the future (McCoy & Burn 2005, Lipovsky et al. 2006).AcknowledgmentsFinancial Support came from NSERC through its NorthernInternship Program and Discovery Grant (to A. Lewkowicz),the Yukon Geological Survey, Northern Scientific TrainingProgram (Indian and Northern Affairs Canada), University ofOttawa (to A. Lewkowicz). We are also grateful for supportfrom Klondike Star, Indian River Farm, EBA EngineeringConsultants Ltd., Cam Arkinstal, Yukon Client Servicesand Inspections, Gimlex Mining, Mike Schultz, GeoffHodgeson, Martina Knopp, Yukon Geological Survey, DrBernd Etzelmüller, Phil Bonaventure, and Emily Schultz.ReferencesDyrness, C.T., Viereck, L.A. & Van Cleve, K. 1986. Firein taiga communities of interior Alaska. In: K. VanCleve, F.S. Chapin III, P.W. Flanagan, L.A. Viereck& C.T. Dyrness (eds.), Ecological Series 57: ForestEcosystems in the Alaskan Taiga. New York: Springer-Verlag, 74-86.Harris, C. & Lewkowicz, A.G. 2000. An analysis ofthe stability of thawing slopes, Ellesmere Island,Nunavut, Canada. Canadian Geotechnical Journal.37: 449-462.Lewkowicz, A.G. 1988. Slope processes. In: M.J. Clarck(ed.), Advances in Periglacial Geomorphology.Chichester: Wiley, 325-368.Lewkowicz, A.G. & Harris, C. 2005a. Morphology andgeotechnique of active-layer detachment failures indiscontinuous and continuous permafrost, northernCanada. Geomorphology 69: 275-297.Lewkowicz, A.G. & Harris C. 2005b. Frequency and magnitudeof active-layer detachment failures in discontinuousand continuous permafrost, northern Canada.Permafrost and Periglacial Processes 16: 115-130.Lipovsky, P.S., Coates J, Lewkowicz, A.G. & Trochim, E.2006. Active layer detachments following the summer2004 forest fires near Dawson City, Yukon. In: D.S.Edmond, G.D. Bradshaw, L.L. Lewis & L.H. Weston(eds.), Yukon Exploration and Geology 2005. YukonGeological Survey, 175-194.McCoy, V.M. & Burn, C.R. 2005. Potential alteration byclimate change of the forest-fire regime in the borealforest of central Yukon Territory. Arctic 58: 276-285.McRoberts, E.C. & Morgenstern, N.R. 1974a. The stabilityof thawing slopes. Canadian Geotechnical Journal11: 447-469.McRoberts, E.C. & Morgenstern, N.R. 1974b. Stabilityof slopes in frozen soil, Mackenzie Valley, N.W.T.Canadian Geotechnical Journal 11: 554-573.Shur, Y., Hinkel, K. & Nelson, F. 2005. The Transient Layer:Implications for Geocryology and Climate-Change Science.Permafrost and Periglacial Processes 16: 5-17.Swanson, D.K. 1996. Susceptibility of permafrost soils todeep thaw after forest fires in interior Alaska, USA,and some ecologic implications. Arctic and AlpineResearch 28: 217-227.Yoshikawa, K., Bolton, W.R., Romanovsky, V.E., Fukuda,M. & Hinzman, L.D. 2003. Impacts of wildfire on thepermafrost in the boreal forests of Interior Alaska.Journal of Geophysical Research 107: 8148.50
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ContentsPreface ...................
Page 8 and 9:
Mapping and Modeling the Distributi
Page 10 and 11:
Satellite Observations of Frozen Gr
Page 13 and 14:
xiiIce Wedge Thermal Regime in Nort
Page 15 and 16:
xivPermafrost Response to Dynamics
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Page 19 and 20: NICOP SponsorsUniversitiesUniversit
Page 22 and 23: Deep Permafrost Studies at the Lupi
Page 24 and 25: Effect of Fire on Pond Dynamics in
Page 26 and 27: Cryological Status of Russian Soils
Page 28 and 29: Acoustical Surveys of Methane Plume
Page 30 and 31: Permafrost Delineation Near Fairban
Page 32 and 33: Preparatory Work for a Permanent Ge
Page 34 and 35: A Provisional Soil Map of the Trans
Page 36 and 37: Martian Permafrost Depths from Orbi
Page 38 and 39: Time Series Analyses of Active Micr
Page 40 and 41: Impact of Permafrost Degradation on
Page 42 and 43: DC Resistivity Soundings Across a P
Page 44 and 45: Modeling Thermal and Moisture Regim
Page 46 and 47: A Provisional Permafrost Map of the
Page 48 and 49: Alpine Permafrost Distribution at M
Page 50 and 51: Cryogenic Formations of the Caucasu
Page 52 and 53: Modeling Potential Climatic Change
Page 54 and 55: A Hypothesis: A Condition of Growth
Page 56 and 57: Modeled Continual Surface Water Sto
Page 58 and 59: Freeze/Thaw Properties of Tundra So
Page 60 and 61: Discontinuous Permafrost Distributi
Page 62 and 63: Thermal Regime Within an Arctic Was
Page 64 and 65: Seasonal and Interannual Variabilit
Page 66 and 67: Twelve-Year Thaw Progression Data f
Page 68 and 69: Continued Permafrost Warming in Nor
Page 72 and 73: A Permafrost Model Incorporating Dy
Page 74 and 75: Seasonal Sources of Soil Respiratio
Page 76 and 77: Greenland Permafrost Temperature Si
Page 78 and 79: The Importance of Snow Cover Evolut
Page 80 and 81: The Account of Long-Term Air Temper
Page 82 and 83: The Combined Isotopic Analysis of L
Page 84 and 85: Adaptating and Managing Nunavik’s
Page 86 and 87: Human Experience of Cryospheric Cha
Page 88 and 89: HiRISE Observations of Fractured Mo
Page 90 and 91: A Soil Freeze-Thaw Model Through th
Page 92 and 93: Mapping and Modeling the Distributi
Page 94 and 95: First Results of Ground Surface Tem
Page 96 and 97: Historical Changes in the Seasonall
Page 98 and 99: Rock Glaciers in the Kåfjord Area,
Page 100 and 101: Snowpack Evolution on Permafrost, N
Page 102 and 103: Climate Change in Permafrost Region
Page 104 and 105: Maximizing Construction Season in a
Page 106 and 107: Pleistocene Sand-Wedge, Composite-W
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