Ninth International Conference on Permafrost ... - IARC Research

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