Contrasting Behaviour of Two Recent, Large Landslides in Discontinuous Permafrost Little Salmon Lake, Yukon, Canada

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1 Contrasting Behaviour of Two Recent, Large Landslides in Discontinuous Permafrost Little Salmon Lake, Yukon, Canada Ryan R. Lyle BGC Engineering Inc. Kamloops, British Columbia D. Jean Hutchinson Queen s University Kingston, Ontario Panya S. Lipovsky Yukon Geological Survey Whitehorse, Yukon Magundy River Landslide YT Landslide Presented by P. Lipovsky at 1st North American Landslide Conference, Vail, Colorado, June 2007

2 Background Increased development is being considered in Canada s permafrost regions. There are major concerns about the potential impacts of climate change on warm permafrost in the discontinuous zones. There has been little slope hazard research completed in the Yukon. The Little Salmon Lake area was an ideal location for landslide susceptibility mapping research. A L A S K A Y U K O N Northwest Territories Continuous Permafrost Sporadic Discontinuous Permafrost Widespread Discontinuous Permafrost British Columbia (after Heginbottom et al., 1995)

3 Study Area Setting Little Salmon Lake YT Slide Drury Ck Magundy River Slide

4 Regional Climate Record

5 Magundy River N Setting Surficial geology Bouldery diamicton of colluvial and glacial origin Up to 25% clay and 50% silt Multiple organic horizons Source Zone 350 m Permafrost Up to 50% massive ice observed in active scarps. Massive ice more common in colluvium than till. Deposition Zones Aspect = North Source Area Slope = Travel Angle (H/L) = 8.7 Elevation Drop (H) = 335 m Runout Length (L) = 2200 m Source Area Width = 350 m Deposit Width = 1300 m Area Impacted = 84 ha Dates Active = 1996 to present

6 Magundy River Mechanism of failure Steep scarp Small disturbance initiated the retrogressive thaw flow by exposing ground ice in a steep scarp. Exposed ice-rich material thaws, releasing large amounts of moisture. Thawed material is removed in low-angle mud and debris flows (highly viscous, low-plasticity) Gently inclined mud and debris flow

7 Magundy River N Between 1998 and 2004, retreat rates averaged m/yr along the SE and SW scarps, and m/yr along the N scarp 350 m SW scarp 1996 Initiation Point SE scarp N scarp m (Modified from Lyle, 2006) 2004

8 Magundy River N In 2006, a 125 m segment of the N scarp retreated between 8-12 m/yr m 2006 Headscarp Retreat 1996 Initiation Point New Mudflow m (Modified from Lyle, 2006) 2004

9 Magundy River Landslide Factors contributing to initiation: Antecedent meteorological conditions Local hydrogeology Post-Little Ice Age climate warming Permafrost strength / ability to confine or cap groundwater pressure Previous landslide activity

10 YT Landslide N Top Scarp Main Scarp Mid Scarp Low Scarp Lake Scarp 250 m 350 m Morphology Aspect = North Adjacent Undisturbed Slope = 16 Travel Angle (H/L) = 21.8 Elevation Drop (H) = 100 m Length (L) = 250 m Width = 350 m Area Impacted = 5.8 ha Dates Active = pre-1989 to present

11 YT Landslide Surficial geology and permafrost >10 m lodgement till with permafrost likely present below 1.2 m. >15 m ice-contact glaciofluvial complex sediments up to 50% massive ice found in pre-glacial colluvial diamicton in bottom 15 m

12 YT Landslide Mechanisms of failure Top Scarp (pre-1989) Main Scarp ( ) Mid Scarp ( ) Top Scarp Main Scarp Mid Scarp R. Lyle photo 2005/08/11 A. Von Finster photo High groundwater pressure produced intrusion ice at base of slope since deglaciation. Slow creep deformation of warm ice-rich material (McRoberts, 1978). Ice-rich material at toe bulged into lake. Thermal erosion by lake caused undercutting, toppling, and unloading of toe of slope to promote translational movement since 2004.

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14 YT Landslide L E G E N D Surveyed Monuments 24-inch rebar pins Ground Movement (June 23 - Sept. 27, 2006) Horizontal Component 9 cm 92 cm Vertical Component -10 cm -20 cm N Main Scarp Mid Scarp Low Scarp Top Scarp -33 cm +39 cm Lake Scarp (Sept 2006) 250 m 350 m

15 YT Landslide Factors contributing to ongoing activity: Hydrogeological setting drainage behind till plateau contributes to high groundwater recharge Massive ice at toe of slope Creep deformation of warm, ice-rich permafrost (McRoberts, 1978) Post-Little Ice Age climate warming accelerates creep rates Antecedent meteorological conditions Thermal erosion by lake unloads toe of slope, causing translational movement

16 Summary and Implications Similarities: Massive ice concentrated in colluvial materials North-facing slope, gentle to moderate slope angle <20 Role of groundwater flow and antecedent weather Influence of post-little Ice Age climate warming on permafrost degradation Rapid landslide growth rates in the last decade Lack of anthropogenic causes Unique set of causal factors and style of failure Differences: Magundy River: midslope position; piping initiated retrogressive thaw slump with material debris removed by highly viscous flows. YT Slide: lower slope position; creep and high groundwater pressure initiated rotational failure; debris removed by translation resulting from thermal erosion and unloading of toe. Implications: Highlights potential for naturally occuring large slope movements on gentle ice-rich slopes. Highlights potential for increasing frequency of similar failures with climate warming. Reminder that any disturbance to ground thermal regime can cause prolonged and rapid slope instability (plan mitigation carefully). Need for detailed permafrost mapping, landslide susceptibility mapping, and subsurface geotechnical characterizations prior to any development.

17 Acknowledgements Technical Contributions J. Bond (Yukon Geological Survey) W.A. Gorman (Queen s University) A. Lewkowicz (University of Ottawa) L. Dyke (Geological Survey of Canada) CCORE EBA Engineering Consultants Financial Support NSERC GEOIDE Government of Canada (Northern Scientific Training Program Knowledge and Innovation Fund) European Space Agency Yukon Geological Survey