ABSTRACT: Regional landslide mapping: Landslide spatial database and case studies in the Mackenzie Valley, Northwest Territories, Canada.

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Abstract

In the last three decades, hundreds of landslides, often affecting areas several hectares in size, have been identified and mapped in the Mackenzie Valley. A first landslide inventory was completed in 2000 in the western Mackenzie District and Yukon (Aylsworth et al., 2000a). However no detailed inventory has been compiled for the new proposed gas pipeline route between Inuvik (68.35°N, 133,72°W) and Norman Wells (65.28°N, 126.83°W) and more importantly no regional studies have been completed depicting the regional landslide hazards along this corridor. To fill the data gap, Natural Resources Canada initiated a regional landslide mapping project along the proposed gas pipeline between Inuvik and Norman Wells (Fig.1) to provide baseline knowledge on types, regional distribution, and control of landslides in the Mackenzie Valley through a compilation of existing and new information. The preliminary Mackenzie Valley Landslide Spatial Database will be presented as well as a detailed description of three case studies. The study area encompasses a corridor extending 20 km to either side of the proposed pipeline route between Inuvik and Norman Wells for a length of 540 km and an area of 24,000 km2 (Fig.1). The study area also includes the Rampart zone (southwest of Fort Good Hope) and the Thunder River region, both known for widespread landsliding. The study area is characterized by cold winters with low precipitation and relatively warm summers with moderate precipitation (Environment Canada, 2004). The vegetation cover is dominated by coniferous forest (medium to low density) and wetland/shrub land near Inuvik (Cihlar and Beaubien, 1998). The study area is covered by unconsolidated sediments (99%), which are dominated by morainal deposits (60%), and encompasses three zones of permafrost: continuous, extensive discontinuous, and intermediate discontinuous (Fig.1). The permafrost thickness ranges from 0 - 60 m in Norman Wells to 100 - 200 m in Inuvik and the active layer thickness ranges from > 1.5 m in Norman Wells to 50 cm in Inuvik (Wolfe, 1998). Using classical photo-interpretation techniques, a preliminary inventory of over 1800 landslides and other natural terrain hazard features (e.g. karstic sink holes, rock glaciers) has been created for the study area. Currently, about 40% of the study area has been mapped using 665 coloured air photos (scale of 1:30,000) acquired in 2004. For all of the 1800 landslide entries forming the preliminary Mackenzie Valley Landslide Spatial Database, the following attributes were recorded: unique identifier (UID), landslide type and size, location, morphological parameters, surface tone and texture, vegetation re-growth in landslide scar, relative age, activity, material type, flight line, air photo number, and topographic map sheet number. A collection of several hundreds of photographs of landslides, taken while carrying out field surveys, completes the database. From the database attributes, desktop landslide mapping and statistical analyses were performed to characterise the landslide distribution. The results indicate an average density of one landslide per 5 km2 and show that the dominant landslide types are retrogressive thaw flows (28%) and active layer detachments (26%). Rock falls (11%), debris flows (10%), earth slides (9%), and retrogressive thaw slides (5%) are second in order of importance. About 47% of all landslides took place in morainal deposits, 19% in lacustrine sediments, 13% in glaciofluvial sediments, and 12% in bedrock. The relative age of landslides was estimated based on tone, texture, and vegetation re-growth parameters, where 38% were classified old (>50 years old), 40% intermediate in age (10-50 years old), and 22% recent (<10 years old). Detailed investigations at eight landslide sites throughout the valley were carried out during the 2005 field campaign including three areas of great interest: 1) east of Travaillant Lake, 2) Thunder River, and 3) northwest of Norman Wells. These three areas were selected as they offer a wide variety of terrain in terms of permafrost condition, geology, relief and slope, vegetation, and characteristic landslide types. East of Travaillant Lake: The area east of Travaillant Lake covers an area of 8.6 km2 in size and is located 6 km east of Travaillant Lake and 120 km southeast of Inuvik. The area is characterised by continuous permafrost with low to moderate ice content (Heginbottom and Radburn, 1992) and silty clay hummocky and ridged morainal deposits (Aylsworth et al., 2000b). Low relief, low slopes, and wetland/shrub land vegetation cover defined the area. In 1998, an important forest fire burned the area of 200 km2 that enclosed the area east of Travaillant Lake (Canadian Forest Service, 2002). In the area, 26 landslides or clusters of landslides were mapped where dominant landslide types are retrogressive thaw flows - RTF (16). Earth flows (5), retrogressive thaw slides - RTS (3), and earth slides (2) are second in order of importance. Fig.2 presents a detailed description of typical RTF and RTS observed in the area east of Travaillant Lake. Thunder River Area: Thunder River area covers an area of 24.9 km2 in size and is located 153 km southeast of Inuvik. The area is mainly characterised by extensive discontinuous permafrost with nil to moderate ice content (Heginbottom and Radburn, 1992). Glaciofluvial plain and complex, hummocky and ridged morainal, and colluvial deposits were mapped in the area (Aylsworth et al., 2000b). Low relief, moderate slopes, and a coniferous forest cover defined the area. In 1986, an important forest fire burned the area of 557 km2 that enclosed the Thunder River area (Canadian Forest Service, 2002). In the area, 111 landslides or clusters of landslides were mapped where dominant landslide types are RTF (81). Active layer detachment (27), debris flows (2), and debris slide (2) are second in order of importance. Northwest of Norman Wells: The area northwest of Norman Wells covers an area of 11.5 km2 in size and is located 40 km northwest of Norman Wells. The area is mainly characterised by extensive discontinuous permafrost with nil to moderate ice content (Heginbottom and Radburn, 1992). An anticline formed by Devonian shale and limestone units cut by thrust faults (Aitken and Cook, 1976) was mapped in the area where exposed bedrock outcrops were observed. Quaternary deposits include rolling morainal and lacustrine plain units (Aylsworth et al., 2000b). Moderate relief, high slopes, and a coniferous forest cover defined the area. In the area, 23 landslides or clusters of landslides were mapped where dominant landslide types are rock falls (11). Rock slides (5), karstic sink holes (4), and debris flows (3) are second in order of importance. Future Steps: Following the first year of work, a detailed report presenting the three case studies will be published as well as the preliminary Mackenzie Valley Landslide Spatial Database which will contains polygon limits, UID points, and an attribute table (including links to field photographs) for each landslide or cluster of landslides. Photo interpretation will continue north of Inuvik, in the Ramparts zone, and in areas with major river crossings along the Mackenzie River. Air photo temporal sequence analyses will be expanded to areas of widespread landsliding such as east of Travaillant Lake and Thunder River. Finally, we will investigate potential influence of environmental factors (e.g. forest fire, climate variability, future global warming) on frequency and magnitude of landslides, monitor zones of potentially unstable slopes using remote sensing technologies, and apply spatial data analysis models to create landslide susceptibility maps.

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