| Citation |
Schwab, J.W. (2002) Catastrohic Rock Avalanche: Howson Range, Telkwa Pass, Forest Sciences, Prince Rupert Forest Region, BC Forest Service. Extension Note #46, March, 2002. |
| Abstract/Description or Keywords |
At 03:00 hours on Saturday Sept. 11, 1999, a large rock avalanche severed Pacific Northern Gas pipeline through the Telkwa Pass, 50 km west of Smithers and a like distance east of Terrace, B.C. (Figure 1). A series of ice-valley glaciers extend northward from FUBAR Glacier in the Howson Range into Limonite Creek valley. Considerable glacial ice loss has occurred over the past 50 to 150 years within the ice valleys, exposing oversteepened valley walls (Figure 2). The ridges between the glaciers extend up to 300 meters in height and are composed of highly fractured and near vertically jointed granodiorite bedrock. Rockcliffs also rise up directly above Limonite valley in the Telkwa Pass area. Rockfalls in the vicinity can often be heard tumbling onto glacier ice or down slopes. The forest within the September 1999 rock avalanche path was scheduled for timber harvesting. Similar sites elsewhere in the valley are also proposed for harvesting. A landslide could be triggered by ongoing deformation and displacement of the rock, seismic activity, snow melt or heavy rainfall. Also, the instability along the unsupported rock slopes adjacent to the ice valleys will likely increase with further melting of glacial ice. We do not know when another rock avalanche will occur or down which ice valley glacier it could travel. Forest development within potential rock avalanche tracks thus places workers at risk. Landslide hazard, in the context of probable occurrence and consequence as related to time of exposure, becomes an important variable in determining relative risk. Should workers be informed of the hazards and possible consequences? At what point is the risk deemed unacceptable? Without calculated probabilities for events, is acceptable or unacceptable risk a personnel decision or should the sites simply be precluded from forest harvesting? |
