| Abstract/Description or Keywords |
Large magnitude landslides occur on an infrequent basis within the interior portion of the Prince Rupert Forest Region. Where these large landslides occur they have considerable impact to forestland. Four recent large landslides were investigated: 1. Windfall main landslide (Mezidian Lake) was caused by the diversion of creek 300 meters down a logging road (7,000 m3 / 20 hr period). Water flowed into a bedrock fault in sedimentary rock. The failure initiated as a bedrock slide of 100,000 m3 that triggered a debris avalanche, which travelled 900 m. to the valley flat—It carried boulders the size of pickup trucks. Total volume transported is in the order of 160,000 m3. Land area covers 14 hectares. 2. Rock avalanche Howson Range (Telkwa pass) originated as a rock topple failure (1.0X106 m3) from a rock ridge. The rock fell about 150 meters onto glacial ice, expanded to cover the ice valley glacier, a width of 300-400 meters, continued down the ice valley and avalanched into the Limonite Creek valley. The rock avalanche travelled a total distance of 2.6 km dropping 1,300 m in elevation and severed the PNG pipeline. The avalanche path through mature forest covered and area 1,200 meters long by 400 meters wide. This area was scheduled for harvest through the SBFEP. Historical rock avalanches have occurred in the Limonite Creek valley. The level of acceptable risk must be considered for forestry operations in potential rock avalanche zones. 3. A retrogressive slide-flow occurred on relatively gentle, completely forested, seemingly benign terrain proposed for timber harvesting (Goat Mountain, Harold Price watershed east of Smithers). Failure was within a deep compact basal till. The landform was dissected by a few minor ephemeral stream channels. There was no evidence of previous large landslides. The landslide covers an area of 6.1 ha. Volume is in the order of 250,000 m3. The headscarp of the failure bowl is 24-28 m deep. Sliding occurred along an assumed failure plane of 24%. Morphological features resemble failures in sensitive clays. The till is similar to glacial tills found through out the Bulkley Valley (LL 33.5, PL 12.1, PI 21.4, sand 41.8%, silt 31.0%, clay 27.2%, and bulk density 1.85). The gentle slope and the lack of discernible field indicators suggest that a failure at this site may not have been anticipated through a routine terrain stability field assessment. 4. A large underwater landslide (3x106m3) occurred off the steep slope of a fan-delta on Troitsa Lake. The visible portion of the landslide measures 400 m by 60 m. A displacement wave 1.5 m high hit the opposite side of the lake one kilometre away. A backwash wave 2 m in height crashed back over the head scarp. The north end of the lake (10 km from the landslide), experienced an initial sharp swell about 60 cm in height; bays and shallows where then sucked empty. A large return wave 2 m high crashed through a shallow bay, tearing sunken logs and debris from the bay floor and hurling the debris on to the beach and into the forest surrounding the bay. Boats and floating wharves were ripped from their moorings. The instability of fans and fan-deltas in freshwater fjord-type lakes is mentioned briefly in published literature. However, little documentation exists of known sites and landslide-tsunami events in British Columbia. |
