The characterization and dating of landslides in the Tsitika River and Schmidt Creek watersheds, northern Vancouver Island, British Columbia
Date
1997
Authors
Guthrie, Richard H.
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Abstract
Forty three natural landslide events at 31 landslide sites were examined in the Tsitika River and Schmidt Creek watersheds on Vancouver Island. The watersheds were chosen as a test case, and it is expected that the results will be applicable to similar biogeoclimatic regions on both Vancouver Island and coastal mainland British Columbia. The 365 km2 study area was divided into eight subdrainages, and landslides were analyzed and compared to one another accordingly.
The landslides examined included debris slides, rockfalls, and rockfall/avalanches. They ranged in size from 7,000 m2 to 600,000 m2, and in volume from 3,200 m3 to 500,000 m3. Approximately 71 % of landslide sites deposited sediment directly into a stream channel. The landslides were distributed over two bedrock types; a granitic intrusive body (Island Intrusives) and layered volcanics (Karmutsen Volcanics). Debris slides were most likely to occur over the Karmutsen Volcanics, and to occur on dip slopes. Debris slides, which accounted for 84% of the landslide types, were strongly associated with morainal deposits, typically failing in the initiation zone at the till/bedrock interface.
Rockfall, excluding active rockfalls (talus buildup), accounted for three of the landslides in the study area. They occurred on steep bluffs as a result of jointed or fractured rock. They were not associated with morainal deposits, nor did they directly affect stream channels. Rockfall/avalanches accounted for only two of the landslides in the study area, but accounted for approximately 63% (1 ,000,000 m3) of the total volume. Rockfall/avalanches in the study area have severely impacted streams. Debris slides are the most frequent contributor of sediment to streams of the landslides examined in this study. Two drainages where this is particularly evident are Schmidt Creek and Thursday Creek.
Landslides in the study area were dated using dendrochronology, air photograph analysis, and archival information. Dates ranged from as young as 1990 to as old as 1386. The landslides examined were typically less than 100 years old. Precipitation and seismic data were analyzed for this period, and where possible, the occurrence of landslides were correlated to severe events. Landslides were tentatively correlated to three seismic events; they were the 1946 magnitude 7. 3 Vancouver Island earthquake, and two great earthquakes, 1300 +/- 130, and 1700. They occurred at similar dates as the rockfall avalanches. A reasonable correlation occurs between landslides and severe storms, where a more severe storm has a greater likelihood of triggering landslides. Specifically, six landslides occurred in 1975, the same year as a severe storm in the study area. Significant dates of other landslides, which occurred the same year as severe storms recorded at Alert Bay a include: 1926, 1946, 1963, and 1977. In addition, a storm in 1990 is thought to have been locally severe and resulted in landslides.