The influence of bedrock type on the magnitude, frequency and spatial distribution of debris torrents on Northern Vancouver Island

• Main Author: Sterling, Shannon M.
• Language: English
• Published: 2009
• Subjects: Floods - British Columbia - Vancouver Island; Flood control - British Columbia - Vancouver Island
• Online Access: http://hdl.handle.net/2429/6563

This thesis examines the influence of lithology on debris torrent occurrence. The analysis covers a thirty-year period in 80 supply-limited basins distributed in the 400 km2 Tsitika River watershed, on northern Vancouver Island, British Columbia. Two bedrock types occur in the watershed, the Igneous Intrusive and the extrusive Karmutsen formations, covering forty-nine and fifty-one percent respectively. The debris torrent source basins are unlogged. The frequency data were obtained in the field using dendrochronological evidence of debris torrents. Field data were compared with data derived from air photographs, the latter were found to be unrepresentative of debris torrent occurrence and were not used. All study basins were digitised from 1 : 20 000 Terrain Resource Inventory Maps (TRIM), and were characterised by selected morphometric parameters. Results show that geology exerts significant control over the temporal and spatial occurrence of debris torrents in the Tsitika watershed; the Karmutsen formation is more prolific. Geology also was found to exert significant control over the runout area and volume of debris torrents. Climate, morphometry and surficial materials do not appear to be confounding parameters. Differences in weathering rates, infiltration patterns and detrital grain-size distribution associated with the two bedrock types are believed to account for the differences in debris torrent behaviour.