Water Stewardship Information Sources
| Citation | Millard, T. 1999. Debris flow initiation in coastal British Columbia Gullies. Forest Research Technical Report TR-002, Vancouver Forest Region. |
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| Organization | FLNRO |
| URL | https://www.for.gov.bc.ca/rco/research/georeports/tr002.pdf |
| Abstract/Description or Keywords | This study investigates debris flow initiation hazard in Coastal British Columbia gullies. Field assessments were conducted in four geographic areas: north of Nitinat Lake on Vancouver Island, south of Nitinat Lake, the Squamish River and Mamquam River drainages in the Coast Mountains (southwestern mainland), and the Deena Creek and Ghost Creek drainages in the Queen Charlotte Islands. A total of 286 slope failures of which 75 were debris flows were analysed. Information was collected on gully and slope failure characteristics, including slope failure location (headwall or sidewall), gully wall slope angle, gully wall slope distance, channel gradient, surficial material and surficial material depth, soil drainage, slope failure dimensions, the volume of slope failure debris delivered to the channel, the planimetric angle of entry of the slope mass into the channel, and whether the slope failure continued to transport as a debris flow down the gully channel. Two types of analysis were carried out: the first to identify any factor that may have affected the initiation of channelized debris flow, and the second to identify factors that could be used in pre-logging assessments to identify gully locations that are prone to debris flow. Both single and multivariate analyses were used to identify factors in debris flow initiation. The study found that slope failures in gully headwalls were much more likely to initiate a debris flow than slope failures in gully sidewalls. Headwall debris flows started on lower gradient slopes than did sidewall debris flows. The minimum initial failure volume required to initiate a headwall debris flow was about half the volume required to initiate a sidewall debris flow (11 m3 compared with 25 m3 ). Angle of entry is highly associated with location. The median headwall slope failure angle of entry is 0°, compared with the median sidewall slope failure angle of entry of 74°. In addition to location within the gully (headwall or sidewall), the initial failure volume and the volume of debris delivered to the channel were the most important factors in determining whether a slope failure would result in a debris flow. Larger initial failures, and larger amounts of debris delivered to the channel, were much more likely to initiate debris flows than small slope failure volumes. Larger volume slope failures are associated with deeper surficial materials on sidewalls but not headwalls. Surficial material depth, or terrain type, divided into shallow and deeper classes, is significant in the prediction models. About 90% of debris flows had at least 80% of the initial failure volume delivered to the gully channel. For both headwall and sidewall locations, a set of minimum criteria could be stated for debris flow initiation in this study. Below these criteria, debris flows did not occur. Above these criteria, debris flows occurred, but not all slope failures resulted in debris flows. For headwalls, minimum debris flow initiation criteria were: • Initial slope failure area of at least 30 m2 • Initial slope failure volume of at least 10 m3 • Debris volume delivered to the channel of at least 10 m3 • Gully wall slope angle of at least 26° for till slopes • Gully wall slope angle of at least 32° for colluvial slopes. For sidewalls, minimum debris flow initiation criteria were: • Initial slope failure area of at least 50 m2 • Initial slope failure volume of at least 25 m3 • Debris volume delivered to the channel of at least 25 m3 • Gully wall slope angle of at least 35° for till slopes • Gully wall slope angle of at least 39° for colluvial slopes (with one exception) • A gully wall slope distance of at least 7 m. The likelihood of debris flow initiation increased with the following factors: • For both headwalls and sidewalls, an increase in volume of debris delivered to the channel • For both headwalls and sidewalls, an increase in initial slope failure size • For both headwalls and sidewalls, an increase in surficial material depth • For sidewall slope failures, an increase in channel gradient Although three of the geographic areas (South Nitinat, Squamish, and Queen Charlotte Islands) did not have significant differences in debris flow initiation, the fourth area, North Nitinat, was different from the other three areas. The most likely explanation for this difference was the presence at North Nitinat of highly sheared bedrock within gullied areas. The results show that debris flow initiation is a complex process. As with most geomorphic processes, a significant degree of uncertainty exists. Although this study better defines the factors which are associated with debris flow initiation, there are limitations to this type of research. Therefore, caution must be used when applying these results. Terrain scientists should examine relationships between local slope failures and debris flows in gullies before applying these results to specific locations within Coastal British Columbia. |
| Information Type | report |
| Regional Watershed | Vancouver Island South, Haida Gwaii, Howe Sound |
| Sub-watershed if known | Nitinat, Squamish, Mamquam, Deena Creek, Ghost Creek |
| Aquifer # | |
| Comments | |
| Project status | complete |
| Contact Name | Tom Millard |
| Contact Email | [email protected] |