Water Stewardship Information Sources
| Citation | Millard, T, Rollerson, TP and Thompson, B. 2002. Post-logging landslide rates in the Cascade Mountains, southwestern British Columbia. Res. Sec., Van. For. Reg., B.C. Min. For., Nanaimo, B.C. Tec. Rep. TR-023/2002. |
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| Organization | FLNRO |
| URL | https://www.for.gov.bc.ca/rco/research/georeports/tr023.pdf |
| Abstract/Description or Keywords | Forestry operations in Coastal British Columbia occur across large tracts of land, often with terrain subject to post-logging landslides broadly distributed throughout the landscape. Some types of terrain have greater numbers of post-logging landslides than other terrain types. Terrain types that have high post-logging landslide rates often have high levels of risk to resources. Therefore, it is important to be able to accurately predict the expected post-logging landslide rate for areas designated for logging. Note that in this study the term “landslide rate” is used as either a spatial rate or as a percentage of samples with landslides – it does not imply a temporal rate. The term “landslide likelihood” is used in this study to indicate a prediction of expected post-logging landslide rates. Methods of identifying terrain types susceptible to landslides are needed that are accurate and yet are rapid enough to be applied to extensive areas. Terrain attributes are landscape features that can be used to identify hillslopes susceptible to landslides. Terrain attribute studies produce statistically derived post-logging landslide rates by identifying significant terrain attributes or groups of terrain attributes associated with landslides in previously logged terrain. Logged hillslopes are divided into terrain polygons, which are small areas that have a consistent set of terrain attributes such as slope gradient, surficial material type, and slope morphology. Terrain attribute data is systematically collected in the field in conjunction with post-logging landslide data and analysed to identify those terrain attributes that are useful predictors of post-logging landslides. The method provides quantification of post-logging landslide rates – information that can be used to predict the likelihood of post-logging landslides in similar unlogged terrain. Such information is an important improvement over qualitative evaluation of landslide likelihood (e.g. undefined high, medium, and low likelihood of landslides). Two types of landslide rates were used in this study. Landslide presence is the presence of one or more landslides within a terrain polygon. This study analysed landslide presence using only landslides >500 m2 in area, as well as the presence of landslides of any size. Landslide density is the average number of landslides in a unit area. In this study, only >500 m2 landslides were used to determine landslide density. Three post-logging landslide initiation locations were evaluated: clearcuts, road fills, and road cuts. The study used a combination of univariate and multivariate statistical procedures. Clearcut areas in the Cascade Mountains of southwestern British Columbia were studied. A total of 617 terrain polygons were sampled, representing an aggregate area of 1808 ha. The mean polygon area was 2.9 ha, with a standard deviation of 2.7 ha and a range of 0.2 to 20.2 ha. Over 44 km of road were sampled. There were 83 >500 m2 clearcut landslides, 39 >500 m2 road fill landslides, and 24 >500 m2 road cut landslides. A total of 54 (9%) of the 617 terrain polygons in the sample population had >500 m2 clearcut landslide presence. Overall, the average >500 m2 clearcut landslide density was 0.05 landslides per hectare (ls/ha). |
| Information Type | report |
| Regional Watershed | Lower Fraser |
| Sub-watershed if known | Chilliwack River, Silverhope Creek, Nicolum Creek |
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| Contact Name | Tom Millard |
| Contact Email | [email protected] |