Coastal Watershed Assessment Procedure Guidebook (CWAP)

[Coastal Watershed Assessment Procedure Guidebook Table of Contents]

Appendix 3

Definition of unstable slopes and those with a potential for landsliding

The potential for slopes to experience landsliding is determined by terrain (surficial geology) mapping, and the subsequent interpretation of the terrain mapping information into slope stability classes using, as defining criteria, slope angle, materials and landforms, material texture, active geomorphological processes, and soil drainage.

Terrain maps provide information about the distribution and characteristics of surficial materials, landforms and geological processes in an area. The terrain classification system used for mapping in British Columbia is defined in Howes and Kenk (1988). The Resources Inventory Committee (1994) provides additional important information, including material on terrain survey intensity levels and interpretive products such as slope stability classification. The Mapping and Assessing Terrain Stability Guidebook provides detailed information on the standard procedures to be used for forestry-related purposes in British Columbia.

Terrain mapping and slope stability classification must be done by a registered professional who has extensive experience in terrain mapping and landslide hazard interpretations. Junior mappers can do this work under the close supervision of such an individual.

In British Columbia a five-class slope stability classification is most commonly used. The slope stability classes are as follows:

I No significant stability problems exist.

II There is a very low likelihood of landslides following harvesting or road construction.

III Minor stability problems may develop in some areas.

IV Terrain polygons contain areas with a moderate likelihood of landslide initiation following harvesting or road construction.

V Terrain polygons contain areas with a high likelihood of landslide initiation following harvesting or road construction.

Locating areas of potential slope instability

Slope stability Class IV and V are used as indicators of potentially unstable terrain. If slope stability maps are available for the watershed of concern, then the polygons identified as Class IV and V should be indicated on the overlay or entered into a GIS.

Where terrain mapping or slope stability classification is not available for a watershed, the potentially unstable terrain may be assessed as (in order of reliability):

Areas properly defined as Es1 and Es2, in environmental sensitivity mapping done as part of forest development planning, where the mapping was done by a registered professional with extensive experience in terrain mapping and landslide interpretation.
There are a large number of Es1 and Es2 maps in existence that were not produced by such individuals. These are not acceptable for use as terrain stability hazard maps, as there is no certainty that all areas of potentially unstable or unstable terrain have been identified.
Areas with slopes greater than 60% (31 degrees). In most cases, using a slope angle classification alone will largely over-estimate the area of potentially unstable terrain.

References

Howes, D. E. and E. Kenk. 1988. Terrain Classification System for British
Columbia. (Revised edition). B.C. Ministry of Environment,
Manual 10, Victoria, B.C.

Resources Inventory Committee. 1994. Guidelines and Standards to Terrain
Geology Mapping in British Columbia. Report prepared for: Terrain
Geology Task Group, Earth Science Task Force, Resources Inventory
Committee, British Columbia, 1993–1994.

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