[Mapping and Assessing Terrain Stability Guidebook Table of Contents]

Classification and mapping conventions

The Terrain Classification System for British Columbia (Howes and Kenk 1997) should be used for all detailed terrain mapping. The recommendations on mapping standards and procedures in the Guidelines and Standards for Terrain Mapping in British Columbia (Resource Inventory Committee, 1996a) are to be followed These approaches can be supplemented with innovative terrain stability mapping strategies on a trial basis where appropriate rationale is presented to, and accepted by, government staff specialists.

Slope gradient and soil drainage labels

Terrain polygon labels should include descriptions of slope gradient and soil drainage. Slope gradient can be derived by field measurements or from detailed large-scale contour maps if available, and should be recorded as an estimated range (or range and average) of typical slope gradients. Slopes of polygons that are not field-checked should be based on air-photo interpretation and extrapolation from similarly checked polygons. Soil drainage classes as described in the Canadian System of Soil Classification (Agriculture Canada 1998) should be used. The drainage class reflects both the soil permeability and site hydrology.

The recommended procedure for presenting slope information is to give a range of the maximum and minimum commonly occurring slopes in the polygon. Two ranges may be given where two disjunct slope ranges exist and the features cannot be identified as a separate polygon; e.g. a terrace face and scarp. See the Standard for Digital Terrain Data Capture in British Columbia (Resource Inventory Committee, 1996a) for details. Another common procedure is to give three slopes-a maximum, minimum and modal. Soil drainage classes should be given as a primary and secondary class, where a range of classes is present.

Whenever possible, polygons should delineate homogeneous terrain and slope. In certain areas of critical or complicated terrain, smaller polygons than the preferred minimum size may be necessary. For example, deep, steep-sided gullies create many forest management problems. The mapper should use specific polygons for these wherever feasible.

Terrain stability classes

Terrain stability classes provide a relative ranking of the likelihood of a landslide occurring after timber harvesting or road construction. They give no indication of the expected magnitude of a landslide or potential downslope/downstream damage. The 5-class terrain stability classification, a tool for forest development planning, is used for flagging potential problem areas. It should not be considered an on-site prescription tool for terrain stability field assessments. Where sufficient data is available, the mapper can use terrain stability classes showing expected numerical frequency or likelihood of failure (e.g., 0.1 landslides/hectare) or other innovative approaches to complement the 5-class terrain stability classification.

For the derived terrain stability map, each map polygon should be labeled with a single terrain stability class (see below). In some places, highly variable terrain types are intermixed and must be mapped as composite polygons. In these cases, the entire polygon must be labeled as the more hazardous class. Similarly, when the interpretation for a map polygon is in doubt, the more hazardous class should always be chosen. This conservative approach to interpretation is necessary, given the moderate intensity of field-checking represented by a TSIL C survey. However, the mapper should not be overly conservative; the criteria used to define terrain stability classes must be justified.

The assignment and interpretation of terrain stability classes is quite subjective. The 5-class terrain stability classification system in Table 3 should be used on detailed terrain stability maps.

The IVR-class terrain stability is used only where terrain responds, with respect to slope stability, very differently to road construction than to timber harvesting. For example, steep, irregular bedrock units can have a high likelihood for road fill-slope failure, but a negligible likelihood of landslide initiation due to timber harvesting. Similarly, in areas of low rainfall, the likelihood of slope failure due to timber harvesting alone may be very low compared with the likelihood of landslides from road construction. In these situations, it is appropriate to identify the terrain stability class with IVR, indicating a low or very low likelihood of landslides initiating after timber harvesting, but a moderate or high likelihood of landslide initiation following road construction. For IVR areas, a terrain stability field assessment would be required for road construction proposed through the IVR polygon or if the proposed harvesting system included bladed or excavated trails.

Table 3. Terrain stability classificationa

Terrain stability class

Interpretation

I

  • No significant stability problems exist.

II

  • There is a very low likelihood of landslides following timber harvesting or road construction.
  • Minor slumping is expected along road cuts, especially for 1 or 2 years following construction.

III

  • Minor stability problems can develop.
  • Timber harvesting should not significantly reduce terrain stability. There is a low likelihood of landslide initiation following timber harvesting.
  • Minor slumping is expected along road cuts, especially for 1 or 2 years following construction. There is a low likelihood of landslide initiation following road construction.

IVR

  • Expected to contain areas with a moderate likelihood of landslide initiation following road construction and a low or very low likelihood of landslide initiation following timber harvesting.

IV

  • Expected to contain areas with a moderate likelihood of landslide initiation following timber harvesting or road construction.

V

  • Expected to contain areas with a high likelihood of landslide initiation following timber harvesting or road construction.

a Modified from: Land Management Handbook 18 (Chatwin et al, 1994). The classification addresses landslides greater than 0.05 ha in size, conventional timber harvesting practices, and sidecast road construction.

On-site symbols may be used to identify features that are important for terrain stability interpretations, but too small to be mapped as distinct polygons (e.g., landslides, gullies or terrace scarps). The use of on-site symbols should not be carried to the point where they result in a cluttered or unreadable map.

Where terrain stability is influenced by bedrock geology, relevant information on bedrock geology may be included on the terrain and/or terrain stability maps (e.g., symbols for strike and dip direction, faults) or be described in the accompanying report.


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