[Mapping and Assessing Terrain Stability Guidebook Table of Contents]

Terrain stability field assessments (TSFAs)

A terrain stability field assessment (TSFA) is an on-site assessment of the potential impact of timber harvesting, road construction, or the construction of excavated or bladed trails on terrain stability. The purpose of a TSFA is to: describe the terrain conditions within a proposed cutblock or along a proposed section of road; evaluate the likely effect of timber harvesting or road construction on terrain stability; and recommend site-specific actions to reduce the likelihood of post-harvesting or road-related landslides including a recommendation not to locate or construct trails on areas where the likelihood of a landslide will be significantly increased or there is a moderate or high likelihood of landslide debris entering fish streams or streams in community watersheds, or cause damage to private property or public utilities. These actions may involve modification of the cutblock layout, harvesting technique, road location, trail location, construction techniques, or rehabilitation techniques.

A TSFA should not be considered mapping, because only areas of immediate concern within a forest development plan (e.g., a cutblock or a road location) are assessed. With few exceptions, TSFAs are based on inspections on the ground, not on stereoscopic air-photo interpretation, although air-photos are often used for background information. They require knowledge of the surficial geology and terrain stability of the area, as well as of the proposed forest development, including the proposed timber harvesting and road or trail construction and rehabilitation methods.

Field assessments may involve several stages, depending on the nature of the terrain or soil problems present, and the state of development planning. Sometimes a recommendation will be that a further assessment or prescription is needed by a different specialist.

Wherever possible, field assessments should be done with the forestry, logging or engineering staff of the licensee or the forest district. Often, useful information that is not in the written plan can be supplied by these people. As well, agreement on prescriptions or changes to the plan on the site can often be reached, which would not be possible if the assessor did the assessment in isolation. Generally, the best results are achieved if a terrain specialist works as part of a planning team, providing advice when needed.
The resulting TSFA reports for cutblocks and road locations may be used and reviewed by a wide range of individuals:

Professionals carrying out TSFAs must ensure that prepared reports provide sufficient technical information to support their interpretations and judgment, and that they are written concisely and in a language that is understandable to a wide readership.

Conducting TSFAs

TSFAs can take several forms:

Pre-layout assessments, especially if done with the layout personnel, can often produce more cost-effective layout and environmental protection. Post-layout reviews ensure that falling boundaries and roads are optimally located, and avoid potential problem areas.

The level of field effort and the information provided in the report will depend on what type of assessment is being done. The most detailed level for a terrain stability assessment is generally a post-layout field assessment of a proposed cutblock or proposed road locations.

In areas where there is no terrain mapping but slopes are greater than 60% and previous studies have shown a low likelihood for landslides, a qualified registered professional who is familiar with the forest development plan area may carry out low-level aerial reconnaissance and stereoscopic air-photo interpretation of the area. If, on the basis of this air-photo interpretation, the professional can describe the terrain and confirm in writing and with supporting logic that certain cutblock areas will have a low likelihood of post-harvesting failure, then a report by the professional may be considered an adequate substitute for an on-the-ground TSFA. Examples of such terrain for which this approach may be adequate include steep, irregular bedrock bluffs and blocky talus slopes. This approach is only suitable for areas where no roads will be built, or harvesting will be by cable or aerial systems and excavated trails will not be constructed.

A TSFA may be combined with other assessments such as those for soil erosion hazard, risk of sediment delivery to streams, gully assessments, windthrow hazards, snow avalanche hazards and stream channel stability. Considerable efficiency can be gained by combining more than one type of assessment and teaming with one or more professionals or specialists to carry out the work.


The following is intended as a guide to professionals carrying out TSFAs. Each professional must exercise their professional judgment in selecting the methodology that best suits the site conditions, the goals of the assignment and the client's needs.

Before conducting a TSFA, the following background information, where available, should be obtained and reviewed:

A TSFA should assess the existing and potential terrain stability hazards of all critical areas within and adjacent to the cutblock and road location such as: unstable areas, moderate to steeply sloping areas, potentially unstable areas, and steep gully headwalls. As well as ground traverses, helicopter overviews can be useful. Areas of highly erodible soils should be assessed in locations where soil erosion or stream sedimentation is a concern.

Traverses along falling boundaries should describe and evaluate the terrain inside and immediately outside the falling boundary. Traverses along road alignments should describe and evaluate the terrain immediately upslope and downslope of the centerline. These assessments should identify stability hazards that could affect the road or cutblock (such as snow avalanche tracks, debris flows, rockfalls upslope of the road, etc.). For small cutblocks, traverses of the falling boundaries and road locations may suffice. In large cutblocks or in complex terrain, additional traverses within the cutblock area may be necessary to fully describe and evaluate terrain conditions. For areas with no topographic mapping or for which the only available mapping does not have adequate resolution (e.g., TRIM), it is best to record geographic features that can be easily identified (e.g., bedrock bluffs, talus slopes, windthrow patches, tall snags).

Traverses of the hillside and valley floor below a cutblock or road location may be necessary for a full assessment of the areas that could be adversely affected by road drainage, potential landslide runout zones and geomorphic consequences of landslide activity.

The assessment should cover the potential on-site and off-site effects of harvesting and road construction, including possible downslope consequences of post-timber harvesting and road construction landslide activity. It should also cover the potential stability hazards to workers carrying out harvesting or road construction, including stability hazards (natural, harvesting and road related) that may originate upslope of the subject area.

Sufficient field time must be allowed to adequately traverse all areas of concern. Complex or difficult terrain may require several days of field investigation. Return trips to pick up missing information are very costly, especially for remote sites or sites requiring helicopter access. TSFAs must be carried out when the area is not covered with snow.

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