[Mapping and Assessing Terrain Stability Guidebook Table of Contents]
Results and recommendations
The results and recommendations should be clearly stated in plain language so that forestry personnel fully understand the planning/management implications. Included should be:
- Expected outcomes of timber harvesting for individual boundary segments and landforms within and adjacent to the cutblock:
- likelihood and/or expected frequency of landslide activity.
- expected landslide runout zone/size and relative geomorphic consequences
- potential for soil erosion/stream sedimentation
- expected outcomes of road construction for individual road sections as outlined above for the cutblock
- downslope/downstream terrain stability concerns related to road or logging trail drainage
- potential effects of windthrow on terrain stability on adjacent slopes
- safety: potential terrain hazards originating both within and upslope of the subject area and which present an undue risk to workers, and an assessment of whether this risk is similar or greater than adjacent areas. The client and professional conducting the assessment should ensure that current Workers' Compensation Board requirements and concerns are addressed.
- downslope/downstream elements at risk and possible consequences
The results and recommendations must also state whether proposals for clearcutting and construction of excavated or bladed trails will be in compliance with the Timber Harvesting Practices Regulation. For example:
- In a community watershed, on areas with a moderate likelihood of landslides and a high risk of landslide debris entering directly into streams, the recommendations must state whether there are reasonable grounds to believe that clearcutting will not significantly increase the risk of a landslide.
- Outside of a community watershed, on areas with a high likelihood of landslides, the recommendations must state whether there are reasonable grounds to believe that clearcutting will not significantly increase the risk of a landslide and that there is a low likelihood of landslide debris:
- entering into a fish stream or perennial stream that is a direct tributary to a fish stream; or
- damaging private property or public utilities.
- Inside or outside of a community watershed, on areas with a moderate likelihood of landslides, the recommendations must state whether there are reasonable grounds to believe that an excavated or bladed trail can be located, constructed and rehabilitated in a manner that will not significantly increase the risk of a landslide, and there is a low likelihood of landslide debris:
- entering into a perennial stream in a community watershed, a fish stream or a perennial stream that is a direct tributary to a fish stream; or
- damaging private property or public utilities.
Documentation of any inherent limitations of the TSFA
The recommendations must be clearly written so that forestry personnel fully understand what needs to be done. They should address by segments/areas along the falling boundary, reserve areas and interior of the cutblock or road sections:
- harvesting issues (e.g., falling boundary relocation, alternative harvesting methods, in-block reserves, gully/stream management)
- windthrow relevant to stability in and around a cutblock (e.g., in-block reserves, falling boundary relocation, feathering, crown modification)
- roads and stream crossings (e.g. relocation, sidecast limitations, ditchwater control, cutslope control, designed fills, gully crossing advice, endhaul sections, spoil site locations, deactivation considerations)
As well, they should identify any additional investigations or other expertise that is needed (e.g., windthrow assessment, geotechnical engineering design of specific sites, snow avalanche assessments, hydraulic design or rock slope assessments).
The results and recommendations should be supported by a rationale, including:
- Geomorphic, hydrologic, geotechnical, geological or pedologic inferences
- Past response of comparable natural or logged areas nearby (the effects of timber harvesting may mimic the response of specific terrain units to natural wildfire or extensive windthrow)
- Local knowledge/history of terrain performance
- Applicable research data/knowledge where available
- Applicable models if any, or new information
- Other rationales
The report should indicate whether a professional or a designate need be on site during road construction or deactivation. Typically an on-site specialist is required if:
- the design and design changes are dependent on actual conditions and materials encountered during construction;
- the design is complex or non-standard and the specialist needs to explain it and consult with on-site forest operations personnel to ensure that it is correctly built; or
- there are significant downslope/downstream elements at risk.
Methods of describing cutblocks, gullies, streams,
roads, and adjacent areas in TSFA reports
- linearly, by falling boundary section (delineated by falling corners or other geographic references), supplemented with map polygons, points, groups of points or spatial descriptions, as necessary, to clearly describe the interior of the block or adjacent areas. This is the preferred and more common approach.
- by map areas (polygon), with an appropriate detailed description for each unit delineated. These descriptions must be accompanied by linear traverse notes that clearly document the extent of field investigation carried out to define the polygons. The use of map polygons as a descriptive tool does not satisfy the requirement for complete foot traverses of all critical areas within, adjacent and above the block. Descriptions based on single point observations or incomplete foot traverses of critical areas are not acceptable. Map area (polygon) boundaries and descriptions must be accurate and must not mislead readers as to the extent, variability or character of the terrain included in each polygon.
Gullies and streams
- linearly, by reach for critical harvesting areas; and by point descriptions at road crossings.
- linearly, by homogenous road design section (delineated by station numbers), or on a map that shows traverse routes.
Accepted standard terminology should be used in TSFA reports to avoid confusion or misinterpretation. The following are the conventions in common use for this type of work in the forest sector:
- Forest soils: Canadian System of Soil Classification (Agriculture Canada 1998)
- Terrain and surficial materials: Terrain Classification System for British Columbia (Howes and Kent 1997)
- Engineering design: Unified Soil Classification System (1953); The Use of the Unified Soil Classification System by the Bureau of Reclamation (Wagner 1957)
- Gully and stream descriptions: use the current versions of the following Forest Practices Code guidebooks: Channel Assessment Procedure Guidebook; Fish-stream Identification Guidebook; Gully Assessment Procedure Guidebook; Riparian Management Area Guidebook. Some habitat information is still in the form of the Coastal Fisheries Forestry Guidelines conventions, and is acceptable for use provided it is properly referenced.
(If the author chooses to use another convention, he or she should state what convention that is.)
Maps should use the cartographic conventions specified by the Terrain Classification System for British Columbia and the Guidelines and Standards for Terrain Mapping in British Columbia to indicate map reliability.
Appendices and attachments
Presentation of information will depend to some extent on the specific requirements of the client (e.g. format requirements) and the manner in which the TSFA report is submitted. For example, if the report is submitted along with the forest development plan documents, other information relevant to the assessment may already be in the forest development plan (e.g., stream classifications, terrain classification/stability mapping, 1:20 000 location maps, etc.). On the other hand, if the TSFA is a stand-alone document, or is being forwarded for review or other information purposes, it may be helpful to attach some of these other documents in full or in part.
Typical attachments include the following:
- 1:5000/1:10 000 maps showing topography, obvious and critical terrain features, cutblock boundaries and road locations, traverse routes and stability hazard areas
- additional sketches to delineate relatively homogenous areas or segments of the cutblock or road location, identify specific terrain hazards, or illustrate recommendations
- 1:20 000 (or in some cases 1:50 000) location map of the cutblock and roads
- photos, where feasible and useful
- summaries of gully assessment data or stream classifications, where relevant
- where measures to maintain terrain stability or water quality are required, any sketches or plans necessary to describe the measures to forest operations personnel
Reports in low hazard areas
A TSFA report can be abbreviated where an on-the-ground inspection (required for road and cutblock location) determines that, for selected road location, the assessed terrain has a likelihood of landslide initiation no more severe than low. A report rationale must be included to explain why an area is determined to have a low likelihood of landslides. Sufficient attachments, such as maps at 1:5000 to 1:10 000 and other items as listed above, must be included to clearly identify the areas traversed.
Limitations of TSFAs
A TSFA depends on surface features and natural exposures observed during the field visit, supplemented by air photo interpretation and evaluation of topographic maps and other available information. This type of assessment does not include subsurface investigation or measurement of the engineering properties of materials. It is, by nature, a qualitative assessment based on the professional's training, observational skills and experience in similar terrain. Prediction of terrain stability is based on an understanding of past and present geomorphic processes and the extent to which they are influenced by forestry operations. Terrain stability predictions are more accurate for some types of terrain and certain types of instability than for others.