Ministry of Environment Terrain

Suggested Methods of Terrain Stability Mapping -- Specific Aspects

Chapter 5 summarized general aspects of terrain stability mapping. This chapter summarizes a number of the specific aspects directed toward the three broad groups of uses described in Section 3.1, resource development planning (Section 6.1), land use and development planning (Section 6.2) and linear project planning (Section 6.3). As in Chapter 5, the suggested methods are highlighted in a series of boxes.

6.1 Resource Development Planning

The British Columbia forest industry is the most frequent user of terrain stability mapping. Similar maps are used by the mining industry, usually associated with mine site development. Since 1995, all resource roads on crown land are now under the jurisdiction of the Forest and Range Practices Act.

Landslide hazard mapping, and to a lesser extent, landslide risk mapping is usually carried out to assist with forest management, and quite often is used to predict the stability of the terrain after road construction or timber harvest. Since forest practices can influence the potential landslide hazards, mappers should be familiar with various logging road construction techniques and timber harvesting practices, and make themselves aware of specific proposed development plans.

In British Columbia three levels of terrain stability mapping are used in the forest industry:

• 'reconnaissance terrain stability mapping' identifies unstable or potentially unstable terrain from a broad perspective;
• 'detailed terrain stability mapping' provides a more comprehensive assessment of terrain stability within a specific forest development area; and
• 'field terrain stability assessments' focus on specific areas of concern for a cutting permit or road location.

BC Ministry of Forests' (1995a) 'Mapping and Assessing Terrain Stability Guidebook' outlines in some detail the standards and procedures for carrying out the three levels of mapping. The reader is referred to that document and Ryder et al (1995) for specific details. The following comments put these three levels of mapping into the broader context of terrain stability mapping as described in this document.

6.1.1 Reconnaissance Terrain Stability Mapping

Reconnaissance terrain stability mapping (BC Ministry of Forests 1995a) identifies unstable or potentially unstable areas following road construction or timber harvest over a large region for long-range planning purposes. Based on the map scales described in Section 3.4, reconnaissance terrain stability mapping is considered to be medium to regional scale mapping. Reconnaissance terrain stability mapping was previously referred to as 'ESA' (Environmentally Sensitive Area) mapping, 'Es1 and Es2' mapping, and/or 'Level 1' mapping.

It is recommended that reconnaissance terrain stability mapping be carried out at terrain survey intensity level (TSIL) D (refer to Table 5.1). The map units are polygons, delineated primarily by air photo interpretation supplemented by limited helicopter reconnaissance and field checking. Linear and point map symbols can be used to show the locations and type of landslides, and other indicators of unstable terrain too small to be mapped as separate polygons.

The entire map area is not mapped, but only those areas predicted to be 'unstable' or 'potentially unstable' following forestry activities. A qualitative, relative subjective rating analysis is used to map two terrain stability/landslide hazard classes: 'unstable and 'potentially unstable'. A third class, 'stable', is assumed for all remaining areas and is often not mapped. Table 6.1 summarizes the three classes.

The mapping is not accompanied by terrain mapping, but terrain symbols, geomorphic processes and slope gradients or classes relevant to terrain stability are recorded for the unstable and potentially unstable polygons. Most reconnaissance terrain stability maps address landslide hazards in the initiation zone, but can be supplemented by codes to indicate effects on the runout zone and thus become landslide risk maps.

Reconnaissance terrain stability mapping as described above can be modified, if necessary, for use with other resource development planning.

6.1.2 Detailed Terrain Stability Mapping

Detailed terrain stability mapping (BC Ministry of Forests 1995a) collects and presents data on a number of terrain attributes using an extended version of the BC Terrain Classification System, then interprets that data for terrain stability following timber harvest or road construction. In community watersheds, this level of mapping is extended further to include interpretations of landslide-induced stream sedimentation and surface erosion hazards. It is considered large scale mapping as described in Section 3.4 and is used for forest development planning and to identify areas requiring on-site assessments prior to the approval of cutblocks and/or road construction. Detailed terrain stability mapping was previously referred to as 'Level 2' mapping.

Detailed terrain stability mapping should be done at TSIL C, or in certain circumstances, TSIL B (refer to Table 5.1). It is recommended that 1:20,000 scale base maps be used wherever possible. Polygons, and linear and point map symbols, are used to delineate the surficial material (genesis), texture, surface expression and geological processes as described by the BC Terrain Classification System (Howes and Kenk 1996; Resources Inventory Committee 1996a), as well as slope gradients or classes and soil drainage classes.

Subjective rating analysis is used to interpret the terrain data, and to place each polygon in one of five terrain stability classes. The five relative and qualitative classes are summarized in Table 6.2. The criteria for classifying the terrain is uniform throughout the map area but can change between map areas due to regional differences such as terrain and climate. An example class criteria is presented in Table 5.7. Table 6.3 is an example of how probabilistic univariate analyses can be used to help define the class criteria.

The terrain stability classes suggested by BC Ministry of Forests (1995a) provide a relative ranking of the probability of occurrence of a landslide in the initiation zone after forestry activities only. They provide no quantitative predictions of probability of occurrence and no indication of magnitude of potential landslides. As mentioned previously, however, they can be extended to include landslide-induced stream sedimentation which does address some aspects of effects in the runout zone. They can also be interpreted for surface erosion potential and the potential for any eroded material to reach a drainage course. The latter two type of maps are examples of simple landslide risk maps.

For uses other than the forest industry and beyond what is suggested BC Ministry of Forests (1995a), it is possible to extend the suggested five class system to include quantitative probabilities of occurrence, potential magnitudes in the initiation zone, and possible effects in the runout zone. These extensions require substantially more data. Detailed terrain stability mapping as described above can also be modified and used for mapping of existing conditions, as opposed to conditions following logging operations.

6.1.3 Field Terrain Stability Assessments

The purpose of an field terrain stability assessments as described by BC Ministry of Forests (1995a) is to identify the probability of occurrence and potential effect of a landslide in a specific area, and to recommend mitigative measures. These assessments are carried out at scales considered to be detailed mapping scale described in Section 3.4, however, the field assessments described by BC Ministry of Forests (1995a) should not be considered as either landslide hazard or risk mapping. The terrain stability of specific areas should be clearly and concisely described on a map and in a report. Terrain stability classes should not be used.

Generally for resource development planning detailed scale mapping is not carried out, but can be done using TSIL A if required. Such terrain stability mapping requires a great deal of field checking, and possibly, depending upon the intended use, surveys of terrain features and map boundaries.

Field mapping along proposed or existing forestry roads at 1:5,000 scale can use the linear mapping methods described in Section 6.3.

6.2 Land Use and Development Planning

Terrain stability mapping in land use and development planning usually delineates areas where existing and/or future land development may be affected by landslide hazards, either under existing or changed conditions. The understanding and confidence of citizens, developers, elected officials and government agencies are required for acceptance of terrain stability maps and any resulting regulations, restrictions or specific recommendations in land use planning process.

The intensity of land use planning can vary dramatically in area from a single property to a subdivision to an entire region. Investigation of a landslide on a particular property is generally considered a geotechnical or geological engineering investigation, rather than mapping, and therefore is not discussed further.

Terrain stability mapping for land use planning can have a great impact on property values and, therefore, to the extent possible, it should be carried out uniformly over the study area and as objectively as possible. Quite often quantitative approaches of mapping are preferred because there is less misunderstanding among citizens, developers, elected officials and government agencies. Complex analytical methods, however, should be simplified for public presentation.

The responsibility of the mapper, as discussed in Section 5.5, should be kept in mind for terrain stability mapping associated with land use planning. It is the responsibility of the mapper to produce terrain stability maps and/or estimates of probabilities of occurrence, magnitude, intensity, elements at risk, vulnerability, consequence and risk. Individuals, agencies or authorities, such as landowners, governments or courts, who incorporate appropriate socio-economic and environmental factors into their decisions, are responsible for determining the acceptability of the landslide hazard or risk.

Unlike the British Columbia forest industry, except for a few provincial regional districts there are no standards or guidelines for terrain stability mapping and reporting for land use planning. A phased approach to mapping is often preferred. In all cases changes in existing conditions must be considered. Will the slopes above be crossed by a road, will they be logged, will development at the top of the slope increase the water discharge onto the slope? It is often difficult to take all possibilities into account, but a range of possibilities should be considered. If necessary a series of terrain stability maps should be produced to show how the hazards and risks change under different changed conditions.

Regional terrain stability mapping, usually carried out at a medium scale of 1:20,000 to 1:50,000, can be used to identify hazardous areas requiring further, larger scale, mapping. Regional mapping is usually focussed on the initiation zone. Subjective geomorphic or rating analyses, or relative univariate, probabilistic univariate or probabilistic multivariate analysis are considered acceptable methods (refer to Section 4.1). The latter three methods are more objective and quantitative, but require more data.

Subdivision terrain stability mapping is usually large scale mapping carried out at scales ranging from 1:5,000 to 1:20,000. Subdivision mapping may be used to establish landslide hazard or risk zones for bylaws and regulations and should consider both hazards and risks in the initiation zone and the runout zone. Subjective geomorphic or rating analyses, or relative univariate, probabilistic univariate or probabilistic multivariate analysis are considered acceptable methods in the initiation zone (refer to Section 4.1). Any of the mapping methods described in Section 4.2 can be used in the runout zone. As mentioned previously, the more objective and quantitative methods are preferred, however, they require a great deal more data, time and resources.

Detailed terrain stability mapping for land use planning, at scales of 1:500 to 1:5,000, is usually associated with surveyed legal boundaries such as restrictive covenants, and almost always considers both the landslide hazards and risks in the initiation zone and the runout zone. Quite often either the initiation or runout zone is outside the study area. Subjective geomorphic or rating analyses, or relative univariate, probabilistic univariate or probabilistic multivariate analysis are considered acceptable methods in the initiation zone (refer to Section 4.1), although at this scale these methods should be calibrated with some form of stability analysis. Any of the mapping methods described in Section 4.2 can be used in the runout zone, although the hazard consequence analysis is less desirable.

For some projects at the large or detailed scales, magnitude(or intensity)-probability of occurrence and/or consequence (or severity)-probability of occurrence relationships are produced in association with the mapping. For many projects these are used to determine the probability of damage to property and resources (structural, environmental, or economic) and probability of death of an individual or group. The BC Ministry of Highways and Transportation suggests that a 10% long-term probability of occurrence of a landslide (other natural hazard) over a 50 year period should be considered as a guideline for approval of subdivision permits. This is equivalent to an annual probability of occurrence of 1:475. The Fraser Valley Regional District applies this same probabilitiy of occurrence to building permit approvals unless remedial or protective works are practical.

Such landslide risk mapping can be used for cost-benefit analyses that can be used to evaluate development alternatives, and compare possible passive versus active mitigative measures. This sort of detailed mapping is developing rapidly in British Columbia at the present time. See for example Morgan et al (1992), Hungr and Rawlings (1995), Sobkowicz et al (1995).

6.3 Linear Project Planning

Terrain stability mapping for the planning of linear projects, such as roads, railways, transmission lines, and pipelines is referred to as corridor mapping. Mapping carried out for planning along linear geomorphic features such as along streams, shorelines and reservoirs is linear segment mapping. Both use methods that are similar to general land use planning, except that terrain attributes and terrain stability are focussed along a linear corridor or a linear segment.

Landslide hazards upslope of a proposed alignment or linear feature should also be mapped because these areas may initiate events that can affect the linear zone of interest. In some cases, such as debris flows moving down pre-existing channels, there is sometimes a tendency to avoid mapping the upslope areas and to concentrate on the assessment of the defined paths. If this approach is taken, it should be carefully justified in the project report. Particular situations, such as undercutting or piping, will also require that downslope hazards and risks should also be investigated.

Both corridor mapping and linear segment mapping should be carried out using a phased approach going from a regional (medium scale) through to a project (large scale) to a detailed scale.

Corridor terrain stability mapping is often used to select the best route for the linear project. Such maps should address the initiation zone and runout zone, and how the landslide hazard will change once the project has been constructed and is in operation. Depending on the requirements of the mapping project, subjective geomorphic or rating analyses, or relative univariate, probabilistic univariate or probabilistic multivariate analysis are considered acceptable methods in the initiation zone (refer to Section 4.1). Any of the mapping methods described in Section 4.2 can be used in the runout zone.

Project or detailed corridor mapping requires consideration of large or detailed scale base mapping, and proposed project alignment plans, profiles and cross-sections. The BC Ministry of Transportation and Highways regularly carries out these sorts of mapping projects for locating new or realigned highways.

For some large scale projects, magnitude (or intensity)-probability of occurrence and/or consequence (or severity)-probability of occurrence relationships are produced in association with the mapping. Such mapping can be used for cost-benefit analyses that can be used to evaluate corridor or alignment alternatives, and to compare possible passive versus active mitigative measures. An example of a relatively simple risk assessment procedure for forestry road corridors is described in BC Ministry of Forests' "Engineering Manual" (1993).

Linear segment terrain stability mapping addresses hazards and risks along linear geomorphic features. From the early 1970s to the early 1990s, such mapping was used by BC Hydro to delineate a 'break line' and a 'safe line' along a proposed reservoir shoreline. The 'break line' was defined as the maximum anticipated extent of shoreline regression by erosion, beaching and/or landsliding. The 'safe line' was defined as a conservatively located line placed along the shoreline, landward of which security or residents and their belongings could be reasonably assured. See Morgan (1982) for an example. The concepts of break line and safe line can be adopted for any other types of linear segment mapping.

In the early 1990s, BC Hydro replaced the 'safe line' concept with the 'impact line' concept (BC Hydro 1993). BC Hydro felt the term 'safe line' could be misunderstood, and its conservative application was originally intended for residential areas and had limited application for agriculture, industry, forestry and recreation. Around the linear segments of a reservoir, BC Hydro's 'impact lines' delineate the potential upslope and downslope extent of various hazards such as flooding, erosion, groundwater and landslides.

For existing or proposed residential areas adjacent to a reservoir where lives may be threatened, the 'landslide impact line' is defined as the boundary landward of which there is less than a 1:10,000 annual probability of occurrence of the area being subject to landsliding, either due to the reservoir, or due to existing instabilities not affected by the reservoir (BC Hydro 1993). The effects of toe erosion and seismic activity on slope stability are also considered. BC Hydro realizes that to determine an annual probability of occurrence of 1:10,000, extensive geotechnical investigations are required. BC Hydro suggests that "different", presumably greater, annual probabilities of occurrence may be selected for specific projects.

In non-residential areas, BC Hydro suggests the landslide impact line be determined in a similar manner as for a residential area, but that it can have a lesser degree of confidence, and presumably a greater probability of occurrence, due to less data and non-life threatening consequences. The degree of confidence and annual probability of occurrence can vary with land use, but if land use changes, the impact line should be reviewed.

BC Hydro (1993) suggests that for reservoir projects, shoreline stability should initially be classified in terms of existing, pre-flooding stability. For preliminary studies, post-flooding shoreline stability can be derived from judgement based on experience from existing reservoirs. As studies advance from preliminary to final design and with more data, numerical factors of safety determined by stability analyses can be determined for typical or critical shoreline segments. These, along with experience, can be used to classify the stability of shoreline segments for first flooding of the reservoir, normal reservoir operation and for rapid reservoir drawdown.

It is possible to extend BC Hydro's 'impact lines' by showing degrees of confidence and annual probabilities of occurrence on the map as a band or series of lines. Impact line maps can also be used as the basis for cost-benefit analyses to aid with the planning and design processes.

Table 6.1 Reconnaissance Terrain Stability Classification

Table 6.2 Detailed Terrain Stability Classification

Table 6.3. Approximate relationship between Terrain Stability Class, frequencies and likelihood of landslides following timber harvesting and road construction for BC coastal conditions.

These relationships are generalized from limited data for several coastal study areas, for the period 5 to 15 years after logging (Howes 1987, Rollerson 1992, and Rollerson and Sondheim 1985). They may not be applicable to other climatic regions or longer time periods. The table addresses landslides * 0.05 ha, and sidecast road construction practices. Some terrain types will have a different likelihood of failure for road-building compared to timber harvesting. (modified from BC Ministry of Forests 1995a