Ministry of Environment Terrain

Suggested Methods of Terrain Stability Mapping -- General Aspects

This Chapter summarizes the general aspects of terrain stability mapping and provides a series of suggested methods, that are highlighted in a series of boxes. There is no one method of producing a terrain stability map, and the suggested methods are intended to aid the work of the mapper, not stifle it by the imposition of rigid procedures. Suggested methods for specific uses of terrain stability mapping are presented in Chapter 6.

5.1 Starting a Project

When starting a mapping project, the ultimate purpose of the project and map should be clearly defined, and the time and resources that are available to produce such a map should be determined. Once these factors are well defined and understood by both the client and the mapper, the appropriate type of terrain stability map (Section 5.2) and the appropriate method of mapping (Sections 5.3) can be selected. In many cases, the use, the available time and resources, the type, and the method of mapping will direct the mapping procedure (Section 5.4). Section 5.5 describes the professional responsibility of the mapper.

Frequently the time and resources available for a mapping project are defined by the client's budget, however, they should be defined by the purpose of mapping. If the final product is not achievable with the time and/or resources available, either the desired final product should be modified, the project area should be modified, or additional time and/or resources should be allocated to the project.

5.2 Type of Map

Seven types of terrain stability maps were reviewed in Section 3.2. The first five types (geology maps, terrain maps, engineering geology maps, terrain attribute maps and process inventory maps) must be interpreted or combined with other information to be used as either a landslide hazard or risk map. The latter two types are produced specifically for landslide hazards and risks.

5.3 Method of Mapping

The method of mapping should be selected based on intended use, time and/or resources available, type of map required, nature of the terrain, regional experience, and experience of the mapper. Before the method is selected, consideration should be given to whether the mapping should be qualitative or quantitative (refer to Section 2.3.4), and whether it should be directed toward the initiation zone or the runout zone (refer to Sections 4.1 and 4.2).

Terrain stability mapping in the initiation zone should have three components:

• an identification of potential type or types of landslides (see Table 2.1) and/or description of the typical behavioural characteristics of the landslide;
• an estimation of a magnitude, or range of magnitudes, expressed in terms of volumes or geographic areas potentially affected by each type of landslide; and
• an estimation of probability of occurrence in temporal terms such as events per year or events per specified time period (such as 50 years, or 5 - 10 years after logging), and/or spatial terms such as events per square kilometre, events per linear kilometre, or hectares of potentially unstable ground per km2.

In many circumstances, these components should consider the landslide hazards and risks after a particular activity, such as road construction or timber harvesting, is carried out within the map unit.

An ideal approach would be to produce a series of terrain stability/landslide hazard classes based on predicted magnitude-probability of occurrence relationships for each landslide type for each map unit (refer to Section 2.3.1 and Figure 2.1). This is rarely practical, and therefore the terrain stability classes are usually simplified depending on the purpose of mapping.

Terrain stability mapping in the runout zone should also have three components:

• an estimation of the magnitude-probability of occurrence relationship for each type of landslide in the initiation zone that could enter the runout zone;
• a runout analysis for each type and magnitude of landslide including an estimation of the characteristics of the landslide movement and debris, such as velocity and thickness; and
• a summary of all elements at risk including the land, resources, buildings, economic activities and people, and their vulnerability

Ideally, each map unit in the runout zone should be assigned a terrain stability/landslide risk class based on a consequence-probability of occurrence relationship (refer to Section 2.3.3 and Figure 2.2). Again, such determinations are exceedingly complex and usually a great deal of simplification is required. Often an assumption is made that the probability of occurrence of a given consequence is the same as the probability of occurrence of the landslide. In other words, the reduction of the probability of occurrence of the landslide, from its initiation to it reaching the runout zone, is not taken into account.

The following provides some other general criteria for selecting and using a particular method of terrain stability mapping:

• the method should be backed by an appropriate amount of reliable data;
• the method should be appropriate to the terrain conditions;
• preference should be given to a method that has been regionally tested;
• if possible, the method should be calibrated by research or experience;
• the method should be compatible with the experience of the mapper; and
• the use of a combination of two or more methods is encouraged.

5.4 Mapping Procedures

Once the above topics have been addressed, the procedures to successfully complete the terrain stability mapping can be established. If the terrain stability mapping is an extension of another form of mapping, such as geology mapping, terrain mapping, engineering geology mapping, terrain attribute mapping and/or process inventory mapping, the accepted procedures developed for those other types of mapping should be followed, or modified as required.

The recently published "Guidelines and Standards for Terrain Mapping in British Columbia" (Resources Inventory Committee 1996a) is a most useful summary for terrain mapping and mapping in general. Of particular interest to terrain stability mapping are sections that refer to selection of map scale, review of previous work including previous mapping, selection of air photos, air photo interpretation, field work, compiling the terrain map and reporting, and reliability of terrain maps The document also briefly discusses derivative maps and uses several terrain stability maps as examples.

The "Terrain Database Manual" (Resources Inventory Committee 1996b) summarizes the standards for collecting terrain information in a data base format and the GIS specifications for map analyses and presentation. It supersedes Kenk et al (1987).

The procedures for terrain mapping and additional procedures that are specific to terrain stability mapping are discussed below. Where applicable, the procedures for terrain mapping (Resources Inventory Committee 1996a and 1996b) have been adopted directly. The reader is referred to those documents for details.

5.4.1 Map Scale and Mapping Intensity

Section 3.4 reviewed four general scales of map presentation. As discussed, the scale of presentation of the terrain stability map is important to communicate the appropriate level of detail for the intended use. The presentation scale should be dependent upon the actual scale of mapping, and methods and intensity of field checking.

The suggested mapping intensity levels, and map scales for terrain stability, are adopted from the 'terrain survey intensity levels' (TSILs) for terrain mapping (Resources Inventory Committee 1996a) and presented as Table 5.1. Refer to that document for further details. The map scales in Table 5.1 are minimums and larger scales are encouraged.

5.4.2 Base Map

The best topographic map available, at the appropriate scale, should be used as the base map. For 1:250,000 or 1:50,000 scales, the National Topographic System (NTS) is recommended. For 1:100,000 scale, British Columbia topographic mapping is recommended. For 1:20,000 scale, the provincial topographic Terrain Resource Inventory Mapping (TRIM) is recommended. TRIM mapping also exists for portions of the province at 1:10,000 and 1:5,000 scales. It should be noted that TRIM maps are produced from small scale air photos, and topographic detail, especially in forested areas, is often lacking. Usually base maps for detailed scales have to be custom produced. Besides topography, the base map should include the latitude and longitude, the main geographic names, the major roads and other cadastral detail.

5.4.3 Previous Work

Before starting a terrain stability mapping project, a thorough review of all relevant mapping and/or studies in the study area and the surrounding region should be carried out. This should include geology maps, terrain maps, engineering geology maps, terrain attribute maps and/or process inventory maps at all scales, and all site-specific geological and/or geotechnical engineering reports. Examples and sources of information are listed in BC Ministry of Energy, Mines and Petroleum Resources (1983), Clague (1987), Grant (1991), Bobrowsky et al (1992), the Canadian Foundation Engineering Manual (1992), Fulton et al (1995) and Resources Inventory Committee (1996a).

Research by the various federal and provincial agencies, and at universities (theses), should not be overlooked. Regional and district offices of the BC Ministries of Energy, Mines and Petroleum Resources; Environment, Lands and Parks; Forests; Municipal Affairs; and Transportation and Highways should be contacted, as should the planning and engineering offices of the appropriate Regional Districts and Municipalities.

5.4 4 Slope Map and Drainage Map

As discussed in Section 3.3, slope gradient is the only terrain attribute common to almost all terrain stability maps. It is suggested that at least a simple slope map should be derived from the topographic base as background data to help direct mapping and field checking of potential critical areas.. The slopes can be classified as 'average slopes', or as 'slope classes' consisting of ranges of slopes gradients. Table 5.2 summaries commonly used slope classes.

A drainage network map can also quickly be produced from the topographic base map. Such a map is useful to highlight permanent and ephemeral drainage paths, drainage divides, watershed areas and drainage density.

5.4.5 Air Photos

Interpretation of vertical air photos is an integral part of a terrain stability mapping project, and therefore the selection of appropriate air photos is most important. Air photos are available from federal and provincial agencies, some regional districts and municipalities, private photogrammetry companies and some private resource companies. Important characteristics of the air photos to consider include: scale, focal length of camera, flying height, year of photography, time of year of photography, type of product (black and white, vs colour, vs black and white from colour negatives), and quality of the air photos. These characteristics are described in Resources Inventory Committee (1996a).

The air photo scale should be the same as, or slightly larger than, the scale of the final map. Ideally the scale should never be smaller. The interpretation of air photos with different scales and dates, and even different types of products, is encouraged, because different terrain attributes and landslide features may be emphasized on different sets of air photos. Air photos taken in different years are most useful for age-bracketing specific landslide events, and/or determining the effects of changed conditions, such as timber harvesting.

5.4.6 Terrain Attributes

Table 3.1 lists 59 terrain attributes assocated with landslides. The ideal terrain stability map would record information on all these terrain attributes, however, the resulting map would of course be unrealistically complex. Furthermore, not all attributes are important in all circumstances. Therefore, for a specific terrain stability mapping project a relatively small group of relevant terrain attributes should be selected. The selection should be left to the discretion of the experienced mapper based on regional conditions, however, the mapper should explain why particular terrain attributes were selected. If at all possible, preliminary work should be carried out to identify those terrain attributes most closely linked with landslide activity in the study area.

The mapper should use, to the extent possible, standard definitions and descriptions of the terrain attributes. Standard definitions and descriptions of many terrain attributes are summarized in International Association of Engineering Geology (1981b), Luttmerding et al (1990), Canadian Foundation Engineering Manual (1992), Howes and Kenk (1996) and Resources Inventory Committee (1996a and 1996b).

As discussed in Section 3.3, slope gradient and evidence of previous landslide activity are the two more common terrain attributes. Based upon a review of the literature, discussions with mappers, and the experience of the authors, the terrain attributes most relevant to terrain stability mapping, and readily mappable, are summarized in Table 5.3. This table also indicates whether the attribute is readily mappable from the topographic base map, other types of maps, air photo interpretation, ground mapping, and/or subsurface methods.

Several terrain mapping systems are capable of recording many of the relevant terrain attributes. The BC Terrain Classification System, for example, (Howes and Kenk 1996 and Resources Inventory Committee 1996a) does this in a descriptive manner. The ''Terrain Database Manual" (Resources Inventory Committee 1996b) provides a data base and procedures for collecting many of the relevant terrain attributes. The headings from that data base are summarized in Table 5.4.

5.4.7 Air Photo Interpretation

Air photo interpretation can involve the interpretation of a single terrain attribute, or more practically, the simultaneous interpretation of several of the relevant terrain attributes selected above. The mapper should be experienced in air photo interpretation of the province's terrain. The BC Terrain Classification System is suggested as a good method to capture many of the relevant terrain attributes. Details of the system, and guidelines and standards for terrain mapping, are summarized in Howes and Kenk (1996) and Resources Inventory Committee (1996a). The mapper, however, should not feel constrained to use only those attributes defined by the above system.

Preliminary delineation of terrain polygons and terrain stability classes, should be completed in the office to guide field work. Further refinement of polygon boundaries and assignment of classes should be made on the basis of field work.

The interpreted geomorphic processes from the terrain maps can be re-interpreted to produce a derived process inventory map. Or alternatively, the standard terrain map can be extended by using additional geomorphic process modifiers and/or relevant feature outline symbols and linear and point symbols, such as landslide headscarps, surface drainage paths, bluffs, and lineaments, to produce a more detailed process inventory map. Such methods are explained in Resources Inventory Committee (1996a) and Schwab (1993). Table 5.5 summarizes the geological process modifiers and subclass modifiers (Howes and Kenk, 1996).

Depending upon the scale of the air photos, the detail of mapping required, the size of the landslides, and the density and height of the forest cover, it may be possible to map the outlines of the landslides and their internal features from air photos.

5.4.8 Other Remote Sensing Data

In the past several decades there has been a dramatic increase in remote sensing technology. Some of this technology has application to terrain stability mapping. Examples are the various types of satellite imagery and radar and include:

• Landsat, SPOT, ERS 1, and ERS 2, which can be used as multispectral scanners and thematic mappers; and
• airborne radar, multispectral scanners and imaging spectrometers, such as synthetic aperture radar.

Depending upon the requirements of the project, these and other remote sensing methods, should be investigated to determine their applicability to the project.

5.4.9 Field Work

Field work is carried out to verify or correct terrain attribute data and polygon boundaries or linear segments determined from map interpretation, air photo interpretation and/or other remote sensing interpretations, and to extend the mapping to beyond the level of detail the above methods provide. As presented in Table 5.3, there are a number of important terrain attributes that cannot be obtained or confirmed accurately without field work.

The intensity of field work varies depending on the terrain survey intensity level (TSIL) as presented in Table 5.1. Field access includes fixed wing aircraft and helicopters, vehicles and on foot. During field work observations are made along the traverse route and at specific observation sites. The amount of data collected can vary depending upon the intensity and purpose of the mapping. It can be collected in hand-written form or on data base forms. The latter method allows for later input into a data base. Table 5.4 summarizes the terrain attributes to be collected for the terrain data base in association with the BC Terrain Classification System (Resources Inventory Committee 1996b).

The field work should be organized so as to direct initial efforts toward more critical areas as determined from the pre-field work, however, it should also be representative of all terrain in the map area. The mapper should take maximum advantage of all available clues, particularly soil or rock exposures in cuts, eroded channels, landslide scars and those provided by windthrown trees. Attention should be directed to signs of incipient landslides. A summary of some of indicators of past and potential slope instability is presented in Table 5.6.

In certain locations and under certain conditions, Global Positioning Systems (GPS) are becoming useful for ground navigation and positioning. Shallow subsurface sampling using available exposures or portable equipment should be carried out as necessary and as the terrain and access will allow.

It is not possible to suggest a 'standard' method for field checking, as this activity has the character of detective work. Some guidelines are provided in Resources Inventory Committee (1996a).

5.4.10 Terrain Stability Class Criteria

Throughout the mapping, the mapper should consider possible criteria for grouping the terrain stability into landslide hazard and/or risk classes. Classes are usually based on a combination of the terrain attributes and the hazard and risk parameters such as probability of occurrence, magnitude, and/or specific risk. To a large extent, the method of selecting a criteria is based on the method of mapping (refer to Sections 4.1 and 4.2). For some methods the selection is objective, while for others it is subjective; for some it is highly systematic and quantitative, while for others it is judgemental and qualitative.

In subjective cases, the criteria depends on the knowledge and experience of the mapper, but the mapper should be guided by all available background data including slope maps, drainage maps, process inventory maps (especially landslide inventory maps), terrain maps, terrain attribute studies and field observations.

In establishing any criteria, it is important that a clear distinction be made between terrain stability class criteria for existing conditions and land use, and the criteria assuming changed conditions and/or land use. Examples of the latter include road construction across a slope, residential development at the top of a slope, timber harvesting of a slope, and reservoir flooding.

Two examples of terrain stability class criteria, one based upon a subjective rating analysis (BC Ministry of Forests 1996a) and the other based upon a probabilistic univariate analysis (Howes 1987), are presented in Tables 5.7 and 5.8.

5.4.11 Map Units

Section 3.5 summarized several different methods of presenting terrain stability data on the associated maps. The grid methods are, in theory, highly objective but allow little opportunity for the use of experience and judgement, and are highly dependent on the reliability of the data -- a handicap in heavily forested terrain.

Polygons which can delineate one or more terrain attributes or hazard or risk parameters are preferred as they allow for the greatest flexibility and the maximum use of the experience of the mapper. The information within a polygon can also be analysed using GIS techniques. At least one of the terrain attributes delineated by the polygons should be slope. The polygon boundaries can be delineated by judgement or by overlaying two or more terrain attribute or terrain parameter maps. The minimum polygon size on the final map should not be less than 1 cm2.

Final polygon boundaries should be such that each polygon can be assigned one unique terrain stability class. If two classes fall within a single polygon, that polygon should identified with the more conservative class. Map scales usually require that small units of one class occur within larger polygons of another class. This should be noted on the map, however, the polygon should be identified with the dominant class.

Linear segments are also a good method to present map units, but are obviously limited to linear geomorphic features, such as shorelines or gullies. A minimum segment length on the final map should not be less than 5 mm.

Feature outline symbols are best suited to larger scale maps to show detailed landslide features that are internal to a landslide. Linear and point symbols indicate features that are important to the terrain stability but are too small to map as polygons or linear segments. Both feature outline symbols and linear and point symbols should complement polygons and linear segments. As much as possible, standard feature outline symbols and linear and point symbols should be used. Suggested symbols and codes are summarized in Resources Inventory Committee (1996a and 1996b). The use of symbols should be limited to avoid cartographic clutter.

5.4.12 Final Terrain Stability Map

Depending upon the type of mapping, the final terrain stability map will be a landslide hazard map or a landslide risk map. Some maps, with detailed legends, may be stand alone documents. All maps, and any subsequent revisions, should be dated, signed and sealed by the mapper and/or the professional responsible for the mapping.

The compilation and presentation of the final map should be similar to the format suggested by Resources Inventory Committee (1996a). Compilation includes transferring the data to the base map, preparation of a map legend and summarizing additional information, and preparation of any accompanying data bases.

The use of digital maps, GIS, accompanying data bases and similar tools is encouraged. These techniques should not, however, be considered as a replacement for established scientific methods, diligence and judgement. Such tools should not be imposed on the mapper. Digital format maps, and those compiled using GIS techniques, should follow the appropriate standards. For details on digital map formats and GIS standards for the BC Terrain Classification System, refer to "Terrain Database Manual" (Resources Inventory Committee 1996b). It is important that digital mapping and GIS do not dictate what information is presented on the map. That is the responsibility of the mapper.

The final map should contain all relevant information, but should not be cluttered or so detailed to make it difficult to interpret and use. In certain circumstances, it may be appropriate to present the information as a series of maps using the same base. The final map should be accompanied by a title block, legend and marginal notes including any limitations to mapping. An example layout of a final map and a list of suggested marginal information is presented in Resources Inventory Committee (1996a). An accompanying index map can serve as a reliability map, and should display location access routes, geographic names, access routes, field traverses and observation/sampling sites.

5.4.13 Report

Most terrain stability mapping projects, and any subsequent revisions, should be accompanied by a report that is dated, signed and sealed by the mapper and/or the professional responsible for the mapping . The following items should be included in the report:

• the terms of reference and who authorized the project;
• the intended purpose of the project;
• the level of effort and detail of study;
• a description of the work performed in mapping and preparing the map;
• a list of references to all background information examined;
• a list of all air photos examined, giving specific dates, scales, flight lines and frame numbers;
• a regional description of physiography, bedrock and surficial geology, vegetation, drainage and geological or geomorphic processes - especially landslides;
• a discussion of the basis of selecting the method of mapping, determining any parameters and any limitations, uncertainties or simplifying assumptions;
• an explanation of the criteria used to develop the terrain stability classes, outlining appropriate assumptions;
• recommendations for follow-up work such as a subsequent level of study or required field checking during site operations or construction.

Examples of other typical report contents are summarized in Resources Inventory Committee (1996a) and BC Ministry of Forests (1995a).

5.5 Professional Responsibility

All terrain stability mapping projects should be carried out under the direction of a professional registered as a member of the Association of Professional Engineers and Geoscientists of the Province of British Columbia (APEGBC), who is qualified by training or experience to engage in this type of work. Junior mappers can carry out this work under close professional supervision.

It is the responsibility of the mapper to prepare terrain stability maps and/or estimates of probabilities of occurrence, magnitude, intensity, elements at risk, vulnerability, consequence, and risk. It is not the responsibility of the mapper to determine the acceptability of the landslide hazards or risks. Such decisions are reserved for those individuals, agencies or authorities, such as landowners, governments or courts who incorporate appropriate socio-economic and environmental factors into their decisions.

Table 5. Terrain Survey Intensity Levels (TSIL)

Table 5.2 Common Slope Classes

Table 5.3 Relevant Terrain Attributes

Table 5.4 Summary of Terrain Attribute Headings on Data Base Form

Table 5.5 Mass Movement and Erosion Modifiers and Subclass Modifiers

(Modified from Howes and Kenk 1996)

Table 5.6 Some Indicators of Past and Potential Slope Instability

Table 5.7 An Example of Terrain Stability Class Criteria, Subjective Rating Analysis

Table 5.8 Example of Terrain Stability Class Criteria, Probabilistic Univariate Analysis