[Mapping and Assessing Terrain Stability Guidebook Table of Contents]

Terrain and terrain stability mapping

Terrain mapping is a method to categorize, describe and delineate characteristics and attributes of surficial materials, landforms, and geological processes within the natural landscape. Terrain stability mapping is a method to delineate areas of slope stability with respect to stable, potentially unstable, and unstable terrain within a particular landscape. Terrain stability map polygons indicate areas or zones of initiation of slope failure.

Both methods are undertaken initially by stereoscopic interpretation of aerial photographs (supplemented with field-checking), and therefore require the mapper to have advanced skills in recognising and interpreting terrain and natural slope processes from both aerial photos and fieldwork. Terrain stability mapping is a derivative of terrain mapping by utilizing the terrain and map polygon attributes of the terrain mapping.

Terrain survey intensity levels

There are five terrain survey intensity levels (TSIL) used for terrain and terrain stability mapping in British Columbia (Table 1). The survey intensity levels represent the extent of field-checking done during mapping, expressed as a scale ranging from A (most checks) to E (least checks). Each level is a measure of the reliability of the mapping. It does not refer to a type of mapping or a map scale.

The ranges used in Table 1 to describe the level of effort required for a given TSIL are provided to help account for the range of mapper experience, terrain complexity, and access difficulties (dense bush, severe topography, limited helicopter landing sites). For example, for TSIL C on relatively gentle plateau terrain with good road access, an experienced terrain mapper may need to ground-check only 20% of the polygons and be able to achieve a high rate of daily progress (e.g., 1200 ha/day). An inexperienced terrain mapper, however, may need to ground-check a significantly higher number of polygons (30-35%)-as well as have an experienced mapper review the results-to achieve a comparable level of accuracy. In areas of difficult or complex terrain, even an experienced mapper may need to ground-check a higher percentage of the terrain units and will have difficulty achieving a progress rate of 600-700 ha/day. Clearly, then, mapper experience, terrain complexity and access difficulties must be accounted for when work plans and budgets are being developed and reviewed for terrain and terrain stability mapping projects.

During ground checks, all terrain attributes relevant to the entire polygon being mapped must be investigated and described. Collecting detailed site data for one point in a polygon rarely qualifies as appropriate mapping procedure. Emphasis should be on walking across polygons and checking their boundaries. Vehicle traverses of existing road networks and low-level helicopter inspections can be used to supplement-not replace-information obtained from foot traverses.

Table 1. Terrain survey intensity levels (TSIL)a for terrain and terrain stability mapping

TSIL

Preferred map scale

Estimated rangeof average polygon sizes (ha)

% of polygons ground-checked

Method of field-checking

Rate of field progress per crew day (ha)

A

1:5000 to 1:10 000

2-5
5-10

75-100

Ground checks by foot traverses

20-100

B

1:10 000 to 1:20 000

5-10
10-15

50-75

Ground checks by foot traverses

100-600

C

1:20 000 to 1:50 000

15-20
50-200

20-50

Ground checks by foot traverses, supported by vehicle and/or flying

500-1200

D

1:20 000 to 1:50 000

20-30
100-400

1-20

Vehicle and flying with selected ground observations

1500-5000

E

1:20 000 to 1:100 000

20-40
200-600

0

No field work, only photo interpretation

n/a

a Modified from "Guidelines and Standards for Terrain Mapping in British Columbia" (Resources Inventory Committee 1995). For forestry applications, typical map scales are: TSIL A, 1:5000; TSILs B and C, 1:20 000; and TSIL D, 1:20 000 or 1:50 000. This table does not apply to terrain stability field assessments. The field investigation and data collection requirements for terrain stability field assessments are structured differently from those for terrain and terrain stability mapping (see section on terrain stability field assessments).

Mapping

It is expected that the person who does the stereoscopic air-photo interpretation work will carry out the field-checking. It is not acceptable to carry out stereoscopic air-photo interpretation in the office and then send less experienced staff out to collect field data. The nature of terrain and terrain stability mapping demands that the mapper walk the ground.

Reconnaissance and detailed mapping

The decision whether to carry out detailed mapping or reconnaissance mapping is based on a combination of factors.

Reconnaissance mapping for the identification of unstable or potentially unstable terrain is most suitable for plateau areas where there are only local occurrences of potentially unstable terrain. In this case, it is not cost-effective to terrain map large areas of stable terrain. The second situation for reconnaissance is where extensive areas of steep terrain need to be mapped in a short time period and as economically as possible. In this instance, reconnaissance mapping can be used to identify areas that have potential hazards. Areas for which the consequence of landslides is high (e.g., fish streams) can also be scheduled for subsequent detailed terrain mapping.

Detailed mapping is most appropriate for areas that have a large proportion of steep, landslide-susceptible terrain, as well as significant resources that might be affected by landslides. Detailed terrain mapping is required to be completed in all existing community watersheds by June 15, 2000.

Development of criteria for terrain stability classes

The criteria used to separate terrain stability classes are usually defined in terms of slope gradient, surficial materials, material texture, material thickness, slope morphology, moisture conditions and ongoing geomorphic processes. Because of regional variations in climate, geology, soils and other factors, few specific criteria apply universally across all regions of the province.

The mapper must develop criteria for terrain stability classes specific to the map area. These criteria and the rationale used to develop them must be documented in the report or legend accompanying the terrain stability maps. The terrain stability class criteria should be applied systematically to all map polygons in a given study area. Any exceptional conditions that change assigned terrain stability classes should be noted in the list of criteria.

The criteria for terrain stability classes are typically qualitative and depend on the knowledge and experience of the terrain mapper. The criteria for terrain stability classes in reconnaissance terrain stability mapping are far less rigorous than in detailed terrain stability mapping and by necessity must be based on factors that can be determined primarily from air-photo interpretation. Polygons containing naturally occurring landslides should be categorised as reconnaissance stability class "U" (Table 2) on reconnaissance terrain stability maps and terrain stability class "V" on more detailed terrain stability maps (Table 3). An example of detailed terrain stability class criteria are provided in Appendix 1 (Table 1B).

For detailed terrain stability mapping projects, it is essential that the mapper investigate areas within the project area (or similar areas nearby) that have been logged, and areas where roads have been built. The mapper should document and report the types of terrain that typically experience landslides related to timber harvesting or road construction. Certain types of terrain will be more prone to failure than others. For example, steep, gullied terrain often experiences higher rates of post-timber harvesting landslide activity than benchy, irregular terrain. The mapper should use this information to develop criteria for ranking the different types of terrain in the map area for the expected likelihood or frequency of development-related landslide activity.

A highly systematic approach to collecting this type of data, termed a terrain attribute study, has been carried out in several areas in the province (Rollerson et al. 1997). A tentative terrain stability mapping methodology has also been developed, using extensive landslide inventory data from the Kamloops Forest Region (Pack 1994). As well, several exploratory approaches exist which use digital elevation models to derive slope and catchment characteristics for terrain stability interpretations (Pack et al. 1998). Studies of this type will be used more in the future for developing terrain stability criteria and maps.

A number of approaches for selecting qualitative or quantitative criteria for terrain stability maps have been developed in various parts of the world. These range from simple slope maps and landslide inventories to complex statistical analyses. A summary describing the most common of these approaches is included in "Terrain Stability Mapping in British Columbia" (Resource Inventory Committee, 1996b). Because terrain stability mapping is still evolving in British Columbia, the development and application of innovative and more quantitative approaches is encouraged.

A final note: The mapper must ensure that the terrain stability criteria and interpretations are not overly cautious. Such interpretations can lead to unnecessary terrain stability field assessments, increased logging and road construction costs and unnecessary prohibitions on some forest practices.


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