Challenges with Terrain Stability Mapping in Northern British Columbia Marten Geertsema and James W. Schwab Introduction I n British Columbia, the basis for most landslide hazard mapping is the terrain stability map (TSM), which combines surficial geology with modifying geomorphic processes (Howes and Kenk 1988, 1997; Resources Inventory Committee 1996). A system of TSM has been developed for the forest sector to identify a polygon-based classification of the likelihood of landslide occurrence following timber harvesting and (or) road building (BC Ministry of Forests 1999). In addition to mapping, forest licens- ees often undertake a terrain stability field assessment (TSFA) in potentially unstable areas. The TSFA is used to assess landslide potential for proposed timber harvesting or road construc- tion. The Forest and Range Practices Act (FRPA; BC Ministry of Forests 2004) states that primary forest activi- ties shall not cause landslides. However, neither TSMs nor on-site studies (TSFAs) that are required as follow-up for areas mapped as unsta- ble or potentially unstable include the potential of upslope hazards normally considered—only the likelihood of ini- tiation of a forestry-related landslide. Sometimes, separate assessments indi- cate the likelihood of sediment entering a stream but hazards in the run-out zone are not usually fully identified. Except for road safety con- siderations, mapping and assessments used in the forest sector were not meant to assess the suitability of sites for infrastructure. In contrast, land- slide hazard mapping is applied to land use zonation and residential development (Hungr et al. 1984; APEGBC 2006). Recently, the Association of Profes- sional Engineers and Geoscientists of British Columbia has adopted guide- lines for terrain stability professionals in the forest sector (APEGBC 2003). Objectives of assessments outlined in section 4.2(a) of the guidelines are “to characterize the existing landslide haz- ards (terrain and terrain stability conditions) in areas within, adjacent to or connected to the Development area.” This objective goes beyond the mandate of simply assessing the landslide hazard of forest development. Terrain stability mapping and TSFAs are generally done in remote regions where little previous work has been done and where there is no knowl- edge of subsurface conditions. The work is therefore subjective, based on air-photo interpretation, on-site obser- vations, and experience of the mapper. Terrain scientists in BC have developed considerable skill and expertise in this field. The mapping was developed primarily for debris slides, debris flows, and gully pro- cesses—relatively simple mass movements. However, sometimes more complex landslides occur. The purpose of this paper is to intro- duce geological conditions common in northern BC that require special considerations in terrain stability map- ping and assessments, where standard forestry-related TSM may not be suffi- cient or is not the appropriate tool for assessing the overall slope stability hazard. More importantly, below-ground (not visible) and off-site conditions should be considered for assessing hazards for large and (or) complex landslides. Slope hazards generally not considered in forestry application are discussed in this arti- cle. Topics include situations where rock slides trigger larger movements in soil, glaciomarine sediments, permafrost areas, and preglacial buried valleys. Background Terrain mapping British Columbia adopted a terrain classification system (ELUCS 1976), based on a system introduced by Fulton et al. (1979). The classification 18 Streamline Watershed Management Bulletin Vol. 10/No. 1 Fall 2006 Table 1. Subclasses for mass movement processes a Subclass name Map symbol Initiation zone “ Soil creep c Rock creep g Tension cracks k Rock spread p Soil spread j Debris fall f Rock fall b Debris flow d Debris torrent t Earthflow e Rock slump m Soil slump u Slump–earthflow x Debris slide s Rock slide r a Table modified from Howes and Kenk (1997).