Landslide Risk Analysis of Historic Forest Development in the Interior of British Columbia—Challenges Encountered at Fall Creek

In June 1990, the Fall Creek area east of Enderby, B.C., experienced about 66 landslides. These landslides impacted forested slopes and forestry roads, destroyed several BC Hydro transmission towers, damaged and destroyed houses, and cut off highway access to the area, resulting in the evacuation of residents. These slides also destroyed a fi sh farm, several licensed water intakes, and impacted on the Shuswap River (salmon spawning habitat). The total cost of the slide events is estimated to exceed $3 million. The 66 landslides occurred during a 3-day period within 12 small watersheds encompassing 3000 ha. The area has experienced logging activity since the 1940s and contains well over 100 km of active and overgrown roads and trails, many of which are no longer visible on recent aerial photographs. The task of sorting out the contribution of the historic logging activities to the landslide events was described by others as “formidable at best.” The challenges in commencing such a project included (1) determination of the potential costs of the assessment work; (2) estimating if any practicable conclusions could result from such a study, given the vast network of roads and overgrown trails; and (3) conducting the fi eldwork during the narrow work window of the spring freshet (the importance of which became very clear shortly after peak runoff as large streams began to disappear). The fi eldwork used an observational approach, which required continuous modifi cation as the work progressed. The fi eldwork began with mapping the natural and road drainages along road corridors. The fi eld information was plotted daily on a 1:10 000 scale base map. The plotted information revealed inconsistencies in stream volume and the location of fl ows. The drainages between road corridors were then traversed. The “piecing together” of the complex series of drainage diversions eventually resulted in the creation of a site drainage map, which showed the interaction between natural drainage and road/trail structures. By overlaying this map with the landslide locations, a link between many of the 66 landslides and drainage diversions was revealed. It is judged that drainage diversions were a signifi cant contributing factor in most of the landslides that occurred in 1990. The creation of this site drainage map proved to be a valuable tool in evaluating the landslide activity associated with historic logging practices. The fi eld studies also conclude that the Forest Practices Code and current forest practices would likely have prevented many of these slides.