Landslide Susceptibility from Watershed and Fan Characteristics, Salmon Arm and Vernon Forest Districts

Landslide occurrence in the Salmon Arm and Vernon forest districts in the last five years has stimulated research to determine how small drainage basins might be classified into debris flow prone, debris flood prone, or flood prone. Alluvial fans situated at the base of several entrenched stream systems may have people dwelling on them. In addition to safety concerns, water quality and habitat are also of importance. A procedure is presented here that uses both map-based (GIS) and field-based (fan and channel mapping) analysis to show relative risk of impact on fans. Bedrock and surficial geology are used to type the basins that are dominated by metamorphic rocks. Basin area, aspect, slope, inner gorge, and gentle-over-steep conditions were determined. GIS-generated channel longitudinal profiles were compared between the data sets. Fan morphology and stratigraphy aided in identifying previous debris flow events and their magnitude. Historical information provided by time-series air photo interpretation, historic maps, and archival research provides clues to recurrence intervals. Some fans will have relict paraglacial deposits that include debris flows and may still support future debris flows. Last, the methods proposed here can be used to signal special concerns for forest development. The use of this template may find application in risk assessment outside the selected forest districts.