Urban landslides in the vicinity of Vancouver, British Columbia, with special reference to the December 1979 rainstorm

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Abstract
Historical landslides in the urbanized Vancouver region, southwestern British Columbia, have almost commonly occurred along escarpments within and at the margins of gently rolling upland surfaces underlain by Pleistocene unconsolidated sediments. The most common and most destructive landslides are debris avalanches and debris flows. They are triggered by intense autumn and winter rainstorms, when water infiltrates and saturates the surficial layer of weathered colluvium. After failure the veneer of debris gains momentum and picks up additional soil and uprooted vegetation. Debris avalanches may temporarily block gullies swollen with runoff water, thus changing into rapidly moving debris flows.A severe rainstorm in December 1979 was accompanied by destructive debris avalanches and debris flows in urban areas in the vicinity of Vancouver. A search of local newspapers and meteorological records back to 1900 indicates that this event was not unique, for at least 26 other comparable storms have triggered landslides in the Vancouver region during this century. Thus it is likely that landslides similar to those of December 1979 will occur repeatedly in the future. The danger of such landslides to life and property will grow if potentially hazardous sites are urbanized without appropriate protective measures.

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