LAUSR

Abstract Previous research in southwest British Columbia (BC) suggested that projected climate change would result in in increases in the frequency of debris flows on the order of a 10% by the end of the century compared to the last approximately 30 years (Jakob & Lambert, 2009). Advances in climate modeling now allow higher resolution simulation of precipitation compared to the work completed in 2009. Regional climate models have become better able to resolve convective activity (i.e., thunderstorms) at a finer temporal scale. This paper quantifies and projects changes in the frequency and magnitude of shallow landslides and resulting debris flows because of the impacts of climate change for the North Shore Mountains of Vancouver, BC. Analysis of a landslide database curated by Metro Vancouver shows a doubling in the frequency of landslides since 1981, prior to which recording was sporadic. How much of the increase since 1981 is attributable to further recording improvements cannot be quantified which implies that it is too early to attribute frequency changes unambiguously to climate change. Prior to 2004, the average volume of shallow landslides was 1800 m3, whereas after 2004, this volume increased 7-fold to 13,000 m3. In particular, landslides with volumes exceeding 10,000 m3 appear to have increased abruptly since in the early 2000s. Using a Poisson model, we project an increase of up to 300% in the frequency of shallow landslides until the end of this century. A multivariate regression analysis suggests that this increase in landslide frequency will be accompanied by a potential increase in the average landslide volume of up to 50% for Relative Concentration Path (RCP) 8.5. Such changes may challenge watershed management attributable to increases in sediment transport rates, widening channels and in some cases may transition from single thread to braiding or anastomosing. The likelihood of landslides impacting and impounding water courses is projected to increase. Turbidity in reservoirs is likely to increase because of the influx of fine-grained sediment. Outside of the Vancouver watersheds that are closed to the public, infrastructures will increasingly be tested by more frequent and sometimes higher magnitude landslides.