Orographic Precipitation Changes and Shallow Landslide-Derived Sediment in Steep Landscapes

Abstract:

Rainfall-triggered shallow landslides are both a hazard and a major source of sediment in steep landscapes. Extreme precipitation events are expected to change under climate warming, but we have limited understanding of their effect on the relative abundance, size, and spatial distribution of landslides. In particular, changes in orographic precipitation may alter landslide distributions in steep landscapes. Recent theory suggests a significant downwind shift in orographic precipitation under a warming climate, causing larger fractional changes on leeward than on windward slopes. This may prove particularly relevant on the west coast of the USA where mountain ranges are oriented perpendicular to the moisture-laden prevailing winds coming from the Pacific. We analyze precipitation data from regional climate models (NARCCAP) under a high-end emissions scenario (IPCC A2). Preliminary results show that mean winter precipitation over the contiguous United States increases by roughly 2% over both low and high elevation regions in the simulations between the periods 1971-2000 to 2041-2070, but much larger fractional changes (~ triple) are found on leeward slopes. For the same periods, we find that extremes have greater fractional changes in intensity than the mean, and that these changes increase with return period. Similar trends emerge in the West Coast, but with bigger differences between windward and leeward slopes, which increase with return period. Moreover, while leeward increases are large for 30-year events (14%-19%), they are marked even for 2-year events (7-12%). This could significantly impact the frequency and magnitude of shallow landslides in steep landscapes affected by orographic precipitation. We test this hypothesis by applying a new catchment-scale model, which predicts both landslide location and size, to an intensively investigated study site in the Oregon Coast Range, where a ten-year landslide inventory has been mapped onto high-resolution topographic data and soil parameters have been well constrained. We perform numerical experiments under a range of orographic rainfall scenarios to quantify the impact of changes in extreme precipitation on landslide-derived sediment volume on steep slopes, and we compare this behavior with the historical observations available at the site.