Landslide Legacies: The cascading impacts of landslides on hazards and landscape evolution

Abstract

In this dissertation I explore the cascading impacts of landslides on both hazards and landscape evolution. For hazards, one of the worst case impacts of a landslide is that it dams a river and leads to a catastrophic outburst flood. In Chapter 1, I develop a new method for estimating where this may occur, called the damability function, validate it in the Oregon Coast Range, and generate the first landslide dam hazard assessment for anywhere in the United States. I find that high landslide dam susceptibility is widespread, so this hazard should be considered in the Pacific Northwest. I also find that in only a handful of large rivers would landslide dams possibly lead to large catastrophic floods. For landscape evolution, landslides work in tandem with downhill soil creep to move sediment from hillslope to rivers as part of the geomorphic landscape system, but when and how shallow landslides fit into this system remain unclear, despite their abundance. In Chapter 2, I develop a new model to simulate shallow landslides that only displace the surface soil layer, called SoilLandslider, that is capable of running in simulations that are long enough for landscapes to reach steady-state. In Chapter 3, I add the SoilLandslider module to sets of numerical experiments that explore what landscape conditions favor persistent shallow landsliding. I find that at low slopes the amount of shallow landsliding is limited by the slope, and the emergent slope angles of the model are set by the ratio between uplift and erosional efficiency of soil creep and fluvial erosion. For models with high slopes landslide erosion is limited by the soil depth, and soil depth is determined by the ratio between uplift and soil production. The ratio between uplift and soil production determines whether a landscape is in a slope-limited landslide domain, or a soil-limited landslide domain. Through landscape evolution modelling I further establish the importance of soil landsliding to landscape evolution, and the importance of soil depth on moderating soil mantled landscapes.