Decreasing Landslide Erosion on Steeper Slopes in Soil‐Mantled Landscapes

Abstract Slope‐stability models predict that steeper hillslopes require smaller hydrological triggers for shallow landslides to occur due to the added downslope pull of gravity, which should result in more frequent landslides and faster erosion. However, field observations indicate that landslide frequency does not consistently increase on steeper hillslopes. Here, we use measurements of 1,096 soil landslides in California and Switzerland, and a compilation of landslide geometries, to show that steeper hillslopes typically have thinner soils and that thin soils inhibit landslides due to enhanced roles of cohesion and boundary stresses. We find that the landscape‐averaged landslide erosion depth peaks near the threshold slope for instability, and it drops to half that value on hillslopes that are just 5° to 10° steeper. We propose that faster rates of soil creep on steeper slopes cause thin and more stable soils, which in turn reduces landslide erosion, despite the added pull of gravity.