Jeff Prancevic

The influence of hillslope angle on landslide initiation and the occurrence of threshold hillslopes

The concept of threshold hillslopes has been widely applied to both soil-mantled and bedrock landscapes, postulating that once hillslopes achieve the critical topographic gradient where landslides dominate erosion they cannot steepen further.  This process transition has been used to explain observations of ubiquitous modal hillslopes that cluster around values of 35° to 40° in steep landscapes globally, despite wide variability of climate, erosion rate, and soil characteristics.  However, there is no clear explanation for why ~35° is the special slope towards which landscapes adjust.  Landslides start occurring at slopes that are much lower (~20°), and the angle of repose of weathered bedrock is generally much steeper (45°+).  In this study we use a series of laboratory landslide experiments and observations of landslide scars to examine how hillslope gradient affects the occurrence of landslides. Surprisingly, we find that similar runoff rates are required to initiate landslides at all hillslope gradients between 25° and 45°.  The ubiquitous modal hillslope gradient of 35° sits in the middle of this range of gradients that are similarly susceptible to landslide initiation.  Because hillslope gradients tend to be normally distributed, a modal hillslope gradient of 35° places the greatest portion of the landscape within the landslide regime.  If landslides, specifically those caused by large rainfall events, are the most efficient mode of erosion, then a distribution of hillslope gradients that sits in the middle of this process regime should generate the fastest possible erosion rates.