HILLSLOPE BOULDER DISPLACEMENT FROM SHALLOW LANDSLIDES

Hillslope boulder (diameter > 25.6 cm) transport by mass wasting events such as rockfall or landslides can pose a serious hazard to human lives and infrastructure, particularly in the mountainous wildland-urban interface typical of Southern California. In this study, we explore a previously undocumented boulder displacement mechanism mediated by grain-supported entrainment from shallow landslide runouts on steep soil-mantled topography. Previous studies on rockfall in more bedrock-dominated terrain have found that boulder volume is an important control on lateral displacement. However, under a shallow landsliding transport regime (i.e. soil slips), we hypothesize that boulder size would be decoupled from lateral displacement magnitude, which we expect to be more influenced by controls on landslide runout, such as runout slope. To test this hypothesis, we examined spherically weathered, granitic boulders that were displaced in the Box Springs Mountains near Riverside, California by extensive saturation-initiated shallow landslides after a series of long duration, high magnitude rainfall events in 2010. Boulder displacement (n = 30) and size was measured using Google Earth historical imagery from 2009 to 2011 and from 2014 USGS aerial orthoimagery. Runout distances were compared against boulder short axis, long axis, estimated nominal diameter, estimated volume, elevation, local slope, and runout slope. A flow accumulation raster, extracted from USGS 10m digital elevation models (DEMs), helped establish an area-slope relationship to objectively identify transitions from hillslopes to channels. Our results revealed that a boulder’s diameter, ellipsoidal volume, slope, and elevation did not significantly affect displacement distance. However, boulders located at the initial failure zones of the headscarps traveled nearly an order-of-magnitude further than those displaced in the runout zones downslope in the channel and hillslope domains. These results reveal an effective transport mode where boulders located in potential landslide initiation zones are the most likely to travel furthest, a potentially important aspect of landslide risk assessment.