An examination of controls on debris flow mobility: Evidence from coastal British Columbia

We characterize and consider controls to runout distance of debris slides and debris flows using 1700 field observations supplemented by air photograph interpretation from coastal British Columbia. We examine the role of slope on deposition and scour and determine that they occur on steeper and flatter slopes (respectively) than previously reported. Mean net deposition occurred on slopes between 18° and 24° for open slope failures and between 12° and 15° for channelized debris flows. We demonstrate a practical method for estimating both entrainment and runout in the field and in a GIS and provide an example entrainment map for Vancouver Island, British Columbia. We consider other controls to post failure landslide mobility, including the role of gullies and stream channels, roads and benches, and intact forests. Shallow landslides that hit streams and gullies at acute angles had a high probability of transforming into a channelized debris flow, whereas landslides that hit streams and gullies at obtuse angles did not. Forests played a substantial role in landslide runout: debris flows travelling through a logged slope deposited much of their load when hitting a forest boundary and stopped entirely within 50 m of that boundary in 72% of the cases examined. Roads also tended to stop or reduce the size of open slope debris flows in 52% of the cases. The results are expected to be useful to land management applications in regions with frequent shallow landsliding.