Effects of terrain on the occurrence of debris flows after forest harvesting

Forest harvesting and subsequent forest regeneration represent widespread changes in land cover in mountain regions. Although impacts of forest harvesting on landslide initiation has been widely reported, the effects of forest harvesting on the occurrence of debris flows remains unclear. We propose that forest harvesting will differentially affect the susceptibility of debris flows amongst catchments with different terrain characteristics. This hypothesis was assessed in the Sanko catchment, Japan, where comprehensive forest harvest records date back to 1913. The frequency of debris flows directly originating from landslides occurred in similar timeframes as the occurrence of landslides. Landslides that reached channels and continued downstream as debris flows were more prevalent in steep channel reaches with small hillslope-channel junction angles. In addition to the increase in the frequency of landslides, especially within 10 years after forest harvesting, debris flows caused by mass movement of channel deposits in steep reaches increased during this period. These relationships between the occurrence of debris flows and channel topography indicate a high susceptibility of debris flow occurrence after forest harvesting in first and zero-order mountain streams. Sediment previously routed into channel networks by landslides is likely a more important factor for in-channel debris flow initiation in the lower channel reaches, while instability of areas proximate to riparian zones, including stream banks and geomorphic hollows, possibly accelerate occurrence of debris flows in upper channel reaches. Consequently, catchment topography should be considered in evaluating debris flow risk after forest harvesting.