Shallow Landslide and Debris Flow Behavior and Linkages in Steep Terrain: Hydrogeomorphic Influences

Our studies in steep channels reveal important processes related to the interconnection of mass wasting processes. Three types of sediment linkages in steep terrain are discussed: (1) landslides that supply sediment directly to channels, which immediately mobilize as debris flows; (2) landslides that deposit sediment in headwater stream junctions, which later mobilize as debris flows; (3) landslides that terminate on hillslopes that later supply sediment to channels via mass wasting and surface erosion. At the Ohya landslide site, frequent debris flows (39 within a 6-yr period) were attributed to rapid sediment supply by dry ravel from steep hillslopes during freeze-thaw. Fluvial processes, important in debris flow initiation in low gradient channels, were minimal and some debris flows occurred during unsaturated conditions, affected by channel gradient. In Miyagawa catchment, percentages of landslides reaching channels varied from 56% to 75% and were correlated with maximum hourly rainfall. The mobility of debris flows was higher during periods with high maximum instantaneous discharge compared to lower discharge, suggesting that water content in initially failed materials and transported sediment controlled their mobility. More than 70% of the landslide sediment that reached channels terminated at hillslope-channel junctions. In catchments > 10 ha, the percentage of stream channel impacted by debris flows declined with increasing catchment size due to gentler gradients. The percentage of landslide volume attributed to roads was highest in 1965 – 1975 when most of the construction occurred; afterwards this declined to 7%. In the managed Sanko forest catchment, the frequency of debris flows originating directly from landslides was greatest 1-5 yr after clearcutting and decreased with increasing forest age. Debris flows that initiated in channel deposits occurred in similar numbers as those that initiated from landslides up through forest stands of 25 yr in age; thereafter, debris flows from landslides dominated.