LANDSLIDES AND DEBRIS FLOWS IN STEEP TERRAIN: HYDROGEOMORPHIC PROCESSES AND LINKAGES

Landslides and debris flows triggered by rain storms supply much of the natural sediment to streams and rivers in mountainous terrain. The temporal connectivity of hillslope landslides with debris flows in steep headwater channels is an important and poorly understood phenomenon. Many models and hazard assessments assume that landslides either mobilize into debris flows or directly impact headwater channels where they convert into debris flows. Studies based on sequential aerial photographs and field measurements in three steep, unstable catchments in Japan indicate that less than 20% of the landslides directly mobilize into debris flows in headwater channels, with larger landslides more readily converting to debris flows. Debris flow initiation in such channels is highly dependent on the dominant geomorphic processes that supply sediment, channel characteristics (e.g., gradient, tributary junctions), and the timing of rain storms which mobilize the sediment. Between 40 to 52% of the landslide sediment in the Sanko and Miyagawa catchments was stored in channels where it was available for later mobilization as a debris flow or bedload transport. In the steep Ohya catchment in the southern Japanese Alps, hillslope geomorphic processes were dominated by ravel and rockfall, especially during active freeze-thaw periods. Due to this rapid infilling of the headwater channel, as many as five debris flows occurred within a year. At Miyagawa, the volume of landslides that proceeded down the main channel as debris flows decreased markedly for catchment areas greater than a few hectares. As such, understanding the interrelationships amongst geomorphic infilling of channels, channel and basin characteristics, and the initiation mechanisms of debris flows is critical for improved sediment disaster predictions.