Hydrogeomorphic processes in a steep debris flow initiation zone

Many debris flows initiate in steep channels (>20 degrees), yet studies have focused on lower-gradient streams where failure is controlled by water height above channel deposits. Multiple debris flows in a steep channel in Ohya landslide, central Japan, were linked to infilling processes (i.e., freeze-thaw and dry ravel) and partial saturation of accumulated sediment. Because of very active geomorphic processes in this catchment, 39 debris flows were observed in the past 6 yr. Pre- and post event imagery indicates selective transport of finer materials during most debris flows that comprise saturated and unsaturated flows; however, fluvial sediment processes, important in relation to debris flow initiation in gentler channels, were minimal. Critical conditions for the movement of such unsaturated materials are derived by equating shear stress with solid friction of the channel bed.