Linkage of sediment supply and transport processes in Miyagawa Dam catchment, Japan

[1] Linkages between sediment supply by episodic geomorphic processes ( obtained from aerial photographs and field surveys) and sediment transport ( estimated from changes in sediment deposition behind a large dam) were investigated in the Miyagawa Dam catchment, central Japan. A total of 6667 landslides were confirmed in the period from 1965 to 2000 on the basis of seven temporal pairs of aerial photographs. Both the occurrence of landslides and discharge into the dam lake affect sediment yield, indicating that fluvial systems in Miyagawa Dam catchment are supply limited with respect to sediment. Sediment yields are affected by not only the initial failed volume of landslides but also the mobility of landslides and debris flows. In Miyagawa Dam catchment, percentages of landslides reaching channels varied from 56% in 1997 - 2000 to 75% in 1976 - 1981 and were correlated with maximum hourly rainfall. In addition, the mobility of debris flows was higher during periods with high maximum instantaneous discharge compared to lower discharge, suggesting that the water content both in initially failed materials and transported sediment controlled their mobility. Topography also affected the mobility of landslides/debris flows. For catchments > 0.1 km(2), the percentage of channel network length impacted by debris flows decreased with increasing catchment area due to reduced channel gradient. Thus both the magnitude of rainfall-runoff events and the catchment topography affect how landslide sediment contributes to sediment yield at the large catchment scale.