2003 Volume 55 Issue 6 Pages 52-58
On devastated hills in the Tanakami Mountains of Shiga Prefecture, rill formation is considered as an important factor that controls the sediment discharge (Suzuki and Fukushima, 1989). Despite of the importance, the processes of rill system development and runoff generation during the heaviest rainfall have not been clearly evaluated on devastated hills in the Tanakami Mountains, mainly due to the infrequency of rainstorm that produces remarkable water and sediment discharges. Because of the difficulty in the observation during the heaviest rainfall, we carried out the slopescale artificial rainfall experiments on a bare slope in the Tanakami Mountains. The temporal variations of rainfall, runoff, sediment discharge, and pore-water pressure were measured, and the processes of a rill system development were monitored by video tape recording during the artificial rainfalls. The artificial rainfall experiments confirmed that : (1) a highly permeable soil surface contributed to a large amount of rainfall infiltration, leading to the generation of subsurface flow, (2) a shallow soil depth allowed the soil layer to readily become a saturation condition, and a saturation-excess overland flow was partially generated on the slope with a shallow soil layer, (3) sediment flushing due to the concentrations of a subsurface flow and a saturation-excess flow occurred at the bottom of the valley, causing a rill initiation, (4) mass failures along the rill sidewalls and headwalls allowed the rill channel widened and extended upstream, resulting in the generation of small-scale debris flows. As a consequence of the generation of small-scale debris flows following the rill system development ; (5) the sediment concentration at the outlet of the slope was increased by 1 to 3 orders of magnitude, even though the intensities of rainfall and runoff were almost steady. These results suggest that individual soil movement driven by raindrop impacts and Hortonian overland flow is relatively minor, and a massive soil movement due to a subsurface flow and a saturation-excess overland flow is the dominant processes of sediment transport on a bare slope in the Tanakami Mountains.
