Katsushige Shiraki

Numerical experiments of watershed-scale soil water movement and bedrock infiltration using a physical three-dimensional simulation model

Numerical experiments of soil water movement and bedrock infiltration based on a simplified simulation method were conducted to analyze watershed-scale rainfall-runoff processes. To verify the model accuracy, it was applied to a Minamitani watershed (0.45 ha). The simulation was performed with 2.5-m space grids horizontally and five cells vertically. Results of long-term calculation of this model proved that this simulation model is robust and demonstrated good computational water mass conservation. Calculation results showed the best agreement with observed hydrographs and the number of groundwater levels simultaneously when laboratory-tested soil hydraulic characteristics for topsoil were used and infiltration into bedrock was included in numerical calculations. Numerical experiments show that bedrock infiltration generated a stable base flow and suppressed the secondary discharge peak. The reproducibility achieved by observed soil hydraulic characteristics with the assumption of bedrock flow demonstrates the effectiveness of the simulation model used in this article for analyses of watershed-scale soil water movements.

Small Flume Experiment on the Influence of Inflow Angle and Stream Gradient on Landslide-Triggered Debris Flow Sediment Movement

Rainfall-induced landslide might transformed into more severe disaster, namely debris flow and natural dam which both holds serious threats on human life and material. The runout distance has crucial role for determining affected areas of a landslide. Our previous research found the correlation of inflow angle and stream gradient to transformation of landslide collapsed sediment either into natural dam or debris flow. This research intended to test our previous research result with a small flume experiment and aimed to analyze the influence of sediment inflow angle and stream gradient to the sediment deposition percentages as representative of runout distance and the possibility of natural dam formation. Soil samples were taken from landslide-triggered debris flow disaster initiation zone in Hiroshima (Hiroshima Pref.) and Izu Oshima (Tokyo Pref.), Japan which were induced by heavy rainfall. The small flume was 10 cm width and 15 cm height, the inflow segment angle was varied to 60° and 90°, and the stream segment gradient was varied to 10° and 15°. From the experiment results, stream gradient influence the sediment movement effectively rather than inflow angle, and it was sufficient to examine the possibility of collapsed sediment to form natural dam or debris flow. Soil samples from natural dam initiation zones and consideration of water content factor are essential for further experiment.