Eruptions at snow covered volcano enhance very intensive snow melt and can trigger mudflow. Although such mudflow sometimes causes severe damage and loss in downstream areas, triggering mechanism of this type of mudflow is not understood yet. In the present study, a fundamental experiment is conducted to examine the processes of snow melting and water infiltration to the snow layer using experimental column and hot sediment (500℃) supplying equipment. The snowmelt and infiltration processes in the experiment were the follows ; 1) hot sediments melt the snow layer, 2) the water infiltrates in snow layer, 3) water table is formed on the bottom of the column, and 4) hydrograph (melting water discharge from the column bottom) shows sharp peak at the early stage of the water discharge. To simulate the melting water discharge, simple mathematical models for heat conductivity between sediment and snow, snow melt and deformation of snow layer, and water infiltration in the snow layer were developed. The model reproduced the observed water discharge due to snow melt well. The model can be applicable for the prediction of mudflow behavior due to the volcanic eruption and snow melt to mitigate the sediment related disasters.
A large landslide dam at Kuridaira, in Totsukawa Village, Nara Prefecture, which resulted from a deep-seated landslide following torrential rain brought on by Typhoon No. 12, had a 100-meter head with a 600-meter long downstream slope,creating a 1 in 5 slope that made it highly vulnerable to erosion by water overflowing the landslide dam reservoir. An issue was what degree of inclination was suitable for the slope to prevent severe erosion. Therefore, we modeled a onedimensional riverbed fluctuation calculation, which indicated that a 1 in 12 inclination would be suitable downstream of the landslide dam to avoid severe erosion. In order to realize such an inclination, we intended to reduce the overflow elevation of the landslide dam, install a sabo dam at the foot of the landslide dam, and reclaim the space above the sabo dam to install a series of groundsels on the slope. The effectiveness of this plan was verified by means of hydraulic experiments. This clarified that the reclaimed portion would be severely eroded, so a channel with protective sides and bottom on the reclaimed portion was added to the planned countermeasures.
Debris flow causes flooding and sediment deposition when they reach debris fan area. Usually, urban development in Japan results in the construction of many houses on debris fan area, and that affects the flow and deposition during flooding. However, few previous studies have considered the effect of houses to disasters associated with debris flow. This study conducted model experiments to determine the influence of houses on debris flow flooding and deposition. We applied uniform sediment, and also applied with coarse grained sediment. We conducted cases without houses ; with houses, with houses and fences ; and with houses can be destroyed. The model experiments showed that when houses are present, the debris flow spreads widely in the cross direction immediately upstream of the houses. Houses located in the debris fan also influence the deposition area. The presence of houses led to flooding and deposition damage in some places and reduced the damage in others. Especially when fences exist around the houses, flow moved down between the fences ; as in the real disaster cases flow moved down towards the roads. When houses destroyed, flow moved down through the destroyed houses ; and changes the flooding and deposition process comparing with non-destroyed houses case. With debris flow containing coarse grained sediment, when houses exist, most of the coarse sediment deposited upstream area of the houses.
It is known that large-scale landslide dam is often formed by landslide due to heavy rain or strong earthquake. On the other hand, landslide dam formation caused by sediment runoff is not known well. Especially sediment transportation from head stream of the mountain is unclear, and it is not certain that landslide dam can be formed in this area. In upstream area, large sediment may flow down even flow depth is small. In that case, it is not clear that how sediment moves ; it may be because of fluid force pushing the sediment or it may be the sediment movement. Same thing can be mentioned when sediment stops moving. Furthermore, flow through the stopped large sediment may cause collapse due to soil infiltration, or sediment may flow down by erosion due to surface flow ; the phenomena is quite complicated. Therefore, we constructed large scale model experiment facilities for Inari-gawa basin, and conducted experiments. Results showed that large-scale landslide dam can be formed due to sediment runoff from the upstream area. However, it cannot occurs in every stream, it appeared because Inari-gawa basin has characteristics of landform (e.g. curve, confluence, waterfall, and large rocks in the river channel) and sediment conditions (e.g. wide range distribution of sediment, 2 m diameter rocks can be included). Landslide dam formed in confluence point collapse scale changed when runoff time is different from the both branches.
The study of landslide dam deformation is important for predicting potential floods, and in the implementation of effective flood-risk management. To understand the deformation processes of landslide dams, and the outflow discharge characteristics under conditions similar to those of an actual landslide dam, we carried out experiments on a landslide dam in the mountainous streambed of the Ashiarai Basin. The results of previous field experiments suggest that the dam height, reservoir volume, and deformation processes influence the outflow discharge ; however, the effects of the scale of the dam in previous experiments were unclear, and the scale of the dam is expected to be significant. For these reasons, our experiments, which were performed in July 2014, were carried out using a larger-scale dam. We carried out preliminary small-scale experiments to identify potential problems prior to carrying out the large-scale experiments. Here we report the results of the preliminary experiments and the large-scale field experiments. In the field experiments,pipes were laid on the riverbed to drain water from the dam reservoir, and a dam was constructed under dry conditions. During the preliminary experiments, a large quantity of water was drained via infiltration around the pipes, and water was not stored. To address this, we stopped the infiltration of water around the pipes using sandbags and putty. In the large-scale field experiments, dam deformation resulted from erosion due to overtopping. We observed dam deformation processes, outflow discharge, and the water level inside the dam. The resulting measurements of the water level inside the dam showed that part of the dam was unsaturated during deformation. The results of the deformation and outflow discharge processes show that the erosion width increased with increasing outflow discharge. The results of the outflow discharge were compared with those of previous experiments.
In debris flows, larger particles are characteristically concentrated at the front of the flow. To elucidate the mechanics of this, we carried out flume experiments with sediment mixtures, where the flow length, bed roughness and angle of the flume were varied. The experiments showed that 1) Larger particles concentrated more at the front of debris flows with a longer flow length and rougher bed. And the angle of flumes which larger particles concentrated the most at the front depends on a flow length. 2) In the flow direction, the velocities of smaller particles in the front of debris flows were larger than that of larger particles, but in the depth direction, larger and smaller particles had similar downward movement and smaller particles had more downward movement than larger particles in some cases. 3) During the flow down, the proportions of particles with larger diameters increased in the front of debris flows, whereas the proportions of particles with smaller diameters decreased. Based on these observations, we postulated that the concentration of larger particles at the front of debris flows is affected in the following manner. 1) First, in the front of debris flows, smaller particles drop toward the lower layer through the spaces between the particles in the front, whereas larger particles cannot drop out because their diameters are greater than the spaces. 2) The smaller particles that drop toward the lower layer have slower velocities in the flow direction than the larger particles, and subsequently move behind the front of the debris flows. 3) Larger particles remain at the front of the debris flow, and smaller particles are removed. 4) Because the proportions of the larger particles at the front increase the spaces size between these particles increase. Consequently, it is possible for larger particles to drop toward the lower layer. By looping through steps 1) to 4), larger particles become concentrated at the front of the debris flows. The mechanics of above show that the concentration of larger particles at the front is mainly caused by decreasing of the proportions of smaller particles because of dropping.