In recent years there has been an increasing concern about the wood in debris flows and floodwaters, which results in extensive destruction of properties and infrastructures along river courses. In order to prevent the damage, appropriate installment of measures to control the outflow of the wood is necessary, with a reliable budget of coarse woody debris underpinned by solid understanding of its behavior. This study presents the budget for the Tottabetsu River catchment (153.4 km²) in Eastern Hokkaido for a record high rainfall brought by Typhoon No. 10 in August 2016. There were no control measures for woody debris in the river before the event. The budget was estimated by the combination of field investigation, an analysis of airborne LiDAR data obtained before and after the event, and an interpretation of a series of aerial photograph images. The storm intensively eroded riverbeds and banks, producing a large amount of woody debris. The wood, some of which remained jammed or captured in sediment deposits along the course after the event, was originated from both hillslopes and riparian forests. The outflow was estimated at 58,000 m³ from the Tottabetsu River in total, which was 70% of the inflow into the course. Dividing the catchment into units based on stream order, the proportion of the outflow of the wood ranged from 0.90 to 1.00 for the mountainous and from 0.17 to 0.86 for the alluvial parts. The outflow was positively correlated with catchment sediment yield for each unit. The outcomes in this study will help plan the control measures of coarse woody debris through the channel network, not only for the Tottabetsu River but also elsewhere with a range of catchment sizes.

By the past studies, tree root system has effects of preventing shallow landslides. In this study, pull-out test for tree root was performed at constant speed, and changes with time of pull-out resistance of tree roots was measured. Pull-out test was carried out for the purpose of grasping effects of tree roots on shallow landslides appropriately. As a result, it became clear that the maximum value of tensile resistance increased and the time to show the maximum value was delayed, in proportion to the diameter of the root. Based on these results, the model considering changes with time of resistance to landslides was created. Comparing model values with measured values, it was confirmed that this model successfully reproduced measured results. In conclusion, compared with the conventional model, it has become possible to greatly correct effects of tree roots on preventing shallow landslides.

Driftwood disasters, which often occur at the same time as sediment-related disasters, have been widely reported in recent years. To prevent and mitigate driftwood disasters effectively, we developed a uniquely shaped driftwood trap that differs from typical driftwood countermeasures. The driftwood trap proposed in this study consists of horizontal rods aligned in the flow direction. When driftwood reaches the trap, it is efficiently scooped up by the horizontal rods and separated from the water and sediment. The driftwood trapping mechanism was verified by flume experiments. River channels are generally distinguished into debris flow and bedload sections by the type of sediment transport, which changes depending on the slope of the riverbed. Channels with slopes greater than two degrees are defined as debris flow sections, while channels with slopes less than two degrees are defined as bedload sections. We expect that this driftwood trap is effective for both debris flow and bedload sections but, in this study, we focused on a flume slope of five degrees to verify the trapping function in debris flow sections. In the flume experiment, water was circulated at a constant flow discharge, driftwood models were supplied into the flow at various conditions, and the trapping ratio of the driftwood trap was measured. In the analysis of the experimental results, we focused on the relative length of the driftwood models with respect to the horizontal rod spacing, the number and density of the supplied driftwood models, and the surface roughness of the horizontal rods. We found that the longer the relative length of the driftwood model, the higher the trapping ratio. The trapping ratio was also high when the number or the supply rate of the driftwood models was high. With straight driftwood models, the trapping ratio did not change with the surface roughness of the horizontal rods. However, in the case of non-uniform driftwood models using natural trees with bends and nodes, the trapping ratio increased with rod surface roughness.

Lava flows from the 2018 Kı¯lauea eruption caused a volcanic disaster on the island of Hawaii. This study summarizes lava flow characteristics, and reviews past volcanic disasters and countermeasures. Based on an interview survey on the 2018 Kı¯lauea Volcano eruption, we discuss the direction of future countermeasures to be used against possible similar lava flows in Japan. Considering the high viscosity and low flow velocity, no significant damage to humans occurred or is expected, except for special cases. However, economic damage to buildings and other structures was significant. A large amount of basaltic lava erupted during the 2018 Kı¯lauea event, causing damage to homes, infrastructure, and farmland. The main eruptive activity began with the fissure eruption on the Lower East Rift Zone, followed by the summit explosion and collapse of the Halema‘uma‘u crater in the K¯ılauea Caldera. Although no structural measures against lava flows such as flow direction control or cooling to reduce the flow velocity were taken, warning information was issued two days before the eruption and a lava-flow hazard map was updated almost every day. To provide effective lava-flow hazard maps, it is important to obtain updated data continuously, including the information on the volcano effusion rate and the latest topographical data during the eruption. These can be utilized in developing and maintaining a lava flow simulation model. The 2018 Kı¯lauea eruption was the largest in the Hawaiian Islands in the last 200 years. Similarly, in Japan the possibility for volcanic eruptions with lava flow exists, even though no event has occurred in hundreds of years. Due to the steep and narrow topography in Japan, it would be difficult to implement structural measures effectively. Furthermore, an eruption involving a large amount of lava may isolate areas, cutting them off by slope failure associated with strong volcanic earthquakes. Therefore, risk management for volcanic lava flow events also necessitates formulating an evacuation plan that considers the occurrence of multiple hazard chains.