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Landscapes disturbed by shallow landslides present several residual risks. For example, residual soils on sliding surfaces may lead to the re-occurrence of shallow landslides, and landslide deposits on valley floors may introduce multiple hazards associated with river blocking or landslide damming. Despite such risks, few studies have investigated the residual risks following shallow landslides, and thus, no methodology has been established for assessing such risks. In this study, we combined portable cone penetration test (PCPT) and electrical resistivity tomography (ERT) to investigate the post-event risks associated with a landslide scar resulting from the 2018 Hokkaido Eastern Iburi earthquake (HEIE) in Japan. The multi-point PCPT identified the interface between a weak volcanic soil layer and underlying weathered sedimentary rocks. This interface exhibited an abrupt spatial change in resistivity, ranging from 20 to 100 Ωm. The thickness of the residual soil layer was 0.3–3.2 m and that of the landslide deposit exceeded 2.0 m. An infinite-slope stability analysis was performed to predict the re-occurrence of shallow landslides due to a lower intensity earthquake than that of HEIE in this region. Considering the area affected by the HEIE and the physical properties of soil layers, our findings imply a widespread risk of re-occurrence of shallow landslides over an area of 20 km × 20 km. Although the areal coverage of the combined ERT/PCPT method needs to be extended, it is effective for assessing the residual risks associated with shallow landslides. [ABSTRACT FROM AUTHOR]

The study of the morphological characteristics of native species that are present in shallow landslide prone areas is an important factor for the selection of species to be used for soil conservation through soil bioengineering techniques. Previous research has assessed hydrological and mechanical effects of plant species; however, an overview of the morphological parameters of plant species focused on soil bioengineering is still lacking, this is crucial, especially when a long-term slope restoration system is required. Therefore, the aim of this research is to quantify the above ground (plant height, plant mass) and below ground (root diameter, root length and root number) morphological characteristics and to establish the relations with the appropriateness of a plant species as a soil bioengineering element. In total, 122 plant specimens distributed in 5 families, 6 genus and 5 species were registered and measured from the study area. Results showed that Boehmeria spicata could be a key species for the soil bioengineering, which morphology was found as follows: plant height of plant (15 ± 6.09 cm); root length (7.58 ± 5.58 cm); plant mass (1.65 ± 1.3 g); root diameter (1.48 ± 0.58 cm); root number (7 ± 2) and pull-out force (28.98 ± 13.96 N), showing a higher inclination to the canonical axe of number of roots, which resulted the variable with highest correlation with pull-out force. [ABSTRACT FROM AUTHOR]

The Miyanaka Intake Dam fishway underwent improvements in 2012, and we established a new rock-ramp fishway called the Seseragi Fishway, cognizant of its utility as a passage and a habitat for bottom-dwelling and small fish with weak swimming ability. However, the fishway is occasionally submerged by floods, causing sediment accumulation that leads to changes in the vegetation composition. In addition, the arrival and inflow of seeds from upstream and the surrounding areas result in vegetation changes. In this study, the inside and outside of the rock-ramp fishway were divided into eight areas, and the vegetation succession after 2012 was determined. A correlation was observed between the results of fish catch surveys during the same period and the vegetation. Based on these results, we reported on the process of steadily operating the rock-ramp fishway while devising and improving specific management methods. Changes in vegetation, such as an increase in upright vegetation and a decrease in flow-obstructing vegetation, contributed to an increase in the population of bottom-dwellers, weak swimmers, and juvenile fish. The existence and management of appropriate vegetation are important for maintaining fishways inhabited by a variety of fish species. [ABSTRACT FROM AUTHOR]

In Japan, repeated driftwood landslide disasters have become a major issue; thus, studies are required to better understand forest function to implement appropriate forest management and prevent such disasters. We investigated the effect of Japanese cedar tree roots on shallow landslide initiation. To incorporate the effect of roots on the two side-flanks of the shallow landslide, we propose a new slope-stability analysis method in which the sliding block is simplified as a three-prism model. The root reinforcement was approximated by the sum of the root pullout forces over a unit area, incorporating changes in the root strength with the growth of the trees after planting and the decay of the stumps after cutting. The reinforced root strength after the stump-cutting decreased linearly with time, with no strength remaining at 9 years. In contrast, the reinforced root strength of the new plants increased according to a logistic curve with time; thus, the root strength increased only slightly up to 9 years after planting, and the minimum total reinforced root strength was observed at this time. The safety factor of the slopes in a forest basin in Ibaraki Prefecture was calculated using the proposed three-prism method at intervals of 5 years on a 1-metre-resolution digital elevation model. The number of unstable grids peaked at 10 years, and a higher risk of slope instability was observed at 5–15 years. Therefore, implementing forest operations for lowering slope instability during this period should be important to prevent landslide disasters. [ABSTRACT FROM AUTHOR]

Tree resistance to uprooting can be estimated as the critical turning moment in tree-pulling experiments. The depth at the center point of rotation (Dcp) in tree-pulling experiments is measured as an indicator of below-ground traits and is related to this critical turning moment. However, few researchers have investigated the relationship between the Dcp and maximum root depth. Our objective in this study was to clarify whether the Dcp in tree-pulling experiments can be estimated as the maximum root depth of Pinus thunbergii Parl. in sandy soils. We also estimated which position of displacement of the center of rotation (Cp) can be applied as the Dcp. We conducted tree-pulling experiments, and compared the Dcp obtained from images with the measured maximum root depth. We found significant positive correlations between the Dcp and maximum root depth. The Cp displacement concentrated immediately below the stem when the maximum critical turning moment was reached. This position should be measured as the Dcp, which is related to the maximum root depth. We found that the Dcp can serve as a parameter, preventing the need for uprooting, when tree-pulling experiments are performed to obtain an important below-ground trait for understanding the critical turning moment. [ABSTRACT FROM AUTHOR]

Topo‐bathymetric surveys of stream channels and floodplains, along with their vegetation distributions, are difficult and expensive to quantify, but they are crucially important for balanced river management, addressing issues such as flood risk and ecosystem management. This study was conducted to demonstrate seasonal accuracy for the first time, appraising results of a novel cost‐effective unmanned aerial vehicle (UAV)‐borne topo‐bathymetric light detection and ranging (lidar) used to examine the vegetated and gravel‐bedded lower Asahi River, Japan. Compared with autumn, the winter findings were almost identical to those from ground‐truth observations with outperformed accuracy ranging from around 2 to 12 cm of root‐mean‐square‐error values. The results were compatible with the corresponding high‐resolution aerial image. During winter, the three‐dimensional laser point clouds identified details of the submerged artificial infrastructure of the river and the continuous topography of underwater (shallower than or equal to 2.08 m depth) and overland areas. In comparison, the autumn's bathymetric performance was limited to a depth of 1.20 m and turbidity of 5.50 NTU because measurements were conducted after a flooding event. Furthermore, this study applied novel approaches for accurate land cover mapping and vegetation height assessment, which can be readily applied in future riverine vegetation appraisal studies. In conclusion, because the point cloud data can be useful as input and validation data in hydrodynamic‐numerical models and riparian vegetation identification, lidar can be regarded as a technique to inform the management of clear‐flowing and shallow streams with vegetated floodplains. [ABSTRACT FROM AUTHOR]

Landslides in forested landscapes have far‐reaching implications, beyond that of just destroying the forest itself, sometimes initiating large‐scale sediment disasters. Although vegetation increases slope stability through its root network, it is hard to evaluate its contribution to slope stability over a wide area. In this study, the relationship between tree height and landslide characteristics in the Ikawa catchment, central Japan, was investigated to develop a method for evaluating the effects of forest cover on slope stability over a regional extent. Catchment‐wide tree height was obtained using airborne LiDAR point cloud data and used in conjunction with the root depth profile, measured for trees of various height by digging trenches. Root tensile strength per unit area of soil was calculated from individual root diameters and empirical power law equations on the relationship between root diameter and root tensile force in order to better understand the effect that tree height has on slope stability. Landslide density in the Ikawa catchment shows that landslides occur more frequently in forests with shorter trees, with occurrence decreasing as tree height increases. This is likely due to the stabilizing features of larger trees having a greater network of roots, which is supported by the general increase in total root area and the deeper penetration of root biomass into the soil as the height of trees surveyed increases. Landslide density was not solely affected by tree height, but also by slope gradient and plane curvature. Decreasing landslide occurrence and landslide area as tree height increases suggests that slope stability increases with tree height, while the random distribution of results when comparing landslide depth to tree height suggests that while tree height has an impact on relative slope stability, the landslide failure depth is independent of tree height, and thus controlled by other factors. © 2020 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This report describes a method of extracting vegetation conditions and topo‐bathymetric data of rivers using airborne LiDAR with near‐infrared and green pulsed lasers. Parameters related to flow resistance attributable to vegetation in rivers were found using point cloud data of LiDAR with support from supervised classification. Comparison of data obtained from simulations and observations was used to assess the applicability of the proposed method for examination of lower Asahi River flooding that occurred in Japan in September 2011. Results demonstrated that the methodology used herein can estimate flow resistance parameterisation of vegetation distributed on a reach scale. This study also used numerical tests to elucidate effects of established vegetation for flood control at the lower Asahi River bifurcation point using a historically important fixed weir. Results of these numerical tests demonstrated that the method can elucidate effective means of managing vegetation removal around the bifurcation point. [ABSTRACT FROM AUTHOR]