Extensive research on landslide susceptibility and landslide-affected areas has been conducted, but many technologies still lack sufficient accuracy and information to predict the movement of collapsed material. Adequate disaster mitigation requires prediction of the movement type and travel distance of collapsed material from deep-seated landslides. This research aims to classify the movement type of collapsed material from deep-seated landslides and to clarify the topographic conditions that influence it. The research area is the Kii Peninsula, south-western Japan, which was severely damaged by sediment-related disasters triggered by Typhoon Talas in 2011. A digital elevation model and aerial photographs were used in ArcGIS analysis, and topographic characteristics were examined to find significant factors that influenced the movement of collapsed material. Collapsed material of deep-seated landslides formed two main outcomes, debris flows and landslide dams. Debris flows were likely in catchments of streams with gradient > 10 and inflow angle < 60 . Landslide dams were likely in catchments of streams with gradient < 10 and inflow angle > 60 . Landslide dams with an upstream watershed exceeding 100 km 2 tended to remain for much shorter periods than those with smaller watershed. The equivalent coefficient of friction, representing the travel distance and degree of fluidization of collapsed material, could be used for predicting the deposition zone of collapsed material.
Connectivity of landslide sediment to and within fluvial systems is a key factor affecting the extent of mobilization of hillslope material. In particular, the formation of landslide dams and the transformation into landslide-induced debris flows represent “end members” of landslide sediment mobility. To quantify sediment connectivity, we developed a two-segment flume representing tributary inflow and the main channel. Mobility of sediment was examined by combinations of various topographic factors, such as tributary inflow angle (0 to 90° in 30° increments) and main channel gradient (10° and 15°), as well as water content of sediment (0 to 100% in 20% increments). We also examined differences of mobility among sediments derived from various lithologies (sand and shale, pyroclastic sediment, weathered granite, and weathered sedimentary rock). Mobility of sediment differed, depending on the water content of sediment, particularly less than saturation or greater than saturation. When all types of unsaturated landslide sediments entered the channel at inflow angles of 60° and 90°, substantial deposition occurred, suggesting the formation of landslide dams. At low inflow angles (0° and 30°) in a steep channel (15°), >50% of landslide sediment was transported downstream, indicating the occurrence of a debris flow. The amount of sediment deposited at the junction angle was greater for pyroclastic sediment followed by weathered granite, weathered sedimentary rock, and finally, sand and shale. Our connectivity index suggests that a threshold exists between landslide dam formation and debris flow occurrence associated with topographic conditions, water content, and types of sediment.
Earthquake-induced landslides are a major sediment disaster that can lead to significant fatalities and destruction of infrastructures [Owen et al ., 2008]. Numerous studies have investigated the characteristics of earthquake-induced landslides worldwide. By studying the 2015 Mw 7.8 Gorkha earthquake in Nepal, Roback et al . [2018] identified that slopes with gradients ranging from 40 to 50°and high-relief Himalayan mountainous topography (2500-5000 m a.s. l.) increased susceptibility to landslides. A high proportion of landslides occurred on hillslopes with gradients of 30-40°in the 2013 Mw 5.9 Minxian earthquake in China [Tian et al ., 2016]. In Japan, Koyanagi et al . [2020] demonstrated that landslides occurred on upwardly convex landforms in the 6.9 km 2 Tokosegawa watershed of the Mount Aso volcano region in the 2016 Mw 7.0 Kumamoto earthquake.The mobility of landslides is an important factor in their characterization. Guo et al . [2014] reported that the 46 landslides caused by the 2008 Mw 7.9 Wenchuan earthquake traveled for 347-4170 m depending on the slope gradient. A rainfall-induced landslide in the 2014 Oso disaster in Washington, United States, was transported for approximately 1 km because of a high water content and liquefied conditions [Iverson et al ., 2015]. Kharismalatri et al . [2017] showed that 33 deep-seated landslides traveled for 130-3310 m depending on the stream gradient and inflow angle.Vegetation ground cover is another key factor for characterizing landslides and their mobilities in
Risk of landslide hazards strongly depends on how far landslide sediment travels, known as landslide mobility. Previous studies mentioned enhanced mobility of earthquake-induced landslides in volcanic deposits compared to those from other geologic/soil settings. A flume apparatus constructed at a 1:300 scale was used to examine the mobility of landslides with pumice. Four pumice samples were collected from landslides induced by the 2018 Eastern Iburi earthquake, Hokkaido, Japan. Laboratory tests confirmed the unique low specific gravity of the pumice (1.29–1.33), indicating numerous voids within pumice particles. These voids allowed pumice to absorb a substantial amount of water (95–143%), about 9–15 times higher than other coarse-grained soils. Our flume experiments using various saturation levels (0–1) confirmed the influence of this inner-particle water absorption on pumice mobility. Because a low value of specific gravity indicates a low strength of soil, grain crushing may occur on the pumice layer, causing water from the internal voids to discharge and fluidize the transported landslide mass. Our findings indicate that such earthquake-induced landslides can be as mobile as those induced by rainfall, depending on the initial water content of the pumice layers. These conditions might be associated with water accumulation from previous rainfall events and the water-holding capability on pumice layers.
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