Collapsed material movement of deep-seated landslides caused by Typhoon Talas 2011 on the Kii Peninsula, Japan

Abstract

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.

Evaluating Factors for Controlling Sediment Connectivity of Landslide Materials: A Flume Experiment

“…Best [26,27] showed that inflow angles of 15-105 • reflect the most common landslide entry conditions into channels; thus, we examined the mobility of sediment of four categories of inflow angles (0 • , 30 • , 60 • , and 90 • ) to encompass a wide range of potential inflow angles. For the gradient of the main channel, we used 10 • and 15 • to examine the 10 • channel gradient threshold between landslide dam formation and debris flow initiation based on a previous study of deep-seated landslides [20]. In addition to the topographic factors, we mixed different amounts of water with input sediments since hydrologic factors alter their mobility [28].…”

“…The total weight of sediment plus water became 440 to 1360 g, differing based on water content and sediment density. Volume of sediment samples (400 cm 3 ) within the 10 cm width of the flume was determined by scaling down the actual volume of sediment moved by deep-seated landslides and the respective river channel width ( [20]; 3.3 × 10 6 m 3 for a mean channel width of 199 m). Therefore, we assumed that our flume experiment was approximately 1/2000 of an actual deep-seated landslide scenario.…”

“…, where a deep-seated landslide occurred in September 2011 due to Typhoon Talas [20]. This sample was collected from a 3.3 × 10 6 m 3 landslide deposit [20] consisting of a highly fractured muddy alternation of sand and shale of the Cretaceous Miyama Formation [35] .…”

“…, where a deep-seated landslide occurred in September 2011 due to Typhoon Talas [20]. This sample was collected from a 3.3 × 10 6 m 3 landslide deposit [20] consisting of a highly fractured muddy alternation of sand and shale of the Cretaceous Miyama Formation [35] . The geology of this area is dominated by late Cretaceous granite (weathered granite) on the east side and a mélange matrix of a middle to late Jurassic accretionary complex (weathered sedimentary rock) on the west side [36].…”

“…To better understand sediment connectivity related to the mobility of landslide sediments, interactions among multiple factors need to be considered. For instance, a GIS investigation of deep-seated landslides in Kii Peninsula in 2011 revealed that the formation of landslide dams and occurrence of debris flows could be classified based on two criteria: a 60 • inflow angle and 10 • channel gradient [20]. Similarly, an investigation of 62 landslides in British Columbia, Canada, found that landslides with inflow angles <40 • and channel gradients >27 • resulted in debris flows; additional factors influencing debris flow occurrence were initial failure volume and sediment stored within the channel prior to the occurrence of landslides [21].…”