Preliminary investigation of the 20 August 2014 debris flows triggered by a severe rainstorm in Hiroshima City, Japan

Abstract

Background: In the early morning of 20 August 2014, a high-intensity/low-duration rainstorm occurred in Hiroshima City, in southwest Japan. Within 3 h, the rainfall exceeded 200 mm, which is more than twice the monthly-average for this area. This heavy rainfall triggered 107 debris flows and 59 shallow slides, which caused 44 injuries, and 74 deaths. 133 houses were destroyed and an additional 296 houses were severely damaged. Most of the debris flows occurred in heavily weathered granite slopes, while others occurred in weathered hornfels slopes. A field investigation on two of the gullies in which the debris flows occurred was conducted in order to better understand the characteristics of the debris flows. Results: The main purpose of this investigation was to understand the geomorphological and geological conditions, the soil properties, and the initiation/traveling mechanisms of the debris flows. The longitudinal and cross-sectional profiles along the two gullies were measured, beginning at the source areas and ending at the downstream limits of the deposition areas. For soil property determination, disturbed and undisturbed soil samples were collected for laboratory tests which included in-situ density measurement, grain size distribution analysis and triaxial compression tests. In the triaxial compression tests, consolidated-undrained compression tests under different confining stresses were conducted to measure the strength parameters of the strongly-weathered granite. Pore-water pressure controlled triaxial test was conducted to simulate the failure process of the slope given an increase of the pore-water pressure. Chemical analyses of the granite samples were also conducted in order to understand the degree of weathering of the granite in the debris flow gully. Conclusions: A high intensity, short duration, localized rainfall event initiated debris flows in very steep slopes. These were initiated as a thin sliding mass in weathered coarse-grained granite and hornfels, and became two different types of debris flow after traveling down the slopes. The slope angle and the cross section of the gully, and the grain size of the debris significantly controlled the motion behavior of the debris flows.

A Semiautomatic Pixel-Object Method for Detecting Landslides Using Multitemporal ALOS-2 Intensity Images

“…Rice paddy fields and urban settlements are also present between these mountainous regions. According to the Geological Survey of Japan, in this area, the primary types of rocks are Late Cretaceous granite (also known as Hiroshima Granite) and Middle to Late Jurassic accretionary complexes [29]. Additionally, heavy rainfall events have triggered debris flows throughout this study area in the past.…”

“…Additionally, heavy rainfall events have triggered debris flows throughout this study area in the past. In terms of property damage and human casualties, the most significant events occurred in July 1982, June 1999, and August 2014, while the most recent occurred in July 2018 [29,30]. In the July 2018 event, approximately 100 mm of rainfall was observed over a period of 3 h, and heavy rainfall was recorded for a total duration of 72 h [31].…”

Comparison of Conventional Deterministic and Entropy-Based Methods for Predicting Sediment Concentration in Debris Flow

“…where η is obtained experimentally. The detailed derivation of Equation (7) can be found in the work of Lien and Tsai [12].…”

“…Consequently, a 550-m-long and 70-m-wide debris dam was formed, and one-third of the city was inundated for five days [5,6]. Debris flows in the southwest areas of Japan resulting from torrential rain in August 2014 led to more than 49 deaths and 47 missing people, while more than 1.6 million people were forced to flee from their hometowns [7]. Therefore, it is urgent to study debris flow to reduce its damage.…”

Investigation of rainfall-induced shallow landslides on the northeastern rim of Aso caldera, Japan, in July 2012

“…Shallow landslides often occur in weathered debris or loose soil materials on steep slopes during heavy rainfall (Maharaj 1993;Zhu and Anderson 1998;Dai et al 1999Dai et al , 2004Dai and Lee 2002;Capra et al 2003;Yu et al 2006, Dahal et al 2009Giannecchini et al 2012;Yang et al 2015;Wang et al 2015). Annual rainfall in Kyushu is quite high, due to stagnation of wet fronts in the area during the rainy season, from May to July.…”