Preliminary investigation of the 20 August 2014 debris flows triggered by a severe rainstorm in Hiroshima City, Japan
© Wang et al. 2015
Received: 13 February 2015
Accepted: 30 June 2015
Published: 24 July 2015
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.
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.
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.
KeywordsHiroshima Heavy rainfall Debris flow Granite Hornfels Investigation
In past decades, many landslide and debris flow events have been triggered by high intensity, short-duration rainstorms, and caused loss of life and infrastructure damage worldwide (e.g. García-Martínez and López 2005; Casagli et al. 2006; Tang et al. 2012; Cevaso et al. 2014; Chen et al. 2014; Ni et al. 2014; Yang et al. 2015). For example, in December 1999, extreme rainfall on the northern Venezuelan coast triggered a disastrous debris flow, which caused about 1,500 fatalities and the destruction of 23,000 houses (García-Martínez and López 2005). Recently, Japan also suffered from many disasters triggered by extreme rainfall. In September 2011, Typhoon 12 (Talas) attacked the Kii peninsula of central Japan and triggered many landslides and floods, causing 97 casualties (Saito and Matsuyama 2012). Then in July 2012, an unprecedented four-day heavy rainfall hit Kyushu in Southwest Japan and formed devastating floods and mudslides, and was associated with 32 people who were reported either dead or missing; 400,000 people were evacuated from their homes (Duan et al. 2014). Disasters triggered by extreme-rainfall have become a critical and urgent issue for society.
In the Hiroshima area, the 2014 events were preceded by other catastrophic sediment disasters in recent years. At the end of June 1999, 139 debris flows and 186 shallow slides were triggered by rainfall, and caused 32 deaths in a nearby area (The Chugoku Shimbun Online 1999; Wang et al. 2003). Mainly because of this disaster, a law referred to as the “Sediment Disaster Countermeasures for Sediment Disaster Prone Areas Act” was enacted in 2000 to prevent sediment problems caused by debris flows and other causes. Despite the protection of the law for the previous 14 years, the loss of life caused by debris-flow sediment disasters triggered by one heavy rainfall event was not prevented; indeed the loss of life in the 2014 events was almost three times that in 1999. These events brought a huge amount of distress, and severely impacted the local community. The public as well as the academic community, are extremely interested in the causes of the disaster and desire to know the triggering factors and field conditions. Many methodologies have been developed and applied to this topic. To reveal the disaster impact in a larger area, statistical analysis is applied to analyze landslide susceptibility in terms of slope inclination, land use, and other factors (e.g., Lepore et al. 2012; Tang et al. 2012; Winter et al. 2013; Cevaso et al. 2014; Chen et al. 2014; Dijkstra et al. 2014). On the other hand, for a localized catchment, the physically-based models, e.g., slope stability analysis and hydrological models, (e.g., Casagli et al. 2006; Tsuchida et al. 2014) or field monitoring and laboratory tests (e.g., Montgomery et al. 2009; Okada and Kurokawa 2015) are used to analyze the initiation mechanism of shallow landslides or mudslides under different rainfall conditions. In order to assess the future risks in the disaster area, field survey and laboratory tests for the analyses of geological and geomorphological conditions, and soil properties are considered in this paper. Therefore, we shall report the hazard background briefly as well as the results of the field survey and laboratory analysis of soil mechanical and geochemical properties. As shown with a black rectangle in Fig. 1, our investigation focuses on two debris flows in the Asaminami Ward, which suffered major life loss due to its higher population density on the narrow alluvial plain, compared to the Asakita Ward.
In the field investigation, observation of the geological conditions, measurement of the longitudinal and cross-sectional profiles, and the collection of soil samples in source and deposition areas were the main objectives. The deposits in the debris flow gullies were also observed. In the geomorphological analysis, we discuss the effect of topography not only along the longitudinal section, but also the cross section. To clarify the initiation mechanism of the debris flows, we discuss the failure process of a granitic soil slope under increased pore water pressure caused by heavy rainfall through a simulation test with triaxial compression equipment. However, as there is an absence of data from the two gullies before the disaster, the entrainment or erosion processes cannot be analyzed in this study. Instead, we only focus on the initiation mechanism of shallow slides in the source area.
Longitudinal and cross section measurement of the debris flow gullies
We investigated the two gullies of Midori-ga-oka debris flow and Abu-no-sato debris flow from the source areas to the downstream limits of the run-out zones (see Fig. 5 for details). Along the whole gully, we measured the longitudinal and cross-sectional profiles at every cliff and slope using laser rangers, ranging rods, inclinometers and GPS trackers. The locations for measurement are marked by solid cyan circles in Fig. 5. At each point, the label was assigned an “S” for a slope or “C” for a cliff, and followed by a sequential number from upstream to downstream in ascending order. The source areas were investigated in more detail. The deposit profile is also shown in the longitudinal sections.
Sampling and mechanical property analysis of soils
During the field survey, we collected undisturbed and disturbed soil samples in both gullies (see Fig. 5 for sampling locations). In the Midori-ga-oka debris flow, three undisturbed samples (Nos. 1 and 2 in the left source area, and No. 3 in the right source area) were collected for the analysis of physical parameters and the mechanical properties of the soil. At the location of sample No. 3, a disturbed soil sample (labeled as No. 3-DIS) was also collected for the analysis of the initiation mechanism, for use with triaxial tests. In the deposition area of the two gullies, three samples (Nos. 4, 5, and 6) were collected for grain size analysis and permeability characterization. Finally, the fresh granite, the weathered granite, and the granitic soil were also collected in the Midori-ga-oka gully for the geochemical analysis. The sample locations are shown in Fig. 5.
Geochemical analysis of the granite sample
Major element compositions of the fresh granite, the weathered granite, and the granitic soil were determined using a Rigaku RIX-2000 X-ray fluorescence spectrometer (XRF). The samples were crushed manually in an iron mortar. The rock chips were then grounded in an agate mortar crusher for 30 min. All analyses by XRF were made on glass beads prepared in an automatic bead sampler, using an alkali flux comprising 80 % lithium tetraborate and 20 % lithium metaborate, with a sample to flux ratio of 1:2. The Analytical procedure is described by Kimura and Yamada (1996).
Consolidated-undrained triaxial compression tests
With sample No. 3-DIS, the consolidated undrained triaxial tests were conducted to measure the effective soil strength parameters. Dry soil passing 2 mm sieving was used to make a cylindrical specimen, and the dry density was adjusted to the same value as the in-situ dry density of the soil. After that, the specimen was fully saturated.
With the fully saturated specimens, three consolidated-undrained compression tests were conducted under three different confining stresses (50, 75 and 100 kPa). After normal consolidation, the specimen was compressed at 1.0 % axial strain per minute under undrained conditions. Through the tests, the shear strength parameters of the Masa-do were obtained.
Pore-water pressure controlled triaxial test
With sample No. 3-DIS, the pore-water pressure controlled triaxial test was performed to simulate the initiation mechanism of shallow sliding at the source area. The slope angle of the source area is about 35°, and the average thickness of the initial sliding mass is about 1 m. The potential sliding surface is located in the granitic soil above the bedrock surface.
Set the specimen and make it fully saturated.
Apply σ1 and σ3 as the initial axial stress and confining stress. Through this step, the initial stress condition of the slope before rainfall (without any pore-water pressure inside the slope) is simulated.
Apply pore-water pressure to the specimen through a pore-water pressure controller at the rate of 0.1 kPa/minute until specimen failure. Through this step, the situation of pore-water pressure accumulation on the potential sliding surface is simulated, and the failure process can be observed.
In this test, the slope angle was assumed to be 35°, and soil thickness to be one meter.
Results and discussions
Field investigation and cross section measurement of the two gullies
Based on the field investigation, the geological conditions were found to be different in the source areas of the two debris flows. The source area of the debris flow in Midori-ga-oka is composed of coarse granite, with intrusive fine granite. In the coarse granite, sheeting joints and micro-sheeting joints are well developed. The average thickness of the granitic soil is about 1 m. In the main channel of the debris flow, all of the granitic soil was removed, and only the gully bed consisting of fresh granite was left. The situation along the gully will be introduced in details later. For the debris flow in Abu-no-sato, the source area is located in hornfels. In this gully source area (where the debris flow originated), the weathered soil layer is thin, with an average thickness of 0.7 m. The bedrock in the channel is stiff hornfels.
The longitudinal profile consisted of gentle slopes (slope angle < 30°, indicated as S) and steep cliffs (slope angle > 40°, indicated as C).
Most of the cliffs were located at the middle and lower part of the gully.
The cross sections were in a V-shape from S-1 to S-3, and became U-shaped from S-5 to the end (S-21). The section became narrow in the area between S-3 and S-17. Notably, the width change from S-15 to S-17 is very sudden. This could have caused a debris dam between S-15 and S-17, and a later collapse of the dam may have caused a debris flow of greater magnitude. This sudden narrowing in the cross section may had a significant effect on the motion of the debris flow as it flowed to the residential area, because of the constricted flow channel.
Nearly all of the debris deposited on the riverbed of the gentle slope in the gully (below S-23 in Fig. 5).
The slope was steep in the hornfels area, and became gentle in the coarse-grained granite area.
Most of the cliffs were distributed in the middle and upper parts of the gully.
The gully was V-shaped in most of the hornfels area, and gradually became wider.
From C-15 to S-17, the cross sections narrowed rapidly. This change may have caused a dam in the debris flow, and the later outburst may be a direct cause of the disaster.
Compared with the Midori-ga-oka debris flow, the distance between the residential area and the starting point of the deposition area is much larger in Abu-no-sato debris flow. It may mean that keeping enough space between the deposition start point and the residential area is very important for disaster reduction.
Soil properties: in-situ density and grain size distribution
Parameters of soil samples in the source area of the Midori-ga-oka gully
Specific gravity, G S
Dry density, ρ d (kg/m3)
Water content, w (%)
Liquid limit, LL (%)
Plastic limit, PL (%)
Plasticity Index, PI (%)
Geochemical analysis of the granite sample
Major element compositions of granite and related samples in the Midori-ga-oka gully
Where CaO* represents Ca in the silicate fraction only. Fresh granitic rocks generally have CIA values near 50. Al2O3 is less mobile during the weathering process, whereas CaO*, Na2O, and K2O are relatively more mobile (e.g., Nesbitt and Young 1982; Harnois 1988; Fedo et al. 1995). Consequently, CIA values gradually increase with increasing intensity of rock weathering up to a value of 100. The CIA values of the fresh granites, the weathered granites, and the granitic soils of Midori-ga-oka are 52.6–54.5, 53.1–55.5, and 56.4–69.9, respectively (Table 2). We especially focused on the lower CIA values of the granitic soils.
Nesbitt and Markovics (1997) reported the CIA values of typical clay-rich weathered granitoids (72.7–81.6) from the outer layer of the spheroidal weathering-boulder in Toorongo granodiorite, Australia. The weathering conditions of the sample location are similar to those of Hiroshima Prefecture. Summers of both regions are warm, averaging 25–30 °C, and winters are cold with temperatures of −5 to 10 °C. Summer precipitation is 200–300 mm with annual precipitation approximately 1,500 mm for both. As a result of the similarities, the CIA values of Toorongo granodiorite would be a good comparison with our samples. However, the granitic soils of Midori-ga-oka have significantly lower CIA values (56.4–69.9) compared with those of the weathered granitoids in Toorongo granodiorite (72.7–81.6). This suggests that the degree of chemical weathering of the granitic soils in the Midori-ga-oka gully is less than that of normally weathered granitic materials. This provides the evidence that the weathered materials in the shallow slide area were in a removal cycle from the steep slope, due to repeated debris flows. Therefore, the lower CIA values of soil materials in debris flows may be a good indicator in order to detect the active area of debris flows in a granitic region.
Consolidated-undrained triaxial compression tests
The stress–strain relation of the consolidated-undrained tests on the saturated specimen (Fig. 14a) shows that the deviatoric stress increases in the beginning. When the axial strain is about 2 %, the deviatoric stress reaches the peak value. After that, the deviatoric stress turns to decrease with the axial strain. Bulging failure was observed for all specimens at the end of the tests. The stress–strain curves indicate that the critical state of the soil will be reached at low strains value.
The relation between excess pore-water pressure and the axial strain in the consolidated-undrained tests on saturated specimens is shown in Fig. 14b. Positive excess pore-water pressure was generated in all three undrained tests. In all soil specimens, the positive excess pore-water pressure increases to a high value with minor strain, and increases slowly as the strain increases continuously. Finally, the value of pore water pressure is close to the confining pressure, indicating that the soil would liquefy if the strain continued to increase.
Pore-water pressure controlled triaxial test
Under extreme high intensity and short duration rainstorm, debris flows occurred on very steep slopes with a thin initiating sliding mass in weathered coarse-grained granite and hornfels.
Cross sectional properties of the debris flow gully, especially gully shape and changes in width, have important implications for debris flow damming, travel distance, and deposition.
During travel, the debris flow may erode the valley and carry the colluvium and valley deposits downslope. Shallow slides in high mountains can cause large-scale debris flow.
Debris flows that are rich in fine particles like granitic soil, tend to travel long distances until they flow onto very gentle slopes, while those with boulders as components started to deposit as soon as the valley became wide and gently-sloped. The permeability and drainage characteristics of the debris flow controlled the travel distance of the debris flow.
The degree of chemical weathering of the granitic soils in the Midori-ga-oka gully is less than that of normally weathered granitic materials. This provides the evidence that the weathered materials in the shallow slide area were in a removal cycle from the steep slope, due to repeated debris flows.
Simulation test with pore-water pressure controlled triaxial test shows reasonable response of soil behavior under different water pressure conditions, which corresponds to groundwater level in a real slope. It is hoped that the groundwater level in a real slope can be used for failure prediction.
Based on the field investigation experiences, we like to make some suggestions for countermeasure works to prevent debris flow disasters: (a) Keeping enough distance between the residential area and the deposition area of the debris flow is essential; (b) For the hornfels area, a ring-net will be effective to stop the travel of boulders; (c) For weathered granite area, the use of a ring-net to stop large boulders and, in addition, building a large catch pit with a check dam will be effective; (d) Because debris flows always move rapidly, early warning should be sufficiently timely, such people have time to take action. Otherwise, delayed warnings are useless when considering the weather condition that can cause debris flows.
This investigation was financially supported by a fund for exploratory research from Shimane University, JSPS KAKENHI Grant Number A-2424106 for landslide dam failure prediction. The students from Department of Geoscience, Shimane University, Ryoichi Tsukamoto, Tomohiro Oda, Norisato Oishi, Naho Yamamoto, Masafumi Yokoyama joined the field investigation and assisted the topographic survey and soil/rock sampling. Lynn Highland of U.S. Geological Survey made constructive comments of the draft. Valuable and constructive comments from anonymous reviewers are deeply appreciated.
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