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

Preliminary investigation of the 20 August 2014 debris flows triggered by a severe rainstorm in... 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. Keywords: Hiroshima; Heavy rainfall; Debris flow; Granite; Hornfels; Investigation Background and 74 deaths. In addition, 133 houses were destroyed In the early morning of 20 August 2014, a high-intensity/ and 296 houses were severely damaged (Ministry of short-duration, localized rainfall event triggered many Land, Infrastructure, Transport and Tourism 2014; shallow slides and debris flows in the Asakita and Yamamoto and Kobayashi 2014). Shallow slides were Asaminami Wards in the northern part of Hiroshima designated as those that moved for a limited distance City, Japan. In all, there were 166 slope failures trig- from thesourcearea. Most of thelossof lifeand prop- gered by the heavy rainfall event, including 107 debris erty was caused by debris flows. In this paper, all of the flows and 59 shallow slides, which caused 44 injuries slope failures are called debris flows for simplification. Figure 1 illustrates the distribution of debris flows and * Correspondence: wangfw@riko.shimane-u.ac.jp fatalities. As indicated, the debris flows were distributed Department of Geoscience, Shimane University, Matsue 690-8504, Japan 2 in an elongated area extending northeast to southwest Project Center on Natural Disaster Reduction, Shimane University, Matsue and covering the Asaminami Ward and Asakita Ward. 690-8504, Japan © 2015 Wang et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 2 of 16 Fig. 1 Distribution of debris flows and landslide-affected area and number of fatalities. The longitude and latitude is in the decimal format in the all figures hereafter. The debris-flow distribution and fatality number are obtained from Ministry of Land, Infrastructure, Transport and Tourism. DEM data is from Geospatial Information Authority of Japan (GSI). The black rectangle indicates the study area (see Fig. 4 for more detailed aerial photograph) Most of the fatalities were concentrated in the Asaminami Southwest Japan and formed devastating floods and mud- Ward (Asa is the name of the region; Asaminami means slides, and was associated with 32 people who were re- Southern Asa, while Asakita means Northern Asa, in ported either dead or missing; 400,000 people were Japanese). evacuated from their homes (Duan et al. 2014). Disasters In past decades, many landslide and debris flow events triggered by extreme-rainfall have become a critical and have been triggered by high intensity, short-duration urgent issue for society. rainstorms, and caused loss of life and infrastructure In the Hiroshima area, the 2014 events were preceded damage worldwide (e.g. García-Martínez and López by other catastrophic sediment disasters in recent years. 2005; Casagli et al. 2006; Tang et al. 2012; Cevaso et al. At the end of June 1999, 139 debris flows and 186 shal- 2014; Chen et al. 2014; Ni et al. 2014; Yang et al. 2015). low slides were triggered by rainfall, and caused 32 For example, in December 1999, extreme rainfall on the deaths in a nearby area (The Chugoku Shimbun Online northern Venezuelan coast triggered a disastrous debris 1999; Wang et al. 2003). Mainly because of this disaster, flow, which caused about 1,500 fatalities and the destruc- a law referred to as the “Sediment Disaster Countermea- tion of 23,000 houses (García-Martínez and López 2005). sures for Sediment Disaster Prone Areas Act” was Recently, Japan also suffered from many disasters trig- enacted in 2000 to prevent sediment problems caused gered by extreme rainfall. In September 2011, Typhoon by debris flows and other causes. Despite the protection 12 (Talas) attacked the Kii peninsula of central Japan and of the law for the previous 14 years, the loss of life triggered many landslides and floods, causing 97 casual- caused by debris-flow sediment disasters triggered by ties (Saito and Matsuyama 2012). Then in July 2012, an one heavy rainfall event was not prevented; indeed the unprecedented four-day heavy rainfall hit Kyushu in loss of life in the 2014 events was almost three times Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 3 of 16 that in 1999. These events brought a huge amount of mechanism of shallow landslides or mudslides under dif- distress, and severely impacted the local community. ferent rainfall conditions. In order to assess the future The public as well as the academic community, are ex- risks in the disaster area, field survey and laboratory tremely interested in the causes of the disaster and de- tests for the analyses of geological and geomorphological sire to know the triggering factors and field conditions. conditions, and soil properties are considered in this Many methodologies have been developed and applied paper. Therefore, we shall report the hazard background to this topic. To reveal the disaster impact in a larger briefly as well as the results of the field survey and la- area, statistical analysis is applied to analyze landslide boratory analysis of soil mechanical and geochemical susceptibility in terms of slope inclination, land use, and properties. As shown with a black rectangle in Fig. 1, other factors (e.g., Lepore et al. 2012; Tang et al. 2012; our investigation focuses on two debris flows in the Winter et al. 2013; Cevaso et al. 2014; Chen et al. 2014; Asaminami Ward, which suffered major life loss due to Dijkstra et al. 2014). On the other hand, for a localized its higher population density on the narrow alluvial catchment, the physically-based models, e.g., slope sta- plain, compared to the Asakita Ward. bility analysis and hydrological models, (e.g., Casagli The main triggering factor of the debris flows was the et al. 2006; Tsuchida et al. 2014) or field monitoring and high intensity and short duration rainstorm. Figure 2 il- laboratory tests (e.g., Montgomery et al. 2009; Okada lustrates the isohyetal map of the cumulative rainfall for and Kurokawa 2015) are used to analyze the initiation 48 h on 19 and 20 August 2014. It shows that most Fig. 2 Isohyetal map of cumulative rainfall on 19 to 21 August 2014. The data found in the isohyetal lines are interpolated using the data from 13 stations (marked by solid black circles) of the Japan Meteorological Agency, Hiroshima Prefecture Government, and Ministry of Land, Infrastructure, Transport and Tourism. The maximum rainfall occurred at the Uebara meteorological station, with an amount of 287 mm. The debris flows and landslides occurred along the area with intense rainfall. The hourly rainfall hydrograph at the Uebara meteorological station, which is located at the center of the area affected by debris flows, is illustrated in Fig. 3 Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 4 of 16 of debris flows occurred in the area of intense rainfall, of coarse Hiroshima granite is very easily weathered. The which the cumulative rainfall is greater than 200 mm. granitic soil (i.e., the residual soil) is locally called Masa- The maximum rainfall was recorded at the Uebara me- do, which means “real sand” in Japanese. Because of the teorological station, with an amount of 287 mm, and the intense wreathing, the Masa-do can easily lose its struc- recorded maximum in 2 days during the disaster is al- ture and collapse during heavy rainfall. most 2.6 times the August monthly-average value of In this study, we investigated two gullies in which the 110.8 mm in Hiroshima (Japan Meteorological Agency greatest number of fatalities occurred, called Midori-ga- 2015). Figure 3 shows the hourly rainfall hydrograph at oka and Abu-no-sato, respectively (see Fig. 5 for the the Uebara meteorological station. The maximum three- affected areas and positions). Both of the debris flows hour rainfall is 236 mm from 2:00 to 5:00 a.m. (local originated from similar elevations and can be catego- time) on 20 August. In particular, the maximum hourly rized as channelized, but their affected areas are quite rainfall is 115 mm at 4:00 a.m. on the same day. Wit- different. The Midori-ga-oka debris flow entered the nesses verified that the debris flows occurred around residential area and persisted over a long distance while 4:00 a.m. on the same day (The Japan Times 2014). We also spreading to cover a wide area. This phenomenon conclude that this unusual and extreme rainfall is the can also be observed in (Fig. 6a). However, the Abu-no- main triggering factor of these events. sato debris flow stopped behind the residential area Figure 4 illustrates the geology of the affected area. (Fig. 6b). The different travel distances may imply dif- Debris flows were distributed in an area 10 km by 2 km, ferent mobility rates of the debris flows. Understanding extending in a NE-SW direction. The elevation in this the factors causing the different mobility may be helpful area reaches 700 m. A valley, which is controlled by a for disaster reduction. For this reason, we selected these NE-SW fault, trends through this area. Late Cretaceous two debris flows for detailed study. granite, also called Hiroshima granite, is the main bed- In the field investigation, observation of the geological rock in this mountainous region. In the valley and lower conditions, measurement of the longitudinal and cross- slopes, deposits of weathered granite are encountered. In sectional profiles, and the collection of soil samples in the area of Hiroshima granite, coarse granite is mainly source and deposition areas were the main objectives. distributed in the middle and lower slopes of the moun- The deposits in the debris flow gullies were also ob- tain, and fine granite overlies the coarse granite. Gener- served. In the geomorphological analysis, we discuss the ally, the grain size in fine granite is around 1 mm, and in effect of topography not only along the longitudinal sec- coarse granite is several mm (Takahashi 2014). In some tion, but also the cross section. To clarify the initiation places, hornfels overlays the coarse granite. Hornfels is a mechanism of the debris flows, we discuss the failure contact metamorphic rock. It is formed when sandstone process of a granitic soil slope under increased pore and shale are indurated and transformed by the heat of water pressure caused by heavy rainfall through a simu- intrusive granite. Relatively, the fine granite and hornfels lation test with triaxial compression equipment. How- are much more resistant to weathering than coarse gra- ever, as there is an absence of data from the two gullies nite. As a result, the slopes in fine granite and hornfels before the disaster, the entrainment or erosion processes are generally steeper than those in coarse granite. The cannot be analyzed in this study. Instead, we only focus Fig. 3 Hourly rainfall at the Uebara meteorological station on 19 and 20 August 2014. The cumulative rainfall is 287.0 mm. The maximum hourly rainfall is 115.0 mm at 4:00 a.m. (local time) on 20 August 2014. The maximum three-hour rainfall is 236.0 mm from 2:00 to 5:00 a.m. on 20 August 2014 (Data source: Hiroshima Prefecture, 2014) Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 5 of 16 Fig. 4 Geological map (1:200,000) and debris flow and landslide distribution. The data are extracted from Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology. The main types of rock in the target area, indicated by a black rectangle, are Hiroshima granite and hornfels on the initiation mechanism of shallow slides in the investigated in more detail. The deposit profile is also source area. shown in the longitudinal sections. Methods Sampling and mechanical property analysis of soils Longitudinal and cross section measurement of the During the field survey, we collected undisturbed and dis- debris flow gullies turbed soil samples in both gullies (see Fig. 5 for sampling We investigated the two gullies of Midori-ga-oka debris locations). In the Midori-ga-oka debris flow, three undis- flow and Abu-no-sato debris flow from the source areas turbed samples (Nos. 1 and 2 in the left source area, and to the downstream limits of the run-out zones (see Fig. 5 No. 3 in the right source area) were collected for the ana- for details). Along the whole gully, we measured the lon- lysis of physical parameters and the mechanical properties gitudinal and cross-sectional profiles at every cliff and of thesoil. At thelocationofsampleNo. 3, adisturbed soil slope using laser rangers, ranging rods, inclinometers sample (labeled as No. 3-DIS) was also collected for the and GPS trackers. The locations for measurement are analysis of the initiation mechanism, for use with triaxial marked by solid cyan circles in Fig. 5. At each point, the tests. In the deposition area of the two gullies, three sam- label was assigned an “S” for a slope or “C” for a cliff, ples (Nos. 4, 5, and 6) were collected for grain size analysis and followed by a sequential number from upstream to and permeability characterization. Finally, the fresh granite, downstream in ascending order. The source areas were the weathered granite, and the granitic soil were also Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 6 of 16 Fig. 5 Aerial photograph and locations for soil sampling (solid yellow crosses) and granite (black diamonds), and locations for measuring longitudinal and cross sections (solid cyan circles). The aerial photograph (extracted from Google Earth) was captured on 13 September 2014 collected in the Midori-ga-oka gully for the geochemical Consolidated-undrained triaxial compression tests analysis. The sample locations are shown in Fig. 5. With sample No. 3-DIS, the consolidated undrained tri- axial tests were conducted to measure the effective soil Geochemical analysis of the granite sample strength parameters. Dry soil passing 2 mm sieving was Major element compositions of the fresh granite, the used to make a cylindrical specimen, and the dry dens- weathered granite, and the granitic soil were determined ity was adjusted to the same value as the in-situ dry using a Rigaku RIX-2000 X-ray fluorescence spectrom- density of the soil. After that, the specimen was fully eter (XRF). The samples were crushed manually in an saturated. iron mortar. The rock chips were then grounded in an With the fully saturated specimens, three consoli- agate mortar crusher for 30 min. All analyses by XRF dated-undrained compression tests were conducted were made on glass beads prepared in an automatic bead under three different confining stresses (50, 75 and 100 sampler, using an alkali flux comprising 80 % lithium tet- kPa). After normal consolidation, the specimen was raborate and 20 % lithium metaborate, with a sample to compressed at 1.0 % axial strain per minute under un- flux ratio of 1:2. The Analytical procedure is described drained conditions. Through the tests, the shear strength by Kimura and Yamada (1996). parameters of the Masa-do were obtained. Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 7 of 16 and micro-sheeting joints are well developed. The aver- age 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 in- troduced 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. Figure 7 shows photographs taken along the gully of the Midori-ga-oka debris flow, from the source area to the bottom. Figure 8 shows the muddy trace of the deb- ris flow on buildings in one of the prefectural housing areas. In the source area, two shallow slides occurred in the granitic soil. In Fig. 7a, the left-hand shallow slide Fig. 6 Photographs of the Midori-ga-oka debris flow (a) and (source area) has, in the upper part, a slope angle of Abu-no-sato debris flow (b) taken on 20 August 2014 (courtesy of 37.7°, a width of 7.9 m and a horizontal distance of Geospatial Information Authority of Japan) 7.4 m, and, in the lower part, a slope angle of 33.9°, simi- lar width, and a horizontal distance of 64.5 m. The right- Pore-water pressure controlled triaxial test hand shallow slide has, in the upper part, a slope angle With sample No. 3-DIS, the pore-water pressure con- of 37.4°, width of 14.4 m, and horizontal distance of trolled triaxial test was performed to simulate the initi- 19 m, and, in the lower part, a slope angle of 32.4°, a ation mechanism of shallow sliding at the source area. similar width and horizontal distance of 47.0 m. In the The slope angle of the source area is about 35°, and the travel path of the debris flow, the gully is V-shaped average thickness of the initial sliding mass is about 1 m. (Fig. 7b) in the upper part near the source area, and be- The potential sliding surface is located in the granitic comes a U-shape in the middle and lower part (Fig. 7c, soil above the bedrock surface. e, f). There are almost no deposits in the flow channel The pore-water pressure controlled test was con- (Fig. 7c, f). All of the debris coming from the weathered ducted following the procedure below. granite has been transported out of the terminal area of the debris flow channel (Fig. 7g), and deposited on the 1) Set the specimen and make it fully saturated. slope where a residential area is located (Fig. 7h). Forty- 2) Apply σ and σ as the initial axial stress and one lives were lost in this area, including those living in 1 3 confining stress. Through this step, the initial stress the prefectural housing and private houses. Among condition of the slope before rainfall (without any them, Midori-ga-oka prefectural housing that consisted pore-water pressure inside the slope) is simulated. of 21 three-floor reinforced concrete residential build- 3) Apply pore-water pressure to the specimen through ings, were directly hit by the debris flow (Fig. 8). It was a pore-water pressure controller at the rate of 0.1 found that in the middle and lower part of the gully, the kPa/minute until specimen failure. Through this sheeting joints were well developed (Fig. 7d). This step, the situation of pore-water pressure caused the weathered granite above the major sheeting accumulation on the potential sliding surface is joint to easily erode away, and the volume of the debris simulated, and the failure process can be observed. flow became larger and the debris flow increased in magnitude and kinetic energy as it flowed down the In this test, the slope angle was assumed to be 35°, steep gully. The material in the debris flow deposits is and soil thickness to be one meter. rich in fine soil particles, being granitic soil, or Masa-do. The fine soil particles may decrease the permeability of Results and discussions the debris flow, and cause greater mobility, in turn con- Field investigation and cross section measurement of the tributing to the longer travel distance and greater lateral two gullies spread. To confirm this inference, soil samples were Based on the field investigation, the geological condi- taken from the deposition area and the grain size distri- tions were found to be different in the source areas of bution was analyzed in the laboratory. the two debris flows. The source area of the debris flow Figure 9 illustrates the longitudinal profile as well as in Midori-ga-oka is composed of coarse granite, with in- the cross-sectional profiles at all measurement locations trusive fine granite. In the coarse granite, sheeting joints (see Fig. 5) in the gully of the Midori-ga-oka debris flow. Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 8 of 16 Fig. 7 Photographs showing the gully of the Midori-ga-oka debris flow. Arrows point in the downstream direction. a three branches in the source area (at cross section S-0 in Fig. 5); (b) strongly weathered granite bedrock exposed by the erosion of debris flow (S-1); (c) shallow bedrock gully after erosion by the debris flow (S-2); (d) erosion along the sheeting joints in granite (C-4); (e) bedrock gully with few debris flow deposits (C-6); (f) granite bedrock gully after extreme erosion, near the bottom of the gully (S-19); (g) the bottom of the debris flow gully, where deposits of different grain size appeared and a temporary ring net countermeasure (at S-23) was finished in December 2014; (h) severely damaged residences beside the pathway of the debris flow, these houses were built on previous debris flow deposits From these measurements, the following aspects were the dam may have caused a debris flow of greater evident: magnitude. This sudden narrowing in the cross sec- tion may had a significant effect on the motion of 1) The longitudinal profile consisted of gentle slopes the debris flow as it flowed to the residential area, (slope angle < 30°, indicated as S) and steep cliffs because of the constricted flow channel. (slope angle > 40°, indicated as C). 4) Nearly all of the debris deposited on the riverbed of 2) Most of the cliffs were located at the middle and the gentle slope in the gully (below S-23 in Fig. 5). lower part of the gully. 3) The cross sections were in a V-shape from S-1 to S- Figure 10 shows the Abu-no-sato debris flow gully 3, and became U-shaped from S-5 to the end (S-21). from the source area to the deposition area. The source The section became narrow in the area between S-3 area is located in hard hornfels rock (Fig. 10a). The slid- and S-17. Notably, the width change from S-15 to ing direction is S20°E. The slope angle is 40°. The width S-17 is very sudden. This could have caused a debris is about 5 m, the length about 30 m, and the depth to dam between S-15 and S-17, and a later collapse of the sliding surface, which is the outcropped bedrock, is Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 9 of 16 Fig. 8 The damaged prefectural housing at the pathway of the Midori-ga-oka debris flow. From the muddy trace on the wall, the flow depth of debris flow was about two-floors high about 0.7 m. The bedrock is fractured with joints, and upper part was V-shaped, and the gully bed consisted of two nearly vertical faults with a width of 0.15 m passed fractured hornfels (Fig. 10c). In some part, colluvium de- through the source area. After a short flow in a wide posits existed in the gully (Fig. 10d). Because of the large gully slope, the debris flow was limited to a narrow gully boulder size, compared with the smaller grain size debris at a cliff C-3 (Fig. 10b), and dropped down, nearly verti- flow material, these would be a higher permeability, thus cally for about 5 m. Lower down, it passed several slopes transportation would be less likely in this area. After and cliffs, and left the hornfels area. The gully in that flowing out of the hornfels bedrock area, and into the Fig. 9 Longitudinal section and cross sections of the Midori-ga-oka debris flow gully Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 10 of 16 Fig. 10 Photographs in the gully of Abu-no-sato. Arrows point in the downstream direction. a source area, the scarp is marked by a yellow line (at cross section S-0); (b) cliff with a joint (C-3); (c) gully-bed with fractured rock, no deposits (S-4); (d) V-shaped gully, with colluvium from one side (S-7); (e) bottom of the gully, coarse-grained deposits of 30 cm average diameter was widely distributed (S-14); (f) narrow neck of the gully (S-16); (g) downstream deposit area with shallow coarse-grained layer (S-19); (h) the houses along a road damaged by the debris flow in the Abu-no-sato community coarse granite bedrock area, the slope angle became gen- destroyed two houses and caused 4 deaths. Some houses tle (17.7°), and most of the boulders deposited at the along the slope road (see Fig. 6b) were damaged to dif- bottom of the gully (Fig. 10e). Because the content of ferent extents, mainly by rolling boulders and rapid the debris flow is composed of boulders, most of the water flow. boulders were deposited in the gentle and wide slope be- Figure 11 illustrates the longitudinal section and cross fore they reached the residential area as pore water pres- sections at all measurement locations (see Fig. 5) in the sure reduced rapidly. However, an old debris flow gully of Abu-no-sato debris flow. From the information deposit terrace caused a dam of the debris flow. Collapse in the figure, the following effects can be observed. occurred at both sides of the terrace, and deep erosion developed (Fig. 10f shows the right side narrow channel). 1) The slope was steep in the hornfels area, and Through the narrow channels, part of the debris flow in- became gentle in the coarse-grained granite area. cluding boulders and fallen trees were deposited on a 2) Most of the cliffs were distributed in the middle and wide gentle slope (Fig. 10g). A small amount of debris- upper parts of the gully. flow material flowed beyond the boundary between the 3) The gully was V-shaped in most of the hornfels area, natural slope and the residential area (a local road), and gradually became wider. Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 11 of 16 Fig. 11 Longitudinal section and cross sections of the Abu-no-sato debris flow gully 4) From C-15 to S-17, the cross sections narrowed rap- all samples is around 2.64. Samples Nos. 1 and 2 are idly. This change may have caused a dam in the deb- from the same landslide, and have similar properties. ris flow, and the later outburst may be a direct cause The dry densities of sample Nos. 1 and 2 are higher than of the disaster. that of sample No. 3. Figure 12 illustrates the grain size distribution of all Compared with the Midori-ga-oka debris flow, the dis- samples (Nos. 1–6). Sample Nos. 1 to No. 5 are from the tance between the residential area and the starting point Midori-ga-oka debris flow gully. Nos. 1, 2, and 3 are of the deposition area is much larger in Abu-no-sato from the source area, and Nos. 4 and 5 are from the exit debris flow. It may mean that keeping enough space be- of the gully, the first deposit of the debris flow. Sample tween the deposition start point and the residential area No. 3 comprises much finer particles than the other is very important for disaster reduction. samples, and the mean grain size D is 0.035 mm. While the mean grain sizes of samples Nos. 1, 2, 4 and 5 Soil properties: in-situ density and grain size distribution are close to 1 mm, their distributions are also similar. The locations of sample Nos. 1–6 are illustrated in Fig. 5. Comparing the sample from the deposition area with Table 1 shows the physical parameters of undisturbed that from the source area, the grain size is not much dif- sample Nos. 1, 2 and 3. The value of specific gravity for ferent. The amount of fine particles is slightly less in the deposition area. This may mean that the finer particles Table 1 Parameters of soil samples in the source area of the are transported for a longer distance. Sample No. 6 is Midori-ga-oka gully from the lower part of the deposition area of the Abu- Sample No. 1 2 3 no-sato debris flow, which has the smallest grain size in Specific gravity, G 2.640 2.643 2.638 the debris flow deposit. In the upstream part, the deposit Dry density, ρ (kg/m ) 1,370 1,390 1,189 d generally consists of boulders with a mean size of 0.3– Water content, w (%) 15.2 19.5 31.5 0.5 m, and a maximum size up to 1.5–2.0 m. From the grain size distribution, it can be seen that the particles in Liquid limit, LL (%) —— 32.1 the Midori-ga-oka debris and the Abu-no-sato debris Plastic limit, PL (%) —— 23.4 flows are different in grain size and, in turn, we can infer Plasticity Index, PI (%) —— 8.7 that they have different permeability and drainage Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 12 of 16 Fig. 12 Grain size distribution of samples Nos. 1–6 characteristics. At a given travel velocity, the low perme- Geochemical analysis of the granite sample ability, which is associated with a slower rate of drainage Major element compositions of the fresh granite, the of the debris at Midori-ga-oka (mainly consisting of weathered granite, and the granitic soil in the Midori- granitic soil), may mean that the flow has greater mobil- ga-oka gully are listed in Table 2. The fresh granites are ity and may travel for a longer distance because it re- hard rocks and the constituent minerals are mostly un- mains in the saturated condition for longer. The altered. The weathered rocks are relatively hard, but are boulders in the Abu-no-sato debris flow mean that de- discolored to yellowish white. Plagioclase has changed position occurs much earlier as the debris drains rapidly color to creamy white. Fe (iron) stains surround the bio- and changes to the unsaturated condition. The critical tite and other minerals. The granitic soils are mainly velocity of a debris flow transformed from saturated to composed of clays, along with less than half the amount unsaturated condition will be of significance in predict- of fine altered minerals. Small quartz grains are some- ing the run-out mobility of debris flow. times visible. Composed of 70 % of particles finer than 0.1 mm, The Chemical Index of Alteration (CIA, defined by sample No. 3 was used to conduct the Atterberg limit Nesbitt and Young 1982) is employed to investigate the tests. The results are shown in Table 1. In addition, degree of chemical weathering of the collected samples, Fig. 13 shows that the soil classification of the sample as it is well known as a functional tool to estimate the No. 3 is between Silt (ML) and Low plasticity Clay (CL) weathering state of granitic rocks (e.g., Fedo et al. 1995; with a low liquid limit (ASTM D2487-11 2011). Kamei et al. 2012). The Eq. (1) is the formula. CIA ¼ 100  Al O =ðÞ Al O þ CaO þNa O þ K O molar basis 2 3 2 3 2 2 ð1Þ Where CaO* represents Ca in the silicate fraction only. Fresh granitic rocks generally have CIA values near 50. Al O is less mobile during the weathering process, 2 3 whereas CaO*, Na O, and K O are relatively more mo- 2 2 bile (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. Fig. 13 Plasticity chart of sample No. 3 (red solid triangular). Nesbitt and Markovics (1997) reported the CIA values Modified from ASTM D2487-11 (2011). CL = Lean clay; ML = silt of typical clay-rich weathered granitoids (72.7–81.6) Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 13 of 16 Table 2 Major element compositions of granite and related samples in the Midori-ga-oka gully Location Type SiO TiO Al O Fe O MnO MgO CaO NaOKOP O LOI Total CIA 2 2 2 3 2 3 2 2 2 5 G1 FG 72.58 0.23 14.97 2.45 0.05 0.50 1.75 3.10 4.10 0.03 0.71 100.47 54.2 G1 GS 71.74 0.31 14.74 3.24 0.08 0.62 1.66 2.79 3.52 0.01 1.96 100.66 56.4 G2 FG 74.59 0.19 13.80 2.21 0.05 0.43 1.18 2.92 4.26 0.02 0.80 100.45 54.5 G2 GS 69.46 0.29 16.43 2.89 0.16 0.54 0.67 1.88 3.71 0.04 5.20 101.27 66.6 G2 GS 67.20 0.38 17.87 3.38 0.19 0.66 0.45 1.71 3.87 0.05 6.09 101.85 69.9 G3 FG 73.69 0.25 13.80 2.87 0.06 0.57 1.64 2.91 3.98 0.04 0.52 100.33 53.5 G3 WG 72.45 0.26 14.55 2.68 0.08 0.56 1.56 2.85 3.84 0.02 1.79 100.65 55.5 G3 GS 72.78 0.26 14.56 2.59 0.13 0.51 1.01 2.43 3.80 0.00 2.75 100.81 59.4 G4 FG 76.52 0.11 12.85 1.65 0.04 0.31 1.08 2.92 4.50 0.01 0.19 100.17 52.6 G4 WG 76.39 0.12 12.87 1.64 0.04 0.32 1.04 2.89 4.37 0.01 0.44 100.14 53.1 G4 GS 72.72 0.23 14.55 2.38 0.08 0.45 0.79 2.25 3.86 0.02 3.44 100.78 61.1 G1 to G4: sample locations, showing in Fig. 5. FG Fresh granite, WG Weathered granite, GS Granitic soil, LOI loss on ignition, CIA Chemical Index of Alteration (Nesbitt and Young, 1982) from the outer layer of the spheroidal weathering- indicator in order to detect the active area of debris boulder in Toorongo granodiorite, Australia. The weath- flows in a granitic region. ering conditions of the sample location are similar to those of Hiroshima Prefecture. Summers of both regions Consolidated-undrained triaxial compression tests are warm, averaging 25–30 °C, and winters are cold with The stress–strain relation of the consolidated-undrained temperatures of −5 to 10 °C. Summer precipitation is tests on the saturated specimen (Fig. 14a) shows that the 200–300 mm with annual precipitation approximately deviatoric stress increases in the beginning. When the 1,500 mm for both. As a result of the similarities, the axial strain is about 2 %, the deviatoric stress reaches the CIA values of Toorongo granodiorite would be a good peak value. After that, the deviatoric stress turns to de- comparison with our samples. However, the granitic soils crease with the axial strain. Bulging failure was observed of Midori-ga-oka have significantly lower CIA values for all specimens at the end of the tests. The stress– (56.4–69.9) compared with those of the weathered gran- strain curves indicate that the critical state of the soil itoids in Toorongo granodiorite (72.7–81.6). This sug- will be reached at low strains value. gests that the degree of chemical weathering of the The relation between excess pore-water pressure and granitic soils in the Midori-ga-oka gully is less than that the axial strain in the consolidated-undrained tests on of normally weathered granitic materials. This provides saturated specimens is shown in Fig. 14b. Positive excess the evidence that the weathered materials in the shallow pore-water pressure was generated in all three undrained slide area were in a removal cycle from the steep slope, tests. In all soil specimens, the positive excess pore- due to repeated debris flows. Therefore, the lower CIA water pressure increases to a high value with minor values of soil materials in debris flows may be a good strain, and increases slowly as the strain increases Fig. 14 Results of the consolidated undrained test: (a) stress–strain relation; (b) relation between pore-water pressure and axial strain Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 14 of 16 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. All of the effective stress paths show a similar trend. Each path moves to the right initially (increasing 0 0 σ þ σ =2), and then moves toward the left (decreas- 1 3 0 0 ing σ þ σ =2 ) until reaching the critical state line 1 3 (CSL) at the end of the test. The critical states of the sat- urated specimens can be represented by the CSL in the stress plane, as shown in Fig. 15. The gradient of the critical state line is 0.404. This corresponds to an in- ternal effective friction angle of 23.8° and cohesion of 5.5 kPa of the granitic soil. Pore-water pressure controlled triaxial test Fig. 16 Relations between deviatoric stress, axial strain and pore- To determine the initial stress condition of soil sample water pressure for the pore-water pressure controlled triaxial test on No. 3-DIS, the thickness (H) of the soil layer and slope sample No. 3-DIS angle (α) are assumed as 1 m and 35°, respectively, based on the field measurement results in the source area of the debris flow. As shown in Table 1, the unit weight (γ) along the critical state line, and ends at point D, where the of the natural soil is 15.6 kN/m . From consolidated pore-water pressure is 8.3 kPa. This simulation test can undrained triaxial tests, the internal effective friction explain the initiation mechanism of the shallow landslide angle (ϕ′) of the soil is 23.8°. The initial maximum prin- that occurred on a steep slope. During heavy rainfall, con- cipal stress (σ ) and minimum principal stress (σ ) are 1 3 tinuous rainfall infiltration can generate a wet front in the 21.7 and 5.7 kPa, respectively. Through the pore-water soil slope and in turn form a saturated zone above the po- pressure controlled triaxial test, the relations between tential sliding surface. Gradually, the saturated zone will deviatoric stress, axial strain and pore-water pressure move upwards in relation to the slope surface, and were obtained (Fig. 16). the pore-water pressure acting on the potential sliding Figure 16 shows that, when the pore-water pressure in- surface will increase. Finally, it will cause shallow land- creased to about 4.0 kPa, the deviatoric stress started to slide. Because this phenomenon occurs in steep slopes, it decrease, while the axial strain increased. The same soil will create a debris flow moving downward for a long dis- behavior also can be found in Fig. 17. The effective stress tance. From the test, we can see that, with low pore-water path begins to go down when pore-water pressure in- creases to about 4.0 kPa (point B). After this point, yield- ing occurs, and the effective stress path moves downward and to the left, then reaches the critical state line (point C, where pore-water pressure is 6.5 kPa). This shows the major failure of the soil. With the continuous supply of pore-water pressure, the stress path moves down and left Fig. 17 Effective stress paths of pore-water pressure controlled Fig. 15 Effective stress paths under different confining pressures (50, triaxial test, the dotted line indicates the critical state line (CSL) 75 and 100 kPa), the dotted line indicated the critical state line (CSL) obtained in undrained tests Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 15 of 16 pressures (i.e., when the rainfall is not so heavy), the slope 6) Simulation test with pore-water pressure controlled can maintain stability. However, if the pore-water pressure triaxial test shows reasonable response of soil behav- exceeds the critical value, as a result of heavy rainfall, for ior under different water pressure conditions, which example, shallow landslide will be triggered. In this simu- corresponds to groundwater level in a real slope. It lation test, the critical value of the pore-water pressure is is hoped that the groundwater level in a real slope about 4.0 kPa. Considering the slope angle of the source can be used for failure prediction. area is 35°, and the thickness of the initial sliding mass is Competing interests 1 m, the pore-water pressure of 4.0 kPa and 6.5 kPa mean The authors declare that they have no competing interests. that the groundwater levels are about 0.60 m and 0.97 m high, respectively, above the sliding surface. So when the Authors’ contributions All authors participated the field investigation; FW prepared the first draft of groundwater level rises to 0.6 m above the potential slid- the paper; HY conducted the soil tests in laboratory; YHW and ST prepared ing surface, instability may occur in the slope, and deform- the sections and profiles; AK made the geochemical analysis on the granite ation may develop. When the groundwater level rises to sample. All authors read and approved the final manuscript. 0.97 m, the slope will completely fail. For a slope with Acknowledgements depth of 1 m, the groundwater level of 0.97 m almost Based on the field investigation experiences, we like to make some means the existence of surface flow along the slope. These suggestions for countermeasure works to prevent debris flow disasters: (a) two values of the groundwater level may be used as a ref- 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 erence for the prediction of shallow landslide initiation be effective to stop the travel of boulders; (c) For weathered granite area, under similar condition. Since the soil sample used for tri- the use of a ring-net to stop large boulders and, in addition, building a large axial tests is disturbed soil, the in-situ critical value of the 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 pore-water pressure may be slightly higher than experi- time to take action. Otherwise, delayed warnings are useless when considering mental value. However, using a smaller value is conserva- the weather condition that can cause debris flows. tive, erring on the side of community safety. 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 Conclusions Geoscience, Shimane University, Ryoichi Tsukamoto, Tomohiro Oda, Norisato Based on field investigations and laboratory tests, the Oishi, Naho Yamamoto, Masafumi Yokoyama joined the field investigation and assisted the topographic survey and soil/rock sampling. Lynn Highland following conclusions are reached. of U.S. Geological Survey made constructive comments of the draft. Valuable and constructive comments from anonymous reviewers are deeply 1) Under extreme high intensity and short duration appreciated. rainstorm, debris flows occurred on very steep Received: 13 February 2015 Accepted: 30 June 2015 slopes with a thin initiating sliding mass in weathered coarse-grained granite and hornfels. 2) Cross sectional properties of the debris flow gully, References ASTM D2487-11 (2011) Standard Practice for Classification of Soils for Engineering especially gully shape and changes in width, have Purposes (Unified Soil Classification System). ASTM International, West important implications for debris flow damming, Conshohocken, PA. doi:10.1520/D2487-11 travel distance, and deposition. Casagli N, Dapporto S, Ibsen ML, Tofani V, Vannocci P (2006) Analysis of the landslide triggering mechanism during the storm of 20th–21st November 3) During travel, the debris flow may erode the valley 2000, in Northern Tuscany. Landslides 3(1):13–21 and carry the colluvium and valley deposits Cevaso A, Pepe G, Brandolini P (2014) The influence of geological and land use downslope. Shallow slides in high mountains can settings on shallow landslides triggered by an intense rainfall event in a coastal terraced environment. Bull Eng Geol Environ 73:859–875 cause large-scale debris flow. Chen SC, Chou HT, Chen SC, Wu CH, Lin BS (2014) Characteristics of rainfall- 4) Debris flows that are rich in fine particles like induced landslides in Miocene formations: a case study of the Shenmu granitic soil, tend to travel long distances until they watershed, Central Taiwan. Eng Geol 169:133–146 Dijkstra TA, Wasowski J, Winter MG, Meng XM (2014) Introduction to geohazards flow onto very gentle slopes, while those with of Central China. Q J Eng Geol Hydrogeol 47(3):195–199 boulders as components started to deposit as soon Duan W, He B, Takara K, Luo P, Nover D, Yamashiki Y, Huang W (2014) as the valley became wide and gently-sloped. The Anomalous atmospheric events leading to Kyushu’s flash floods, July 11–14, 2012. Nat Hazards 73:1255–1267 permeability and drainage characteristics of the deb- Fedo CM, Nesbitt HW, Young GM (1995) Unraveling the effects of potassium ris flow controlled the travel distance of the debris metasomatism in sedimentary rocks and paleosols, with implications for flow. paleoweathering conditions and provenance. Geology 23:921–924 García-Martínez R, López JL (2005) Debris flows of December 1999 in Venezuela. 5) The degree of chemical weathering of the granitic In: Jakob M, Hungr O (eds) Debris-flow Hazards and Related Phenomena, soils in the Midori-ga-oka gully is less than that of 519–538, Springer normally weathered granitic materials. This provides Harnois L (1988) The CIW index: a new chemical index of weathering. Sediment Geol 55:319–322 the evidence that the weathered materials in the Hiroshima Prefecture (2014) Monitoring information in Hiroshima Prefecture., shallow slide area were in a removal cycle from the http://www.bousai.pref.hiroshima.jp/info/disp?disp=R60100&fmode= steep slope, due to repeated debris flows. 1&year=2014&month=8, Accessed 20 Dec 2014 (In Japanese) Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 16 of 16 Japan Meteorological Agency (2015) The Query of Past Meteorological Data., http://www.data.jma.go.jp/obd/stats/etrn/index.php. Accessed 5 Jan 2015 (In Japanese) Kamei A, Fukushi K, Takagi T, Tsukamoto H (2012) Chemical overprinting of magmatism by weathering: a practical method for evaluating the degree of chemical weathering of granitoids. Appl Geochem 27:796–805 Kimura J-I, Yamada Y (1996) Evaluation of major and trace element analyses using a flux to sample ratio of two to one glass beads. J Min Petrol Econ Geol 91:62–72 Lepore C, Kamal SA, Shanahan P, Bras RL (2012) Rainfall-induced landslide susceptibility zonation of Puerto Rico. Environ Earth Sci 66:1667–1681 Ministry of Land, Infrastructure, Transport and Tourism (2014) Report of countermeasure of slopeland disasters in Hiroshima triggered by heavy rainfall in August, 2014., http://www.mlit.go.jp/river/sabo/H26_hiroshima/ 141031_hiroshimadosekiryu.pdf. Accessed 5 Jan 2015 (in Japanese) Montgomery DR, Schmidt KM, Dietrich WE, McKean J (2009) Instrumental record of debris flow initiation during natural rainfall: implications for modeling slope stability. J Geophysics Res-Earth 114:F01031 Nesbitt HW, Markovics G (1997) Weathering of granodioritic crust, long-term storage of elements in weathering profiles, and petrogenesis of siliciclastic sediments. Geochim Cosmochim Acta 61:1653–1670 Nesbitt HW, Young GM (1982) Early Proterozoic climates and plate motions inferred from major element chemistry of lutites. Nature 199:715–717 Ni H, Zheng W, Song Z, Xu W (2014) Catastrophic debris flows triggered by a 4 July 2013 rainfall in Shimian, SW China: formation mechanism, disaster characteristics and the lessons learned. Landslides 11(5):909–921 Okada Y, Kurokawa U (2015) Examining effects of tree roots on shearing resistance in shallow landslides triggered by heavy rainfall in Shobara in 2010. J For Res 20:230–235 Saito H, Matsuyama H (2012) Catastrophic landslide disasters triggered by record- breaking rainfall in Japan: their accurate detection with normalized soil water index in the Kii peninsula for the year 2011. SOLA 8:81–84 Takahashi Y (2014) The 2014.8.20 sediment disasters in Hiroshima, especially on the geological factors., http://www.geosociety.jp/hazard/content0082.html, Accessed 3 Nov 2014 (In Japanese) Tang C, van Asch TWJ, Chang M, Chen GQ, Zhao XH, Huang XC (2012) Catastrophic debris flows on 13 August 2010 in the Qingping area, southwestern China: the combined effects of a strong earthquake and subsequent rainstorms. Geomorphology 139–140:559–576 The Chugoku Shimbun Online (1999) Hazardous area expands while 26 people dead and 9 missing., http://web.archive.org/web/20080316224507/http:// www.chugoku-np.co.jp/News/990629_gouu/Tn99070101.html. Accessed 17 Jan 2015 (in Japanese) The Japan Times (2014) Mudslides kill 36 in Hiroshima., http:// www.japantimes.co.jp/news/2014/08/20/national/least-eight-dead-hiroshima- landslides-floods/#.VKpOensXd0Z. Accessed 5 Dec 2014 Tsuchida T, Kano S, Nakagawa S, Kaibori M, Nakai S, Kitayama N (2014) Landslide and mudflow disaster in disposal site of surplus soil at Higashi-Hiroshima due to heavy rainfall in 2009. Soils Found 54(4):621–638 Wang G, Sassa K, Fukuoka H (2003) Downslope volume enlargement of a debris slide-debris flow in the 1999 Hiroshima, Japan, rainstorm. Eng Geol 69:309–330 Winter MG, Harrison M, Macgregor, Shackman L (2013) Landslide hazard assessment and ranking on the Scottish road network. Proceedings, Institution of Civil Engineers (Geotechnical Engineering), 166(GE6), 522–539.. doi:10.1680/geng.12.00063 Yamamoto H, Kobayashi H (2014) Characteristics of heavy rainfall and debris flow disaster in Hiroshima City by Akisame-front, 20 August 2014. J Jpn Soc Nat Disaster Sci 33(3):293–312 (in Japanese with English Abstract) Submit your manuscript to a Yang H, Wang F, Miyajima M (2015) Investigation of shallow landslides triggered by heavy rainfall during typhoon Wipha (2013), Izu Oshima Island, Japan. journal and benefi t from: Geoenvironmental Disasters 2:15. doi:10.1186/s40677-015-0023-8 7 Convenient online submission 7 Rigorous peer review 7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the fi eld 7 Retaining the copyright to your article Submit your next manuscript at 7 springeropen.com http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Geoenvironmental Disasters Springer Journals

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

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Environment; Environment, general; Earth Sciences, general; Geography (general); Geoecology/Natural Processes; Natural Hazards; Environmental Science and Engineering
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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. Keywords: Hiroshima; Heavy rainfall; Debris flow; Granite; Hornfels; Investigation Background and 74 deaths. In addition, 133 houses were destroyed In the early morning of 20 August 2014, a high-intensity/ and 296 houses were severely damaged (Ministry of short-duration, localized rainfall event triggered many Land, Infrastructure, Transport and Tourism 2014; shallow slides and debris flows in the Asakita and Yamamoto and Kobayashi 2014). Shallow slides were Asaminami Wards in the northern part of Hiroshima designated as those that moved for a limited distance City, Japan. In all, there were 166 slope failures trig- from thesourcearea. Most of thelossof lifeand prop- gered by the heavy rainfall event, including 107 debris erty was caused by debris flows. In this paper, all of the flows and 59 shallow slides, which caused 44 injuries slope failures are called debris flows for simplification. Figure 1 illustrates the distribution of debris flows and * Correspondence: wangfw@riko.shimane-u.ac.jp fatalities. As indicated, the debris flows were distributed Department of Geoscience, Shimane University, Matsue 690-8504, Japan 2 in an elongated area extending northeast to southwest Project Center on Natural Disaster Reduction, Shimane University, Matsue and covering the Asaminami Ward and Asakita Ward. 690-8504, Japan © 2015 Wang et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 2 of 16 Fig. 1 Distribution of debris flows and landslide-affected area and number of fatalities. The longitude and latitude is in the decimal format in the all figures hereafter. The debris-flow distribution and fatality number are obtained from Ministry of Land, Infrastructure, Transport and Tourism. DEM data is from Geospatial Information Authority of Japan (GSI). The black rectangle indicates the study area (see Fig. 4 for more detailed aerial photograph) Most of the fatalities were concentrated in the Asaminami Southwest Japan and formed devastating floods and mud- Ward (Asa is the name of the region; Asaminami means slides, and was associated with 32 people who were re- Southern Asa, while Asakita means Northern Asa, in ported either dead or missing; 400,000 people were Japanese). evacuated from their homes (Duan et al. 2014). Disasters In past decades, many landslide and debris flow events triggered by extreme-rainfall have become a critical and have been triggered by high intensity, short-duration urgent issue for society. rainstorms, and caused loss of life and infrastructure In the Hiroshima area, the 2014 events were preceded damage worldwide (e.g. García-Martínez and López by other catastrophic sediment disasters in recent years. 2005; Casagli et al. 2006; Tang et al. 2012; Cevaso et al. At the end of June 1999, 139 debris flows and 186 shal- 2014; Chen et al. 2014; Ni et al. 2014; Yang et al. 2015). low slides were triggered by rainfall, and caused 32 For example, in December 1999, extreme rainfall on the deaths in a nearby area (The Chugoku Shimbun Online northern Venezuelan coast triggered a disastrous debris 1999; Wang et al. 2003). Mainly because of this disaster, flow, which caused about 1,500 fatalities and the destruc- a law referred to as the “Sediment Disaster Countermea- tion of 23,000 houses (García-Martínez and López 2005). sures for Sediment Disaster Prone Areas Act” was Recently, Japan also suffered from many disasters trig- enacted in 2000 to prevent sediment problems caused gered by extreme rainfall. In September 2011, Typhoon by debris flows and other causes. Despite the protection 12 (Talas) attacked the Kii peninsula of central Japan and of the law for the previous 14 years, the loss of life triggered many landslides and floods, causing 97 casual- caused by debris-flow sediment disasters triggered by ties (Saito and Matsuyama 2012). Then in July 2012, an one heavy rainfall event was not prevented; indeed the unprecedented four-day heavy rainfall hit Kyushu in loss of life in the 2014 events was almost three times Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 3 of 16 that in 1999. These events brought a huge amount of mechanism of shallow landslides or mudslides under dif- distress, and severely impacted the local community. ferent rainfall conditions. In order to assess the future The public as well as the academic community, are ex- risks in the disaster area, field survey and laboratory tremely interested in the causes of the disaster and de- tests for the analyses of geological and geomorphological sire to know the triggering factors and field conditions. conditions, and soil properties are considered in this Many methodologies have been developed and applied paper. Therefore, we shall report the hazard background to this topic. To reveal the disaster impact in a larger briefly as well as the results of the field survey and la- area, statistical analysis is applied to analyze landslide boratory analysis of soil mechanical and geochemical susceptibility in terms of slope inclination, land use, and properties. As shown with a black rectangle in Fig. 1, other factors (e.g., Lepore et al. 2012; Tang et al. 2012; our investigation focuses on two debris flows in the Winter et al. 2013; Cevaso et al. 2014; Chen et al. 2014; Asaminami Ward, which suffered major life loss due to Dijkstra et al. 2014). On the other hand, for a localized its higher population density on the narrow alluvial catchment, the physically-based models, e.g., slope sta- plain, compared to the Asakita Ward. bility analysis and hydrological models, (e.g., Casagli The main triggering factor of the debris flows was the et al. 2006; Tsuchida et al. 2014) or field monitoring and high intensity and short duration rainstorm. Figure 2 il- laboratory tests (e.g., Montgomery et al. 2009; Okada lustrates the isohyetal map of the cumulative rainfall for and Kurokawa 2015) are used to analyze the initiation 48 h on 19 and 20 August 2014. It shows that most Fig. 2 Isohyetal map of cumulative rainfall on 19 to 21 August 2014. The data found in the isohyetal lines are interpolated using the data from 13 stations (marked by solid black circles) of the Japan Meteorological Agency, Hiroshima Prefecture Government, and Ministry of Land, Infrastructure, Transport and Tourism. The maximum rainfall occurred at the Uebara meteorological station, with an amount of 287 mm. The debris flows and landslides occurred along the area with intense rainfall. The hourly rainfall hydrograph at the Uebara meteorological station, which is located at the center of the area affected by debris flows, is illustrated in Fig. 3 Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 4 of 16 of debris flows occurred in the area of intense rainfall, of coarse Hiroshima granite is very easily weathered. The which the cumulative rainfall is greater than 200 mm. granitic soil (i.e., the residual soil) is locally called Masa- The maximum rainfall was recorded at the Uebara me- do, which means “real sand” in Japanese. Because of the teorological station, with an amount of 287 mm, and the intense wreathing, the Masa-do can easily lose its struc- recorded maximum in 2 days during the disaster is al- ture and collapse during heavy rainfall. most 2.6 times the August monthly-average value of In this study, we investigated two gullies in which the 110.8 mm in Hiroshima (Japan Meteorological Agency greatest number of fatalities occurred, called Midori-ga- 2015). Figure 3 shows the hourly rainfall hydrograph at oka and Abu-no-sato, respectively (see Fig. 5 for the the Uebara meteorological station. The maximum three- affected areas and positions). Both of the debris flows hour rainfall is 236 mm from 2:00 to 5:00 a.m. (local originated from similar elevations and can be catego- time) on 20 August. In particular, the maximum hourly rized as channelized, but their affected areas are quite rainfall is 115 mm at 4:00 a.m. on the same day. Wit- different. The Midori-ga-oka debris flow entered the nesses verified that the debris flows occurred around residential area and persisted over a long distance while 4:00 a.m. on the same day (The Japan Times 2014). We also spreading to cover a wide area. This phenomenon conclude that this unusual and extreme rainfall is the can also be observed in (Fig. 6a). However, the Abu-no- main triggering factor of these events. sato debris flow stopped behind the residential area Figure 4 illustrates the geology of the affected area. (Fig. 6b). The different travel distances may imply dif- Debris flows were distributed in an area 10 km by 2 km, ferent mobility rates of the debris flows. Understanding extending in a NE-SW direction. The elevation in this the factors causing the different mobility may be helpful area reaches 700 m. A valley, which is controlled by a for disaster reduction. For this reason, we selected these NE-SW fault, trends through this area. Late Cretaceous two debris flows for detailed study. granite, also called Hiroshima granite, is the main bed- In the field investigation, observation of the geological rock in this mountainous region. In the valley and lower conditions, measurement of the longitudinal and cross- slopes, deposits of weathered granite are encountered. In sectional profiles, and the collection of soil samples in the area of Hiroshima granite, coarse granite is mainly source and deposition areas were the main objectives. distributed in the middle and lower slopes of the moun- The deposits in the debris flow gullies were also ob- tain, and fine granite overlies the coarse granite. Gener- served. In the geomorphological analysis, we discuss the ally, the grain size in fine granite is around 1 mm, and in effect of topography not only along the longitudinal sec- coarse granite is several mm (Takahashi 2014). In some tion, but also the cross section. To clarify the initiation places, hornfels overlays the coarse granite. Hornfels is a mechanism of the debris flows, we discuss the failure contact metamorphic rock. It is formed when sandstone process of a granitic soil slope under increased pore and shale are indurated and transformed by the heat of water pressure caused by heavy rainfall through a simu- intrusive granite. Relatively, the fine granite and hornfels lation test with triaxial compression equipment. How- are much more resistant to weathering than coarse gra- ever, as there is an absence of data from the two gullies nite. As a result, the slopes in fine granite and hornfels before the disaster, the entrainment or erosion processes are generally steeper than those in coarse granite. The cannot be analyzed in this study. Instead, we only focus Fig. 3 Hourly rainfall at the Uebara meteorological station on 19 and 20 August 2014. The cumulative rainfall is 287.0 mm. The maximum hourly rainfall is 115.0 mm at 4:00 a.m. (local time) on 20 August 2014. The maximum three-hour rainfall is 236.0 mm from 2:00 to 5:00 a.m. on 20 August 2014 (Data source: Hiroshima Prefecture, 2014) Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 5 of 16 Fig. 4 Geological map (1:200,000) and debris flow and landslide distribution. The data are extracted from Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology. The main types of rock in the target area, indicated by a black rectangle, are Hiroshima granite and hornfels on the initiation mechanism of shallow slides in the investigated in more detail. The deposit profile is also source area. shown in the longitudinal sections. Methods Sampling and mechanical property analysis of soils Longitudinal and cross section measurement of the During the field survey, we collected undisturbed and dis- debris flow gullies turbed soil samples in both gullies (see Fig. 5 for sampling We investigated the two gullies of Midori-ga-oka debris locations). In the Midori-ga-oka debris flow, three undis- flow and Abu-no-sato debris flow from the source areas turbed samples (Nos. 1 and 2 in the left source area, and to the downstream limits of the run-out zones (see Fig. 5 No. 3 in the right source area) were collected for the ana- for details). Along the whole gully, we measured the lon- lysis of physical parameters and the mechanical properties gitudinal and cross-sectional profiles at every cliff and of thesoil. At thelocationofsampleNo. 3, adisturbed soil slope using laser rangers, ranging rods, inclinometers sample (labeled as No. 3-DIS) was also collected for the and GPS trackers. The locations for measurement are analysis of the initiation mechanism, for use with triaxial marked by solid cyan circles in Fig. 5. At each point, the tests. In the deposition area of the two gullies, three sam- label was assigned an “S” for a slope or “C” for a cliff, ples (Nos. 4, 5, and 6) were collected for grain size analysis and followed by a sequential number from upstream to and permeability characterization. Finally, the fresh granite, downstream in ascending order. The source areas were the weathered granite, and the granitic soil were also Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 6 of 16 Fig. 5 Aerial photograph and locations for soil sampling (solid yellow crosses) and granite (black diamonds), and locations for measuring longitudinal and cross sections (solid cyan circles). The aerial photograph (extracted from Google Earth) was captured on 13 September 2014 collected in the Midori-ga-oka gully for the geochemical Consolidated-undrained triaxial compression tests analysis. The sample locations are shown in Fig. 5. With sample No. 3-DIS, the consolidated undrained tri- axial tests were conducted to measure the effective soil Geochemical analysis of the granite sample strength parameters. Dry soil passing 2 mm sieving was Major element compositions of the fresh granite, the used to make a cylindrical specimen, and the dry dens- weathered granite, and the granitic soil were determined ity was adjusted to the same value as the in-situ dry using a Rigaku RIX-2000 X-ray fluorescence spectrom- density of the soil. After that, the specimen was fully eter (XRF). The samples were crushed manually in an saturated. iron mortar. The rock chips were then grounded in an With the fully saturated specimens, three consoli- agate mortar crusher for 30 min. All analyses by XRF dated-undrained compression tests were conducted were made on glass beads prepared in an automatic bead under three different confining stresses (50, 75 and 100 sampler, using an alkali flux comprising 80 % lithium tet- kPa). After normal consolidation, the specimen was raborate and 20 % lithium metaborate, with a sample to compressed at 1.0 % axial strain per minute under un- flux ratio of 1:2. The Analytical procedure is described drained conditions. Through the tests, the shear strength by Kimura and Yamada (1996). parameters of the Masa-do were obtained. Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 7 of 16 and micro-sheeting joints are well developed. The aver- age 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 in- troduced 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. Figure 7 shows photographs taken along the gully of the Midori-ga-oka debris flow, from the source area to the bottom. Figure 8 shows the muddy trace of the deb- ris flow on buildings in one of the prefectural housing areas. In the source area, two shallow slides occurred in the granitic soil. In Fig. 7a, the left-hand shallow slide Fig. 6 Photographs of the Midori-ga-oka debris flow (a) and (source area) has, in the upper part, a slope angle of Abu-no-sato debris flow (b) taken on 20 August 2014 (courtesy of 37.7°, a width of 7.9 m and a horizontal distance of Geospatial Information Authority of Japan) 7.4 m, and, in the lower part, a slope angle of 33.9°, simi- lar width, and a horizontal distance of 64.5 m. The right- Pore-water pressure controlled triaxial test hand shallow slide has, in the upper part, a slope angle With sample No. 3-DIS, the pore-water pressure con- of 37.4°, width of 14.4 m, and horizontal distance of trolled triaxial test was performed to simulate the initi- 19 m, and, in the lower part, a slope angle of 32.4°, a ation mechanism of shallow sliding at the source area. similar width and horizontal distance of 47.0 m. In the The slope angle of the source area is about 35°, and the travel path of the debris flow, the gully is V-shaped average thickness of the initial sliding mass is about 1 m. (Fig. 7b) in the upper part near the source area, and be- The potential sliding surface is located in the granitic comes a U-shape in the middle and lower part (Fig. 7c, soil above the bedrock surface. e, f). There are almost no deposits in the flow channel The pore-water pressure controlled test was con- (Fig. 7c, f). All of the debris coming from the weathered ducted following the procedure below. granite has been transported out of the terminal area of the debris flow channel (Fig. 7g), and deposited on the 1) Set the specimen and make it fully saturated. slope where a residential area is located (Fig. 7h). Forty- 2) Apply σ and σ as the initial axial stress and one lives were lost in this area, including those living in 1 3 confining stress. Through this step, the initial stress the prefectural housing and private houses. Among condition of the slope before rainfall (without any them, Midori-ga-oka prefectural housing that consisted pore-water pressure inside the slope) is simulated. of 21 three-floor reinforced concrete residential build- 3) Apply pore-water pressure to the specimen through ings, were directly hit by the debris flow (Fig. 8). It was a pore-water pressure controller at the rate of 0.1 found that in the middle and lower part of the gully, the kPa/minute until specimen failure. Through this sheeting joints were well developed (Fig. 7d). This step, the situation of pore-water pressure caused the weathered granite above the major sheeting accumulation on the potential sliding surface is joint to easily erode away, and the volume of the debris simulated, and the failure process can be observed. flow became larger and the debris flow increased in magnitude and kinetic energy as it flowed down the In this test, the slope angle was assumed to be 35°, steep gully. The material in the debris flow deposits is and soil thickness to be one meter. rich in fine soil particles, being granitic soil, or Masa-do. The fine soil particles may decrease the permeability of Results and discussions the debris flow, and cause greater mobility, in turn con- Field investigation and cross section measurement of the tributing to the longer travel distance and greater lateral two gullies spread. To confirm this inference, soil samples were Based on the field investigation, the geological condi- taken from the deposition area and the grain size distri- tions were found to be different in the source areas of bution was analyzed in the laboratory. the two debris flows. The source area of the debris flow Figure 9 illustrates the longitudinal profile as well as in Midori-ga-oka is composed of coarse granite, with in- the cross-sectional profiles at all measurement locations trusive fine granite. In the coarse granite, sheeting joints (see Fig. 5) in the gully of the Midori-ga-oka debris flow. Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 8 of 16 Fig. 7 Photographs showing the gully of the Midori-ga-oka debris flow. Arrows point in the downstream direction. a three branches in the source area (at cross section S-0 in Fig. 5); (b) strongly weathered granite bedrock exposed by the erosion of debris flow (S-1); (c) shallow bedrock gully after erosion by the debris flow (S-2); (d) erosion along the sheeting joints in granite (C-4); (e) bedrock gully with few debris flow deposits (C-6); (f) granite bedrock gully after extreme erosion, near the bottom of the gully (S-19); (g) the bottom of the debris flow gully, where deposits of different grain size appeared and a temporary ring net countermeasure (at S-23) was finished in December 2014; (h) severely damaged residences beside the pathway of the debris flow, these houses were built on previous debris flow deposits From these measurements, the following aspects were the dam may have caused a debris flow of greater evident: magnitude. This sudden narrowing in the cross sec- tion may had a significant effect on the motion of 1) The longitudinal profile consisted of gentle slopes the debris flow as it flowed to the residential area, (slope angle < 30°, indicated as S) and steep cliffs because of the constricted flow channel. (slope angle > 40°, indicated as C). 4) Nearly all of the debris deposited on the riverbed of 2) Most of the cliffs were located at the middle and the gentle slope in the gully (below S-23 in Fig. 5). lower part of the gully. 3) The cross sections were in a V-shape from S-1 to S- Figure 10 shows the Abu-no-sato debris flow gully 3, and became U-shaped from S-5 to the end (S-21). from the source area to the deposition area. The source The section became narrow in the area between S-3 area is located in hard hornfels rock (Fig. 10a). The slid- and S-17. Notably, the width change from S-15 to ing direction is S20°E. The slope angle is 40°. The width S-17 is very sudden. This could have caused a debris is about 5 m, the length about 30 m, and the depth to dam between S-15 and S-17, and a later collapse of the sliding surface, which is the outcropped bedrock, is Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 9 of 16 Fig. 8 The damaged prefectural housing at the pathway of the Midori-ga-oka debris flow. From the muddy trace on the wall, the flow depth of debris flow was about two-floors high about 0.7 m. The bedrock is fractured with joints, and upper part was V-shaped, and the gully bed consisted of two nearly vertical faults with a width of 0.15 m passed fractured hornfels (Fig. 10c). In some part, colluvium de- through the source area. After a short flow in a wide posits existed in the gully (Fig. 10d). Because of the large gully slope, the debris flow was limited to a narrow gully boulder size, compared with the smaller grain size debris at a cliff C-3 (Fig. 10b), and dropped down, nearly verti- flow material, these would be a higher permeability, thus cally for about 5 m. Lower down, it passed several slopes transportation would be less likely in this area. After and cliffs, and left the hornfels area. The gully in that flowing out of the hornfels bedrock area, and into the Fig. 9 Longitudinal section and cross sections of the Midori-ga-oka debris flow gully Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 10 of 16 Fig. 10 Photographs in the gully of Abu-no-sato. Arrows point in the downstream direction. a source area, the scarp is marked by a yellow line (at cross section S-0); (b) cliff with a joint (C-3); (c) gully-bed with fractured rock, no deposits (S-4); (d) V-shaped gully, with colluvium from one side (S-7); (e) bottom of the gully, coarse-grained deposits of 30 cm average diameter was widely distributed (S-14); (f) narrow neck of the gully (S-16); (g) downstream deposit area with shallow coarse-grained layer (S-19); (h) the houses along a road damaged by the debris flow in the Abu-no-sato community coarse granite bedrock area, the slope angle became gen- destroyed two houses and caused 4 deaths. Some houses tle (17.7°), and most of the boulders deposited at the along the slope road (see Fig. 6b) were damaged to dif- bottom of the gully (Fig. 10e). Because the content of ferent extents, mainly by rolling boulders and rapid the debris flow is composed of boulders, most of the water flow. boulders were deposited in the gentle and wide slope be- Figure 11 illustrates the longitudinal section and cross fore they reached the residential area as pore water pres- sections at all measurement locations (see Fig. 5) in the sure reduced rapidly. However, an old debris flow gully of Abu-no-sato debris flow. From the information deposit terrace caused a dam of the debris flow. Collapse in the figure, the following effects can be observed. occurred at both sides of the terrace, and deep erosion developed (Fig. 10f shows the right side narrow channel). 1) The slope was steep in the hornfels area, and Through the narrow channels, part of the debris flow in- became gentle in the coarse-grained granite area. cluding boulders and fallen trees were deposited on a 2) Most of the cliffs were distributed in the middle and wide gentle slope (Fig. 10g). A small amount of debris- upper parts of the gully. flow material flowed beyond the boundary between the 3) The gully was V-shaped in most of the hornfels area, natural slope and the residential area (a local road), and gradually became wider. Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 11 of 16 Fig. 11 Longitudinal section and cross sections of the Abu-no-sato debris flow gully 4) From C-15 to S-17, the cross sections narrowed rap- all samples is around 2.64. Samples Nos. 1 and 2 are idly. This change may have caused a dam in the deb- from the same landslide, and have similar properties. ris flow, and the later outburst may be a direct cause The dry densities of sample Nos. 1 and 2 are higher than of the disaster. that of sample No. 3. Figure 12 illustrates the grain size distribution of all Compared with the Midori-ga-oka debris flow, the dis- samples (Nos. 1–6). Sample Nos. 1 to No. 5 are from the tance between the residential area and the starting point Midori-ga-oka debris flow gully. Nos. 1, 2, and 3 are of the deposition area is much larger in Abu-no-sato from the source area, and Nos. 4 and 5 are from the exit debris flow. It may mean that keeping enough space be- of the gully, the first deposit of the debris flow. Sample tween the deposition start point and the residential area No. 3 comprises much finer particles than the other is very important for disaster reduction. samples, and the mean grain size D is 0.035 mm. While the mean grain sizes of samples Nos. 1, 2, 4 and 5 Soil properties: in-situ density and grain size distribution are close to 1 mm, their distributions are also similar. The locations of sample Nos. 1–6 are illustrated in Fig. 5. Comparing the sample from the deposition area with Table 1 shows the physical parameters of undisturbed that from the source area, the grain size is not much dif- sample Nos. 1, 2 and 3. The value of specific gravity for ferent. The amount of fine particles is slightly less in the deposition area. This may mean that the finer particles Table 1 Parameters of soil samples in the source area of the are transported for a longer distance. Sample No. 6 is Midori-ga-oka gully from the lower part of the deposition area of the Abu- Sample No. 1 2 3 no-sato debris flow, which has the smallest grain size in Specific gravity, G 2.640 2.643 2.638 the debris flow deposit. In the upstream part, the deposit Dry density, ρ (kg/m ) 1,370 1,390 1,189 d generally consists of boulders with a mean size of 0.3– Water content, w (%) 15.2 19.5 31.5 0.5 m, and a maximum size up to 1.5–2.0 m. From the grain size distribution, it can be seen that the particles in Liquid limit, LL (%) —— 32.1 the Midori-ga-oka debris and the Abu-no-sato debris Plastic limit, PL (%) —— 23.4 flows are different in grain size and, in turn, we can infer Plasticity Index, PI (%) —— 8.7 that they have different permeability and drainage Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 12 of 16 Fig. 12 Grain size distribution of samples Nos. 1–6 characteristics. At a given travel velocity, the low perme- Geochemical analysis of the granite sample ability, which is associated with a slower rate of drainage Major element compositions of the fresh granite, the of the debris at Midori-ga-oka (mainly consisting of weathered granite, and the granitic soil in the Midori- granitic soil), may mean that the flow has greater mobil- ga-oka gully are listed in Table 2. The fresh granites are ity and may travel for a longer distance because it re- hard rocks and the constituent minerals are mostly un- mains in the saturated condition for longer. The altered. The weathered rocks are relatively hard, but are boulders in the Abu-no-sato debris flow mean that de- discolored to yellowish white. Plagioclase has changed position occurs much earlier as the debris drains rapidly color to creamy white. Fe (iron) stains surround the bio- and changes to the unsaturated condition. The critical tite and other minerals. The granitic soils are mainly velocity of a debris flow transformed from saturated to composed of clays, along with less than half the amount unsaturated condition will be of significance in predict- of fine altered minerals. Small quartz grains are some- ing the run-out mobility of debris flow. times visible. Composed of 70 % of particles finer than 0.1 mm, The Chemical Index of Alteration (CIA, defined by sample No. 3 was used to conduct the Atterberg limit Nesbitt and Young 1982) is employed to investigate the tests. The results are shown in Table 1. In addition, degree of chemical weathering of the collected samples, Fig. 13 shows that the soil classification of the sample as it is well known as a functional tool to estimate the No. 3 is between Silt (ML) and Low plasticity Clay (CL) weathering state of granitic rocks (e.g., Fedo et al. 1995; with a low liquid limit (ASTM D2487-11 2011). Kamei et al. 2012). The Eq. (1) is the formula. CIA ¼ 100  Al O =ðÞ Al O þ CaO þNa O þ K O molar basis 2 3 2 3 2 2 ð1Þ Where CaO* represents Ca in the silicate fraction only. Fresh granitic rocks generally have CIA values near 50. Al O is less mobile during the weathering process, 2 3 whereas CaO*, Na O, and K O are relatively more mo- 2 2 bile (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. Fig. 13 Plasticity chart of sample No. 3 (red solid triangular). Nesbitt and Markovics (1997) reported the CIA values Modified from ASTM D2487-11 (2011). CL = Lean clay; ML = silt of typical clay-rich weathered granitoids (72.7–81.6) Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 13 of 16 Table 2 Major element compositions of granite and related samples in the Midori-ga-oka gully Location Type SiO TiO Al O Fe O MnO MgO CaO NaOKOP O LOI Total CIA 2 2 2 3 2 3 2 2 2 5 G1 FG 72.58 0.23 14.97 2.45 0.05 0.50 1.75 3.10 4.10 0.03 0.71 100.47 54.2 G1 GS 71.74 0.31 14.74 3.24 0.08 0.62 1.66 2.79 3.52 0.01 1.96 100.66 56.4 G2 FG 74.59 0.19 13.80 2.21 0.05 0.43 1.18 2.92 4.26 0.02 0.80 100.45 54.5 G2 GS 69.46 0.29 16.43 2.89 0.16 0.54 0.67 1.88 3.71 0.04 5.20 101.27 66.6 G2 GS 67.20 0.38 17.87 3.38 0.19 0.66 0.45 1.71 3.87 0.05 6.09 101.85 69.9 G3 FG 73.69 0.25 13.80 2.87 0.06 0.57 1.64 2.91 3.98 0.04 0.52 100.33 53.5 G3 WG 72.45 0.26 14.55 2.68 0.08 0.56 1.56 2.85 3.84 0.02 1.79 100.65 55.5 G3 GS 72.78 0.26 14.56 2.59 0.13 0.51 1.01 2.43 3.80 0.00 2.75 100.81 59.4 G4 FG 76.52 0.11 12.85 1.65 0.04 0.31 1.08 2.92 4.50 0.01 0.19 100.17 52.6 G4 WG 76.39 0.12 12.87 1.64 0.04 0.32 1.04 2.89 4.37 0.01 0.44 100.14 53.1 G4 GS 72.72 0.23 14.55 2.38 0.08 0.45 0.79 2.25 3.86 0.02 3.44 100.78 61.1 G1 to G4: sample locations, showing in Fig. 5. FG Fresh granite, WG Weathered granite, GS Granitic soil, LOI loss on ignition, CIA Chemical Index of Alteration (Nesbitt and Young, 1982) from the outer layer of the spheroidal weathering- indicator in order to detect the active area of debris boulder in Toorongo granodiorite, Australia. The weath- flows in a granitic region. ering conditions of the sample location are similar to those of Hiroshima Prefecture. Summers of both regions Consolidated-undrained triaxial compression tests are warm, averaging 25–30 °C, and winters are cold with The stress–strain relation of the consolidated-undrained temperatures of −5 to 10 °C. Summer precipitation is tests on the saturated specimen (Fig. 14a) shows that the 200–300 mm with annual precipitation approximately deviatoric stress increases in the beginning. When the 1,500 mm for both. As a result of the similarities, the axial strain is about 2 %, the deviatoric stress reaches the CIA values of Toorongo granodiorite would be a good peak value. After that, the deviatoric stress turns to de- comparison with our samples. However, the granitic soils crease with the axial strain. Bulging failure was observed of Midori-ga-oka have significantly lower CIA values for all specimens at the end of the tests. The stress– (56.4–69.9) compared with those of the weathered gran- strain curves indicate that the critical state of the soil itoids in Toorongo granodiorite (72.7–81.6). This sug- will be reached at low strains value. gests that the degree of chemical weathering of the The relation between excess pore-water pressure and granitic soils in the Midori-ga-oka gully is less than that the axial strain in the consolidated-undrained tests on of normally weathered granitic materials. This provides saturated specimens is shown in Fig. 14b. Positive excess the evidence that the weathered materials in the shallow pore-water pressure was generated in all three undrained slide area were in a removal cycle from the steep slope, tests. In all soil specimens, the positive excess pore- due to repeated debris flows. Therefore, the lower CIA water pressure increases to a high value with minor values of soil materials in debris flows may be a good strain, and increases slowly as the strain increases Fig. 14 Results of the consolidated undrained test: (a) stress–strain relation; (b) relation between pore-water pressure and axial strain Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 14 of 16 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. All of the effective stress paths show a similar trend. Each path moves to the right initially (increasing 0 0 σ þ σ =2), and then moves toward the left (decreas- 1 3 0 0 ing σ þ σ =2 ) until reaching the critical state line 1 3 (CSL) at the end of the test. The critical states of the sat- urated specimens can be represented by the CSL in the stress plane, as shown in Fig. 15. The gradient of the critical state line is 0.404. This corresponds to an in- ternal effective friction angle of 23.8° and cohesion of 5.5 kPa of the granitic soil. Pore-water pressure controlled triaxial test Fig. 16 Relations between deviatoric stress, axial strain and pore- To determine the initial stress condition of soil sample water pressure for the pore-water pressure controlled triaxial test on No. 3-DIS, the thickness (H) of the soil layer and slope sample No. 3-DIS angle (α) are assumed as 1 m and 35°, respectively, based on the field measurement results in the source area of the debris flow. As shown in Table 1, the unit weight (γ) along the critical state line, and ends at point D, where the of the natural soil is 15.6 kN/m . From consolidated pore-water pressure is 8.3 kPa. This simulation test can undrained triaxial tests, the internal effective friction explain the initiation mechanism of the shallow landslide angle (ϕ′) of the soil is 23.8°. The initial maximum prin- that occurred on a steep slope. During heavy rainfall, con- cipal stress (σ ) and minimum principal stress (σ ) are 1 3 tinuous rainfall infiltration can generate a wet front in the 21.7 and 5.7 kPa, respectively. Through the pore-water soil slope and in turn form a saturated zone above the po- pressure controlled triaxial test, the relations between tential sliding surface. Gradually, the saturated zone will deviatoric stress, axial strain and pore-water pressure move upwards in relation to the slope surface, and were obtained (Fig. 16). the pore-water pressure acting on the potential sliding Figure 16 shows that, when the pore-water pressure in- surface will increase. Finally, it will cause shallow land- creased to about 4.0 kPa, the deviatoric stress started to slide. Because this phenomenon occurs in steep slopes, it decrease, while the axial strain increased. The same soil will create a debris flow moving downward for a long dis- behavior also can be found in Fig. 17. The effective stress tance. From the test, we can see that, with low pore-water path begins to go down when pore-water pressure in- creases to about 4.0 kPa (point B). After this point, yield- ing occurs, and the effective stress path moves downward and to the left, then reaches the critical state line (point C, where pore-water pressure is 6.5 kPa). This shows the major failure of the soil. With the continuous supply of pore-water pressure, the stress path moves down and left Fig. 17 Effective stress paths of pore-water pressure controlled Fig. 15 Effective stress paths under different confining pressures (50, triaxial test, the dotted line indicates the critical state line (CSL) 75 and 100 kPa), the dotted line indicated the critical state line (CSL) obtained in undrained tests Wang et al. Geoenvironmental Disasters (2015) 2:17 Page 15 of 16 pressures (i.e., when the rainfall is not so heavy), the slope 6) Simulation test with pore-water pressure controlled can maintain stability. However, if the pore-water pressure triaxial test shows reasonable response of soil behav- exceeds the critical value, as a result of heavy rainfall, for ior under different water pressure conditions, which example, shallow landslide will be triggered. In this simu- corresponds to groundwater level in a real slope. It lation test, the critical value of the pore-water pressure is is hoped that the groundwater level in a real slope about 4.0 kPa. Considering the slope angle of the source can be used for failure prediction. area is 35°, and the thickness of the initial sliding mass is Competing interests 1 m, the pore-water pressure of 4.0 kPa and 6.5 kPa mean The authors declare that they have no competing interests. that the groundwater levels are about 0.60 m and 0.97 m high, respectively, above the sliding surface. So when the Authors’ contributions All authors participated the field investigation; FW prepared the first draft of groundwater level rises to 0.6 m above the potential slid- the paper; HY conducted the soil tests in laboratory; YHW and ST prepared ing surface, instability may occur in the slope, and deform- the sections and profiles; AK made the geochemical analysis on the granite ation may develop. When the groundwater level rises to sample. All authors read and approved the final manuscript. 0.97 m, the slope will completely fail. For a slope with Acknowledgements depth of 1 m, the groundwater level of 0.97 m almost Based on the field investigation experiences, we like to make some means the existence of surface flow along the slope. These suggestions for countermeasure works to prevent debris flow disasters: (a) two values of the groundwater level may be used as a ref- 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 erence for the prediction of shallow landslide initiation be effective to stop the travel of boulders; (c) For weathered granite area, under similar condition. Since the soil sample used for tri- the use of a ring-net to stop large boulders and, in addition, building a large axial tests is disturbed soil, the in-situ critical value of the 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 pore-water pressure may be slightly higher than experi- time to take action. Otherwise, delayed warnings are useless when considering mental value. However, using a smaller value is conserva- the weather condition that can cause debris flows. tive, erring on the side of community safety. 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 Conclusions Geoscience, Shimane University, Ryoichi Tsukamoto, Tomohiro Oda, Norisato Based on field investigations and laboratory tests, the Oishi, Naho Yamamoto, Masafumi Yokoyama joined the field investigation and assisted the topographic survey and soil/rock sampling. Lynn Highland following conclusions are reached. of U.S. Geological Survey made constructive comments of the draft. Valuable and constructive comments from anonymous reviewers are deeply 1) Under extreme high intensity and short duration appreciated. rainstorm, debris flows occurred on very steep Received: 13 February 2015 Accepted: 30 June 2015 slopes with a thin initiating sliding mass in weathered coarse-grained granite and hornfels. 2) Cross sectional properties of the debris flow gully, References ASTM D2487-11 (2011) Standard Practice for Classification of Soils for Engineering especially gully shape and changes in width, have Purposes (Unified Soil Classification System). 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J Jpn Soc Nat Disaster Sci 33(3):293–312 (in Japanese with English Abstract) Submit your manuscript to a Yang H, Wang F, Miyajima M (2015) Investigation of shallow landslides triggered by heavy rainfall during typhoon Wipha (2013), Izu Oshima Island, Japan. journal and benefi t from: Geoenvironmental Disasters 2:15. doi:10.1186/s40677-015-0023-8 7 Convenient online submission 7 Rigorous peer review 7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the fi eld 7 Retaining the copyright to your article Submit your next manuscript at 7 springeropen.com

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Geoenvironmental Disasters – Springer Journals

Published: Jul 24, 2015

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