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Ulleung-do is a volcanic island located 130 km east of the Gangwon-do province of South Korea. It is characterized by steep slopes covered with effusive rocks released from multiple volcanic activities. Having accumulated deep colluvium, Ulleung-do Island manifests a high vulnerability to landslides frequently caused by heavy rains or typhoons, debris flow, rockfall, and other disaster hazards in mountainous areas. Therefore, facilities and residential areas located in the lower areas of the island sustain widespread damage. Hence, the installation of check dams designed to reflect the area’s local conditions is required to avoid further damage. In line with that, this study analyzes the disaster cases in Ulleung-do’s mountain areas and the effect of check dams in debris flow reduction for each type using the KANAKO-2D model. At observation point 1, the result shows that the maximum rate of debris flow is reduced by 48.5% with an open-type check dam installed and 62.9% with a closed-type check dam installed from the level without a check dam. For observation point 2, the maximum flow depth decreases by 49.7% with an open-type check dam and 77.4% with a closed-type check dam. Thus, this study suggests that the simple installation of check dams in a mountain stream that has experienced debris flow effectively mitigates damage brought by various disasters.

The topographical characteristics of South Korea are coupled with heavy rain and typhoons concentrated from July to September, which take up two-thirds of the country’s annual precipitation, causing substantial damage from slope failures, landslides, debris flows, and other mountain disasters. This study uses the Land-Atmosphere Modeling Package (LAMP) data to study the area, which has been affected by numerous landslides in the past. Specifically, to analyze the likelihood of slope failure, this study simulates the rainfall events in the area at the time of the landslides as well as changes in the volumetric water content in the soil layer and uses the changes in the volumetric water content per unit time, which are affected by the effective cumulative rainfall within the soil layer. To that end, this study assesses the risk of landslides based on how rainwater infiltration in the saturation depth changes the saturation depth ratio and compares the results with the landslide spots in the past. Moreover, this study analyzes the landslide risk by applying the saturation depth ratio (Ht) to an infinite slope stability analysis model. The landslide threshold proposed in this study was obtained by analyzing the rainfall events that caused the landslides in the study area using the LAMP model. It can then be concluded that, when compared with the spatial information on the previous landslides, if the change in the volumetric water content per unit time during the first and second rainfall events is 0.7, a landslide may occur. These results indicated that the landslide risk increases the volumetric water content in the soil layer, along with the congruence between past landslide risk points and spatial landslide risks.

Landslides, which often occur on natural slopes of mountainous areas and artificial slopes around urban areas during summer in South Korea, are mostly caused by localized heavy rainfalls and typhoons. A survey was conducted, and the characteristics of landslide occurrences in different geological conditions&mdash, in this case, granite soils in Sangju area and gneiss soils in Yangu area&mdash, were analyzed. Soil characteristics in the landslide and non-landslide areas and the surroundings of each geological condition were also evaluated. Triggering factors, namely permeability coefficients (k), shear strength with cohesion (c), and internal friction angle (&phi, ) of soils that are closely linked to landslides around weathered soil layers were extracted based on the examined characteristics and a statistics method. The study used regression analysis to formulate equations to estimate the permeability coefficients and shear strength. Ultimately, the permeability coefficients showed significant results in terms of void ratio (e), the effective size of grains (D10), and uniformity coefficient (cu), while shear strength correlated with the proportion of fine-grained soil (Fines), uniformity coefficient (cu), degree of saturation (S), dry weight density (rd), and void ratio (e).

In South Korea, landslides are caused by localized heavy rainfall and typhoons, which often occur in the summer season at natural slopes in mountainous areas and artificial slopes in urban surroundings. Flow-type landslides frequently occur in mountainous areas. To evaluate flow-type landslides, it is essential to identify the physical characteristics of soil, giving focus to the soil on the top layers of various types of slope. This study conducts a survey and an analysis of the characteristics of landslides that occurred in the study area with different geological conditions of granite and gneiss. The characteristics of soil in the area and its surroundings that have or have not undergone landslides for every geological condition is also evaluated. Based on these characteristics and a statistics method, it extracts the triggering factors, permeability coefficients (k), and shearing strength with cohesion (c) and internal friction angel (φ) of soils that are highly related to landslides around weathered soil layers. As a result, the permeability coefficients show significant relevance with void ratio (e), the effective size of grains (D10), and uniformity coefficient (cu), while the shearing strength with the proportion of fine-grained soil (Fines), uniformity coefficient (cu), degree of saturation (S), dry weight density (rd), and void ratio (e). By obtaining this result, the study uses the regression analysis to suggest models to estimate the permeability coefficients and shearing strength. For the gneiss area, the statistics-based estimation model (SEM) is proposed as kgn = (1.488 × 10-02 × e) + (1.076 × D10) + (-1.629 × 10-04 × cu) - (1.893 × 10-02) for permeability coefficients; cgn = (-0.712 × Fines) + (-0.131 × cu) + 15.335 for cohesion; and φgn = (27.01 × rd) + (-12.594 × e) + 6.018 for internal frictional angle of soils. For the granite area, the statistics-based estimation model (SEM) is proposed as kgr = (8.281 × 10-03 × e) + (0.639 × D10) + (-2.766 × 10-05 × cu) - (9.907 × 10-03) for permeability coefficients; cgr = (-0.689 × Fines) + (-0.0744 × S) + 18.59 for cohesion; and φgr = (33.640 × rd) + (-0.875 × e) - 9.685 for internal frictional angle of soils. The use of statistics-based estimation models (SEMs) for landslide-triggering factors that trigger landslides will support the simple calculation of permeability coefficient and shearing strength (cohesion and internal frictional angle), only requiring information about the physical properties of soil at the natural slopes that have different geological features such as gneiss and granite areas.

Recently, human and property damages have often occurred due to various reasons&mdash, such as landslides, debris flow, and other sediment-related disasters&mdash, which are also caused by regional torrential rain resulting from climate change and reckless development of mountainous areas. Debris flows mainly occur in mountainous areas near urban living communities and often cause direct damages. In general, debris flows containing soil, rock fragments, and driftwood temporarily travel down to lower parts along with a mountain torrent. However, debris flows are also often reported to stream down from the point where a slope failure or a landslide occurs in a mountain directly to its lower parts. The impact of those debris flows is one of the main factors that cause serious damage to structures. To mitigate such damage of debris flows, a quantitative assessment of the impact force is thus required. Moreover, technologies to evaluate disaster prevention facilities and structures at disaster-prone regions are needed. This study developed two models to quantitatively analyze the damages caused by debris flows on structures: Type-1 model for calculating the impact force, which reflected the flow characteristics of debris flows and the Type-2 model, which calculated the impact force based on the topographical characteristics of mountainous regions. Using RAMMS a debris flow runoff model, the impact forces assessed through Type-1 and Type-2 models were compared to check reliability. Using the assessed impact forces, the damage ratio of the structures was calculated and the amount of damage caused by debris flows on the structures was ultimately assessed. The results showed that the Type-1 model overestimated the impact force by 10% and the Type-2 model by 4% for Mt. Umyeon in Seoul, compared to the RAMMS model. In addition, the Type-1 model overestimated the impact force by 3% and Type-2 by 2% for Mt. Majeok in Chuncheon, South Korea.

Landslides and debris flows that occur around residential areas are considered, globally, as significant disasters that cause damage to human life and property. With terrain slope defining the flow characteristics of debris flows, flow depth, flow velocity, and impact force vary by time and distance. In particular, when a structure is located in the flow path of debris flows, the flow characteristics of debris flows vary by terrain slope and direction angle. To simulate the flow characteristics of these debris flows, the simulation results obtained by FLO-2D were analyzed with six-stage conditions for the research area. In the analysis, the flow depth, flow velocity, and impact force were estimated on the basis of the outlet of the research area in the presence and absence of structure(s) at certain distances. With this, the variation of the impact force in accordance with the variation of the flow depth of the debris flows was highly similar to the simulation results obtained by FLO-2D, when the correction index (α) of the suggested dynamic impact force equation was 0.3–0.4. There were sections where the estimated value of the impact force was overestimated near the outlet, and it was judged that the fixed values of the terrain factors (width, roughness coefficient, slope, etc.) caused the impact force to be overestimated. However, the correlation analysis showed that the correlation index was above the normal ranges in the suggested dynamic impact force equation for debris flows with the application of the terrain factors.

Kim, Suk-Woo; Chun, Kun-Woo; Otsuki, Kyoichi; Shinohara, Yoshinori; Kim, Man–Il; Kim, Min–Seok; Lee, Dong–Kyun; Seo, Jung–Il; Choi, Byoung–Koo.February, 2015.Heavy rain types for triggering shallow landslides in South Korea,Article,[Fukuoka, Japan]Kyushu University

An early warning system can be an effective measure to reduce the damage caused by landslides by facilitating the timely evacuation of residents from a landslide-prone area. Early detection of landslide triggering across a broad range of natural terrain types can be accomplished by monitoring rainfall and the physical property changes of soils in real time or near-real time. This study involved the installation of a real-time monitoring system to observe physical property changes in soils in a valley during rainfall events. This monitoring included the measurement of volumetric water content, which was compared with the results of laboratory flume tests to identify landslide indicators in the soils. The response of volumetric water content to rainfall events is more immediate than that of pore-water pressure, and volumetric water content retains its maximum value for some time before slope failure. Therefore, an alternative method for landslide monitoring can be based on the observation of volumetric water content and its changes over time at shallow soil depths. Although no landslide occurred, the field monitoring results showed a directly proportional relationship between the effective cumulative rainfall and the gradient of volumetric water content per unit time (t/t max). This preliminary study thus related slope failure to the volumetric water content gradient as a function of rainfall. Laboratory results showed that a high amount of rainfall and a high gradient of volumetric water content could induce slope failure. Based on these results, it is possible to suggest a threshold value of the volumetric water content gradient demarcating the conditions for slope stability and slope failure. This threshold can thus serve as the basis of an early warning system for landslides considering both rainfall and soil properties.

Landslides induced by debris flow have been discussed in relation to the geotechnical properties of soil developed on bedrock, together with an emphasis on the importance of mineralogy comprising precursor soils. Three areas composed of different types of geology were compared to relate landslide with soil compositions: Precambrian gneiss (Jangheung area), Jurassic granite (Sangju area), and Tertiary sedimentary rocks composed of shale and mudstone (Pohang area) in Korea. X-ray diffraction for mineral identification and quantitative analysis, Scanning Electron Microscope for observation of microtexture, and laser size analysis for very fine particles ranging from micrometer were performed, with conventional measurements of particle size, porosity, density, permeability, and consistency for the soils. Soils at landslide sites containing a large amount of finer particles have higher uniformity and gradation coefficients, but lower consistency than those at non-landslide sites. Landslide areas are characterized by higher porosity and lower density. Soil from the gneiss area shows a high plasticity index while that of mudstone has high water content. Main clay minerals contained in soils of the sites where landslides took place are illite, chlorite, kaolinite, and montmorillonite. Mineralogical information on the constituents and microtexture of soils aids in better understanding the causes and patterns of landslide, together with mechanical properties of soils.

A geophysical survey was undertaken at Wiri area of the Andong in southeastern Korea to delineate subsurface structure and to detect the fault zone, which affected the 1997 mountain–hill subsidence and subsequent road heaving initiated by the intense rainfall. Electrical resistivity methods of dipole–dipole array profiling and Schlumberger array sounding and seismic methods of refraction and reflection profiling were used to map a clay zone, which was regarded as the major factor for the landslide. The clay zone was identified in electrical resistivity and seismic sections as having low electrical resistivity (