Earthquakes and heavy rainfall result in the formation of landslide dams following a massive collapse or landslide. Clarifying geomorphic changes to landslide dams following overtopping is important for mitigation of future disasters. However, past modeling experiments have not provided adequate information about geomorphic changes at landslide dams in actual scale. Recently, detailed geomorphic data before and after the landslide dam overtopping erosion by LiDAR was acquired, and it came to be able to understand the detailed shape of the water channel that has been caused along with the erosion. Therefore, we used aerial photographs, and LiDAR data sets to analyze the geomorphic changes of some landslide dams formed during the 2004 Chuetsu earthquake, the 2008 Iwate and Miyagi inland earthquake and so on. In this study, we also presented new method for describing geomorphic change of water channel. We used slope stability analysis for slope collapse at side slope of water channel. Then, we verified this method with the LiDAR data sets.
SABO soil cement method is to utilize site generated soil as material. INSEM (IN-situ Stabilized Excavation Materials) method is one of the SABO soil cement method. Construction method of INSEM is to screed and compact the material that is made by mixing site generated soil with cement powder, and to construct SABO structures. Because of the advantages, such aspect as cost, environmental and workability etc., applying of INSEM method is increasing in the field of SABO works in recent. However, basic characteristic of INSEM is not always clear enough. One of them is hexavalent chromium elution from INSEM. There are several reports that hexavalent chromium elution from INSEM exceed the limited value (0.05mg/L) regulated by Environmental standards on soil pollution formulated by Environment Agency. If such incident takes place after initiation of construction, unexpected time is consumed for managing to decrease hexavalent chromium elution value or even to modify the construction method itself, resulting serious influence to term of work or cost. Expecting more widely use of INSEM method that have many advantages in terms of zero emission, environmental and cost merit, it is necessary to clarify characteristic of chromium hexavalent elution from INSEM as well as coping strategy to decrease excess value of chromium hexavalent elution. This study is to clarify characteristic of chromium hexavalent elution from INSEM and coping strategy by making use of different type of site-generated soil and changing amount of cement use in indoor examination. From results and considerations of study, it is found that to power up sufficiently reaction of cement for solidification is important as a fundamental coping policy, when excess value of hexavalent chromium elution from INSEM is found.
It is necessary to deal with transitional flow from debris flow through sheet flow to bedload flow for estimating the effect of debris flows on downstream area. Previously, numerical simulations were conducted using a model combining each type's flow model simply. In regard to riverbed shear stress, Manning's theory is applied in bedload region, an empirical formula is applied in sheet flow region, and existing theory of debris flow is applied in debris flow region. In regard to equilibrium sediment concentration, existing bedload discharge formula is applied in bedload region, an experimental formula is applied in sheet flow region, and existing theory of debris flow is applied in debris flow region. However, in this type numerical simulation, the calculated riverbed shear stress and equilibrium sediment concentration often discontinuously transit at transition boundaries because physical background of each flow model is different. These discontinuities may cause unreasonable simulation results and numerical oscillation. Therefore, a seriallyconnected model was developed in this study. In our model, riverbed shear stress formula was developed by applying existing theory of debris flow to the sediment layer and Manning's theory to the water layer. Also, we proposed a new method for determining transitional condition of sediment transport region to be continuous transition of equilibrium concentration. Numerical simulations were performed by using an existing model along with our model. The results for the condition where the riverbed gradient changes gradually showed that existing model caused irregular deposition deformation. Moreover, simulation results of flow depth and sediment concentration oscillated around transition boundaries. When discontinuities of riverbed shear stress and equilibrium sediment concentration coincided, numerical oscillation became larger. Our model could yield smooth and stable simulation results without these problems. Overflow sites and flooding area may be underrated or overrated using existing model, therefore, our model is effective.
We analyzed the occurrence of torrential rain in low rainfall areas by comparing sediment disasters between low and high rainfall areas. Additionally, to clear conditions where torrential rain causes slope failure in mountain soil surfaces, we investigated hydrological processes, including rainfall initiation, storage, runoff and drainage. From this investigation we have shown the usefulness of the same one tank model for effective rainfall. We chose heavy precipitation (daily rainfall R1/2), which has a probability of occurring once every two years, as a factor to represent resilience against sediment disaster, and considered this daily rainfall R1/2 to be a local peculiarity rainfall (R1/2). The results can be summarized as follows. Mountains in both high and low rainfall areas involve the same mechanisms for slope failure. Following this assumption, we concluded that for mountains in high rainfall areas, hydrological basement drains well. However, mountains in low rainfall areas have poor drainage. Based on these results, we propose a “model of constant critical effective rainfall amount” concerning slope failure. This model indicates that the minimum amount of accumulated precipitation in low rainfall areas, which is where disasters occur, is the most appropriate factor for determining the critical effective rainfall amount (Dc). Additionally, our results indicate that Dc is 270 mm. We calculated the runoff and drainage factor (b) of the one tank model, for each disaster area based on the hourly precipitation and the effective rainfall amount leading up to the time of the disaster. The results suggest a linear relationship between the runoff and drainage factor (b), which is represented by the one tank model for runoff and drainage, and local peculiarity rainfall (R1/2).
This study examined factors that affected the geographic distribution of shallow landslides caused by typhoon 0310 (Etau) in the Appetsu River basin in the Hidaka region of Hokkaido. We established a 7.5 km2 study area in the basin of the Itarakki River, a tributary of the Appetsu River. Using mainly field surveys, we investigated the properties of shallow landslides and geology and then mapped shallow landslides and geology type. Slope degree, profile curvature, plan curvature, geology type, and forest type were chosen as factors that may influence shallow landslide occurrence. We divided the study area into 10-m grids using geographic information system software and analyzed the relationship between each factor and landslide occurrence in each grid using logistic regression analysis. Landslide probability of steep or concave slope was higher than that of gentle or convex slopes. Plan curvature was more important factor than profile curvature. Some types of geology and forest were significant factors that influence landslide occurrence.
By Miyagi offing earthquake occurred on May 26, 2003, a landslide-related collapse occurred in a gentle slope of around 10 degrees. This experiment study carried on for make clear the mechanism of collapse at gentle slopes coming about, using a shaking table. The following things became clear from the experiment. By the movement of the sand materials due to vibration, cavities increased, and the pore water pressure rose by repeating decrease, and the result to repeat the decline was provided. As the result of having considered a rise in pore water pressure, the rise type of two patterns was confirmed. As for the first, the volume shrinkage happens by vibration, and the excessive pore water pressure is generated by a cavity decreasing, and a rise of the sudden pore water pressure was recognized. When, as for the second, movement of the groundwater happens by a shake, and the rise of the water table happens. The difference of change for pore water pressure due to earth materials was thought of as distinction of size and water permeance coefficient. Because pore water is hard to move to other cavities by the decrease in cavity generated in the case of the volume shrinkage if a water permeance coefficient is small, the pore water pressure is easy to increase. On the contrary, if a water permeance coefficient is big, groundwater is easy to move, and it is thought that a factor to raise an underground water table becomes. The cause for collapses on a gentle slope was thought to be the rapid change of pore water pressure by seismic movement.
Many debris flows and slope failures occurred concentratively in the Northern Kyushu Region in July 2012, which led the damage of 25 people dead or missing in the Aso district in Kumamoto Prefecture. Although these disasters are calamity, but they provide useful information for prefectural governments to establish warning and evacuation system for sediment-related disasters,such as debris flow or slope failure prone area. We held an interview to the residents of the Aso city which affected landslide disaster to obtain information about disaster characteristics and how they evacuated on September 20-21, 2012. There found several causes which make their evacuation difficult as a result. －Danger by intensive downpour and thunderbolt. －Difficulty to watch out surrounding situation through darkness and rain. －Overflow of muddy water to the evacuation route and debris as obstruction. From the actual condition of evacuation in this time, four important points are suggested to improve warning and evacuation system in future. －Improvement of accuracy to predict precipitation amount which promotes prior evacuation. －The set-up of the timing of evacuation in consideration of a regional peculiarity. －Development of evacuation for primary refuge and secondary to help safe evacuation. －Improve laws/regulations of local land-use for improvement of evacuation routes.