Reducing the potential hazard of sediment-related disasters is a raging issue in Japan. In reality the counter-measures against the disasters are not progressing fast enough as expected. Therefore, the most important matter is to find a way to measure that can attain maximum effect with minimum expense by making precise evaluation on the level of potential danger of each dangerous spot and then comparing the measured effects with one and another. Takemoto et al. set the level of potential danger of debris flow, using the rough set theory based on desk work data and applied it to the debris-flow torrent in Nagasaki caused by heavy rain in 1982. However, their evaluation of the level of potential danger using the rough set theory has its own limitations -- only applicable to the data when the data of debris flow falls under the definition of the occurrence or non-occurrence. Furthermore, the effect of the measures was not mentioned. In this study, using a Support Vector Machine, accurate evaluation of potential danger on debris flow, which includes the effects of present measures, is carried out.
For sediment management from an ecological point of view, it is essential to evaluate the quantitative and qualitative changes of riverbeds. Particularly the importance of assessing the change in void structure of bed material has recently been pointed out. Some bed variation models are available to the numerical analysis on topographical features of rivers, but they are not useful for such an assessment at all. This study aims at developing a framework of bed variation model considering porosity change in riverbed material. Relating the porosity to the grain size distribution of a sediment mixture and installing the relation into a standard bed variation model, we have developed a framework of advanced bed variation models able to assess changes of porosity. Then, a simple analytical model for a mixture of two particle groups with much different grain sizes is presented in this framework. A flume experiment was conducted to realize the transformation processes of void structure for two situations ; one is that only fine sediment is removed from a sediment mixture and another is that fine sediment deposit into a coarser bed material. After the validity of the presented model was verified using a data set provided by the experiment, the model was applied to a calculation of reservoir sedimentation and its performance was examined in detail for these two situations. The simulation results have showed the model can produce a reasonable distribution of porosity of the riverbed material in the longitudinal and vertical directions for both situations.
The low altitude aerial photograph is used for the investigation and the research in various fields as well as satellite photograph and general aerial photograph, and it is used in the river fields today. This photography method includes several kinds and it is necessary to select the best method for the riverbeds gravel investigation. However, very few attempts have been made to examine these several methods at simultaneity and to verify using photographic measurement method. Therefore, we have taken photographs of several fields and have analyzed digital images using photographic measurement technique of CAD software. As a result, we have concluded that we can separate 10mm gravel of digital image at altitude 10 m and this method is effective for riverbeds gravel investigation, especially investigation of large gravel. In this paper, we describe accuracy of digital image and practicality of low altitude aerial photograph.
Flow-direction changes in the structure of deposits and grain size distribution of hot ash clouds from the pyroclastic flows that occurred on June 3, 1991(Kitakamikoba district) and June 24, 1993(Senbongi district) in the Unzen Volcano were investigated. Changes in the flow velocity, eddy scale, and the eddy rotational speed of the hot ash clouds were also revealed by VTR- analysis. The typical structure of a flow unit of the hot ash cloud deposits showed the alternation of a sand layer and an ash layer. Only sand layers were identified in the area where the hot ash clouds separated from the main body of pyroclastic flow. The depth and grain size of the sand layer decreased rapidly in the lower area where the hot ash clouds separated. The ranges of the critical velocities of traction and suspension for the 60% grain size of the hot ash cloud deposits of the Senbongi district were calculated to be 4.5 to 9.5 m/sec and 6.7 to 13.6 m/sec, respectively, using Bagnold's critical friction velocity equation and the logarithmic distribution law (flow velocity : 18 to 37 m/sec). Above results might be expected to offer that hot ash cloud flow structure is the combination of a high- density sand suspension prevailing layer as a lower part and a low- density ash suspension layer as a upper part.
Proper warning and evacuation are essential for mitigating sediment-related disasters during heavy rainfall. For this purpose, a scientific and rational rainfall index should be established in accordance with regional geological and topographical characteristics. The authors proposed Rainfall index R′ for warning against sediment-related disaster. It became possible to express influences of both preceding and present rainfall by a single value of R′ . This rainfall index was developed by using the past disaster cases in Masado (decomposed granite soil) area in Hiroshima district. In this paper, several cases of slope failure and debris flow including non-granite zone in 2005 were also examined to confirm the effectiveness of this newly-proposed rainfall index. It has been understood that the rainfall index R′ will be able to use widely, if the coefficient and the reference value are chosen properly for each region considering its geological and topographical characteristics. Also, in this paper, we proposed the method for easily converting the usual rainfall index into the R′.
A large landslide caused by Typhoon 0514 formed a landslide dam on the Mimi River, Miyazaki Prefecture, Japan, during the night of September 6, 2005. Fortunately, the dam collapsed shortly afterwards and the resulting flood did not cause damage downstream because the flood discharge was small. We estimated the discharge rate downstream from the landslide dam using the discharge rate from hydroelectric dams and the time that the landslide dam collapsed based on the noise that local inhabitants heard. We also studied communications between disaster -prevention personnel regarding the landslide dam. Because they could not see the landslide dam as it formed during the night, and could not predict the flow when the dam burst, local inhabitants could not be suitably evacuated. In this study, we calculated the peak discharge rate when the landslide dam burst using a new simulation model. We found that the peak discharge rate in this model was greatly affected by the longitudinal form of a landslide dam, and had a tendency to increase when the longitudinal length was short. To predict the peak discharge rate rapidly when a landslide dam forms, a system that determines the longitudinal form of the landslide dam and the inflow rate is needed.