Structure and age of sediments play an important role in determining the sedimentation of a river. We plan on combining a nondestructive survey on the structure of riverbed and its effective dating to investigate the soil movements, specifically during flood events. Here, we applied ground-penetrating radar (GPR) to analyze the sedimentary structure of a riverbed. At the Ida River in Toyama city, the GPR prospecting with a 250 MHz antenna was applied to investigate the sedimentary structure down to a few meters depth from the riverbed surface. After the survey, the sediment of the riverbed was bored into, and the boundary between sand and gravel, and cobble stones was identified at a depth of 1.4 meters. In the GPR profile, the difference of reflections was detected at about the same depth, which showed that GPR prospecting was useful for studying the shallow riverine subsurface sedimentary. Formerly, very few studies dealt with the age of the riverbed, especially for the flood sediments. We applied a geomagnetic dating method on the flood sediments found at the Imaichi archaeological ruin of Toyama city, around the old Jinzu River. The magnetic study using remanent magnetization successfully revealed that the sediments were formed in 1900- 1950, which suggests a correlation with the 1914 flood event at the old Jinzu River. The geomagnetic dating method is useful for the past flood events independent from the study by paleography. We will further improve the efficiency of magnetic dating and GPR prospecting methods to examine the deposition history of flood events.

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The Asakura et al. formula is used to estimate tsunami wave force acting on the land structure on the non-overflowing flow. The Matsutomi formula and the Ikeno et al. formula are used for the estimation of the collision force of the driftage thrust by the tsunami acting on the seawall. It is not clear whether these estimation formulas can be applied even in different situations from the proposed experimental conditions. In this experiment, the tsunami wave force was measured in slope and upright topography. It is also done installing obstacles representing sandy beach or protective forest on the front of the seawall. We examined whether the experimental value can be evaluated by the past formula under these conditions different from the original Asakura et al. experiment. In addition, we verified the collision force of the driftage acting on the seawall can be evaluated by the past formula by a different wave generation method from the Matsutomi and Ikeno et al. experiments.

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In the event of a disaster that might cause severe and extensive damage, such as natural damming due to large-scale mass wasting or out-of-channel flooding of a river due to a breach in a bank, disaster prevention organizations need to take emergency response actions as promptly as possible by making effective use of limited personnel. In order to complete planned activities within a short period of time, it is good practice to estimate the time required for accomplishing a network of emergency response-related activities as a factor to take into account when making decisions as to details such as efficient staffing and whether or not to request for assistance. In this study, a simple formula was developed for estimating the time needed to accomplish a multipath network of emergency response activities by deriving a mathematical model based on the Kolmogorov forward equations and approximating Gaussian integrals to find analytical solutions. The simple formula thus derived was applied to disaster response training associated with an emergency study to be conducted in the event of natural dam (landslide dam) formation to estimate two parameters of the mathematical model, namely, the work done ratio,μ_k, and the rework ratio,λ_k.

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This research aims to reveal purposes of disaster revitalization plan as slogans. City offices often draw up their disaster revitalization plan after disaster. Some slogans which represent purposes of disaster revitalization are written by city offices in that plan. The author analyzed eighty one city offices ‚ disaster revitalization plan from the Great East Japan Earthquake through text analysis. All plans expressed “concept” and “purpose” in short term as slogans. The results show that slogans represent seven main purposes as below‚ “safe and peaceful society” “disaster prevention” “ back to normal life” “ social inclusion” “build back better” “the world’s most advanced Tsunami prevention model”.

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