During the 2008 Iwate-Miyagi Nairiku earthquake, many slope failures occurred in mountainous areas in southwestern parts of Iwate Prefecture and northwestern parts of Miyagi Prefecture. In this study, in order to fully understand the mechanism of slope failures occurring in the Koei district, Kurihara City, we carried out drilling survey at the investigation site and some soil tests on the drilled cores and soil samples obtained from the ground surface. Various aspects of soil characteristics were multilaterally investigated including physical and chemical properties, mineral compositions, and mechanical properties under both static and dynamic conditions. As a result, depth distribution of clay minerals and N-values of the drilling sites revealed that weathering and rock degradation of the soft pumiceous tuff are significant just under the strata boundary. This strongly indicates the role of underlying weak weathered layer and its destruction during the earthquake possibly triggering slope failures of the steep cliff locating near the flank of the caprock layer. Furthermore, Long-traveling flow-type failures occurring on the gentle slope could be caused by excess pore pressure generation by loading of collapsed materials on the top part of the slope. In addition, results of dynamic shear tests on the slip surface soils also suggest the possibility of liquefaction of previously collapsed and deposited materials which are composed of loosely sedimented volcanic ash soils.
Local torrential rain frequently causes serious damage to hillslopes in mature stand. In this study, we examined relationship between stand age and collapsed area ratio, collapse depth in reforested sites in Miyagawa-dam basin, Mie prefecture, Japan. We report our findings as follows : 1) Area ratio of landslides was highest after 6 years of reforestation, and declined after 20 years. After 30 years of reforestation, there occurred little landslides. 2) However many landslides occurred by typhoon T 0421 in forestation sites after 15-20 years of reforestation. 3) Positive regression was shown between the stand age and the landslide depth. 4) In Japanese cedar stands, landslide depth was greater than those of Japanese cypress stands.
This study used two conceptual models to examine the effects of rainstorms on landslides after an earthquake : 1) a tank model for calculating the soil water index and 2) a process-based model. The process-based model used a digital terrain model with 10-m resolution to calculate the regional potential for shallow landslides, based on the distribution of shallow infiltration water, Darcy's law, and a safety factor estimated by infinite slope stability analysis. We used this process-based model as a conceptual model, rather than as a physically based model, and calculated safety factors for each cell area ; values less than 1 were compiled as the potential landslide area index, after eliminating cells that were unstable under no-rainfall conditions. The two models were applied to the Funyu Experimental Forest of Utsunomiya University in Tochigi Prefecture, Japan. At the end of August 1998, a heavy rainfall event caused many shallow landslides in the study area, whereas other heavy rainfall events from 1994 to 2003 did not cause severe landslides. The relationships between the two indices (soil water index and potential landslide area index) showed a close correlation, confirming the validity of both approaches for the estimation of rainfall properties. Response analysis of the effect of earthquakes on soil strength parameters indicated that landslide-triggering rainfall thresholds varied with decreasing values of effective soil cohesion. This implies that the relationships between effective soil cohesion and the soil water index triggering landslides derived from two conceptual models were valid for the re-estimation of the influence of rainfall properties on landslide occurrence after an earthquake.
Recently, several disasters have been caused by sudden heavy rain in short time intervals and ranges. X-band multiparameter radar had been installed across the country since 2009 to improve the precision of precipitation observations. The integration of this X-band multi-parameter radar into early warning systems that the user can judge intuitively is a key challenge. In this study, we developed techniques that use the radar precipitation difference to detect rain clouds that develop suddenly. This technique is aimed at the development of an index that allows users to judge the severity of the situation. To detect new rain clouds and not simply those in motion, this technique calculated the velocity of the cloud and extracted the areas of clouds that suddenly developed. In addition, we examined the effectiveness of some examples. Using X-band multi-parameter radar, we found that we could provide information on both precipitation and suddenly developing rain clouds.
In the observation of bed load transport using a hydrophone, a bed load measuring pit is often installed for the calibration of the hydrophone's pulse data. A small pit, however, poses a problem as it tends to fill up before a flood event reaches its peak. Because past observations have revealed a tendency for bed load to exhibit different flow patterns before and after the peak of a flood, we need to establish a relationship between the hydrophone pulses and bed load transport after the peak. For this purpose, a device for measuring bed load transport has been developed with a mechanism that opens the slit of the measuring pit automatically at the peak of a flood. This device uses water-level measurements obtained by an ultrasonic water gauge as a trigger to activate a hydraulic jack which moves the lid installed over the slit so as to allow the pit to start collecting bed load in the middle of a flooding event.