Hydrophones have been used to measure bedload transport intensity by counting the number of pulses generated when bedload sediments such as sand or gravel strike a steel pipe. However, this method has disadvantages ; when sediment rate is high, contact with the pipe is continuous, and therefore, readings rarely drop below a count of one or zero. In contrast to this method, we proposed an analysis method using sound pressure data obtained with a hydrophone in a past study. Flume experiments show that bedload sediment discharge can be calculated quantitatively when using this method. In this study, the applicability of the method was analyzed using field observations. The method was first tested and then modified in field experiments at the Bouzudaira sabo dam in the Yotagiri River within the Tenryu River System. It was determined that a relational expression between the number of grains and the detection rate could be computed for conditions with grain sizes of mixed diameter. To compute the number of grains, the geometric mean of values calculated with multiple division numbers using sound wave data can be used for a high-accuracy calculation during sound wave interference analysis. Field observations using the modified method and direct measurements made by the observation facility were conducted at the same point for the 12 th and 15 th typhoons in 2011. It was confirmed that our method has an analytical limit, but this limit is considered to be negligibly low. Analysis results of bedload transport intensity were in accordance with field observations results. However, when the bedload transport intensity was high, analysis results of the grain diameter were larger than field observations results. These results indicate that the increase of flow velocity and the decrease in the rate of impact are balanced.

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A warning and evacuation system is very important for reducing damage that may occur during sediment disasters. An accurate, high-resolution, real-time hazard map system is very useful for supporting an evacuation decision. In this study, we developed a refined real-time hazard map system that comprised a distributed rainfall-runoff model, slope failure model, sequential learning model, and debris flow inundation model. First, a physically based detailed distributed rainfall-runoff model was constructed. To achieve real-time predictions, parallel computation was applied to the distributed model to reduce the calculation time. As verification, the model was then applied to past heavy rainfallrunoff events and wide-scale slope failure events in the Sumiyoshi basin, Rokko Mountains. Finally, a WebGIS-based graphical display screen image and an alarm system were constructed to inform the user of the predictive calculations of the model. We found that the model could adequately describe sediment disasters and was quite useful as a prediction tool.

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The air-borne magnetic surveys are reported to determine the distribution of susceptible layers to landslide in the southwestern Toyama, including the boundary between the Hida belt and the green tuff region. The results show the strong magnetic anomaly for the igneous rocks but the weak magnetic anomaly for the hydrothermal alternation zone or thick colluvial deposits that are susceptible to landslide, and therefore determine the landslide areas in the Funatsu granitic rocks and the colluvial deposits in the Neogene formations. Further, there is a strong positive correlation between the boundary of the anomalies and the group of faults that divide granitic rocks and Neogene formation in the area. The air-borne magnetic survey is not affected by small variation such as intrusive rocks due to its relatively low resolution of less than several meters, suggesting that this technique could be a powerful tool for the first screening survey for the susceptible layers to landslide, although the magnetic properties of the regional rocks have to be clarified in advance.

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The use of In situ Stabilized Excavated Material (INSEM) is expected to continue in various types of Sabo constructions. Few studies, however, have investigated the quality variation and strength characteristics of the INSEM on the basis of long-term data, and the research has not yet been systematized. This report summarizes the results of studies on the factors that contribute to weak parts and weak layers, apparent quality variation, and long-term strength and durability when using INSEM, which were obtained on the basis of test construction, quality control tests, and construction site monitoring at the Yuzawa Sabo Dam No. 3. In parts and layers, the occurrence of weakness due to defects, known to be dependent on the maximum gravel size, could be alleviated by using a maximum gravel size of 80 mm and using bedding mortar for joint processing. In this construction, with regard to the variation of apparent quality, dispersion of about 20% was found in the variation coefficient of compressive strength and the overdesign factor could be set to 1.49. Assuming quality control based on 1-week age strength, the target test piece strength was achieved in all cases by setting the 1-week age strength equal to the target test piece strength divided by 1.6. Measurement of longterm strength and durability showed that the compressive strength remained high up to about 5 years and that the durability increased with compressive strength. In addition, although the durability obtained by the INSEM was poor in an indoor examination, a certain level of quality was observed under the actual conditions of the natural environment.

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A hydrophone was used to measure the intensity of transported bedload. In a past study, we proposed a method of analysis for use in field monitoring using sound pressure data obtained by a hydrophone and showed that bedload sediment discharge can be calculated quantitatively in flume experiments. In our design, the sampling frequency needed to be several times greater than the circumferential direction frequency. We therefore used a sampling frequency of approximately 20 times the value, in order to ensure a fair design. However, the large volume of sampling data is a disadvantage of our method. Therefore, in this study, the optimum sampling frequency and sampling time for a reduction in data volume were considered using field-monitoring data at Harukidaini sabo dam in the Haruki River within the Fujigawa River System. If the sampling frequency was around four times the circumferential direction frequency, the accuracy of the analysis remained nearly constant ; however, for less values of sampling frequency, the analysis results were entirely different. Therefore, a sampling frequency of around four times the circumferential direction frequency provides an adequate amount of data. We also found that the analysis accuracy remained nearly steady if the sampling time was changed from 0.5 to 5 s. However, since the issue of sampling time depends on the temporal stability of the bedload, the setting of sampling time requires consideration in other observing sites. This is because the temporal stability of bedloads is affected by the terrain conditions and upstream sediment supply conditions.

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To evaluate seasonal changes in forest floor cover percentage (FCP ; total percent cover of understory vegetation and litter), we continuously observed daily variation in forest floor cover on steep hillslopes in two catchments using automatic interval cameras. FCP values in six 0.5 x 0.5-m plots varied in their response to meteorological factors, such as precipitation and air temperature. The cover percentage of understory vegetation in the plots decreased from 65% to 13% and from 54% to ＜ 1% following decreases in daily air temperature during the fall and early winter, respectively. Seasonal variation in the cover percentage of litter was associated not only with precipitation during typhoon events and decline in daily air temperature but also with the species of overstory trees. Under mixtures of broad-leaved trees and Cryptomeria japonica, the cover percentage of litter increased from 35% to 87% together with a decrease in understory vegetation. Under broad-leaved canopies, the cover percentage of litter increased from 27% to 64% as the daily air temperature declined after November. When daily air temperature fluctuated around 0℃ after mid-December, surface soil creep occurred from freezing and thawing in plots with sparse litter cover. Continuous observation using interval cameras allowed us to evaluate not only seasonal variation but also dynamic alteration of forest floor cover. Our approach could be used to examine the processes of soil erosion and sediment production on forested hillslopes, based on critical conditions for forest floor cover and air temperature combined with monitoring of surface soil movement.

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