Debris flow capturing function of open type steel sabo dam depends on the maximum boulder diameter and net interval of steel pipe members that composes the opening portion. This maximum boulder diameter (D95), deduced from the boulder diameter examination, is actually the 6th largest boulder, and members' interval is decided just by this boulder diameter. This means that disposition characteristic of boulder diameter at the actual site is not being put in consideration at all. So we examined the characteristics of boulders blocking the opening portion, based on debris flow capturing examples with boulders as their main capturing object. As a result, we found out that arch effect is being demonstrated by the 1 to 2 boulders captured between the members (primary blockage), and then these captured boulders capture small size boulders by arch effect (secondary blockage). This chain reaction was how the opening portion was being blocked. The area being occupied by boulders within the opening portion was approximately 70% for both primary and secondary blockage (blockage rate). Then we estimated how much the boulders sought out by the previous boulder diameter examination contributes to the blockage of opening portion, based on blockage rate of 70% and decreasing rate of debris flow's peak discharge, when setting the members' interval at 2 times the maximum boulder diameter. As a result, we found out that half of the subjected boulders contributed to capture debris flow when setting the members' interval at 2 times the maximum boulder diameter, and almost all the subjected boulders when setting the members' interval at 1 times the maximum boulder diameter. But there weren't many examples of opening portion being blocked just by boulders, and most debris flow capturing examples were those being blocked by driftwood. So we examined the boulder capturing function from the blockage condition of open type steel sabo dam capturing debris flow mixed with driftwood. As a result, we found out that the opening portion was blocked by arch effect, just like the boulder capturing result. But no chain reaction like primary blockage and secondary blockage could be observed with driftwood mixed, so it was made clear that the blockage of opening portion was done immediately by small size boulders.
This study introduces a new method for evaluating storm events which triggered slope failures, by using antecedent precipitation indices （APIs）. The method uses novel one-dimensional and two-dimensional diagrams, which examine excesses of APIs over their past maxima. While each of APIs with various half-life times（HLTs）is examined in the one-dimensional diagram, the two-dimensional diagram evaluates each pair of APIs with various HLTs. The method was applied to ５sediment disasters, and results indicated that the method could detect excesses of APIs over their past maxima at occurrences of slope failures for all disasters except for Maniwa disaster. Especially, the two-dimensional diagram was thought to be effective because it can detect anomaly in rainfall more credibly than the widely-used warning systems which use hourly rainfall, water storages in a tank model, and APIs with HLTs of 72 and 1.5h, as rainfall indices. For Maniwa disaster, some slope failures occurred before excesses of APIs were detected by the proposed method. This indicated increases in slope failure vulnerabilities in the region which suffered from treewindfall disasters.
The difficulties in directly monitoring bedload have been recognized and prompted research into surrogate monitoring technologies, including acoustic methods and seismic methods. In Japan, in the last decade, hydrophones (pipe geophone) were widely used in mountainous rivers to monitor bedload. Thus, recently, monitoring data has been dramatically increased. It has been widely recognized a part of bedload might not hit pipe due to complex sediment transport processes, likes saltation and so on. Thus, the observed bedload amounts might be underestimated. However, there was no adequate information about the degree of this underestimation. Here we proposed a method for quantifying the ratio of sediment hit hydrophone to total sediments. Then, we confirmed our proposed method using comparison between our estimated value using hydrophone data and high speed video images.
We have been developing a system called a watershed-management system (WMS). It consists of a rainfall-runoff model, a sediment discharge model and a deformation of riverbed model. WMS was applied to the Sumiyoshi River, and the parameters of the model were determined by considering the observation results. By conducting simulations under various conditions, for instance it was found that several Sabo dams in a downstream area are enough to control the amount of sediment caused by rainfall over a period of 10 years if their sediment trapping capacity is maintained by removal works. This paper explains the model, and describes the validation results.
For the purpose of detecting a premonitory phenomenon involving shallow landslides due to heavy rainfall, we observed the process of rainfall infiltration in soil on a slope of weathered granite during artificial rainfall experiments. Measurements of lateral flow discharge, volumetric water content, and matric potential were taken during conditions where the artificial rainfall intensity was the same but the initial soil moisture contents were variable. The following results were obtained. (1) In the case where the ground was initially dry, a large discharge of lateral flow occurred from the litter and the root mat layer that was shallower than 20 cm in depth. The discharge of lateral flow decreased with increases in the matric potential and matric potential gradient in the lower layer. (2) In the case where the ground was wet prior to rainfall, lateral flow from the litter and root mat layer that was shallower than 20 cm in depth did not occur. This may have been caused by enhanced vertical rainfall infiltration. (3) In the case where the ground was wet prior to rainfall, in comparison to the case where the ground was initially dry, lateral flow was slight in the layer of decomposed granite soil with many roots from 20 to 45 cm in depth. When the moisture conditions approached saturation, and when the difference in the volumetric water content decreased, the lateral flow tended to flow out easily.
The diameter of bed load sediment is measured directly using bed load samples. However, it is expensive to transport sediment from the field to the laboratory and to measure the diameter using the sieve test in the laboratory. Indirect methods of estimating gravel size have been evaluated, including photographs of a dry river bed and using the sound of gravel colliding with a steel pipe called a hydrophone. However, these methods are difficult to evaluate gravel size. Therefore, we proposed a simple, low cost method for measuring the diameter of bed load sediment in a river. Proposed method requires the contact time between sediment and a square acrylic pillar. The contact time was measured in a flume experiment. We examined whether the diameter of sediment could be estimated using the relationship between the contact time and gravel diameter. In addition, the maximum signal wave height and collision velocity were used to estimate the gravel diameter. The results were as follows : (1) the relationship between the maximum signal wave height and gravel diameter was not clearly observed in this experiment ; (2) the collision velocity was not correlated with the contact time ; and (3) the nature of the gravel movement, i.e., meandering or straight, changed with the depth : diameter ratio.