A Japanese pipe hydrophone has been used for bedload measurements in mountain torrents. This measurement sensor is an indirect method using an acoustic signal. Bedload discharge rate can be estimated using the Japanese pipe hydrophone with a linear relationship between a maximum amplitude of eigen frequency of circumferential component of acoustic wave and a momentum of colliding sediment particle with a pipe. Acoustic wave is influenced by several factors, for example, one of factors is an installation condition of a pipe, however there were few studies concerning the acoustic wave deformation. We conducted flume tests on acoustic wave deformation focusing on the pipe fixed condition and re-collision of sediment particles to a pipe. Results show as follows : (1) The maximum amplitude of circumferential component of acoustic wave is changed by the pipe fixed condition, but an inclination of the linear relationship did not changed. We proposed that the linear relationship should be checked every installation location. (2) A re-collision of sediment particle occurred by wake. It influences a number of count sediment particles with the Japanese pipe hydrophone at the beginning and the end of flooding stage, and there is a little influence at high bedload discharge which is exceeded the limitation of a number of countble sediment particles. We proposed the method not to re-collide focusing on the relationship between a step height and a reattachment point length from an experimental data in the condition of high Reynolds number and supercritical flow.
The main volcano on Miyake Island, one of the Izu Islands, erupted in 2000, and all islanders evacuated because of the subsequent debris flows and volcanic gas. Various actions had been taken including the construction of sabo dams to cope with debris flows. Under these circumstances, a questionnaire survey was conducted twice, in 2012 and 2014, to find out what the residents around its disaster stricken area think about the sabo, or sediment control, and how they rate it. During the two surveys, a debris flow disaster occurred on Izu Oshima Island in October 2013. In analyzing its result, the relation between recognition and opinions are currently used. However, the survey shows it is efficient to add evaluation to them in analyzing. It is considered useful for future sabo projects if the authorities can take actions based on having a good understanding of the residents' recognition, opinion and evaluation of sabo.
In recent years, there has been increased use of INSEM (in-situ stabilized excavated material)-based Sabo structures. Because INSEM is a low-quality material, one concern about structures built using INSEM is whether they will prematurely degrade in natural environments. There have been few investigative case studies, so it is currently impossible to clearly evaluate durability with regards to freeze-thaw cycles and so forth. The aim of this study was to provide information related to INSEM aging, durability, and degradation characteristics, based on the results of an aging study performed from 1999 to 2014 by test pieces and a structure at an altitude of 1300 m. The aging study investigated external appearance, compressive strength, relative dynamic modulus of elasticity, and mass loss rate. The lowest temperature at the site was －12.8℃, the freezing period was 91 days per year, and the observed materials were 8-15 years old, yet barring exceptional circumstances, no remarkable degradation due to freeze-thaw cycles or other meteorological phenomena was seen. The results of this investigation therefore suggest a low risk of degradation of INSEM and INSEM-based structures of this age due to the effects of meteorological phenomena such as freeze-thaw cycles. In other words, we confirmed that INSEM has sufficient durability to resist premature degradation under natural conditions for at least 8-15 years
The Act for Prevention of Disasters Due to Collapse of Steep Slopes (Steep Slope Act) was established in 1969 to complement the relevant acts mainly including the Sabo Act and Landslide Prevention Act, with “life protection of the Japanese people” as the direct aim and “stabilization of people's livelihood and national land conservation” as the ultimate aim. The Act was pioneering and epoch-making in that it required that people who are reasonably expected to suffer damage due to steep slope failure, be advised to take appropriate measures against such slope failure and lists new nonstructural measures, including the designation of disaster danger areas, the establishment of a warning and evacuation system, and relocation of houses, not mentioned by conventional acts such as the Landslide Prevention Act. However, the Act did not necessarily serve its purpose as the nonstructural measures mentioned therein were not intended to apply to all of the areas expected to be damaged, it left building restrictions to the relevant ordinances, and it could not be applied to the debris flow disasters that frequently occur. In response, the notifications were issued in the early part of the 1980 s to compensate for the drawbacks of the Act by reinforcing nonstructural measures against debris flow. Such improvement efforts were taken up by the Act on Promotion of Sediment Disaster Countermeasures for Sediment Disaster Prone Areas (Sediment Disaster Prevention Act) enacted in 2000. Clarifying the characteristics of the Steep Slope Act in comparison with the aims of other acts and having an overview of the roles the Act played in nonstructural measures against sediment disasters are considered to support measures against sediment disasters.