Journal of the Japan Society of Erosion Control Engineering

In this study, we investigated effects of interval of measurement of soil thickness on prediction of shallow landslide susceptibility. Study area is the Aratani district, the West Hiroshima mountain range, where 4 landslides occurred in a heavy rain of June, 1999. We prepared five datasets about soil thickness which have different spatial resolution. We calculated spatial distribution of critical steady-state rainfall (r_c) required to cause slope instability. If interval of measurement of soil thickness was high enough, the spatial distribution of critical steady-state rainfall (r_c) were consistent with the spatial pattern of shallow landslides triggered by the heavy rainfall of June, 1999. While, the critical steady-state rainfall (r_c) could not explain the location of shallow landslides at the point where we did not measure soil thickness. Based on these analysis, we considered measurement intervals of soil thickness have to be shorter than width of the old shallow landslide scars.

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In Japan, many of the recent sabo dam constructions have been of the open-type in consideration of the continuity of sediment routing from upstream to downstream reach in a river basin. A lot of experimental and numerical research has been conducted on sediment control using concrete slit dam, slit dam with steel grids and so on. However, there are a few researches which discuss sediment runoff and its control near upper parts of the grids, while channel shifting and bars movements can be active due to deposition after huge volume of sediment is captured in the storage area of sabo dam
In the present study, the sediment control function of the “grid-type sabo high dam (GHD)”, which is defined as a gridtype sabo dam with a height over 15 m, was examined preliminarily using experimental data sets such as dimensionless sediment runoff volume passing through the check dam. Flume experiments for sediment control using a kind of “steel wire-nets” near the vertically upper parts of the grid were conducted focusing on the simple and the effective countermeasures for the sediment storage in a sabo dam, and sediment control in present easy way, in which steel wire nets could be flexible and removable, was confirmed.

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We set the rules for occurrence and non-occurrence of debris flow using the existing site survey data for the sediment disaster in Hofu, Yamaguchi, Japan on July 21, 2009. Using only separable factors for the occurrence and nonoccurrence events, we could improve the rule's accuracy rather than in case of using matching rates based on rough set theory. Compared with the accuracy of the rules extracted from topographical or geological maps etc, these two rules' accuracy was equivalent. Therefore, we can detect high-risk torrents of debris flow without interpreting topographical or geological maps. As the results, it is able to reduce the risk of damage at future heavy rainfall by using the extracted rules based on the existing site survey data. Moreover, we can apply this rule to another area with similar conditions. It will be very effective for setting up a plan of disaster-prevention measures.

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Flexible protection barriers have not been as widespread as retaining walls in Japan, and it is difficult to confirm their effectiveness using full-scale tests because of safety management issues. However, the flexible protection barrier will become more useful tool in slope failure protection as new designs are developed. Here, we report on an experiment conducted in Switzerland in 2008. This experiment utilized full-scale collapse where soil flowed down-slope and the load was measured at the barrier. The flexible protection barrier safely caught 140 m³of soil. According to the calculated flow velocity determined using high-speed video cameras, flexible protection barriers could endure loads on many slopes in Japan, although they require a ‘catching space' downstream of the barrier to collect collapsed soil.

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