It is important to estimate the occurrence probability of deep-seated catastrophic (rapid) landslides (DCL) in order to plan measures to counter future disasters. We attempted to estimate the occurrence frequency at two sites using three methods. Firstly, we estimated the occurrence frequency based on a literature survey. Secondly, we estimated the occurrence frequency based on the result of the interpretation of multiple aerial photographs taken at different times to clarify the timing of DCL occurrence. Thirdly, we used a relationship between rainfall magnitude and DCL to evaluate the return period of rainfall-magnitude-induced DCL. This method also used the interpretation of multiple aerial photographs to clarify rainfall-magnitude-induced DCL. Here, we show that these three methods could estimate the occurrence probability of DCL, although the frequency based on the literature survey was underestimated by one order of magnitude. The rainfall magnitude and frequency relationship is effective for areas for which there is limited information about DCL occurrence.
A very rare and heavy rainstorm occurred between 15 and 18 o'clock on July 16, 2010 and induced over 1,500 shallow landslides and debris flows in Shobara, Hiroshima. The three-hour event was 173 mm of rain with a maximum hourly rainfall of 72 mm. In this study, the relationship between geographical features, vegetations and shallow landslides was investigated in relation to the landslides caused by the heavy rain. In order to analyze this disaster, aerial photographs, a 1-m-mesh Digital Elevation Map created by airborne laser survey, and vegetation data created using a Geographical Information System were employed. It was determined that the landslides happened at altitudes between 300 and 650 m. The mean value of the slope gradient on which they occurred was 28.1 degrees, which is steeper than the average slope of the entire analysis area ; landslides on slopes in excess of 40 degrees were widespread. Moreover, the slope-failurearea rate of the landslides was highest on northeast slopes. The vegetation coverage by area in the areas that experienced landslides existed in the following order : Sawtooth Oak (Quercus acutissima), Japanese cedar (Cryptomeria japonica) and Japanese cypress (Chamaecyparis obtusa). The landslide which was locating the Japanese oak existed in the steep place as compared with other forest type. 95% of the forest of the study area was less than the 16 age class and the forest of the 11 age class occupied the largest area. The slope-failure-area rate was very high for forests of age class 1, 2, and 6.
In Kanayama-sawa, which is located in the Hime-kawa basin, unstable soil masses, with an estimated soil volume of
720,000 m3, and movement of soil were observed via on-site inspection and from data acquired from the laser profilers(LPs). The unstable soil masses might cause damage downstream, since they are prone to fluidization due to snowmelt runoff or heavy rain. Therefore, in this study, we observed the soil movement in the field and employed debris flow numerical simulation to determine the effect of debris flow caused by fluidized soil masses. From the on-site observation, we found that the unstable soil-mass moves 1.6 m per month at maximum, and that the soil mass can be
divided into four blocks. In simulations, we applied three scenarios of debris flow occurring from this soil mass by
varying the soil volume-small, medium, and large. The debris flow outreach varied with soil volume, and was thus
different in each case ; however, flooding was limited to the current river channel in all cases.
During Typhoon No.12 (September, 2011), a large landslide dam resulted from a deep-seated landslide in Akadani area, in Gojo, Nara Prefecture. Subsequently, Kii Mountain District Sabo Office, Ministry of Land Infrastructure, Transport and Tourism, developed countermeasures. However, frequent debris discharge from the deep-seated landslide slope has disturbed the progress of the countermeasure works. On September 21, 2013, although no rainfall was observed, a large re-slide, with an estimated collapsed soil volume of 420,000 m3, occurred suddenly after 5 continuous days of fine weather following the passage of Typhoon No. 18. Slope stability analysis using airborne laser scanner data obtained on September 18, just before the re-slide, showed that its mechanism involved gully erosion at the foot of the remaining collapsed soil, which made it unstable. As it occurred before morning, no people were injured. Nevertheless, conducting countermeasures safely became more difficult, even in fine weather. Therefore, we plan to conduct a slope stability analysis using airborne laser scanner data before resuming work after a heavy rainfall.
In Taiwan, there are over 1,600 debris flow prone torrents, and various sediment-related disaster prevention facilities have been installed on these torrents in the mountain areas. However, most of them are closed-type concrete Sabo dams and open-type steel Sabo dams are rare. One of the reasons is that their effect or effectiveness has not been duly recognized by Taiwan's authorities. Therefore, we installed a full-scale test piece (Open-type steel Sabo dam) on the Landow torrent in Huisun forest, National Chung Hsing University, in order to verify its debris flow capture effect. On July 13, 2013, the seventh typhoon of the season (Typhoon Soulik) struck northern Taiwan and made substantial debris flow occur in this torrent. The installed open-type steel Sabo dam also got buried under the enormous amount of sediment and gravel following the debris flow, but significantly reduced the impact of debris flow and protected the downstream bridge. This paper describes the consideration on debris flow capture effect of open-type steel Sabo dam, by analyzing the debris flow capturing and depositing process and capture form based on the monitoring camera data and field survey. From the monitoring camera data, we recognized that this dam got buried because the sediment trap capacity exceeded due to the continuous sediment flowing down even after the capturing of debris flow and subsequent flow, leading to subsequent sediment and gravel being deposited to downstream by flowing over the dam, but not because boulders and sediments passed between the members. In addition, it was also confirmed by the field survey that the opening of open-type steel Sabo dam got closed up by arched stacking efficiency between boulders to capture debris flow.
To maintain the torrent environment under construction of erosion control facilities such as check dams and channel works, evaluating their influence on the torrent environment such as torrent flow velocity or water depth and their space -time fluctuations is essential. The field investigation and statistical analysis were conducted to clarify the influence of the artificial structures on the flow conditions in torrents. We found that the artificial structures tend to simplify the flow conditions and reduce the fluctuation and/or turbulence of flow that associates complex natural flow condition.
In terms of the SABO soil cement business which is aiming at zero emission, it is a big subject to utilize 100% cohesive soil and we need to consider for measures utilize the earth and sand effectively. In SABO dam filled INSEM materials, due to be worse than concrete in terms of freeze-thaw durability and wear resistance etc, generally, a plan, application of outside protective texture and the section are wider than usual, is constructed. however, there is few study case related to degradation of filled material before, and it is necessary to comprehend the relation among many kinds of influencing factor about long-term stability for two cases. one is field-generated soil consists of cohesive soil and another is bad condition in the field. As a result of the SABO soil cement (INSEM), about the cohesive soil of this survey sample, compressive strength : 1.2 N/mm2 is confirmed as threshold by repeat of drying and wetting. In the case of dispersion into consideration, if it aims at site average compressive strength : 3.0 N/mm2, it is considered that it ensures long term durability by repeat of drying and wetting. Moreover, the value exceeds threshold : 2.6 N/mm2 by freezing and thawing and it was considered as an aim value to construct more stable dam structure.