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Volume 55 (2014 - 201･･･
- Issue 6 Pages 279-
- Issue 5 Pages 216-
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Volume 55, Issue 6
Special Issue
Displaying 1-5 of 5 articles from this issue
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General Article
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Hideki INAGAKI2015 Volume 55 Issue 6 Pages 279-289
Published: February 10, 2015
Released on J-STAGE: July 19, 2015
JOURNAL FREE ACCESSVolcano areas account for one fourth of the area of Japan. In the volcano areas, many debris disasters occur due to earthquakes, heavy rains and volcanic eruptions.
The author explains six natural disasters in the volcano areas at the time of heavy rain which occurs often in recent years. The feature of slope failure at the heavy rain indicates mainly surface slope failure based on geographical and geological structure. For example, the volcanic ash layer is weak to erosion and collapse. The layer collapses easily at the heavy rain because the layer deposits in parallel with a slope.
Moreover, there is a cap rock type slope failure as a serious disaster in the volcano area.
Where surface slope failure occurs in the forest-rich volcano areas, driftwood increases in debris flow relatively. The numerousness of driftwood gives a lot of downstream site damage and the subject of disaster waste treatment.
Finally, the author discusses the land use and legal system to decrease natural disaster damage in the volcano areas.View full abstractDownload PDF (5125K)
Original Article
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Nobuo HAYASHI, Kazuhiro TANAKA, Hiroaki YOSHITAKE2015 Volume 55 Issue 6 Pages 290-306
Published: February 10, 2015
Released on J-STAGE: July 19, 2015
JOURNAL FREE ACCESSBrecciated rocks of several centimeters to several meters in size are distributed on the surface and underground of a deep-seated landslide that occurred in 2005 at the Tsukabaru site in the Mimikawa basin of Miyazaki Prefecture, Japan. A breccia-predominant formation (BPF) composed of breccia and a fine-grained matrix is distributed above the fresh basement rock. We carried out detailed core logging and field observation of the BPF, classifying areas into five grades based on the brecciation process. From investigation of the grain size distribution and assuming a continuous distribution of the BPF, we conclude that BPFs are composed of crushed fine-grained breccia distributed parallel to the slope. BPFs are formed not by faulting due to a lack of sliding plane and clay formation but by the concentration of gravitational strain. As evidence of this, basement rock was crushed along the deformation zone where the gravitational stress is concentrated underground of the unstable slope. That slope resulted from rapid river erosion, and several blocks were formed with river erosion. A deep-seated landslide occurred in 2005 along the boundary between fresh basement rock and the BPF. It is reasonable to assume that BPF is the precursor of the deep-seated landslide and significant tectonic features should be evaluated to elucidate the formation mechanism and make predictions of deep-seated landslides.View full abstractDownload PDF (8907K) -
Takahito KUROKI, Nozomi ISO, Keisuke KURODA, Tatsuroh SOH, Kensuke GOT ...2015 Volume 55 Issue 6 Pages 307-316
Published: February 10, 2015
Released on J-STAGE: July 19, 2015
JOURNAL FREE ACCESSWe researched geomorphic quantities on micro landforms at our study area damaged by overland flood due to the typhoon no.0514 in the Miyazaki Plain. Many micro landforms can be classified in the downstream area of Ooyodo River and Honjo River. The study area adjacent to both rivers was classified into the landforms such as dike, slightly elevated area, lowland and river bed from the interpretation of shade maps made by 5mDEM. The geomorphic quantities of the landforms are average height and relative height to water level calculated by using 5mDEM. The characteristics of the longitudinal profiles on both geomorphic quantities were discussed.
In the profiles, some sections become convex curve caused by aggradations in elevation of micro landforms. The form of convex curves in the profiles on relative height is clearer than that in the profiles on average height. In the profiles on relative height, we can identify large convex curves with approximately 5 km wavelength and small ones with approximately 1 km wavelength. The areas with the maximum value in the large convex curves are close to the river mouth to the main river and the sea, the flection of the rivers, and the bottleneck section. Such areas in the small convex curves are close to the confluence of tributaries. Such areas of the landforms outside of the dike are located downstream those of the landforms inside of the dike. At the areas, it is considered that water velocity decreases, sediment accumulates, and flood level is easy to go up. In the inundated area, the relative height of landforms increases outside of the dike and it decreases inside of the dike. In the non-inundated area, the height of them decreases outside of the dike and it increases inside of the dike. Therefore, the relative height difference between lowland and river bed from the each profiles, we can distinguish the sections where are easy to be inundated by overland flood and where are hard to be inundated.
Consequently, in this study, we clarified that it is easy for us to recognize the vulnerable area for inundation from the analysis of geomorphic quantities on micro landforms.View full abstractDownload PDF (5913K)
Report
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Masaki ISHIKAWA, Toshinori SAKAI, Kenji OKAJIMA, Tatsuo KONEGAWA, Yasu ...2015 Volume 55 Issue 6 Pages 317-324
Published: February 10, 2015
Released on J-STAGE: July 19, 2015
JOURNAL FREE ACCESSIn the Higashi-Kishu area of Mie prefecture, many slope failures occurred by the typhoon No.12 which attacked 2011. As a result of examining the situation of landslides, based on the aerial photographs, geological maps, and DEM, about 90% of 290 landslides had occurred in the Kumano acidic rock area.
Many landslides were concentrated on slopes directed from southwest to southeast. The gradient of slopes concentrated 30 to 40 degrees with the Kumano acidic rock south block, 35 to 45 degrees with the Kumano acidic rock north block. And, the slope forms before failures consisted of valley-type slopes (47%), straight slopes (48%), and ridge-type slopes (5%). As a result of field work, landslides had occurred at grade DL (strongly weathered saprolite) and/or grade DM (moderately weathered granitoid) or the boundary of a grade DH (decomposed granitoid) and talus sediment. In the collapsed places, sheeting joints were confirmed and inclination to valley side at part of columnar joints were confirmed. It was guessed that landslides generated at the remarkable part of deformation by gravity. Along joints exists grade DH. The coefficients of permeability of grade DH were about from 10−5 to 10−6 m/s. And, the electrical conductivities of springs were as low as from 3 to 4 mS/m. It was thought that shallow groundwater at rain was participating in landslides.View full abstractDownload PDF (4517K)
Review
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Ken-ichi NISHIYAMA, Tsuyoshi WAKATSUKI2015 Volume 55 Issue 6 Pages 325-333
Published: February 10, 2015
Released on J-STAGE: July 19, 2015
JOURNAL FREE ACCESSDownload PDF (793K)
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