地形
Online ISSN : 2759-2529
Print ISSN : 0389-1755
36 巻, 4 号
選択された号の論文の4件中1~4を表示しています
論説
  • 古市 剛久
    原稿種別: 研究論文
    2015 年 36 巻 4 号 p. 231-252
    発行日: 2015/10/26
    公開日: 2024/12/25
    ジャーナル フリー

    In the hilly terrain in the wet-temperate zone, evolution of hillslopes has been driven largely by scouring the surface by rapid landslides at various spatio-temporal scales and filling or modifying the scours by continuous soil creep. The spatial pattern of landform units in the valley-head areas can be recognized as a result of repetition of the scouring-filling processes at various spatio-temporal scales. Typically, the head hollow, whose initial shape was formed by a relatively large, deep landslide, is located upslope of or close to the present channel head, which was formed by a smaller and shallower landslide. This spatial pattern of landform units is observed in the valley-head areas in the Tomiya Hills, northeastern Japan, where large, deep landslides initially formed head hollows probably after 30000 years BP. However, the occurrence of associated shallow landslides has not been adequately examined. This study investigated patterns of hillslope micro-landform units and regolith structure across the valley-head areas in the Tomiya Hills to identify spatio-temporal characteristics of shallow landslides. Then, a conceptual framework of hillslope evolution by multi-scale, nested landslides in the valleyhead areas was constructed and discussed. In the eleven valley-head areas investigated, conduit-shaped, narrow and shallow depressions (subhollows) were commonly observed upslope of the present channel heads. Regolith beneath the depressions is unconsolidated and weak (N10≤1.5). These characteristics suggest that the depressions were formed initially by shallow landslides and have been filled by materials transported from the surrounding slopes. Two 14C dates for buried charcoal collected close to the bottom of the weak deposits in a subhollow are 927-787 cal yr BP and 688-673 cal yr BP, indicating that the newest scour was formed by a shallow landslide before these ages. The storage capacity of the newest scour of the TK-1 valley-head area was estimated to be about 70 m3. Under the climate of the last 800-1000 years, the range of the critical depth of the weak deposits for slope failure in the TK-1 valley-head area is estimated to be 0.5-1.3 m. Two discrete spatio-temporal scales of scouring processes in the Tomiya Hills are characterized by large, deep landslides with a spatial scale (i.e. volume) of an order of 103 m3 and a temporal scale of an order of 104 years, and shallow landslides with volume of an order of 101-102 m3 and a temporal scale of an order of 102-103 years.

  • 丹羽 雄一, 須貝 俊彦, 松島 義章, 松崎 浩之
    原稿種別: 研究論文
    2015 年 36 巻 4 号 p. 253-270
    発行日: 2015/10/26
    公開日: 2024/12/25
    ジャーナル フリー

    This paper discussed the formation process of small alluvial plain at an embayment of ria coast as an example of the Kesennuma Okawa Plain, northeast Japan, where existing boring logs are easily obtainable because of many drilling surveys related to earthquake disaster reconstruction after 2011 Tohoku-oki earthquake, based on analysis results of one borehole core and utilization of information from existing boring logs. Total over fifty radiocarbon ages, most of which is from soil samples of existing boring logs related to 2011 Tohoku-oki earthquake disaster reconstruction, enable us to describe sediment-accumulation curves corresponding to delta system. Depositional rate is high (10 ~ 20 mm/yr) in the lower part (tidal flat to estuary), low (0.4 ~ 3 mm/yr) in the middle part (inner bay), and high (3.5 ~ 20 mm/yr) in the upper part (deltafront). Geomorphic/geologic cross section with 1000 year isochrones shows that study area is characterized by delta prgradation since 5,000 cal BP at the latest with progradation rate of less than 1 m/yr, which is consistent with smallness of river and size of alluvial plain. Depositional rate change and sediment stacking pattern corresponding to Holocene delta sequence probably reflect narrow incised valley and closed inner bay environment. This study is first example in the alluvial plain in the Sanriku coast which shows accumulation rate changes and sediment stacking pattern similar to those identified in relatively large typical coastal plain.

  • Kiyoharu HIROTA
    2015 年 36 巻 4 号 p. 271-284
    発行日: 2015/10/26
    公開日: 2024/12/25
    ジャーナル フリー

    This paper describes predominant factors related to slope failure of two neighboring slopes with differing geomorphic forms through investigation and experiments. The study site is an undercut slope of the Oyodo River (Oyodo gawa), Miyazaki Prefecture, Japan, which has two different slopes. Slope A is at the upper stream near the small catchment and the other Slope B is at the lower stream on the river. The geology of this site is composed of the Mesozoic mudstone and fluvial terrace deposits at the left bank of the river. Experiments are carried out with the ring shear test, X-ray diffraction test, pH and electronic conductivity (EC) of clay test, and ion chromatography of the water seeping from the slope. In the results found that the undercut slope has two forms, landslide form at the upper stream as Slope A, and denudation form with gully erosion at the lower stream as Slope B. Slope A has “valley-like inner structure” with joint system in mudstone, retaining water in the slope and further weathering rock from the joints. Meanwhile, Slope B has a “valley-like inner structure” erosion on the slope is caused by gully erosion thorough some joints.

研究ノート
  • 宇多 高明, 三牧 純子, 大中 晋, 三波 俊郎
    2015 年 36 巻 4 号 p. 285-308
    発行日: 2015/10/26
    公開日: 2024/12/25
    ジャーナル フリー

    Tuvalu is located at 8°31’S and 179°11’E in south Pacific Ocean with the capitol on Fongafale Island in Funafuti Atoll. On October 21, 1972, Tropical Cyclone Bebe hit Funafuti Atoll, and during the cyclone a large amount of coral debris was deposited on the reef flat (Maragos et al., 1973; Baines et al., 1974). Although this example shows that coral islets could be developed by storm waves during cyclones, the formation and deformation of the rampart on the reef flat and resulting deformation of the coral islets are important in the conservation of the atoll, when future sea level rise is considered. The long-term shoreline changes in Funafuti Atoll was investigated by using aerial photographs in 1971 and 1984, and satellite image in 2010. Also the photographs of the formation of the rampart immediately after Tropical Cyclone Bebe were collected. On June 6, 2012, site observations were carried out on Fatato, Funangongo and Funamanu Islands located southwest of Fongafale Island, as well as the beach surveys using a total station at the east and west ends of Funamanu Island on June 12 and 13, 2012. Maragos et al. (1973) concluded that although severe tropical cyclones may be considered to be infrequent phenomena, from a geological time perspective, they may play a significant role in the accretion and formation of atoll islets. In fact, coral debris was transported shoreward due to wave action, and this rampart was becoming incorporated into the fabric of Funafuti (Nunn, 1994). However, it was concluded that the net increase in atoll islets is small, suggesting that almost all coral debris was lost due to the southwestward longshore transport along the reef edge in the long period, and finally they are assumed to have been fallen into reef gap.

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