地形
Online ISSN : 2759-2529
Print ISSN : 0389-1755
35 巻, 1 号
選択された号の論文の2件中1~2を表示しています
総説
  • 堀田 新太郎
    2014 年 35 巻 1 号 p. 1-24
    発行日: 2014/01/25
    公開日: 2024/12/25
    ジャーナル フリー

    Sand fences are commonly employed structures to control wind-blown sand on beaches. The accumulation characteristics of wind-blown sand around single-row sand fences are identified and examined based on a literature survey. An attempt is made to describe the two-dimensional accumulation processes and forms until the sand fences are buried. These processes and forms are complicated and depend on the fence properties (e.g., porosity, shape of openings, and height), sand characteristics, and wind conditions. Experimental data show three different types of accumulation processes and forms. The first type involves sand accumulation on the upwind side of a solid fence with a porosity of 0%. The second type implies sand accumulation both at the upwind and downwind sides of a permeable fence with porosity below about 30%, resulting in dunes forming with a narrow width and a steep back-slope corresponding to the angle of repose. The third type encompasses sand accumulation mainly at the downwind side of a permeable fence with porosity higher than about 30%, causing a dune to form downwind, at the initial stage having two peaks (bimodal), but at the final stage the dune becomes monopeaked with a mild back-slope and a wide width. With increased wind speed, general trends found for single-row fences of a constant fence porosity are: (1) the upwind dune slope becomes steeper, (2) the crest of a dune moves further downwind, and (3) sand accumulation downwind the fence becomes greater. For constant wind speed, the following observations were made: (1) fence porosity controls accumulation speed in front of the fence, but the forms that develop after the fences are buried are almost the same, (2) with increase in fence porosity, the crest of a dune moves further downwind, (3) sand mainly accumulates in front of the fence and the deposition downwind the fence forms a steep slope near the angle of repose of the sand, if the fence porosity is less than 30%, (4) sand mainly accumulates behind the fence and the deposition downwind the fence will create wide dunes with mild slopes, if fence porosity is higher than 30%, (5) a fence porosity of about 20% produces rapid dune crest growth, and (6) a fence porosity of about 40% rapidly leads to accumulation of sand in large quantities.

研究ノート
  • 木下 祐介, 八反地 剛, 八木 勇治, 江崎 隼輝, 奥村 大輔
    2014 年 35 巻 1 号 p. 25-39
    発行日: 2014/01/25
    公開日: 2024/12/25
    ジャーナル フリー

    The 2011 East Japan Earthquake caused landslides across the Northern Kanto and Tohoku Regions. The purpose of the present study is to reveal the characteristics of shallow landslides induced by the earthquake and subsequent rainfall in granitic mountains located in northern Ibaraki Prefecture, Japan. Total 140 shallow landslides were identified through interpretation of aerial photographs and fieldwork. Among the 140 landslides, 65 were directly triggered by the main shock (March 11 to 18, 2011), and the rest 75 were triggered by rainfall events or other earthquakes from March 19 to December 15, 2011. The most of them are shallow landslides with a slip plane of granitic soil (grus). Bedrock or weathered bedrock is exposed on some landslide scars in the area underlain by biotite granite where soil is thinner. Among the 87 landslides, which occurred on natural hillslopes, 50 landslides slid eastward (NE to SE). The bias in landslide direction on natural hillslopes would correspond to the direction of crustal movement of the main shock.

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