Morphology and evolution of an isolated sand dune affected by the intersection angle of alternating bidirectional flows: results of flume experiments

摘要

We used a hydraulic flume to conduct analog model experiments to elucidate the effects of alternating orientations of bidirectional flows (intersection angle θ) on the morphology and evolution of isolated sand dunes. An initial cone of fine-grained quartz sand was exposed to bidirectional water flows with a range of intersection angles (45°, 90°, 135°, and 180°) for 20 cycles of flow alternation, with a cycle duration of 2 min. Experimental trials showed two types of deformation of the crest line: (1) “independence type”, in which a new crest line formed at a different location from the existing one; and (2) “reverse type”, in which an existing crest line reversed its migration direction and a new slip face was generated on the new stoss side of the existing crest. The independence type was observed at θ = 45° and the reverse type was observed at θ = 180°. At intermediate values of θ, the two types occurred contemporaneously. Due to the cumulative effect of deformation of the crest line, three types of topographies developed, with features characteristic of dome, longitudinal (seif), and reversing dunes. These results yield insights into the processes and conditions required for the formation of sand dune types. 移動砕屑物量が少ない場で発達する孤立砂丘地形と流れ環境との関係を調べる水中モデル実験をおこなった．初期の孤立地形に作用させる流れを交互二方向流としたところ，峰線の挙動にふたつの類型があることが分かった．既存の峰線とは異なる位置に新たな峰線が形成される独立型と，既存の峰線に沿って逆方向のスリップフェイスが形成され，峰線の移動方向が逆転する反転型である．両者は二方向流の方位差θに対応し，θ＝45°の場合は独立型のみが，θ＝180°の場合は反転型のみが生じた．θ＝90°とθ＝135°の場合，独立型と反転型とが地形上の異なる部位で同時に進行した．これらの峰線の挙動が長い時間に渡って繰り返されると，その累積的な影響の結果として，3種類の地形が発達した．これらの地形は，それぞれ天然のドーム砂丘，縦列（セイフ）砂丘，リバーシング砂丘と共通する形態を持っていた．この成果は，天然の各種孤立砂丘の形成要因解明に寄与するものである．