Observations of flow and grain movement near bed in the ripple and the dune regimes from high-speed video pictures.

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  • 高速ビデオによるリップル領域およびデューン領域での砂床付近の流れと粒子の挙動の観察

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Abstract

The mode of grain movement is a significant factor in sedimentology because it influences the grain fabric of sediments. Laboratory studies regarding the relation between bedform and grain fabric have been carried out. However, grain movement and grain fabric of small bed waves formed immediately after the flow velocity decrease from the upper-plane-bed regime to the dune regime ("upper ripples") have not been investigated so far. This study investigated grain movement near the bed using high-speed video system in the ripple and dune regimes. Grains avalanched on lee slopes in the ripple regime. In the dune regime, however, grains moved as sheet flow everywhere on the surface of "upper ripples", which had wave heights comparable to that of ordinary ripples. These grain movements suggest that the dominant direction of long axes of grains in ripple deposits is similar to cross laminae, while the grain fabric of "upper ripples" shows imbrication with laminae as bases. Grain fabric can be a clue to judge whether small-scale cross-stratification was formed in the ripple regime or in early stage of the dune regime following the upper-plane-bed regime.

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