Effect of grain concentration on bedforms under the upper flow regime

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  • 高領域ベッドフォームに対する流砂濃度の影響
  • コウリョウイキ ベッドフォーム ニ タイスル リュウサ ノウド ノ エイキョウ

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Abstract

Under shallow flow conditions with a Froude number of about 1.4, the bedform and the direction of migration are affected by grain concentration in an experimental flume. With a decreasing feeding rate, the bedform changed sequentially from a plane bed to antidunes steadily migrating upstream, antidunes with smaller wavelengths showing occasional breakers, standing antidunes, antidunes migrating downstream, and finally fast-migrating “ripples” with an inharmonious water surface in the upper flow regime. Under a higher feeding rate, antidunes are stable with longer wavelengths and with a lower frequency of breaking waves (hydraulic jump). Wave breaking, which is more frequent at lower grain concentrations, also produces a local flat surface via downward filling of the deepened trough. The sheet flow layer, which has a high sediment concentration and is identified from a transmitted light image, is thicker and denser over upstream-migrating antidunes than over downstream-migrating antidunes. Therefore, the sediment concentration is likely to influence bedforms under shallow supercritical flow. The fact that larger wavelengths and less-frequent wave breaking are associated with higher sediment concentrations suggests that the sediment load acts to stabilize the flow. The velocity gradient near the bottom and the turbulent component (velocity deviation) of the vertical and horizontal velocities of the flow are large compared with those of the sediment grains. The grain velocity structure does not change within a range of sediment concentrations between 0.55 and 3.3 wt%. Therefore, the concentration of sediment at the lower boundary can give rise to a steeper density gradient and a shallower velocity gradient for the flow, both resulting in a higher Richardson number (i.e. more stable boundary flow). The sediment concentration can suppress flow oscillation or wave breaking of the flow and thereby produce upstream-migrating stable antidunes or a flat bed.

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