(30) Experimental study of bedforms formed by supercritical density currents
Hajime Naruse1, Norihiro Izumi2, Miwa Yokokawa3 and Tetsuji Muto4
1Division of Earth and Planetary Sciences, Kyoto University, Kyoto 606-8502, Japan.
2Graduate school of Engineering, Hokkaido University, Hokkaido 060-8628, Japan
3Faculty of Information Science and Technology, Osaka Institute of Technology, Osaka 573-0196, Japan
4Faculty of Environmental Studies, Nagasaki University, Nagasaki 852-8521, Japan
This study reveals characteristics and formative conditions of plane bed and cyclic steps formed by saline density flows with suspended sediments in supercritical flow condition. Experimental density flows were produced by mixtures of salt water (1.01-1.04 in density) and plastic particles (1.5 in specific density, 140 or 240 μm in diameter). Salt water and plastic particles are analogue materials of muddy water and sand particles in turbidity currents respectively. Acrylic flume (4.0 m long, 2.0 cm wide and 0.5 m deep) was submerged in an experimental tank (6.0 m long, 1.8 m wide and 1.2 m deep) that was filled by clear water. Features of bedforms were observed when the bed state in the flume reached equilibrium condition. The experimental conditions range 1.5-4.2 in densimetric Froude number and 0.2-0.8 in Shields dimensionless stress.
We reports two major discoveries as a result of the flume experiments: (1) Plane bed under Froude-supercritical flows and (2) Geometrical characteristics of cyclic steps formed by density flows. (1) Plane bed was formed under the condition of supercritical flow regime. In previous studies, plane bed has been known that plane bed can be formed by subcritical unidirectional flows. However, this study implies that plane bed can also be formed by supercritical conditions with high Shields dimensionless stress (>0.4) and very high Froude number (> 4.0). This discovery may suggest that previous estimations of paleo-hydraulic conditions of parallel lamination in turbidites should be reconsidered. (2) This study also revealed geometrical characteristics of cyclic steps. Cyclic step is a type of bedform that is frequently observed in flanks of submarine levees. This study proved that cyclic steps of density flows show different geometry to those formed by open channel flows. Cyclic steps formed by open channel flows have generally asymmetrical geometry in which lee side is short, whereas cyclic steps formed by density flows are relatively symmetrical and varies their morphology remarkably depending on flow conditions.