Japan Geoscience Union Meeting 2015

Presentation information


Symbol H (Human Geosciences) » H-CG Complex & General

[H-CG35] Interdisciplinary approach to earth's changing surface

Wed. May 27, 2015 11:00 AM - 12:45 PM 105 (1F)

Convener:*Naofumi Yamaguchi(Center for Water Environment Studies, Ibaraki University), Hajime Naruse(Department of Geology and Mineralogy, Graduate School of Science, Kyoto University), Koji Seike(Atmosphere and Ocean Research Institute, University of Tokyo), Hideko Takayanagi(Institute of Geology and Paleontology, Graduate School of Science, Tohoku University), Masayuki Ikeda(Shizuoka University), Chair:Koji Seike(Atmosphere and Ocean Research Institute, University of Tokyo)

12:00 PM - 12:15 PM

[HCG35-11] Equilibrium condition for high-concentration turbidity currents

*Hajime NARUSE1 (1.Graduate School of Science, Kyoto University)

Keywords:turbidity current, hindered settling, turbulence, suspension, turbidite

This research theoretically explore formative conditions and characteristics of high-concentration turbidity current. Generally, it has been suggested that suspended load even in very high-velocity flows such as tsunamis or turbidity currents cannot exceed 1-5 vol.%. This is because density stratification due to suspended load inhibit turbulence in flows.
However, it was revealed that flow can attain very high concentration (~30%) of suspension because of hindered settling. When calculation starts from very high-concentration and high turbulent kinetic energy, positive feedback between sediemnt entrainment from a bed and hindered settling occurs, and finally flows reach the equilibrium condition in which suspended load is around 30 vol.%. This equilibrium condition requires (1) small grain-size (< 200 micron meter), very high initial concentration (> 20 vol.%), (3) high flow velocity (> 5 m/s).
The origin of two types of turbidity currents, i.e. low- and high-concentration flows, has been subject to debate for sedimentologists. This research implies that two types of flows are generated from different initial conditions. For example, tsunami-generated turbidity currents are supposed to be low-concentration initally, so that they cannot increase their concentration even if the self-acceleration mechanism works. On the other hand, turbidity currents generated by subaqueous debris-flows are supposed to have very high concentration of suspension, and therefore they may sustain their high-energy and concentration for long distance.