[M-IS15] Geophysical particulate gravity current

[MIS15-02] Grain size characteristics of cyclic steps formed by multiple surge turbidity currents in an experimental flume: a discussion of downstream coarsening

*Miwa Yokokawa¹, Ren Nagano¹, Kazuma Matsunami¹, Atsuki Fukuoka¹ (1.Osaka Institute of Technology)

Keywords:Cyclic Step, Turbidity currents, Grain size distribution, Downstream coarsening, Flume experiment

Grain-size distribution in cyclic steps was investigated in a flume, formed by supercritical surge-type turbidity currents. The flume is 7m long, 8cm wide, 50cm high, with 7 degrees gradient. Turbidity currents were generated using a mixture of salt water and two sizes of plastic particles, supplied from a header tank. After 141 surges, two steps were present. The wave steepness of Step 1 on the upstream side was 0.06. The median grain size of the surface sediment showed downstream coarsening. In Step 1, grain size on the downstream side tends to be coarser, while in Step 2, that on the upstream side tends to be coarser. Considering the results of grain size analysis of the suspension, the sedimentation occurs from the body in Step 1, whereas from the head in Step 2. The grain size distribution is bimodal in all samples. The two modal diameters do not change significantly, but the D₅₀ differs because the frequency varies from place to place. Comparing the geologic example, it is suggested that the high discharge and sediment concentration in this experiment may have resulted in the accumulation of a large amount of fine material from the body of the turbidity current at the upstream side and generating the downstream coarsening in median grain size. The D₅₀ of the sample taken vertically in Step 1 shows that the median grain sizes are fine near the bottom, coarsen on the way upward, and become fine again toward the uppermost level. This may also be the result of the large cumulative velocity due to the deposition of fine-grained material described above.