11:30 AM - 11:45 AM
[HCG20-04] The role of gravel in subaqueous debris flow: a preliminary report from flume experiments
Keywords:Flume experiments, Subaqueous debris flow, gravel
Subaqueous gravity flows such as subaqueous debris flows and turbidity currents are important sediment transport phenomena and also have a significant impact on submarine infrastructure, and their behavior has been the subject of various studies. Various mixing patterns of hybrid event beds have been reported from strata, and there is much debate on the behavior of the flow and how it is reflected in the strata. We are currently conducting experiments to gain basic knowledge on the effects of gravels on high mud content flows, and here, we report a preliminary experimental results.
The experimental conditions of Ilstad et al. (2004) were used as a reference, and gravel was mixed with that condition. The ratios of water and clay to entire slurry were fixed at 35wt% and 32.5wt% respectively, and the remaining 32.5% by weight was varied in three ways: sand only (0% gravel), 5wt% gravel, and 15wt% gravel, to investigate the flow behavior and sediment distribution. Kaolinite (mean grain size 0.4 μm) was used as clay, silica sand (average grain size 330 μm) as sand, and commercially available Oiso gravel (from Oiso gravel beach) (3-5 mm) as gravel.
These materials were mixed well and poured into a submerged flume 700 cm long, 8 cm wide, 50 cm high, and with a slope of 6 degrees. The behavior of the flow was observed using digital cameras and a high-speed camera. The distribution of sediments after the flow passed through the flume was also measured.
The results showed that when gravels were added, the flow accelerated along the way and the time required to flow 7 m was significantly shortened (21 s for 0% gravels, 15 s for 5% gravels, and 16 s for 15% gravels). The distribution of the sediments along the flume also differed depending on the presence or absence of gravels, and the sediments with gravels were deposited more upstream than those without gravels. The distribution of gravel in the sediments showed that the gravel distribution ended 0.5-1 m downstream of the point where the flow with gravel began to accelerate and then switched to deceleration. Observation with a high-speed camera fixed at 460 cm from the upstream end showed that the shape of the bottom of head differed depending on the presence or absence of gravel. Without gravels, the shape of the bottom of head is a high-angle wedge shaped, whereas with gravels, the shape of the bottom of head is a low-angle.
Although the mechanism of flow alteration due to gravels remains to be clarified, the fact that the velocity of subaqueous gravity flow increases when gravels are mixed suggests the need to be careful not to underestimate the effect of gravity flow when gravels are supplied near the point of gravity flow generation.
The experimental conditions of Ilstad et al. (2004) were used as a reference, and gravel was mixed with that condition. The ratios of water and clay to entire slurry were fixed at 35wt% and 32.5wt% respectively, and the remaining 32.5% by weight was varied in three ways: sand only (0% gravel), 5wt% gravel, and 15wt% gravel, to investigate the flow behavior and sediment distribution. Kaolinite (mean grain size 0.4 μm) was used as clay, silica sand (average grain size 330 μm) as sand, and commercially available Oiso gravel (from Oiso gravel beach) (3-5 mm) as gravel.
These materials were mixed well and poured into a submerged flume 700 cm long, 8 cm wide, 50 cm high, and with a slope of 6 degrees. The behavior of the flow was observed using digital cameras and a high-speed camera. The distribution of sediments after the flow passed through the flume was also measured.
The results showed that when gravels were added, the flow accelerated along the way and the time required to flow 7 m was significantly shortened (21 s for 0% gravels, 15 s for 5% gravels, and 16 s for 15% gravels). The distribution of the sediments along the flume also differed depending on the presence or absence of gravels, and the sediments with gravels were deposited more upstream than those without gravels. The distribution of gravel in the sediments showed that the gravel distribution ended 0.5-1 m downstream of the point where the flow with gravel began to accelerate and then switched to deceleration. Observation with a high-speed camera fixed at 460 cm from the upstream end showed that the shape of the bottom of head differed depending on the presence or absence of gravel. Without gravels, the shape of the bottom of head is a high-angle wedge shaped, whereas with gravels, the shape of the bottom of head is a low-angle.
Although the mechanism of flow alteration due to gravels remains to be clarified, the fact that the velocity of subaqueous gravity flow increases when gravels are mixed suggests the need to be careful not to underestimate the effect of gravity flow when gravels are supplied near the point of gravity flow generation.