4:30 PM - 4:45 PM
[MIS16-04] Experimental study for features of turbidity currents constructing cyclic steps
Keywords:cyclic steps, turbidity currents, grain size
In the submarine canyon and the vicinity, cyclic step-like morphology is often observed. Many of them are inferred to be formed by turbidity currents. For example, in Toyama Deep Sea canyon, sediment waves are formed on the natural levee due to the overflow of turbidity current (Nakajima & Satoh, 2001). The grain size analysis of the coarsest part turbidite constituting this sediment wave showed that in the some case, the grain size of the upstream slope is coarser than that of the downstream slope, and the opposite tendency is obtained in the other location. Moreover, in an experiment in which a cyclic step was formed by a surge turbidity current (Fujita et al., 2019), the size of the particle size on the upstream side and the downstream side changed depending on the conditions. In this experiment, when the upstream side was coarse, downstream granulation was observed in the entire channel, and when the downstream side was coarse, downstream coarse graining was observed in the entire channel. There are few experimental examples of the formation process of cyclic steps due to turbidity current and its particle size distribution, and the details are often unknown. Therefore, in this study, we formed a cyclic step by a surge turbidity current in the experimental channel, and investigated the particle size distribution and the characteristics of the turbidity current in detail.
In this experiment, a flume with a length of 700 cm, a width of 8 cm, and a height of 50 cm was installed in a water tank with a length of 760 cm, a width of 30 cm, and a height of 120 cm with a gradient of 7 degrees. In addition, a slope (180 cm in length) with an inclination of 7 ° was installed at the upstream end with an acrylic plate so that the cyclic step could be easily formed. Salt water (density 1.16 g / cm3) and two types of plastic particles were mixed at a weight ratio of 20: 1: 1 using an automatic stirrer and supplied from the head tank to generate turbidity current. The particle size range of plastic particles is 75-150 µm and 180-325 µm. One surge was 3s, and a flow rate of about 17.7L was flowed 141 times. As a result, two steps were formed. The wave steepness of the step was 0.06.
The moving speed of the head reached a maximum of about 45 cm / s at a point 400-500 cm from the upstream end. As a result of PIV measurement, the average flow velocity at the tip of the head was about 50 cm / s. Furthermore, the concentration and particle size of suspended sand in the turbidity current were investigated at 400 cm and 590 cm from the upstream end. As a result, it was found that the volume concentration of floating sand near the bottom surface (height 0.5 cm from the bottom surface) was about 4% at both 400 cm and 590 cm points, and the concentration was higher in the head than in the body. Both the body and head have higher concentrations at the 400 cm point than at the 590 cm point. The particle size was about 230 μm at both 400 cm and 590 cm points near the bottom of the head, and the body was about 180 μm near the 400 cm point and about 160 μm near 5910 cm.
In this experiment, a flume with a length of 700 cm, a width of 8 cm, and a height of 50 cm was installed in a water tank with a length of 760 cm, a width of 30 cm, and a height of 120 cm with a gradient of 7 degrees. In addition, a slope (180 cm in length) with an inclination of 7 ° was installed at the upstream end with an acrylic plate so that the cyclic step could be easily formed. Salt water (density 1.16 g / cm3) and two types of plastic particles were mixed at a weight ratio of 20: 1: 1 using an automatic stirrer and supplied from the head tank to generate turbidity current. The particle size range of plastic particles is 75-150 µm and 180-325 µm. One surge was 3s, and a flow rate of about 17.7L was flowed 141 times. As a result, two steps were formed. The wave steepness of the step was 0.06.
The moving speed of the head reached a maximum of about 45 cm / s at a point 400-500 cm from the upstream end. As a result of PIV measurement, the average flow velocity at the tip of the head was about 50 cm / s. Furthermore, the concentration and particle size of suspended sand in the turbidity current were investigated at 400 cm and 590 cm from the upstream end. As a result, it was found that the volume concentration of floating sand near the bottom surface (height 0.5 cm from the bottom surface) was about 4% at both 400 cm and 590 cm points, and the concentration was higher in the head than in the body. Both the body and head have higher concentrations at the 400 cm point than at the 590 cm point. The particle size was about 230 μm at both 400 cm and 590 cm points near the bottom of the head, and the body was about 180 μm near the 400 cm point and about 160 μm near 5910 cm.