掲載種別：研究論文（学術雑誌）  

DOI： 10.1063/5.0232852

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掲載種別：研究論文（学術雑誌）   出版者・発行元：AIP Publishing  

In channelized alluvial rivers, alternating sandbars are formed by alternating scouring and sedimentation in the downstream direction. The conditions for the occurrence of sandbars are obtained in the stability analysis as when the ratio of channel width to depth, $\beta$, exceeds a critical value due to innate instability of bottom. However, neither the mechanism nor the conditions for the occurrence of sandbars are well understood because it is difficult to quantify bars before or early in the occurrence of that using conventional measurement methods. We conducted experiments in a laboratory flume with slope as a variable to obtain dense data on the geometry of the water surface, bottom surface, and flow depth. We then performed a wavenumber analysis of the measured data to verify the validity of the results estimated by the stability analysis. We calculated the temporal variation of the correlation coefficients between the depth--water surface $r_{wl}$ and depth--bottom $r_{bl}$ along the development stage of the sandbar since flow state could be estimated from the relationship between the flow depth and the water surface/bottom surface. The temporal variation of the correlation suggests that the development process of sandbars can be divided into three phases, and the flow state is different for each phase. We also found that the occurrence time of sandbars would be defined based on the intersection of the temporal variation of $r_{wl}$ and $r_{bl}$. Furthermore, we show that the formation process of the sandbar is independent of Fr.

DOI： 10.1063/5.0171731

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担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）   出版者・発行元：AIP Publishing  

In channelized alluvial rivers, alternating sandbars are formed by alternating scouring and sedimentation in the downstream direction. The conditions for the occurrence of sandbars are obtained in the stability analysis when the ratio of channel width to depth, β, exceeds a critical value due to innate instability of bottom. However, neither the mechanism nor the conditions for the occurrence of sandbars are well understood because it is difficult to quantify bars before or early in the occurrence of that using conventional measurement methods. We conducted experiments in a laboratory flume with slope as a variable to obtain dense data on the geometry of the water surface, bottom surface, and flow depth. We then performed a wavenumber analysis of the measured data to verify the validity of the results estimated by the stability analysis. We calculated the temporal variation of the correlation coefficients between the depth–water surface rwl and depth–bottom surface rbl along the development stage of the sandbar since flow state could be estimated from the relationship between the flow depth and the water surface/bottom surface. The temporal variation of the correlation suggests that the development process of sandbars can be divided into three phases, and the flow state is different for each phase. We also found that the occurrence time of sandbars would be defined based on the intersection of the temporal variation of rwl and rbl. Furthermore, we show that the formation process of the sandbar is independent of Fr.

DOI： 10.1063/5.0128760

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担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）   出版者・発行元：AIP Publishing  

Hydraulic quantities governing the generation of bedforms (formed spontaneously on the bottoms of rivers) have been investigated through geomorphological methods, laboratory experiments, stability analyses, numerical analyses, and other research methods. Recently, numerical analysis has been conducted using fine spatial grids to accurately describe morphodynamics. However, numerical analysis cannot always describe the real phenomena, because it is based on assumptions. One effective way to verify physical assumptions is to compare numerical analysis with actual measurements of equivalent resolution. Measurement data with high resolution enable the construction of a duplicate of the measurement target on a computer called a “digital twin.” To construct a digital twin of the process of bedforms generation and development, geometries of the water surface and the bed surface must be simultaneously measured. We developed and verified a measurement method for the construction of a digital twin during the generation and development of bedforms. The measurement system uses two cameras and a line laser to simultaneously measure the water surface and bed surface. Accurate refraction correction at the water surface is possible by acquiring the shape of the water surface, allowing the bottom shape to be determined by geometric processing. The method provides sub-millimeter-accurate measurements of the water surface and bottom with a spatial resolution of 0.95 cm longitudinally and 0.038 cm transversely in a 12 m × 0.45 m flume and takes only 1 min per measurement. This method can provide measurement results that contribute to the understanding of the generation and development of bedforms.

DOI： 10.1063/5.0085574

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