Abstract The world’s largest hydropower dam, the Three Gorges Dam (TGD), spans the upper Yangtze River in China, creating a 660-km long and 1.1-km wide reservoir upstream. Several recent studies reported a considerable decline in sediment load of the Lowermost Yangtze River (LmYR) and a rapid erosion in the subaqueous delta of the river mouth after the closure of the TGD in 2003. However, it is unknown if the TGD construction has also affected river channel and bed formation of the LmYR. In this study, we compared bathymetric data of the last 565 kilometers of the Yangtze River’s channel between 1998 and 2013. We found severe channel erosion following the TGD closure, with local riverbed erosion up to 10 m deep. The total volume of net erosion from the 565-km channel amounted to 1.85 billion m 3 , an equivalent of 2.59 billion metric tons of sediment, assuming a bulk density of 1.4 t/m 3 for the riverbed material. The largest erosion occurred in a 100-km reach close to the Yangtze River mouth, contributing up to 73% of the total net eroded channel volume.
Predictive mapping of seabed sediments based on multibeam bathymetric (BM), and backscatter (BS) data is effective for mapping the spatial distribution of the substrate. A robust modeling technique, the random forest decision tree (RFDT), was used to predict the seabed sediments in an area of the Joseph Bonaparte Gulf, Northern Australia, using the multibeam data and seabed sediment samples collected simultaneously. The results showed that: (1) Using multibeam bathymetry data in addition to multibeam backscatter data improves the prediction performance of the RFDT. In comparison to only multibeam backscatter data, the prediction performance achieved a ~10% improvement in sediment properties; it achieved a ~44.45% improvement of overall accuracy in sediment types, and a ~0.55 improvement in Kappa. (2) The underlying relationships between sediment properties and multibeam data show that there is an opposite non-linear correlation between sediment property-BS and sediment property-BM. For example, there is an obvious negative relationship between %mud-BS at incidence angles of 13° and 21°, but the relationship between %mud-BM is positive. As such, the RFDT is a useful and well-performing method in predicting the relationship between sediment properties and multibeam data and in predicting the distribution of sediment properties and types. However, the sediment prediction method in deep-water areas with high gravel content needs to be further evaluated.
Abstract The operation of the world's largest hydropower dam, the Three Gorges Dam (TGD), has significantly influenced the middle and lower reaches of the Yangtze River (YR). However, research has not been conducted on the impact of local engineering projects on the evolution of the tidal channel of the YR. This study used historical water depth and level data to analyze the factors influencing riverbed deformation in the Nanjing reach of the tidal channel of the YR. Moreover, the effect of local engineering projects on riverbed deformation in the study area was quantitatively analyzed. Our results indicated that between 1998 and 2013, ~78.26 × 10 6 m 3 of sediment accumulated in the study area. The goose‐head bifurcated section of this reach exhibited the most significant change in riverbed deformation during this period. Between 1998 and 2013, 46.97 × 10 6 m 3 of sediment accumulated in this section, the riverbed elevation increased by 1.01 m, and the thalweg increased by 2.78 m. Quantitative analysis of the volume of sediment accumulated in the near‐bank area indicated that revetment engineering accounted for >40% of the accumulation. The results of this study provide a more accurate understanding of channel deformation in the lower reaches of the YR. Moreover, the results will improve the prediction accuracy of river channel evolution and assist in channel regulation and future local engineering construction projects.
The Deflector Jet Servo Valve (DJSV) holds a crucial role within the electro-hydraulic servo system, as its performance directly influences the equipment reliability. DJSV operates within a wide temperature range, and the temperature fluctuations result in the magnetomotive force variations of the permanent magnets and changes in the working air gaps, which influence the operating characteristics of the torque motor and contribute to temperature drift. Consequently, a theoretical model is proposed to comprehensively analyze the impact of these factors on the operating characteristics and temperature drift of the DJSV. The results indicate that as the temperature increases, the magnetic flux of working air gap decreases, while the temperature drift increases. Additionally, among the five initial air gap errors discussed, a centrosymmetric error of less than 0.02 l can significantly reduce the temperature drift. The proposed modeling and analysis method of temperature drift can be used to guide the design and analysis of DJSV in wide temperature range.
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