Based on four years of spatially synchronous topographic data of the Oujiang Estuary from 2002 to 2019, the variations and physical mechanisms of estuarine evolution driven by natural conditions and human activities since the beginning of this century were analyzed. The results show that the evolution of the Oujiang Estuary has changed dramatically in the past 20 years. From 2002 to 2019, the net sediment erosion of the estuary was 163.44 million m³, and the average bed elevation of the river-dominated section (RDS), transition section (TS) and tide-dominated section (TDS) decreased by 4.61 m, 1.30 m and 2.14 m, respectively. In addition, the pattern of the shoal channel had changed, and the river facies coefficient (width–depth ratio) decreased by 16–64%. The evolution of the Oujiang Estuary is mainly caused by human activities (such as sand mining, reclamation). Sand mining is the direct cause of riverbed undercutting, and the large undercutting of riverbed terrain causes the increase in tidal power in the estuary and further causes river channel scouring. In the last 20 years, the average annual tidal range of the estuary increased by 0.19–1.14 m, and the flood discharge increased by about 17–80%, with an average value of 58%. The impact of tidal power on the evolution and development of the estuary has increased significantly. Apart from sand mining, reclamation projects such as the Wenzhou shoal outside the mouth also cause the local velocity of the tidal current section to increase, which aggravates the scouring trend of local river sections downstream. The scouring and silting changes in the Oujiang Estuary since the beginning of this century are the result of the adjustment of the estuarine system in response to the strong intervention of human activities.

Local scour around offshore wind turbine foundations is a common engineering challenge. It changes the hydrodynamic loads and affects the foundation’s load-bearing capacity. This study investigates the field scour characteristics and wave–current force characteristics under local scour effects using field data, physical modeling, and numerical simulations. The results show that the field scour hole slope is more gradual than that observed in laboratory settings, and Zhang’s scour depth equation proves more accurate for practical engineering. In addition, under wave–current conditions (Keulegan–Carpenter number, 2 < KC ≤ 15), the relative maximum post-scour wave–current force increases with the relative post-scour water depth but decreases as the KC rises. An equation is developed to predict the relative maximum post-scour wave–current force. This provides key insights for improving load assessments of offshore wind foundations on scoured seabeds.

The vertical distribution of sediment concentration in estuaries is the core issue of suspended sediment transport. At present, there are many researches on the vertical distribution of sediment concentration in rivers, and a lot of results have been achieved, but that in estuaries is seldom studied. The Yellow River Estuary (YRE) and the Qiantang River Estuary (QRE) are typical tidal estuaries with strong erosion/deposition and high concentration in China. Under the action of alternating tidal flow, the suspended sediment transport shows significant time-varying and non-equilibrium transport characteristics. The vertical distribution of sediment concentration is quite different from that of ordinary rivers. This paper analyzes the vertical distribution of sediment concentration under the action of tidal currents based on the theory of sediment movement and diffusion. Compared with equilibrium sediment transport, the vertical distribution of sediment concentration only differs by one vertical distribution coefficient. Based on the analysis of the measured data, it is found that the vertical distribution of sediment concentration in the YRE and QRE is mostly exponential. Firstly, the multivariate relationship between the vertical distribution coefficient and the sediment saturation and suspension index is established by means of the measured water and sediment data of the QRE. The correlation coefficient R² was above 0.64, which has a good correlation. Finally, the established relationship was validated using measured water and sediment data from the YRE, and the results were basically consistent, indicating that the established formula for vertical distribution of sediment concentration is suitable for general tidal estuaries, and has a certain reference value for the simulation of sediment transport in the YRE and QRE.

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