Study on the Morphological Evolution of the Oujiang Estuary, China, in the 21st Century

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.