The Recent Morphodynamic Evolution Of The South Passage,Changjiang (Yangtze) Estuary

Estuary is a rapidly changed and dynamic coastal system. As an important link between the land-based fluvial process and coastal ocean process, estuary is the most active area where the land, ocean, atmosphere and biology interactions. This region is often affected by tides, waves and runoff and a series of natural forces, all kinds of human activities such as large-scale water conservancy projects have also generated serious impacts on it. Morphdynamic evolution of the estuary has been the core of research of the land-ocean interactions in the coastal zone. As one of the four outlets of the Changjiang Estuary into the East China Sea, South Passage (SP) was the main mouth channel before the Deepwater Navigation Regulation Project operation. But in recent years, under the influence of changes in water and sediment discharge and large water conservancy projects, the morphodynamic processes of the SP has undergone tremendous changes and therefore the SP is adjusting to adapt itself. Obvious, analysis and determination of the morphodynamic processes of the SP in the recent decades is of significance to understanding of response of the Changjiang Estuary to anthropogenic actions and natural forcing.Here, the field observed hydrological and suspended sediment data were collected in the SP in October2012, and surface sediments were also collected with covered area of the whole SP in March2013. Based on these data, the suspended sediment transport processes, diffusion mechanisms and dynamic sedimentation characteristics, and morphodynamic processes were analyzed. The main conclusions are summarized as follows:From the view of water dynamic and suspended sediments distribution in the SP, the flow is mainly reciprocated, the direction of the tides is mainly consistent with that of the main channel and the mean ebb current velocity is greater than that of the flood current. The lasted time of the ebb is longer than that of the flood. Meanwhile, water velocity and suspended sediment concentration was increased from the neap tide to the spring tide. The residual current of Euler direction is seaward, showing a significant increase from neap tide to spring tide. Stokes residual current direction is landward, which is consistent with the direction of the tide, and the flow velocity also showed an increasing trend.The deposition of the SP shows relatively coarsens information, which basically consists of sandy silt. Minor types of the deposition of the SP consist of silt, silty sand, mud and sand. Judging from the distribution of sediment types, the study area shows the coarse-fine-coarse pattern from the upward river segment to the downward river segment, reflecting the corresponding dominant hydrodynamic process with transformation from strong via weak to strong phase. According to sediment fractal theory, the sediment grain size is of fractal structure, and the fractal dimension can show the evolution of the sediment. Moreover, it can be found that the sediment transport patterns of the SP is seaward in the upward river segment, and obvious transport directions from the deep to shallow area can be observed in the downward river segment (mouth segment). This could induce Jiuduan Shoal remained accreted states due to the movement of the sediment on the seaward side to the trail of the Jiuduan shoal and Nanhui Eastern shoal.Distributary mouth between the South and North Passage (DMSN) had a related short evolutionary time with two different changed phases before and after1998. DMSN changes had been experienced three stages, i.e. instability, related stability, and dynamic equilibrium before1998. During these stages, shoal head located at DMSN was appeared as downward decrease, upward shift, and downward decrease again. The thalweg depth was experienced similarly changes shifted from location situated at the south of the DMSN, the north, and the south of the DMSN again. The appearance of river section was from the originated ’U’ to the present ’W’ formation. However, during the interval of flood occurrences of the Changjiang River, DMSN geomorphology is under adjustment from the unstable to stable stage. Due to construction of submarine dike at the shoal head in1998, there were minor shifts of the shoal head from the upward to downward change. However, the shoal head was in general leaned to the north which caused decrease in water and sediment load into the North Passage. The shoal head located at the DMSN has experienced slowly upward development since2002due to DMSN accretion. Meanwhile, the northward part of the submarine dike located in the upstream of the North Passage had been serious accreted. But the southward part of the submarine dike located in the upstream of the SP had been serious eroded which induced increased in the water and sediment load into the SP and a further extension of the ebb channel located at the DMSN to the SP.There was two evolutionary phases of the SP associated with connection of the Jiangyanan shoal to the Jiuduan shoal (CJSJS) in1989. Before the CJSJS in1989, because there is the relatively stable water and sediment supply conditions, the SP was widely occurred siltation on both sides of the shallows tidal flats, and the overall river channel was narrowed. However, after the CJSJS in1989, the upper and lower segments of the SP were constantly silted up, and widely erosion can be found in the southern bank of the SP. The SP had showed abnormal appearance as a’deepened erosion of the upper segment, slightly erosion of the middle segment, and siltation in the two sides’. Meanwhile, there is an obvious seasonal characteristic with accretion in the flood season and erosion in the drought season. In other words, the upward river segment experienced erosion during the flood and drought seasons due to the impacts of the intensively runoff and ebb current. However, the middle segment especial around Jiuduan shoal occurred obvious erosion in the flood season and accretion in the drought season. In conversely, accretion in the flood season and erosion in the drought season can be found in the downward river segment. Moreover, there are closely relationships between the morphodynamic processes of the SP in recent years and operation phase of the engineering distributed at the north and south sides. Thereafter, due to the serious impacts from anthropogenic actions, it can be expected that further investigations of the morphodynamic processes of the SP with possible effects of the anthropogenic activities must be carried out.