Abstract:  

In the past 30 years, the North Branch of the Yangtze River Estuary has undergone significant changes in sediment characteristics and transport trend under the influence of natural and human activities. Based on the survey data of grain size of sediment in 2015 and the 1980s, we conducted grain size comparison and transport trend analysis, etc. and discussed sedimentary characteristics and transport trend of the North Branch & offshore area of the Yangtze River Estuary. The results shows that the types of sediments in the North Branch & offshore area of the Yangtze River estuary are dominated by clay silt, sandy silt and sand, and the mean median grain size decreases by about 20% in the past 30 years. Among them, the grain size decreases in the North Branch by 30%, indicating that the entire North Branch is in a siltation environment; the grain size in the zone with the depth of 0 to 10 m outside the mouth increases by about 25%, manifests as the characteristics of erosion under the action of wind waves and tides; in the zone with the depth of over 10m, the grain size decreases slightly and the sediment characteristics are basically stable. From the perspective of sediment transport trend, the sediments from Chongtou to Lingdinggang sections of the North Branch are more coarse，which is mainly effected by runoff; for the sediment transport trend from Lingdiangang to Santiaogang sections, go downward Santiaogang, in addition, the sediments in the main channel from Lingdinggang to Sanhegang are coarse, the sediments in the beach are fine and the intersection of the tidal currents is obvious; In the lower reaches of Santiaogang, there are mainly sands from the sea in the sediments, which are fine and the transport trend goes upward; Outside the North Branch, sediments in the zone with the depth of 0 to 10 m are coarsened and become finer with the increase of the depth. In the zone with the depth of 0 to 2 m, the sand content accounts for more than 90%; and the sediment transport is not significant in the offshore area with the depth over 10 m. The reduction of the sands from the river basin and the construction of the North Branch reclamation result in insufficient recharge of sediment along the coast. Under the action of wind, waves and tides, some sediments in the shallow outside the mouth are transported into the North Branch with the rising tides, others spread towards the open seas with the falling tides, constituting the sediment transport pattern of the North Branch and offshore area.