近30年来长江口北支及口外沉积特征及输移趋势

近30 a来长江口北支在自然和人类活动影响下，沉积物特征及输移趋势发生显著变化。研究基于2015年和20世纪80年代沉积物粒度调查资料，通过粒度对比、趋势分析等方法，探讨长江口北支及口外沉积特征与输移趋势。结果表明：长江口北支及口外沉积物类型以粘土质粉砂、砂质粉砂和砂为主，近30 a来平均中值粒径减小20%左右。其中，北支口内粒径减小约30%，表明北支口内整体为淤积环境；口外0~10 m水深区域粒径增加约25%，表现为风浪和潮流作用下的侵蚀特征；10 m水深外，粒径略有减小，沉积特征基本稳定。从沉积物输移趋势上看，北支崇头至灵甸港河段，沉积物较粗，以径流作用下的沉积为主；灵甸港至三条港河段，沉积物输移趋势向下指向三条港，其中灵甸港至三和港河段边滩粗、主槽细，径潮流交汇作用明显；三条港以下河段，沉积物以海域来沙为主，颗粒细，输移趋势指向上游；北支口外，0~10 m水深区域，沉积物粗化，沉积物随水深增加变细，其中0~2 m水深区域，砂含量占比超过90%；10 m以深离岸水域，沉积物输移趋势不显著。流域来沙减少和北支围垦工程建设，致使沿岸泥沙补给不足，口外浅滩泥沙在风浪和潮流作用下，一部分随涨潮流向口内输运，一部分随落潮流向外海扩散，构成北支及口外沉积物输移趋势格局。

Sediment Characteristics and Transport Trend in North Branch and Offshore Area of Yangtze Estuary in the Last 30 Years

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.