长江入海泥沙的交换和输移过程——兼论泥质区的“泥库”效应

Sediment Exchange and Transport Processes in the Yangtze River Estuary:Concurrent Discussion on the Effects of Sediment Sink in the Muddy Area

采用粒度谱的计算方法分析高浊度河口混合环境下悬浮泥沙和表层沉积物的交换过程,研究悬浮泥沙在河口—陆架系统的输移和归宿问题。分析表明:(1)大约有47%的悬浮泥沙沉积在长江口拦门沙海域及水下三角洲前缘,超过50%的悬浮泥沙摆脱河口的"束缚"进入杭州湾以及向南沿浙闽海岸输运,122.5°～123°E是长江悬浮泥沙向东扩散的重要界限。本文提供的粒度谱计算方法可适用于多种潮汐环境的泥沙输运和沉积的定量计算。(2)长江口悬浮泥沙与表层沉积物高交换区(交换率>0.6)主要分布在南槽口外的泥质区和杭州湾附近海域,其中长江口外泥质区的交换率高达0.9以上,说明该区域悬浮泥沙大量参与供沙和造床过程。在10～100年尺度上,长江口南槽口外的泥质区和杭州湾存在着一个显著的沉积中心,其泥沙交换速率可高达2～3cm/a以上。(3)由于受沿岸流和台湾暖流的影响,从长江口输出的大部分悬浮泥沙首先沉积在长江口南槽口外的泥质区,随后在潮流的作用下向长江口、杭州湾和沿海岸向南输运,泥质区充当了长江口系统泥沙输运和入海泥沙向南输送的"泥库"。长江口泥质区既是长江入海泥沙向口外输送的泥沙"汇",也是涨潮输入长江河口和杭州湾、浙闽沿岸泥质带的泥沙"源"。(4)由于长江口外余流向南槽口外汇聚,南槽口外含沙量较高,而且长江口水下三角洲前缘潮滩仍处于缓慢淤涨状态,上述水动力和地貌的综合分析证实了长江口外泥质区的"泥库"效应。在长江入海泥沙减少的背景下,"泥库"对河口地貌发育的贡献明显;随着流域减沙的不可逆转,口门附近的"泥库"将发挥愈来愈重要的向河口系统供沙的功能。

The suspended surface sediments exchange processes under the high-turbidity and mixing estuarine environments were analyzed based on the grain size spectral calculation,to explore the rate and transport of suspended sediments in the Yangtze estuary-shelf systems.The results showed that：（1） approximately 47% of the sediments from the Yangtze River was accumulated in the mouth bar area and subaqueous delta,while the rest was transported to the Hangzhou Bay and southern Zhejiang and Fujian coasts.122.5°-123°E was the important limits of the Yangtze suspended sediment diffusing eastward to the East China Sea.This method of grain size spectral calculation provided by this paper can be used for the study of quantitative calculation of sediment transport and deposition in other tidal environments.（2） Higher sediment exchange ratio （p 0.6） occurred in the muddy area and Hangzhou Bay,and the ratio of the muddy areas outside the South Passage was higher than 0.9,which indicates that most suspended sediment contributed to the seabed evolution and sediment supply.On the decadal to centennial time scale associated with our results,a clearly defined depocenter of Yangtze sediment with the exchange rates greater than 2-3 cm/a is present in the muddy area and Hangzhou Bay.（3） Due to the impact of longshore currents and the Taiwan warm current,the suspended sediment from the Yangtze River deposited firstly in the muddy area,and then was transported to the inner Yangtze River Estuary,Hangzhou Bay and the south coasts by tidal current.Muddy area was the ＂sediment sink＂ of sediment processes in the estuary and moved southward.（4） Under the background of decrease in the Yangtze River sediment supply,the delta toward tidal flats was still deposited slowly,indicating that the sediments in the sediment sink made great contribution to the estuarine geomorphologic evolution.