近期长江口崇明岛周边岸滩沉积特征及影响机制

大河入海河口由于巨量泥沙堆积以及潮汐作用,往往发育着河口沙岛。崇明岛是世界上典型的河口沙岛,近年来,随着长江流域泥沙来源锐减以及河口大型水利工程建设等,给崇明岛周边岸滩沉积环境带来深刻影响。研究依据2015年9月～2018年4月对崇明岛周边14个岸滩断面的表层沉积物采样数据和定点水文观测资料,分析崇明岛周边岸滩沉积环境特征和区域性差异,探讨不同岸滩断面沉积过程和作用机制。结果表明:崇明岛岸滩沉积物以砂质粉砂和粉砂质砂为主,沉积物中值粒径总体表现为南岸北岸东滩,平均为48μm。南、北岸岸滩总体表现为坡陡、滩窄,沉积物自西向东逐渐变细的特征,北岸岸滩坡度略缓,南岸崇头至庙镇之间存在局部细颗粒物质沉积区;东部岸滩滩宽、坡缓,整体表现为"北细南粗"的沉积特征。岸滩沉积断面上,上细下粗,高潮滩处粘土组分含量相对较高,水下斜坡和河槽底部粉砂含量较高。南、北支河势演变形成了崇明岛"南坍北涨"沉积环境的分布格局;径、潮流作用对岸滩断面沉积物分布特征影响明显;风浪对崇明东滩浅滩沉积物具有强烈的分选作用;潮滩植被主要影响细颗粒沉积物在潮间带的分布;流域来沙减少对崇明东滩前沿沉积环境趋势影响显著。

Recent Sedimentary Characteristics and Impact Mechanism ofTidal Flats in Chongming Island,the Yangtze Estuary

Estuary sand islands are often developed in the estuaries of large rivers due to massive sediment accumulation and tidal action. Chongming Island is a typical estuary sand island in the world. In recent years, with the sharp decrease of sediment discharging in the Yangtze River Basin and the construction of many large-scale water conservancy projects, the sedimentary environment around Chongming Island have been profoundly affected. Based on the sampling data of surfaced sediment of 14 beaches’ cross sections between September 2015 and April 2018, we analyzed the characteristics of the tidal flat sedimentary environment around Chongming Island and its differences between regions and discussed the processes of sedimentation and mechanisms of different tidal flat sections. The results are shown as follow: The tidal flat sediment around Chongming Island is dominated by sandy silt and silty sand. The average of the median size of sediment is 48 μm, which displayed that the size sediments in south coast are largest while those in east coast are the smallest, and the sediment in north coast is between the above. In general, the south and north coasts have steep slope and narrow beach, and the slope of tidal flat on the north coast is slower than that of the south coast whose sediments become finer from west to east. The slope of the north coast is slow, and the area in the south coast between Chongtou and Miaozhen exists fine sediment. However, the east coast has slow slopes and wide beaches whose sediments become finer from the south to north. On the tidal flat profile, the content of clay components at high tidal flat is relatively high, and the sediments become finer with the water depth decreases. The river regime evolution of the Yangtze Estuary forms the sedimentary environment of the tidal flat in the north and south coasts of Chongming Island. It is obvious to influence the distribution of sediments because of runoff and tidal current. The wind wave has a strong sorting effect on the sediments of the Eastern Chongming Island. Tidal vegetation mainly affects the distribution of fine sediments in the intertidal zone. The decrease of sediment discharging has a significant impact on the sedimentary environment in the Eastern Chongming Island.