长江河口河槽近期沉积特征及影响因子分析

随着流域和河口水利工程建设,长江河槽沉积环境发生了巨大改变,对河势演变和河槽冲淤均产生重要影响。依据长江河口河槽大面积表层沉积物采样和各河槽定点水文观测资料,分析各河槽沉积特征,探讨其影响因子及作用机制。结果表明:河槽沉积物类型以砂质粉砂和粉砂质砂分布最广,粒径分布纵向上呈自西向东减小、横向上自北向南减小趋势,河槽总体主槽粗、边滩细。涨落潮泥沙输运和沉降过程影响河槽纵向沉积分布特征,风浪作用强化了口门段河槽南北沉积环境的差异,北支、北港口门段河槽受到偏北方向风浪作用强烈,沉积物粗化明显。不同泥沙来源是造成河槽整体沉积环境差异的主导因素,南支、南港上段表现为流域来沙的沉积特征,北港、南槽、北槽则表现为流域与海域来沙的混合沉积特征,口外沉积物对口内河槽的影响主要是为口内河槽提供细颗粒物质来源。

Recent sediment characteristics and their impact factors in the Yangtze Estuary riverbed

The sedimentary environments of the Yangtze Estuary riverbed have changed a lot with the construction of water conservancy projects in the drainage basin and the estuary, which have significantly influenced the river regime evolution and accretion/erosion of the estuary riverbed. However, less has been reported on detailed sedimentary characteristics of the current estuary riverbed, which restrains further evaluation on the impact on both hydrology and sedimentology of the Yangtze Estuary. Therefore, based on the large-scale surface sediment samples and associated flow-sediment data collected during September 2015, we analyzed the sedimentary characteristics in the seven riverbeds within the estuary and discussed the influencing factors and mechanisms. The particle sizes of the sediment samples were analyzed by Model LS-13320 Coulter laser particle size analyzer and the types of the surface sediments were classified by Folk’s classification. The results are shown as follows. The riverbed sediments of the whole estuary is dominated by sandy silt and silty sand. The coarsest surface sediments are observed in the outer estuary of north branch/north channel above 10 m and upper north/south branch, while the finest surface sediments are found in the north passage, lower south passage and the outer estuary of north/south passage deeper than 5 m. Generally, the particle sizes of the river sediments are coarser in the main channels and finer on the shoals, and decrease from west to east and from north to south. The lateral sedimentation distribution characteristics are affected by the sediment transportation and deposition in ebb and flow process. The longitudinal sedimentation distribution reflects the differences of the sedimentary environment strengthened by the wind wave. The sediments in the north branch are coarsened remarkably due to the intense north wind wave action. The difference of sediment origins is the factor leading to deviation of the sedimentary environment. The sediment in the south branch and upper south channel show a characteristic of riverine sediments, while the sediments in the north channel, north passage and south passage show a mixture of riverine and marine sediments. Sediments in the outer estuary mainly provide fine-grained particles to the inner estuary.