长江河口区上段水、悬沙通量及其对北支演化的意义

2007年9月在长江河口3条控制断面的全潮观测结果表明,长江口南支均以落潮流和落潮输沙占优,而北支大、小潮期间的水沙输运特征迥异; 其特征与长江径流、河口地形地貌特征密切相关。小潮期间,长江口北支以落潮流占优; 大潮期间,则以涨潮流占优,且悬沙输运率比小潮期间增大一个数量级。分析结果进一步表明,从1958年至今,长江口北支的分流比呈下降的趋势,已由11.8% 降至目前的1.9%; 长江口北支也由早期的悬沙输入(与径流方向相反)通道变为输出通道,目前其分沙比仍达6.4% ~7.9% 左右。总体上,分流分沙比呈显著减小趋势,这是长江口北支萎缩的重要特征之一。此外,北支分沙比显著大于分流比,将可能造成北支的进一步淤积。

THE WATER AND SUSPENDED SEDIMENT FLUXES IN UPPER BRANCHES OF CHANGJIANG ESTUARY AND THEIR IMPLICATION ON CHANNEL EVOLUTION OF NORTH BRANCH

The North Branch,which is an individual channel of Changjiang estuary entering the sea,has been experienced great siltation since 1950s for the significantly changes in hydrodynamics and channel geomorphology.In Sep.2007,sediment dynamic parameters were surveyed during the neap and spring tides at North Branch,South Branch and Xuliujing cross-section in the upper Changjiang estuary.The hydrodynamic characteristics are observed by ADCP at North Branch、South Branch and ADP for the Xuliujing cross-section.The acoustic signals recorded have a significant relationship with the suspended sediment concentrations(SSCs)obtained by in-situ water sampling during the measurements,then the acoustic SSC models established were used to convert ADCP(ADP)acoustic signals into high-resolution SSCs along the cross-sections at North Branch,South Branch and Xuliujing.Thus,the water and suspended sediment fluxes could be estimated.The results show that in South Branch,the current velocity and sediment flux were larger during the ebb than those during the flood both in the neap and spring tides.Comparison with the South Branch,the hydrodynamic and sediment transport characteristic of North Branch were of great difference.During the neap tide,the current velocity was larger during the ebb than that during the flood; on the contrary,during the spring tide,the maximum current velocity was smaller during the ebb than that during flood,and the net suspended sediment fluxes was more than 10 times during the flood period than that the ebb.These features were consistent with the water and sediment discharge division ratio variations in North Branch.The water discharge division ratio decreases from 11.8% to 1.9% during 1958~2007,and the suspend sediment discharge division ratio is about 6.4% ~7.9% at present.Thus,the higher sediment discharge division ratio than the water division ratio could result in the further siltation in North Branch.In addition,the North Branch acted as the importing channel in terms of the suspended sediment(transport landward)in the early stage; however,it changed into the outputting channel at present.As a whole,the water and sediment discharge ratio decreased significantly since 1950s,which is an important characteristics for the North Branch degeneration.