近年来长江河口北支水沙特性与河槽稳定性分析

Analysis of the river channel stability and the characteristics of suspended sediment and water in the North Branch of the Changjiang River Estuary in recent years

根据崇头、青龙港、三条港和连兴港1950-2001年潮汐潮流观测资料和2005年6～7月的大、中、小潮及2007年5月5～10日沿北支河槽进行的两个连续潮周期水、沙、盐的现场观测数据并结合参考2005年的实测地形图。通过对长时间序列（1950-2001年）的分流比、年均流量、潮位、潮历时等数据的整理分析并与近期（1998～2005年）的水文特征值比较，发现随着北支分流量逐渐减少，河槽内强烈的潮波已成为影响北支河槽不稳定的主要因素；青龙港近年来（1998-2005年）平均高、低潮位出现增高，比多年（1950～2001年）平均高、低潮位分别高出15和17cm，是北支河床近年来持续性淤浅的体现；大潮期间北支河槽涨潮含沙量无论量值还是扩散强度均大于落潮，明显表现出河口涨潮槽具有的泥沙运动规律。自2001年以来除少数年份的小潮汛外，均表现出明显的涨潮优势，其中2002年3月大潮的潮量优势流仅为9．42％，显示出涨潮优势十分明显，说明北支河槽近几年来正朝着衰退的过程发展；2005年北支河槽-2、-4和-6m等深线均呈喇叭状，闭合端指向上游，符合涨潮槽的几何形态；再从河槽类型系数（λ）看，大潮期间入值均大于1，最大值为31．1，涨潮槽特征显著；而洪季中、小湖各测站入值均小于1，最小值为0．0003，A、B两测站在大潮时入值均大于1，而小潮时又都小于1，显示北支河槽虽主要表现为涨潮槽的特征，但洪季受径流和潮汛影响，其涨潮槽特性不稳定。总体而言，目前北支河槽为一条不稳定的废弃型涨潮槽。

The North Branch had been the main channel into the sea of the Changjiang River. Its runoff has decreased gradually since the river took the South Branch as the main channel into the sea in 18^th century, and the North Branch has been transferred from a runoff-dominated river branch to a tidal-dominated one. However, the evolution of the natural river regime upsteam, changes of incoming water and silt resulted from large-scale engineering projects and human activities such as reclamation on both banks has resulted in some new features of the channel and its water and silt movement. Based on the data of the water and suspended sediment of the North Branch from four hydrologic stations （Chongtou, the Qinglong port, the Santiao port and the Lianxing port） and in situ measurements of water, silt and salinity during spring tide, middle tide and neap tide from June to July in 2005 and from 5^th to 10^th of May 2007 and the observed topographic data in 2005, and through the analysis of long time series （1950-2001） of the annual distributary ratio, annual mean discharge, tidal level and tidal duration, and comparison with short-term hydrologic characteristic value （1995-2001）, it is shown that together with de decease of distributary discharge of the North Branch the strong deformation of internal tidal wave has been a main factor that effects the stability of the North Branch channel. As a result of accretion along the riverbed of the North Branch, the mean high tidal level and low tidal level of the Qinglong port in recent years （1998-2005） are 15 cm and 17 cm higher than that of the annual average values respectively. The suspended-sediment concentration of the North Branch during flood tide is higher than that in ebb tide, which shows the rule of the silt movement in flood channels. The intensity of the flood current of the North Branch along the Changjiang river has been stronger than that of the ebb current except in neap tidal in some years since 2001, which shows that the channel of the North Branch is trending towards a recession process. Moreover, River channel of the North Branch is attributed to the type of flood channel which is varied from spring and neap tide. The measured topography data of the North Branch in 2005 show that the isobaths of 2 m, 4 m and 6 m are all in the trumpet shape with the closed ends pointing to the upper reaches, which is in accord with the geometrical configuration of the flood channel. The channel type coefficient λ in spring tide at different stations is all over 1, with a maximum of 31. 1, which presents obvious characteristics of a flood tide channel. However, the value of ）, in middle tide and neap tide of the different stations during flood season are all below 1, with a minimum of 0. 000 3 and those at A and B stations during spring tide are both over 1 and less than 1 during neap tide, which shows that although the North Channel is mainly presenting the characteristics of a flood tide channel, it is also influenced by the runoff and tide, its feature of flood tide channel is not stable. On the whole it can be suggested that the North Branch of the Changjiang River Estuary is a type of abandoned unstable flood channel.