【作者】 刘高峰；

【导师】 朱建荣；

【作者基本信息】 华东师范大学 ， 自然地理学， 2011， 博士

【摘要】 河口的泥沙运动是河口最复杂的问题,物理模型、数学模型和原型观测是研究河口泥沙运动最重要研究手段,近几十年来,这三种研究手段都得到了长足的发展。河工物理模型试验仍然是现在河口工程中越来越重要的研究手段,能用来研究目前许多无法使用数学表达来准确描述泥沙运动过程的问题,如底沙运动、建筑物稳定性等问题。数学模型正成为研究泥沙问题的重要手段,由于自然界中,泥沙的对流、扩散、沉降、再悬浮,这些现象都是三维的物理过程,所以要准确描述泥沙运动过程,三维泥沙数值模型必然是泥沙研究的发展方向。原型观测是研究泥沙运动规律的最直接、最有说服力的研究手段,原型观测往往能够观测一些尚未被认识的水流泥沙运动规律。本文主要采用这三个研究手段研究长江口泥沙运动的一些规律。长江口滩槽交替,河势极为复杂。沙头冲刷后退、沙尾淤积下移是长江口沙体运动的普遍形式,底沙运动在此过程中扮演重要角色。通过现场取样,获得长江口南支、南北港的底沙资料,并且通过水槽试验选择合适模型沙,在完成水流验证的物理模型上系统地研究了长江口南支、南北港分汉口各沙体头部和主要河槽泥沙输移路径和淤积位置。结果表明南北港分汉口三个沙体冲刷下泄的泥沙,大部分进入南港,只有中央沙北侧和新浏河沙北侧少部分泥沙进入北港。沙体冲刷下泄泥沙对南港影响比对北港影响大。新浏河沙冲刷下泄泥沙对南港影响最大,接下来依次为中央沙和新浏河沙包。南北港分汉口的六条分流通道：宝山南、北水道、南沙头通道冲刷下泄泥沙进入南港,另外三条通道冲刷下泄泥沙进入北港。南沙头通道下泄泥沙对南港影响最大,接下来依次为宝山北水道、宝山南水道,分流北港的三条通道基本不对南港造成影响。中子活化示踪是一种高灵敏度、适用范围广、对环境无污染、能真实模拟天然沙运动的一种泥沙运动观测方法。通过中子活化示踪沙技术对拟选抛泥区的泥沙运移扩散规律进行现场试验研究,研究发现在北槽S7-S8丁坝之间航道南侧拟选抛泥区示踪砂的运移扩散方向在导堤内基本上与航道平行,近似呈长带形分布,出导堤后示踪沙向东南方向运移。示踪砂主要在与航道平行的区域内运移和沉积,在导堤内进入和越过航道的量很少,进入外航道的量亦很少,出导堤后泥沙主要向东南方向运动,而外航道向东方向伸展。长江口深水航道遇到了回淤强度大,回淤分布集中的问题。在深水航道回淤严重的W3节点上下游布置了4个座底式近底观测系统观测了水沙盐度过程,研究发现W3节点上下游近底层余流出现相反方向并且W3节点附近出现了一个富集区域。实测数据揭示了W3节点下游测站出现了经典的河口环流结构,并且余流的垂直剖面存在螺旋结构。上下游测站在涨急时刻存在最大的盐度梯度,证明了存在强锋面的存在,从而导致了泥沙捕集的发生。在W3节点附近的四个站点的近底层泥沙余通量产生一个富聚环流。近底悬沙通量的富集区域的出现说明了泥沙不能立即直接输移到海域。通过在南导堤和南滩地上开展一系列观测,研究结果表明在南导堤被淹没期间,除S3-S4丁坝段有微弱的由北槽指向九段沙的泥沙净输移运动外,其余区段均有较强的由九段沙指向北槽的泥沙净输移运动,从整个南导堤来看,指向北槽的净输沙运动占绝对优势。越堤进坝田的相对高含沙水流并不能直接输向航道。本文在ECOM-si模型的水动力和盐度模块的基础上,并建立了三维泥沙输移模块,采用TVD格式来计算泥沙沉降过程,从而更加准确地刻画了泥沙的沉降过程,并考虑了盐度和含沙量对絮凝沉降速度的影响,构造了便于计算使用的絮凝沉降公式,考虑了波浪对紊动的作用和对底部切应力的作用影响。采用现场座底式三角架近底观测系统观测的含沙量、流速剖面和河床冲淤变化资料计算了北槽临界起动切应力、临界淤积切应力、侵蚀率和沉积率,为计算泥沙场提供了关键性参数,通过这些工作,本泥沙模型计算的含沙量验证结果良好,可以用来进一步研究长江口泥沙运动过程。采用建立并验证的泥沙模型系统研究了长江口泥沙输移机制。研究结果表明长江口主槽的泥沙Eular输移通量主要指向海域,而滩地上的泥沙Eular输移通量主要指向上游或者滩面。Stocks输移在主槽削弱指向口外泥沙Eular输移,而在滩地上增强泥沙向滩面输移。Lagrange输送通量主要和泥沙Eular输移的模式相似。以长江口拦门沙以界,拦门沙以上泥沙潮泵输送主要指向河口上游,而拦门沙以外,泥沙潮泵输送主要指向东南方向。也就是说在拦门沙附近泥沙潮泵输移出现辐散的现象,这条辐散带呈现由西北往东南方向分布。垂向切变输送仅在拦门沙区域较大,其他区域要远小于Lagrange余流输送和潮泵输送。更多还原

【Abstract】 The sediment transport is the most complex problem in estuary. The physical model, numerical model and field observation are the most important methods to research sediment transport, which have developed enormously in recent years. The physical model is still a main means for research estuarine engineering, which can be used to research some mechanisms that can’t be described by numerical expression, such as bed load transport, stabilization of hydraulic engineering and et al. The numerical model has been developed to be a more and more important method to research sediment problems. These mechanisms of sediment advection, diffusion, deposition and resuspension are three dimension processes, so three dimension sediment numerical model is need to research those mechanisms. The field observation is the most direct and convincing method for research on sediment transport, which can be used to reveal some new mechanisms of sediment transport. In this paper, these three methods were used to research sediment transport in Changjiang Estuary.There are many interlaced shoals and channel and the river pattern is very complex in Changjiang Estuary. The bed load transport play a key role in the general process of erosion in shoal head and deposition in shoal tail. Based on the in-situ samples collected by sampling in field and flow flume experiment, the appropriate model sediments were selected to simulate the bed load sediments in a validated physical model. The path line and deposition of sediment from several shoal head and main channel in South Branch, North channel and South channel in Changjiang Estuary were investigated in physical model.To investigate the dispersal pattern and the fate of dredged material disposed in pre-selected disposal site, a field tracer experiment of simulation of dredged material dispersion was conducted in North Passage of Changjiang Estuary during 2005 flood season. Three tons dredged material and 2.792kg Sodium hexachloroiridate (Ⅳ) hexahydrate (SHH) were used to make tracer sediment. Three round sampling were carried out in pre-selected sections on the third, fourth and fifth day after the release of dredged materials. All samples were analyzed by neutron activation analysis and results showed disposed dredged material mainly dispersed and deposited between navigation channel and south dike in the North Passage, whose path line were nearly parallel to navigation channel. Only few dredged material entered or came across navigation channel and the ratio of back silting in navigation channel was about 5%. Disposed dredged material dispersed southeasterly beyond two dike heads.Four bottom-mounted instrument-equipped tripods were deployed at two sections spanning the region characterized by severe sedimentation rates in the Deepwater Navigation Channel (DNC) along the North Passage of Changjiang Estuary in order to observe currents, near-bed suspended sediment, and salinity. Seaward residual currents predominated in the up-estuary section. In contrast, a classical two-layered estuarine circulation pattern occurred in the down-estuary section. Flow moved seaward in the upper layer and a heavier inflow, driven by the salinity gradient, moved landward in the lower layer. The near-bed residual currents in the up-estuary section and the down-estuary section acted in opposing directions, which implies that the region is a convergence zone of near-bed residual currents that trap sediment at the bottom. The maximum salinity gradient at the maximum flood current indicates the presence of a strong front that induces sediment trapping and associated near-bottom convergence of sediment.Observation on south dike and shoal showed huge net sediment transport flux from Jiuduansha shoal to North Passage expect slight inverse flux from S3 to S4 groins during the south dike was submerged. The net sediment transport flux from Jiuduansha shoal to North Passage was dominant. The high SSC flow from Jiuduansha shoal can’t transport directly to navigation channel in North Passage.A three dimensional sediment transport model was setup based on ECOM-si model. A TVD scheme was adopted to calculate the sand deposition process and a convenient flocculation formula was reduced to express the effect of salinity and SSC on sediment setting velocityso sediment deposition process can be simulated more accurately. The effect of wave on turbulent and bottom shear shtree were adopted to include wave. A bottom-mounted instrument-equipped tripods was deployed to observe near bed SSC, current and bed level to calculate several key parameter for sediment model, such as critical initial shear, critical deposition shear, erosion rate and deposition rate. Based on those work, the sediment model can be validated well, which can be used to study sediment transport in Chanjiang Estuary.The sediment transport process was investigated by the validated sediment model. The study results show that the sediment Eular transport flux in main channel is seaward but that on shoal is landward. The sediment Stocks transport flux decrease the seaward Eular transport flux in main channel but increase the landward sediment Eular transport flux on shoal. The sediment Lagrange transport flux pattern is similar to Eular transport pattern. The tidal pumping flux is landward at up-estuary of mouth bar but seaward at down-estuary of mouth bar. The tidal pumping flux is a divergence pattern over mouth bar. The vertical shear sediment transport flux is intensive over mouth bar and is much less than that of Lagrange flux and tidal pumping flux at other area.更多还原