【作者】 刘杰；

【导师】 陈吉余；

【作者基本信息】 华东师范大学 ， 自然地理学， 2008， 博士

【摘要】 河口是海陆相互作用的界面,受到流域来水来沙和外海波浪、潮汐、盐淡水混合等多种因子的叠加作用,形成了复杂的水流动力和泥沙运动条件。经过长期自然演变,河口形成了与自然水流和泥沙条件相适应的平衡地貌形态。人类进行的航道整治工程,即使调整或改变其中的一个因子,都将引起其它因子的变化,航道整治工程与河口动力、沉积和地貌形态之间存在复杂的响应过程。长江口深水航道治理是我国水运交通行业关注的热点,又是世界河口治理的难点。周边河势的新变化、河口细颗粒泥沙运动理论的不完备和现有模型试验技术的不完善,使得工程前难以精确预测工程后的实施效果。长江口深水航道治理一、二期工程取得了巨大的成功,但在二期工程完成后,北槽水流、泥沙和地形边界条件出现了新的不利航道建设维护的方向变化,上航道淤积强度增大。为此,本论文收集和整理了1998年长江口深水航道治理一期工程开工以来系列的原型水文、泥沙、地形和航道回淤资料,试图对长江口深水航道整治工程的河床冲淤调整作用、深水航道回淤的时空变化规律及二期工程后上航道集中回淤的机理和减淤对策等几个问题开展研究。获得的主要认识如下:“宽间距双导堤+长丁坝群”河床调整作用长江口深水航道整治工程采用“宽间距双导堤+长丁坝群”的平面布置形式,其设计思想为“导流、挡沙、减淤”。一、二期工程实施后,北槽(丁坝段)河床冲淤演变特点为“丁坝缩窄河道、主槽冲刷加深、坝田边滩淤涨”。一、二期工程引起的(丁坝段)主槽河床冲刷效应持续至整治工程完工1年左右,调整后的航道自然水深保持在8～9m。在一、二期工程实施阶段,(丁坝段)主槽河床冲刷量与丁坝长度具有较好的正相关关系,平均每km丁坝长度可引起主槽冲刷量达761万m~3。综上可知,“宽间距双导堤+长丁坝群”整治建筑物在工程实施阶段(1998—2005)展示出良好的“导流、挡沙、减淤”效果,具体表现为北槽水流向中泓汇聚,不同阶段工程实施后相应的(丁坝段)主槽冲刷增深,航道成槽率高,在疏浚工程的配合下,深水航道一、二期工程的预定目标(航道水深8.5m和10m)顺利实现。另一方面,“宽间距双导堤+长丁坝群”在北槽形成大范围的丁坝坝田。一、二期工程实施后,北槽丁坝坝田经历了持续的淤积过程,平均淤积速率约0.5m/a。随着北槽丁坝坝田淤积量增加,2002年之后北槽河槽总容积(坝田+主槽)呈现减小趋势。北槽河槽总容积减小同时伴随着潮间带面积的增大,造成北槽纳潮能力下降,落潮分流比不断减小,这不仅导致北槽进口段(非丁坝)淤积,而且使丁坝段主槽由冲转淤。若按丁坝坝田淤积至0m水深时估算,在二期工程完成后上述河床的淤积调整尚需持续6～7年。因此,二期工程完成后,伴随着丁坝坝田淤积引起的主槽河床淤积调整,整治工程实施阶段对主槽河床冲刷调整的效果将有所弱化。深水航道回淤的时空变化规律航道回淤与水流、泥沙运动和河床边界条件有关。由于整治工程引起的北槽水流、泥沙和河床边界条件的改变,长江口深水航道治理一、二期工程实施后,北槽航道回淤发生了明显的变化,表现为全槽回淤强度减小,局部区段回淤强度增大。2000～2005年,尽管航道维护水深有所增加,但北槽航道回淤强度却呈现下降趋势,航道年淤积强度由工程前的2.93m,减小至一期工程的1.46m和二期工程的1.00m,一、二期工程航道治理效果明显。从空间上看,一、二期工程航道主要回淤部位经历了下移和上提的过程。航道回淤强度与地形因素即滩槽高差有关,各单元回淤强度与航道两侧7m、8m河槽的宽度呈负相关关系。相同的河槽宽度,随着航道维护水深的增大,航道的回淤强度将明显增大。当8m河槽宽度等于1000m时,维护8.5m和10m航道水深,航道回淤强度分别约为5m/d和20mm/d。伴随着二期工程北槽整治段主槽的冲淤调整,航道回淤分布呈现向上航道集中的特点,回淤强度大的区段位于W3以上及附近区域。从时间上看,航道回淤的年际、年内变化与流域的来水来沙关系密切,航道回淤呈洪季多淤、枯季少淤;洪季淤积位置下移,枯季淤积位置上提的特点,洪季(5～10月)北槽航道回淤量约占全年回淤量的80%,8月通常为航道回淤强度最大的月份。回淤量的年内分布与来沙量的相关关系好于来水量的相关关系,径流变化可引起航道回淤重心的变化,洪枯季北槽航道回淤的重心上、下移动约7～10km。北槽上航道(W2～W3)淤积的机理地形因素。二期工程后北槽上航道两侧地形冲淤的非对称性,即上航道上段(G单元以上)北侧淤积,上航道下段(G单元以下)南侧淤积,致使上航道轴线与深泓线交错,滩槽高差加大。动力机制。①落潮分流比的减小。二期工程完成后,北槽的落潮分流比仍在减小,导致主槽落潮流优势降低,上航道下段(G单元以下)出现滞流点;②涨、落潮动力轴线分异。上航道上段受科氏力、横沙通道和长兴岛涨潮沟落潮流水流下泄的影响,落潮主流偏南。受科氏力、横沙东滩窜沟封堵的影响,上航道下段涨潮流偏北,上航道区段涨、落潮动力轴线分异,水流分散;③水流与上航道存在夹角。④盐淡水混合。上航道处在盐淡水交会活动的范围,航道主要淤积部位与洪季5‰盐度等值线、最大混浊带核心的摆动范围基本吻合,进一步说明细颗粒泥沙絮凝沉降形成近底高含沙量带,促使了上航道的集中淤积。泥沙来源。近几年南导堤南侧的九段沙不断淤高,部分滩面已呈跨越南导堤进入北槽之势,九段沙与北槽之间的泥沙交换明显增强。九段沙滩面相对较高浓度的含沙水体涨潮越堤进入北槽,演变成为上航道淤积的重要泥沙来源。北槽上航道(W2～W3)的减淤对策根据上航道回淤机理分析,结合国内、外河口航道治理的理论与经验,进一步的减淤对策有:①加高南导堤,阻挡九段沙越滩泥沙进入北槽:②修复瑞丰沙,减小上游泥沙来源;③调整航道轴线,减小上航道与深泓线之间的夹角;④延长丁坝,缩小河宽,减小主要回淤段治导线的放宽率;⑤增大治导线曲率半径,避免水流过分弯曲跨越航道;⑥南槽进口段进行防护与限流,以增加北槽的落潮流动力和落潮流优势。更多还原

【Abstract】 By using bathymetric chart,discharged flow data,hydrological data and back-siltation data,recent change of the riverbed and silting pattern of the deep waterway in the North Passage of the Yangtze Estuary has been studied in this paper. The main contents could be reviewed as follows:The regulation engineering of deep waterway,including two wide spacing jetties and nineteen long groins,had led to an apparent morphological evolution in the North Passage.The main channel of the up,middle and down side in the North Passage had successively eroded and its groin field had significantly deposited.At the same time,sediment has been deposited on the entrance region.Erosion and deposition had responded rapidly to the construction of the regulation engineering.There was about one year of lagging between the time of the erosion happened in the deep channel and the time of the construction of the regulation engineering finished in different stage.The siltation lag of time in the groin field varied with the initial depth, but the average deposited thickness was about 0.Sm per year.Volumetric analysis demonstrates there is an increasing trend of the siltation in the North Passage after 2002,because of the difference in duration and quantity between erosion in the deep channel and deposition in the groin field.The water volume of the North Passage had reduced by≈9%（280 million m³）between 2002 and 2006.Sediment budget reveals that the main sediment deposited in North Passage sourced from the river and the ocean.The decreasing water volume was attributable to shoaling in the groin field. Its triggering factors for the increased sedimentation are the navigational improvements（jetties and groins）after 1998,which altered the passage boundary and destroyed the equilibrium state on the average ebb and flood sediment fluxes.The establishment of a stable estuary is attributed to a reduction rate in depth of the groin field.The forecast on the deposited sediment quantity and continuous infilling time in the groin system is about 325×10⁶m³ and 6-7 years respectively.The South Passage and North Passage are the third branched reach of Yangtze Estuary.After implementing Yangtze Estuary Deepwater Navigational Improvements, there are a decreasing trend in the ratio of ebb flow diversion through the North Passage and an increasing trend through the South Passage.Sediment has deposited on the entrance region in the North Passage,with an length of about 12km long,and eroded on the upper region in the South Passage,with an length of about 18km.But the total cross-section area of the entrance reach in the North Passage and South Passage had decreased from 1998 to 2006.There is a good correlation between the total water volume and the yearly averaged ebb flow ratio in the North Passage,and between the cross-section averaged depth of the entrance reach in the North Passage and South Passage and their yearly averaged ebb flow ratio respectively.According to the relationship formula,the change of the ebb flow ratio and the cross-section averaged depth of the entrance reach in the North Passage and South Passage are evaluated.Due to change of boundary conditions,Back-siltation in the deep channel had undergone an obvious change after the implementing Yangtze Estuary Deepwater Navigational Improvements,which had a decrease in the back-silting intensity of the whole navigational channel and increase in local reach.The yearly back-silting thickness of the whole navigational channel decreases from 2.93m to 1.46m of the 1^ststage and l.00m of the 2^ststage,respectively.The location of the high back-siltation moves from the downstream channel during 1^ststage to the upstream channel during the 2^ststage.The spatial distribution of back-silting intensity along the navigational channel was closely related to its elevation difference between dredged trough and riverbed.The time distribution of back-silting intensity was closely related to the flow runoff.The back-silting amount increased and moved downward in the flood season and decreased and moved upward in the dry season.The main back-silting location centralized in the upstream channel after the 2^ststage.The main factors resulting in back-silting include riverbed topography,tide currents,sediment load and salinity.The mechanism of back-silting in upstream channel involves:①an increase in the elevation difference between the dredged trough and the riverbed.②the worse dynamic conditions.③the rich source of sediment.④mixing of the salt water and the fresh water.The dredging maintenance became difficult due to the centralization of back-silting in the upstream channel.It is necessary to take further measures to reduce the back-silting intensity.According to the theory and practical experiences of the navigational improvements,and the analysis of back-silting mechanism,six steps were proposed,including:①Heightening south jetty.②Stabilizing the upstream riverbed.③Adjusting channel axis.④Revising the width of regulation line.⑤Adjusting the radius of curvature.⑥Protecting the entrance riverbed of South Passage from further scouring.更多还原