Modeling the Water-Flushing Properties of the Yangtze Estuary and Adjacent Waters

As a multi-branch estuary system, the Yangtze Estuary presents distinctive characteristics of hydrodynamic processes through co-action among river runoff, tides, wind-waves, and gravitational circulation. To study the pathways of flushing water along all of the estuary's branches and analyze their differences, especially those due to the influence of seawater intrusion and discharge variations, a free surface flow modeling suite TELEMAC-MASCARET involving passive tracers was applied to the Yangtze Estuary and the adjacent waters. The open boundary conditions were provided by the Nao.99 b model(Matsumoto et al., 2000), which was calibrated using observed velocity and salinity data obtained in March 2002. The water age, which was used as the diagnostic tool to study the flushing efficiency of the water body across the estuary, was solved by additional advection-diffusion-reaction equations implemented in the TELEMAC modeling system. The transport properties were investigated under different river discharge scenarios, which represented seasonal impacts; aspects relating to the influence of tide, surface wind stress, and density-induced circulation on age were also investigated. Model results showed that river runoff is one of the dominant factors influencing the spatial distribution of the mean age, while tidal force is another important factor. The horizontal freshwater age distribution demonstrated similarity compared with the salinity distribution; the vertical age distribution resembled the stratification pattern of salinity in all branches where stratification persists. An experimental numerical simulation of tracing saltwater age from the lower reaches of the estuary was conducted, and implicated the connectivity with transport processes of freshwater from upstream. Additionally, a particle tracking algorithm was used to analyze the dynamic characteristics of the four passages. The South Passage and South Channel were found to be significant as main water flow passages, while salinity intrusion in the North Branch was found to cause a return flow that partially joins the South Branch flushing water.

Three Dimensional Baroclinic Numerical Model for Simulating Fresh and Salt Water Mixing in the Yangtze Estuary

For simulating fresh and salt water mixing in estuaries, a three dimensional nonlinear baroclinic numerical model is developed, in which the gradients of horizontal pressure contain die gradient of barotropic pressure arising from the gradient of tidal level and the gradient of baroclinic pressure due to the gradient of salinity. The Eulerian-Lagrangian method is employed to descretize both the momentum equations of tidal motion and the equation of salt water diffusion so as to improve the computational stability and accuracy. The methods to provide the boundary conditions and the initial conditions are proposed, and the criterion for computational stability of the salinity fields is presented. The present model is used for modeling fresh and salt water mixing in the Yangtze Estuary. Computations show that the salinity distribution has the characteristics of partial mixing pattern, and that the present model is suitable for simulation of fresh and salt water mixing in the Yangtze Estuary.

Filling of the Three Gorges Reservoir to the 135-m Level: Instant Effects on the Yangtze Discharge and Suspended Sediment Concentration Entering the Estuary

Via the valuable opportunity of the Three Gorges Reservoir (TGR) 135-m filling in June 2003, the Yangtze discharge and suspended sediment concentration (SSC) entering the estuary during the period from 15 May to 15 July 2003 were analyzed to examine the instant effects of the filling on them. The Yangtze discharge and SSC entering the estuary in the periods before, during and after the filling clearly indicated three phases: 1) the pre-storage phase characterized by natural conditions, in which the SSC increased with increasing water discharge; 2) the storage phase, during which the SSC decreased dramatically with decreasing water discharge; and 3) the post-storage phase, during which both the SSC and water discharge remained at relatively low levels first until the end of June, then the SSC increased gradually with increasing water discharge. It seems that the times for the instant effects of the decreasing discharge downstream from the upper Yangtze on the Yangtze discharge and SSC entering the estuary due to the TGR 135-m filling to take place were about 5 d and 1 d respectively, while both were about 18 d for those of the increasing discharge. This probably reflects the buffering and resultantly hysteresis of the 1800-km stretch from the upper Yangtze to the estuary. The results are helpful for scientific and hydrological investigation of the Yangtze mainstream downstream from the TGR Dam and of the estuarine and adjacent coastal waters.

A numerical study on water diversion ratio of the Changjiang(Yangtze)estuary in dry season

We studied the flood, ebb and tidal averaged along (net) water diversion ratio (WDR) during dry season in the Changjiang (Yangtze) estuary, China, along with the effects of northerly wind, river discharge, tide and their interactions on WDR using the improved version of three-dimensional numerical model ECOM. Using data for annual mean wind speed and river discharge during January, we determined that the flood, ebb, net WDR values in the North Branch of the estuary were 3.48%, 1.68%,-4.06% during spring tide, and 4.82%, 2.34%,-2.79% during neap tide, respectively. Negative net WDR values denote the transport of water from the North Branch into the South Branch. Using the same data, the corresponding ratios were 50.09%, 50.92%, 54.97%, and 52.33%, 50.15%, 43.86% in the North Channel and 38.56%, 44.78%, 103.96%, and 36.92%, 43.17%, 60.97% in the North Passage, respectively. When northerly wind speed increased, landward Ekman transport was enhanced in the North Branch, increasing the flood WDR, while the ebb WDR declined and the net WDR exhibited a significant decrease. Similarly, in the North Channel, the flood WDR is increased, the ebb WDR reduced, and the net WDR showed a marked decrease. In the North Passage, the flood WDR also increased while the ebb and net WDR declined. As the river discharge increased, the flood and ebb WDR of the North Branch increased slightly and the net WDR increased markedly. In the North Channel the flood and ebb WDR changed very slightly, while the net WDR declined during spring tides and increased during neap tides. The WDR in the North Passage changed slightly during flood and ebb tides while the net WDR showed a marked increase. The WDR values of different bifurcations and the responses to northerly wind, river discharge, and tide are discussed in comparison with variations in river topography, horizontal wind-induced circulation, and tidal-induced residual current.

Impact of water and sediment discharges on subaqueous delta evolution in Yangtze Estuary from 1950 to 2010

In order to determine how the subaqueous delta evolution depends on the water and sediment processes in the Yangtze Estuary, the amounts of water and sediment discharged into the estuary were studied. The results show that, during the period from 1950 to 2010, there was no significant change in the annual water discharge, and the multi-annual mean water discharge increased in dry seasons and decreased in flood seasons. However, the annual sediment discharge and the multi-annual mean sediment discharge in flood and dry seasons took on a decreasing trend, and the intra-annual distribution of water and sediment discharges tended to be uniform. The evolution process from deposition to erosion occurred at the -10 m and -20 m isobaths of the subaqueous delta. The enhanced annual water and sediment discharges had a silting-up effect on the delta, and the effect of sediment was greater than that of water. Based on data analysis, empirical curves were built to present the relationships between the water and sediment discharges over a year or in dry and flood seasons and the erosion/deposition rates in typical regions of the suhaqueous delta, whose evolution followed the pattern of silting in flood seasons and scouring in dry seasons. Notably, the Three Gorges Dam has changed the distribution processes of water and sediment discharges, and the dam＇s regulating and reserving functions can benefit the subaqueous delta deposition when the annual water and sediment discharges are not affected.

Analysis of the Synoptic Situation Field on the Red Tide Occurred in Yangtze River Estuary Water Area

The occurrence of the red tide is an extremely complex process, which is considered as the comprehensive result of various factors. The Yangtze River estuary water area is always in high incidence area of red tide. In this paper, according to the events of red tide and meteorologic and hydrologic data in Yangtze River estuary water area from 2000 to 2010, by using mathematical statistics methods, we analyze the relevance between the occurrence of the red tide and the synoptic situation field, and probe into the regular patterns.

Numerical simulation of water quality in Yangtze Estuary

In order to monitor water quality in the Yangtze Estuary, water samples were collected and field observation of current and velocity stratification was carried out using a shipboard acoustic Doppler current profiler （ADCP）. Results of two representative variables, the temporal and spatial variation of new point source sewage discharge as manifested by chemical oxygen demand （COD） and the initial water quality distribution as manifested by dissolved oxygen （DO）, were obtained by application of the Environmental Fluid Dynamics Code （EFDC） with solutions for hydrodynamics during tides. The numerical results were compared with field data, and the field data provided verification of numerical application： this numerical model is an effective tool for water quality simulation. For point source discharge, COD concentration was simulated with an initial value in the river of zero. The simulated increments and distribution of COD in the water show acceptable agreement with field data. The concentration of DO is much higher in the North Branch than in the South Branch due to consumption of oxygen in the South Branch resulting from discharge of sewage from Shanghai. The DO concentration is greater in the surface layer than in the bottom layer. The DO concentration is low in areas with a depth of less than 20 m, and high in areas between the 20-m and 30-m isobaths. It is concluded that the numerical model is valuable in simulation of water quality in the case of specific point source pollutant discharge. The EFDC model is also of satisfactory accuracy in water quality simulation of the Yangtze Estuary.

Environmental significance of biogenic elements in surface sediments of the Changjiang Estuary and its adjacent areas

Biogenic elements and six phosphorus （P） fractions in surface sediments from the Changjiang Estuary and adjacent waters were determined to investigate the governing factors of these elements, and further to discuss their potential uses as paleo-environment proxies and risks of P release from sediment. Total organic carbon （TOC） and leachable organic P （Lea-OP） showed high concentrations in the estuary, Zhejiang coast and offshore upwelling area. They came from both the Changjiang River and marine biological input. Biogenic silicon （BSi） exhibited a high concentration band between 123 and 124°E. BSi mainly came from diatom production and its concentration in the inshore area was diluted by river sediment. Total nitrogen （TN） was primarily of marine biogenic origin. Seaward decreasing trends of Fe-bound P and Al-bound P revealed their terrestrial origins. Influenced by old Huanghe sediment delivered by the Jiangsu coastal current, the maximum concentration of detrital P （Det-P） was observed in the area north of the estuary. Similar high concentrations of carbonate fluorapatite （CFA-P） and CaCO3in the southern study area suggested marine calcium-organism sources of CFA-P. TOC, TN and non-apatite P were enriched in fine sediment, and Det-P partially exhibited coarse-grain enrichment, but BSi had no correlation with sediment grain size. Different sources and governing factors made biogenic elements and P species have distinct potential uses in indicating environmental conditions. Transferable P accounted for 14%-46% of total P. In an aerobic environment, there was low risk of P release from sediment, attributed to excess Fe oxides in sediments.

Water quality model with multiform of N/P transport and transformation in the Yangtze River Estuary

As the Yangtze River Estuary and adjacent sea have been classified as a problem area with regard to eutrophication, it is important to explore the spatial and temporal variations of nitrogen and phosphorus （N/P） nutrients in this area. Based on danish hydraulic institute （DHI）＇s open platform Ecolab, a hydrodynamic and water quality model was developed for the Yangtze River Estuary, in which the transport and transformation processes of different forms of N/P nutrients were considered. Validations against measured data show that the model is overall reliable. Preliminary application of the model suggests that the model can simulate the characteristics of high phosphorus concentration area in the Yangtze River Estuary, and the high concentration area is closely related to the resuspension process of particulate phosphorus.

Optimization of Shanghai Marine Environmental Monitoring Sites in the Identification of Boundaries of Different Water Quality Grades

Water quality is critical to ensure that marine resources and the environment are utilized in a sustainable manner. The objective of this study is therefore to investigate the optimum placement of marine environmental monitoring sites to monitor water quality in Shanghai, China. To improve the mapping or estimation accuracy of the areas with different water quality grades, the monitoring sites were fixed in transition bands between areas of different grades rather than in other positions. Following bidirectional optimization method, first, 18 candidate sites were selected by filtering out specific site categories. Second, three of these were, in turn, eliminated because of the rule defined by the changes in the areas of water quality grades and by the standard deviation of the interpolation errors of dissolved inorganic nitrogen(DIN) and phosphate(PO_4-P). Furthermore, indicator kriging was employed to depict the transition bands between different water quality grades whenever new sampling sites were added. The four optimization projects of the newly added sites reveal that, all optimized sites were distributed in the transition bands of different water grades, and at the same time in the areas where the historical sites were sparsely distributed. New sites were also found in the overlap region of different transition bands. Additional sites were especially required in these regions to discriminate the boundaries of different water quality grades. Using the bidirectional optimization method of the monitoring sites, the boundaries of different water quality grades could be determined with a higher precision. As a result, the interpolation errors of DIN and PO_4-P could theoretically decrease.

变化条件下长江口防洪御潮及供水保障研究进展与展望

近一个世纪以来,全球气候变暖导致海平面持续上升,增加了整个长江口的水深,抬高了潮汐和风暴潮的基础水位,使得河口咸潮入侵,洪水和风暴潮发生的频率和强度增加。同时,由于长江流域水土保持和水库群建设,进入河口的泥沙量锐减,加剧了河口咸潮上溯,对区域人群生产生活产生一定影响。从全球气候变暖背景下长江口海平面上升和演变规律、长江口人类活动与气候变化影响下的动力要素变化、河口咸潮入侵时空分布和水盐交换机制及供水安全风险、海平面上升对海堤防洪御潮能力的影响、长江口地区防洪御潮韧性等5个方面总结归纳了现有研究成果,提出未来研究的方向,以期对保障河口地区水资源安全利用和防洪御潮能力提升提供参考。

基于空间插值的长江口邻近海域春季悬浮物和叶绿素a空间分布分析

针对目前利用实测数据对长江口邻近海域水质状况进行分析研究相对偏少的情况,基于“淞航”号2018年春季航次对该海域的综合观测,利用实验室水样分析数据对船载温盐深仪(CTD)的测量数据进行校正,并对该区域2018年春季时节悬浮物(TSM)和叶绿素a(Chl-a)浓度的空间插值结果进行分析。研究结果表明:CTD观测数据与水样分析数据呈较强线性相关关系。反距离权重插值对TSM和Chl-a浓度空间分布具有整体最优的效果。TSM浓度在近岸和近海底较高,在观测区域内出现两个高值中心;Chl-a浓度在近岸海域较高,有较明显的片状高值结构,垂向上表层较高。TSM与Chl-a浓度分布在长江口南北表现出不同的特征,且两者具有一定的负相关性。长江径流、外海洋流、潮汐混合等水动力过程是影响该区域TSM和Chl-a浓度分布的主要因素。

咸潮影响下长江口水源地优化布局及白茆沙水库建设可行性分析

为提升远期上海原水水量、水质供应能力和应对突发咸潮入侵和水污染风险,在分析长江口水源现状和咸水入侵形势基础上,论证了长江口水源地优化布局的必要性,结合咸水入侵监测数据及反演成果提出在白茆沙新建水库方案及原水系统优化调度方案,并分析了白茆沙水库建设对河势变化、行洪、通航、渔业资源及生态环境等方面的影响。结果表明:在白茆沙新建水库是较可行的方案,可对上海境内的长江口水源地进行优化布局,扩大供水规模并上移取水口;拟建的白茆沙水库位于白茆沙整治工程内侧,既可确保围堤安全稳定,也不影响行洪、通航,对河势变化、渔业资源及生态环境影响微小;布置于水库南侧微弯水道处的取水口,既可避免淤积对取水口的影响,也可避免北支咸潮倒灌的影响;上岸后的输水管与崇明岛原水复线共建,增强了崇明岛原水的供水安全,与现有长江口和黄浦江水源地的原水系统互联互通,可实现上海原水系统“双源供水、一网调度”的目标。

长江口鱼类群落粒径结构特征及群落稳定性评估

为了解长江口鱼类群落粒径结构特征及群落稳定性状态,实验基于2018年长江口四季鱼类资源与环境调查数据,利用标准化粒径谱、群落稳定性评估模型对鱼类群落特征进行了探究,并利用营养指数法对长江口水体生态环境质量状况进行了综合评价。结果显示:①长江口鱼类粒级主要分布在−2~6范围内,曲线穹顶位于0~3粒级范围,鱼类群落以小型鱼类为主,整体处于受干扰状态;②北支和南支的鱼类群落存在季节变化,北支粒径谱曲率夏季最大(–0.04),秋季最小(–0.41),生物量与数量相对关系统计量W值为–0.11~0.12;南支粒径谱曲率夏季最大(–0.04),冬季最小(–0.24),W值为–0.03~0.37;③南支鱼类群落稳定性系数均值为0.57,北支均值为0.43,4个季节南支与北支鱼类群落稳定性程度不同。水质营养指数表明,北支富营养化程度(18.89)较南支(5.43)高,且富营养化指数和群落稳定性系数呈显著负相关,水体富营养化或对长江口鱼类群落系统偏离稳定状态有重要影响。研究表明,长江口鱼类群落粒径偏小,鱼类群落稳定性不高,水质呈富营养化状态。本研究探究了全面禁捕前长江口鱼类粒径结构特征并评估了鱼类群落稳定性,为长江口禁渔效果评价提供了前期研究依据。

长江口一次持续性强浓雾过程特征和成因分析

使用船载观测资料、FNL再分析资料和葵花八号云图资料,对2022年3月11—14日发生于长江口的持续性浓雾过程进行研究分析,并归纳总结了预报的实用性方法。结果表明:本次浓雾过程包含辐射雾和锋面平流雾,持续时间长,影响范围广,雾区在锋面的前后部都相继出现,且锋面移动往往影响着雾的移动。在雾的生消和维持过程中,气温和相对湿度通常呈负相关,温度露点差和能见度呈正相关,当温度露点差大于3℃时,长江口也会有雾生成,但能见度不低于3 km。雾生时段通常盛行偏南风和偏北风,偏西风和西北风则会导致相对湿度迅速下降,不利于雾的形成和维持。可见光云图上雾的特征明显,颜色纹理均匀,边界清晰整齐,外围无丝状或纤维状云系扩散,而红外云图上对应处是一片灰暗区,只有边界隐约可见。

长江口徐六泾水文站极值水位研究

在气候变化背景下,极端天气和水位事件频发。为了分析长江口极端潮位的长周期特征,研究基于极值理论相关方法,采用近33 a实测资料对长江口极值潮位趋势性和重现期进行分析,结果表明:长江口徐六泾站年极值水位呈轻微下降趋势,直接原因是大通流量减少导致当地年均海面的下降;区分组模型结果显示,台风“温妮”期间长江口造成的风暴潮位为100 a一遇,而POT模型计算结果仅为60 a一遇,100 a一遇重现期潮位约7.0 m;潮位重现期的计算本质是一种用频率代替概率、从样本推求整体的过程,具有一定不确定性。其意义在于以可获取的数据为基础,计算事件发生的风险率,旨在为工程设计或风险管理提供依据。

秋季长江口及邻近海域有色溶解有机物的分布及河口混合行为

【目的】探究秋季长江口及邻近海域有色溶解有机物(chromophoric dissolved organic matter,CDOM)的分布特征与河口混合行为及影响CDOM分布的因素,以深入了解本研究区域CDOM动态变化机制。【方法】测定研究海域溶解有机碳(dissolved organic carbon,DOC)质量浓度、吸收系数a(350)等参数,分析CDOM含量水平;使用三维荧光光谱-平行因子分析(EEMs-PARAFAC)技术鉴别研究海域CDOM荧光组分。运用物理稀释模型分析CDOM河口混合行为。【结果】研究海域b-a4断面DOC质量浓度的变化范围为0.92~2.36 mg·L^(-1),c-a6断面DOC质量浓度的变化范围为1.00~2.40 mg·L^(-1)。b-a4断面a(350)的变化范围为0.22~1.60 m^(-1),c-a6断面a(350)的变化范围为0.15~1.42 m^(-1)。b-a4断面光谱斜率S275-295的变化范围为0.014~0.022 nm^(-1),c-a6断面S275-295的变化范围为0.016~0.024 nm^(-1)。鉴定出4个荧光组分:类腐殖质组分C1(激发波长265 nm、发射波长445 nm)、类蛋白质组分C2(275、325 nm)、类腐殖质组分C3(245、390 nm)和类蛋白质组分C4(235、340 nm)。【结论】长江口及邻近海域DOC质量浓度和a(350)均呈现出由长江口内向口外逐渐减小的趋势,陆源输入是影响CDOM分布的主要因素;远岸S275-295值高于近岸,主要受浮游生物生产活动的影响,陆源输入影响小;4个荧光组分的高值都出现在河口处,且随离岸距离增加而降低,这是受到陆源输入的影响;两个断面的4个荧光组分均符合理论稀释线,CDOM整体呈现保守混合行为。

极端干旱下长江口咸潮入侵成因及压咸补淡措施

2022年长江上游遭受极端干旱,长江口咸潮入侵加剧,区域用水安全受到严峻考验。为探究极端干旱天气下咸潮入侵成因和应对策略,基于长江口2019~2023年枯水期的实测盐度、潮水位、风速风向数据与同期上游大通站径流量数据,采用Spearman相关性分析、卡方分析等方法,分析了2022年9月起长江口咸潮入侵过程与长江口咸潮入侵的主要影响因素。分析结果显示:①2022年8月起长江干流枯水期提前,叠加台风和天文大潮情况,使得2022年9月至2023年4月长江口地区崇明洲头站、杨林站和六滧站的盐度大幅超标,分别达到1.922‰,0.585‰和2.134‰。②长江口3个主要盐度监测站点受上游来水影响时间存在前后差异,在大通站流量低于15000 m^(3)/s时,不同站点所属区域发生了不同程度的咸潮入侵情况。③2022年10月的压咸补淡措施在小潮期间作用于长江口地区,使得大通站流量高于12000 m^(3)/s,在南支上半段产生较好的压咸效果。研究成果可为长江口流域不同地区咸潮上溯成因分析和压咸补淡应对策略提供参考。

长江口深水航道浮泥运移规律及精细化维护疏浚

基于2016—2019年长江口深水航道施工区段月度测图和2016—2020年北槽深水航道洪枯季常态天气条件下的浮泥观测资料,研究浮泥输移特征及回淤、高频水深等变化特征,提出航道精细化疏浚维护方案。结果表明,常态天气条件下洪季的高低频水深差幅度大于枯季,北槽航道中段洪季的浮泥厚度要远大于枯季,航道回淤量具有随高低频水深差的增加而增加的特征。建议优先在洪季实施精细化维护疏浚方案,试应用后能有效减小浮泥、高低频水深差等变化过程对航道考核测量和航道维护疏浚的干扰,提高航道维护疏浚的精细化管理水平。