Transport of suspended matter in a bar-built Danish estuary

The river Varde Å discharges into the bay of Ho Bugt on the western coast of Jutland forming a small, bar-built estuary. This paper deals with tidal fluxes of water and sediment in the Varde Å estuary.The inflowing water at flood tide is part of a turbidity maximum in the northern part of the bay. At high tide slack water the suspended material deposits inside the estuary. During ebb-tide it is resuspended, and the estuary bottom is washed clean coinciding with the influx of relatively pure freshwater from the drainage area.From one station in the estuary mouth, current velocities and concentrations of suspended material have been measured during 10 tidal periods covering all four seasons. It is shown how these data can be used in a quantitative calculation of the transport of water and suspended material through the cross-section of study.A model has been formulated which—based on half-tidal periods—quantifies the transport of water and suspended material through the estuary mouth.The model is calibrated on the basis of measurements made during the above-mentioned 10 tidal periods. The rather small number of measurements is to some extent compensated for by a carefully pre-arranged selection of tidal periods.The model is discussed in relation to the prediction of net suspended transport through the estuary mouth in different weather and tidal situations.     

Effect of sea level rise on tidal circulation in the Hooghly Estuary,Bay of Bengal

The issue of sea level rise is receiving considerable attention all over the world. Although the Indian stations have shown mixed trends, a positive sea level trend has been noticed in the Hooghly Estuary, situated on the east coast of India. The Hooghly River serves as a navigable waterway to Calcutta and Haldia ports. The river is tidal for nearly 250 km. To study the water levels and tidal currents in the lower part of the Hooghly Estuary, from sea face at Sagar to Hooghly Point, a vertically integrated numerical model has been used. The model is fully nonlinear and uses a semiexplicit finite‐difference scheme to solve the basic hydrodynamic equations on a staggered grid. This model is coupled with a one‐dimensional model, which has been used for the upper estuary from Hooghly Point to Swarupganj, where the flow is unidirectional. The computed water levels and currents are found to be in good agreement with the available observations. This model is applied to study the alterations in tidal circulation for a rise and fall in the sea level. The results have shown a substantial increase in the amplitude and velocities of the tidal wave due to the sea level rise.     

Modelling Salt Intrusion and Nitrate Concentrations in the Ythan Estuary

A one-dimensional salt intrusion model is used to investigate the hydrography of the Ythan estuary, a small shallow macrotidal estuary in the north-east of Scotland. The model simulates the longitudinal distributions of water level, salinity and total oxidized nitrogen (TON) in the estuary. The model employs upstream differencing and the Smolarkiewicz anti-diffusion scheme to avoid the numerical difficulties typically encountered when modelling strong tidal flows using centred differences. The physical mechanisms driving the simulations are the tide at the entrance to the estuary and freshwater discharge at the head. The model was calibrated against measurements of water level made at three locations in the estuary, salinity observations made at a central platform and axial salinity distributions. At both spring and neap tides, the full range of salinity observed at the central platform was simulated. However, at the midway stage between springs and neaps, the simulated peak salinity was less than that observed. This was probably due to the sensitivity of the model to the digitisation of the estuarine bathymetry.The model successfully simulated salinity distributions for periods of high and low river flow, and was used to illustrate how TON concentrations fluctuated in response to variations in river flow. The potential implications of variations in the bathymetry of the estuary on salinity and nutrient distributions were predicted to be slight. However, the four fold increase in riverine TON concentrations that has occurred over the past 30 years was shown to increase TON distributions along the entire length of the estuary. The calculated estuary flushing time was strongly dependent on river flow and varied between 11–60 h.     

A modelling study of residence time in a macro-tidal estuary

This paper outlines a numerical modelling study to predict the average residence time of a conservative tracer in a macro-tidal estuary, namely the Mersey Estuary, UK. An integrated hydrodynamic-dispersion model was used to predict the average residence time in the estuary for various tidal level and freshwater discharge conditions. The numerical model was verified against six sets of field measured hydrodynamic data, with the model-predicted water elevations and salinity levels generally agreeing well with the field measurements. The numerical model results show that in the Mersey Estuary both the tidal level and river discharge affect significantly the predicted average residence time. The value of the average residence time is also shown to be closely linked to the intensity of the residual tidal current. This is due to the fact that a large proportion of the Upper and Inner Estuary dries out during low tides, thus a significant amount of the tracer material is transported through the deep channels. An increase in the freshwater discharge causes a considerable increase in the intensity of the residual current along the main channels and thus a reduction in the average residence time. The predicted overall tracer residence time for the whole estuary is relatively short for a relatively large estuary, ranging from less than 1 day to 4 days for various tidal level and freshwater flow combinations. When the tidal range and freshwater discharge are both small, then the local tracer residence time in the upper part of the estuary can be significantly longer than the values predicted for the middle and lower reaches of the estuary.     

Hydrodynamic Effect of the Regulation Project of Yangtze River Deepwater Channel Downstream of Nanjing

A two-dimensional flow numerical model of the tidal reaches, which total length is more than 700 km, is established from Datong to the Yangtze River estuary. The tidal levels, velocities, diversion ratios and dynamic axes before and after the separate regulation of each reach and combined regulation of all reaches are obtained. The comparative analysis shows that the regulation project of a separate reach basically has no impact on velocity distributions and variations of diversion ratios of upper and lower reaches, the variations of dynamic axes are only within the local scope of the project. The regulation project of a separate reach also has less impact on the water level in the lower adjacent reaches, but will make the water levels in the upper reaches rise. After the implementation of the regulation projects for all reaches, the rise of water level in the upstream reaches will have a cumulative impact.     

Three-dimensional Layer-integrated Modelling of Estuarine Flows with Flooding and Drying

Details are given of the refinement and application of a thee-dimensional (3-D) layer-integrated numerical model of tidal circulation, with the aim of simulating severe tidal conditions for practical engineering applications. The mode splitting strategy has been used in the model. A set of depth-integrated 2-D equations are first solved to give the pressure gradient, and the layer-integrated 3-D equations are then solved to obtain the vertical distributions of the flow velocities. Attention has been given to maintaining consistency of the physical quantities derived from the 2-D and 3-D equations. A TWO=layer mixing length turbulence model for the vertical shear stress distribution has been included in the model. Emphasis has been focused on applying the model to a real estuary, which is geometrically complicated and has large tidal ranges giving rise to extensive flooding and drying. The model has been applied to three examples, including: wind-driven flow in a rectangular lake, tidal circulation in a model rectangular harbour, and tidal circulation in a large estuary. Favourable results have been obtained for both the simple and complex flow beds.     

河口地貌对潮汐不对称性影响的数值模拟研究

河口地貌形态对潮汐不对称性的产生和发展有着至关重要的作用。本文根据英国Humber河口数据建立了概化模型,研究了在同一纳潮量情况下,主槽断面形态、平面形态和河口收缩率对河口潮汐不对称性的影响。结果表明,较深的主槽能使相位差峰值出现较晚且峰值更大,从而影响局部区域的涨潮流强弱,主槽越浅,最大落潮流速越小,落潮所需历时越长,河口更倾向于涨潮主导,窄潮滩倾向于涨潮主导型,宽潮滩倾向于落潮主导型;平面形态沿程收缩且长度较长的河口涨潮主导型最强,此外,河口宽度沿程缩窄会加大主槽的余流流速,减小潮滩的余流流速;随着河口平面收缩率的增强,主槽的余流流速减小,潮滩余流流速增大,潮滩更倾向于涨潮主导。本文进一步丰富了河口地形地貌变化对潮汐不对称性影响的认识,可为河口区工程建设和管理维护提供科学依据。     

垂向二维潮流数值模型及其在长江口北槽的应用

应用变网格有限元方法，采用任意四边形等参单元，离散横向积分的Navier—Stokes方程，建立河口潮流垂向二维数学模型。应用此模型，对长江口北槽水域两个观测站的潮流水位、流速垂线分布和整个北槽潮流流速纵向分布进行了模拟。潮流水位、流速垂线分布的模拟值分别与观测站的实测值可以较为吻合，证明了本文模型的实用有效。模拟得到的涨急、落急时刻整个北槽潮流流速纵向分布给出了定性的结果。     

Three-Dimensional Tidal Model and Its Application to Numerical Simulation of Water Quality in Coastal Waters

- The turbulence mechanism plays an important part in the mixing process and momentum transfer of turbulence. A three-dimensional Prandtl mixing length tidal model has been developed to simulate tidal flows and water quality. The eddy viscosities and diffusivities are computed from the Prandtl mixing length model. In order to model the water quality of an estuary or coastal area many interdependent processes need to be simulated. These may be conveniently separated into three main groups: transport and mixing processes, biochemical interaction of water quality variables and the utilization and re-cycling of nutrients by living matter. The model simulates full oxygen and nutrient balance, primary productivity and the transport, reaction mechanism and fate of pollutants over tidal time-scales. The model is applied to numerical simulation of tidal flows and water quality in Dalian Bay. The model has been calibrated against a limited data set of historical water quality observations and in general demonstra     

钱塘江河口潮波特性变化及原因剖析

为掌握钱塘江河口潮波特性的响应机制，采用小波分析法研究了该河口 6 个潮位长期观测站的年平均高、低潮位及年平均涨潮历时等时间序列的变化特征，结合该河口实施的治江缩窄工程及径流周期性变化，探讨了该河口潮波特性时空变 化的因果关系。结果表明：1953—2017 年，钱塘江河口呈现出高潮位显著抬升、涨潮历时缩短，低潮位在河口上段略有抬高、中段大幅抬升、下段变化不大的趋势；受治江缩窄工程进展、位置以及径流周期性变化等影响，各站潮波特性的变化在时间上存在差异；钱塘江河口的治江缩窄工程加剧了河口下段的潮波变形和潮波反射，增强了河口上、中段的河流特性，导致高潮位抬升、涨潮历时缩短，是造成潮波特性变化的主要原因；径流直接影响河口上、中段的潮位和潮流，此外，还通过流速变化改变河床地形而间接影响潮波特性。     

Numerical modelling of suspended sediment fluxes in estuarine waters

A two-dimensional finite difference numerical model, capable of predicting depth-averaged tidal flow fields in coastal and estuarine waters, has been extended to include tide-induced non-cohesive sediment transport processes. The partial differential equations governing the conservation of mass, momentum and suspended sediment in an incompressible turbulent flow are included in a depth-integrated form in the model. For the representation of the processes of erosion and deposition of sediment from the bed an empirically based source-sink term was refined, based on the results of three mobile bed flume studies. The model has been tested by simulating tidal flows and suspended sediment fluxes in two estuaries, with particular application to the Humber estuary in the U.K. The model was calibrated and found to produce an encouraging degree of agreement between the numerical predictions and corresponding field measurements for this estuary. Furthermore, the predicted gross deposition and erosion features of the estuary were found to be in close agreement with interpretations from Eulerian tidal residual predictions.     

强潮河口水下地形测量潮位控制研究

钱塘江河口属于强潮河口,具有潮差大、流急、地形复杂和局地潮汐变化大等特点,对潮位控制设计与实施造成较大困难。为解决钱塘江河口水下地形测量潮位控制难题,针对强潮河口的水下地形和潮汐特征进行分析,提出了该区域潮位控制的潮位站布设方案与作业时间要求。实测数据表明,只要合理布设潮位站、选择合适的时间段作业,强潮河口水下地形测量的潮位控制可以达到规范精度要求。     

Using salt intrusion measurements to determine the freshwater discharge distribution over the branches of a multi-channel estuary: The Mekong Delta case

The fresh water discharge is an important parameter for modelling salt intrusion in an estuary. In alluvial converging estuaries during periods of low flow, when salinity is highest, the river discharge is generally small compared to the tidal flow. This makes the determination of the fresh water discharge a challenging task. Even if discharge observations are available during a full tidal cycle, the fresh water discharge is seldom much larger than the measurement error in the tidal discharge. Observations further upstream, outside the tidal region, do not always reflect the actual flow in the saline area due to withdrawals or additional drainage. Discharge computation is even more difficult in a complex system such as the Mekong Delta, which is a multi-channel estuary consisting of many branches, over which the freshwater discharge distribution cannot be measured directly. This paper presents a new approach to determine the freshwater discharge distribution over the branches of the Mekong Delta by means of an analytical salt intrusion model, based on measurements made during the dry season of 2005 and 2006. It appears that the analytical model agrees well with observations and with a hydraulic model. This paper demonstrates that with relatively simple and appropriate salinity measurements and making use of the analytical salt intrusion model, it is possible to obtain an accurate discharge distribution over the branches of a complex estuary system. This makes the analytical model a powerful tool to analyze the water resources in tidal regions.     

河口潮位在频域内的一种插补模型

若把河口潮位时间序列简单地看为单输入和输出线性系统模型的输入输出信号，借助频谱分析可以将它们在频域内的关系建立起来，用来进行潮位序列的插补，带通滤波技术的应用可使模型的计算精度大大提高     

基于局部时间步长方法的潮流数值模型研究及应用

为了准确快速地模拟潮流过程,将局部时间步长(LTS)方法应用于基于黎曼近似解通量——有限体积法的浅水模型。LTS方法和传统整体最小时间步长方法的计算性能对比表明,两种方法的计算结果定量差异远小于模拟值与实测值之间相对误差;但由于LTS方法对每个网格采用了与稳定性条件相适应的尽可能大的时间步长,计算效率大幅度提高。本模型计算所得渤海、黄海、东海的潮流传播过程、潮汐调和常数的空间分布、无潮点位置,以及长江口和杭州湾区域的潮流过程与已有认识和实测资料吻合良好。     

长江口北港口门海域悬沙输运机制分析

根据2012年10月24日至11月1日在长江口北港口门海域获取的沉积动力学数据,采用输运通量分解方法分析水沙输运机制。结果表明,长江口北港口门附近海域涨、落潮期间底部悬沙浓度与近底部流速呈显著线性相关,存在显著的再悬浮作用;潮周期内的悬沙输运具不对称特征,涨潮悬沙浓度大于落潮悬沙浓度,悬浮泥沙有向陆输运的趋势。拉格朗日平流输运是影响悬沙输运的主要贡献项;潮泵效应(尤其是潮汐捕捉效应)以及河口垂向环流作用是两个次要影响因素,在影响程度上前者比后者略大。观测发现,长江口北港口门海域潮流除了具有涨落潮流速、历时等不对称现象外,还具有流速结构不对称的特征,进而导致涨、落潮底部湍流混合程度不对称与输沙不对称,这可能是造成悬沙向河口内输运形成最大浑浊带的重要因素。     

径流量变化对长江口水动力特性的影响

为了研究径流量变化对长江口水动力特性的影响,利用MIKE21建立长江口水动力数学模型,分别模拟计算长江口不同径流量影响的潮流场,统计分析不同径流量时各汊道涨落潮量和净泄量以及分配比例;统计分析不同径流量对各汊道潮位变化的影响;统计分析不同径流量对北槽中下段水动力特性的影响。得到如下结论:1)径流量的变化对于涨潮分流的影响显著,随着径流量的增加,更多的潮流从北支、北港、南槽进入长江口内。当上游的径流动力增强,外海潮流受上游径流的阻挡,难以继续上溯。2)越是靠近上游,径流量的变化对平均潮位的影响程度越高,入海口处受径流的影响微弱;上游径流量越大,潮波向内陆传播越受阻,能量消耗越大,致使其动力减弱。3)径流量的变化对平均流速的影响与上述潮位变化类似,上游径流量越大,各汊道流速越大;越靠近海洋,径流影响越小。4)长江口北槽中下段转流时刻会出现平面对流现象。横向方向上普遍存在越堤水流,且南北导堤均为北向越堤流占优势。     

灌河口潮汐不对称数值模拟研究

建立灌河口平面二维潮流数字模型,并在灌河口口门至陈家港河段沿程布置5个采样点和3个横向断面,通过对各采样点一个月数值模拟结果进行调和分析,得出灌河口M2分潮及其主要倍潮的基本特征,然后利用潮位-流速关系图及通过对动量方程中各项的统计计算,研究了灌河口潮波运动的基本动力平衡、M2的主要倍潮的产生机制及其对潮汐不对称现象的影响。研究认为:灌河口潮汐潮流均属正规半日潮型。M4、M6分潮对于灌河口潮波变形有着重要的影响,M8分潮除在浅滩处外对潮汐不对称的影响很小。在深槽处M4分潮导致最大落(涨)潮流速减小(增大),最大涨(落)潮流速与最高(低)潮位之间的相位差增大。M6分潮的影响与M4基本一致,但其作用明显小于M4分潮。在浅滩处M4分潮导致最大涨落潮流速均大于M2分潮的值,M6和M8分潮使最大涨落潮流速均出现在高潮位附近,但M6、M8使最大涨潮流速减小,而使最大落潮流速增大。外海边界处倍潮波的传入对于口门附近的潮汐不对称有一定影响。M4和M6分潮导致灌河口的潮汐不对称表现为涨潮主导型,潮波运动主要受到压强梯度力、非线性对流和底摩擦力,三者与局地惯性力构成灌河口的基本动力平衡,摩擦力并不是最重要的作用力。     

长江口外潮汐精细化模型研究与精度分析

长江口外海上测量除受风浪影响较大外,最重要的问题是潮位控制非常困难。文中简要阐述了开展长江口外潮汐精细化模型研究的方法,介绍了利用潮汐精细化模型对长江口外航路任意点进行潮汐预报的方法,并通过实测数据进行了精度分析,提出了建议。     

Suspended sediment transport from the Gironde estuary (France) onto the adjacent continental shelf

Current velocity and suspended sediment concentration measurements at anchor stations in the downstream extremity of the Gironde estuary indicate that during periods of high river discharge, a significant amount of suspended sediment is transported out of the estuary onto the adjacent continental shelf. The vertical profile of the residual (non-tidal) suspended sediment flux is similar to that of the residual current velocity, with a net upstream flux near the bottom and an overlying seaward-directed transport. The overall, depth-integrated result is a net seaward transport of suspended sediment out of the estuary. It appears that this net seaward transport varies directly with tidal amplitude.Aerial photography and water sampling indicate that during high river inflow, the downstream extremity of the turbidity maximum extends onto the continental shelf at ebb tide. The tidal and coastal current patterns of the inlet and inner shelf induce a northward transport of the turbid estuarine water, and at each tidal cycle, a certain amount of suspended sediment leaves the estuary; part of this sediment is deposited in a silt and clay zone on the continental shelf.