Transport in the Hudson estuary: A modeling study of estuarine circulation and tidal trapping

The effects of estuarine circulation and tidal trapping on transport in the Hudson estuary were investigated by a large-scale, high-resolution numerical model simulation of a tracer release. The modeled and measured longitudinal profiles of surface tracer concentrations (plumes) differ from the ideal Gaussian shape in two ways: on a large scale the plume is asymmetric with the downstream end stretching out farther, and small-scale (1–2 km) peaks are present at the upstream and downstream ends of the plume. A number of diagnostic model simulations (e.g., remove freshwater flow) were performed to understand the processes responsible for these features. These simulations show that the large-scale asymmetry is related to salinity. The salt causes an estuarine circulation that decreases vertical mixing (vertical density gradient), increases longitudinal dispersion (increased vertical and lateral gradients in longitudinal velocities), and increases net downstream velocities in the surface layer. Since salinity intrusion is confined to the downstream end of the tracer plume, only that part of the plume is effected by those processes, which leads to the largescale asymmetry. The small-scale peaks are due to tidal trapping. Small embayments along the estuary trap water and tracer as the plume passes by in the main channel. When the plume in the main channel has passed, the tracer is released back to the main channel, causing a secondary peak in the longitudinal profile.     

Nitrogen budget for low and high freshwater inflows, Nueces Estuary, Texas

Nueces Estuary is a relatively shallow, microtidal estuary which receives inputs from a significant industrial and urban area and from a semi-arid drainage basin. Nitrogen (N) loadings were compiled using FLUX model and flow x concentration calculations for four annual periods spanning a range of inflow volumes. For each annual period, N budgets were developed, supported by water and total dissolved solids dudgets. Budget compilations include materials transported in tidal exchange with the Gulf of Mexico and with neighboring bays. Wastewater discharge contributes twice as much N to the bay than inflows from the Nueces River. Net flows and tidal entrainment of materials from neighboring bays contribute 24–32% of total loadings except during flood flows of the Nueces River. Atmospheric deposition to the estuary surface provides 8% of N loading. Despite low tidal amplitude and a restricted Gulf pass tidal entrainment losses are the main vehicle of export, except during flood flows, when net flows to the Gulf become important. The system exports much more of its total N load than would be expected from its water residence time, possibly facilitated by ship channel longitudinal transport. Denitrification accounts for 30–40% of all N lost. Uncertainties in components of the budget are estimated and included in compilations of confidence bounds for N budget components.     

Effects of Tidal-Forcing Variations on Tidal Properties Along a Narrow Convergent Estuary

A 1D analytical framework is implemented in a narrow convergent estuary that is 78 km in length (the Guadiana, Southern Iberia) to evaluate the tidal dynamics along the channel, including the effects of neap-spring amplitude variations at the mouth. The close match between the observations (damping from the mouth to ～ 30 km, shoaling upstream) and outputs from semi-closed channel solutions indicates that the M₂ tide is reflected at the estuary head. The model is used to determine the contribution of reflection to the dynamics of the propagating wave. This contribution is mainly confined to the upper one third of the estuary. The relatively constant mean wave height along the channel (<?10% variations) partly results from reflection effects that also modify significantly the wave celerity and the phase difference between tidal velocity and elevation (contradicting the definition of an “ideal” estuary). Furthermore, from the mouth to ～ 50 km, the variable friction experienced by the incident wave at neap and spring tides produces wave shoaling and damping, respectively. As a result, the wave celerity is largest at neap tide along this lower reach, although the mean water level is highest in spring. Overall, the presented analytical framework is useful for describing the main tidal properties along estuaries considering various forcings (amplitude, period) at the estuary mouth and the proposed method could be applicable to other estuaries with small tidal amplitude to depth ratio and negligible river discharge.     

Flux of particulate matter in the tidal marshes and subtidal shallows of the Rhode River estuary

There was a net influx of suspended particulate matter to the uppermost part of the Rhode River estuary during the several years of this study. Most of the influx was due to episodic discharges of suspended sediment from the watershed during heavy rains. In contrast, tidal exchange of particulate matter was not related to rainstorms. Sediment composition data and historical records indicate that marsh accretion accounts for only 13% of the sediment trapping although marshes occupy 60% of the study area. Influx of particulate matter to the marshes is directly related to the amount of time they are submerged during tidal cycles.     

Residual Exchange Flows in Subtropical Estuaries

Observations of residual exchange flows at the entrance to four subtropical estuaries, two of them semiarid, indicate that these flows are mainly tidally driven, as they compare favorably with theoretical patterns of tidal residual flows. In every estuary examined, the tidal behavior was that of a standing or near-standing wave, i.e., tidal elevation and tidal currents were nearly in quadrature. The pattern of exchange flow that persisted at every estuary exhibited inflow in the channel and outflow over the shoals. Curiously, but also fortuitously, this pattern coincides with the exchange pattern driven by density gradients in other estuaries. The tidal stresses and the residual elevation slopes should be the dominant mechanisms that drive such tidal residual pattern because the Stokes transport mechanism is negligible for standing or near-standing waves. Time series measurements from the semiarid estuaries showed fortnightly modulation of the residual flow by tidal forcing in such a way that the strongest net exchange flows developed with the largest tidal distortions, i.e., during spring tides. This modulation is opposite to the modulation that typically results in temperate estuaries, where the strongest net exchange flows tend to develop during neap tides. The fortnightly modulation on tidal residual currents could be inferred from previous theoretical results because residual currents arise from tidal distortions but is made explicit in this study. The findings advanced herein should allow the drawing of generalities about exchange flow patterns in subtropical estuaries where residual flows are mainly driven by tides.     

长江口泥沙运动形式分类及对深水航道淤积影响

潮汐河口泥沙运动复杂多变,科学划分泥沙运动形式并评估其对航道淤积的影响,是厘清航道淤积泥沙来源、制定有效减淤措施的关键。基于长江口深水航道所处南港—北槽河段2015年和2018年洪季、枯季表层沉积物和近底悬沙的现场采样数据,分析提出潮汐条件下推移质、悬移质和时推时悬泥沙3类泥沙运动形式的粒径划分方法,量化3类泥沙对深水航道淤积的贡献比例。结果表明：近底悬沙级配曲线上拐点粒径对泥沙由推移质向悬移质转化具有较好的指示意义;长江口南港—北槽悬沙、底沙交换显著,深水航道淤积物中除仅做推移质或悬移质运动的泥沙外,还包括大量的时推时悬泥沙,其在航道淤积泥沙中的占比最高,约达50%～60%;南港段航道洪季、枯季推移质淤积占比分别为36%和26%,高于悬移质的6%和13%;北槽段航道悬移质落淤泥沙占比为44%～48%,明显较推移质3%～6%的占比高。3类泥沙运动形式粒径划分方法为深化潮汐河口泥沙运动规律认识、判别航道淤积泥沙来源提供了新途径。     

Thermal Variability in a Tidal River

In this paper, we discuss observations of temperature variability in the tidal portion of the San Joaquin River in California. The San Joaquin River makes up the southern portion of the Sacramento San Joaquin Delta, the eastern end of San Francisco Bay. Observations made in August 2004 and August 2005 show significant diurnal variations in temperature in response to surface heat exchange. However, to account for observed changes in heat content a sizeable downstream heat flux (approximately 100 W m⁻²) must be added to the surface heat flux. To account for this flux via Fickian dispersion, a flow-dependent dispersion coefficient varying from 500 to 4,000 m² s⁻¹ is needed. These values are much larger than would be predicted for a river of this size, suggesting that the complex topology of the Delta greatly enhances longitudinal dispersion. Building on these observations, we present a simple theory that explores how the subtidal temperature field varies in response to changes in flow rate, dispersion, and heat exchange.     

Flow paths of water and sediment in a tidal marsh: Relations with marsh developmental stage and tidal inundation height

This study provides new insights in the relative role of tidal creeks and the marsh edge in supplying water and sediments to and from tidal marshes for a wide range of tidal inundation cycles with different high water levels and for marsh zones of different developmental stage. Net import or export of water and its constituents (sediments, nutrients, pollutants) to or from tidal marshes has been traditionally estimated based on discharge measurements through a tidal creek. Complementary to this traditional calculation of water and sediment balances based on creek fluxes, we present novel methods to calculate water balances based on digital elevation modeling and sediment balances based on spatial modeling of surface sedimentation measurements. In contrast with spatial interpolation, the presented approach of spatial modeling accounts for the spatial scales at which sedimentation rates vary within tidal marshes. This study shows that for an old, high marsh platform, dissected by a well-developed creek network with adjoining levees and basins, flow paths are different for tidal inundation cycles with different high water levels: during shallow inundation cycles (high water level <0.2 m above the creek banks) almost all water is supplied via the creek system, while during higher inundation cycles (high water level >0.2 m) the percentage of water directly supplied via the marsh edge increases with increasing high water level. This flow pattern is in accordance with the observed decrease in sedimentation rates with increasing distance from creeks and from the marsh edge. On a young, low marsh, characterized by a gently seaward sloping topography, material exchange does not take place predominantly via creeks but the marsh is progressively flooded starting from the marsh edge. As a consequence, the spatial sedimentation pattern is most related to elevation differences and distance from the marsh edge. Our results imply that the traditional measurement of tidal creek fluxes may lead in many cases to incorrect estimations of net sediment or nutrient budgets.     

Vegetation and peat accumulation steer Holocene tidal–fluvial basin filling and overbank sedimentation along the Old Rhine River,The Netherlands

In the transformation from tidal systems to freshwater coastal landscapes, plants act as eco-engineering species that reduce hydrodynamics and trap sediment, but nature and timing of the mechanisms of land creation along estuaries remains unclear. This article focuses on the Old Rhine estuary (The Netherlands) to show the importance of vegetation in coastal landscape evolution, predominantly regarding tidal basin filling and overbank morphology. This estuary hosted the main outflow channel of the river Rhine between ca 6500 to 2000 cal bp , and was constrained by peat during most of its existence. This study reconstructs its geological evolution, by correlating newly integrated geological data and new field records to varying conditions. Numerical modelling was performed to test the inferred mechanisms. It was found that floodbasin vegetation and resulting organic accumulation strongly accelerated back-barrier infill, by minimizing tidal influence. After tidal and wave transport had already sufficiently filled the back-barrier basin, reed rapidly expanded from its edges under brackish conditions, as shown by diatom analysis and datings. Reed growth provided a positive infilling feedback by reducing tidal flow and tidal prism, accelerating basin infilling. New radiocarbon dates show that large-scale crevassing along the Old Rhine River – driven by tidal backwater effect – only started as nutrient-rich river water transformed the floodbasin into an Alder carr in a next phase of estuary evolution. Such less dense vegetation promotes crevassing as sediments are more easily transported into the floodbasin. As river discharge increased and estuary mouth infilling progressed, crevasse activity diminished around 3800 to 3000 cal bp , likely due to a reduced tidal backwater effect. The insights from this data-rich Holocene study showcase the dominant role that vegetation may have in the long-term evolution of coastal wetlands. It provides clues for effective use of vegetation in vulnerable wetland landscapes to steer sedimentation patterns to strategically adapt to rising water levels.     

Longitudinal dispersion processes in the upper tamar estuary

Measurements of velocity, salinity, and suspended solids concentration have been used to investigate the intra-tidal variation of vertical and transverse shear-induced dispersion. For the study research the interaction of the longitudinal density gradient and vertical shear during the early part of the ebb tide accounted for much of the net longitudinal dispersion of solute landward. The same mechanism also is shown to lead to a net particulate transport landward. The landward flux, however, takes place during the flood tide. The field data are also used to elucidate the tidally averaged tidal pumping mechanism.     

Effect of a broad shallow sill on tidal circulation and salt transport in the entrance to a coastal plain estuary (Mira—Vila Nova de Milfontes, Portugal)

We describe the tidal circulation and salinity regime of a coastal plain estuary that connects to the ocean through a flood tide delta. The delta acts as a sill, and we examine the mechanisms through which the sill affects exchange of estuarine water with the ocean. Given enough buoyancy, the dynamics of tidal intrusion fronts across the sill and selective withdrawal (aspiration) in the deeper channel landward appear to control the exchange of seawater with estuarine water. Comparison of currents on the sill and stratification in the channel reveals aspiration depths smaller than channel depth during neap tide. During neap tide and strong vertical stratification, seawater plunges beneath the less dense estuarine water somewhere on the sill. Turbulence in the intruding bottom layer on the sill promotes entrainment of fluid from the surface layer, and the seawater along the sill bottom is diluted with estuarine water. During ebb flow, salt is effectively trapped landward of the sill in a stagnant zone between the aspiration depth and the bottom where it can be advected farther upstream by flood currents. During spring tide, the plunge point moves landward and off the sill, stratification is weakened in the deep channel, and aspiration during ebb extends to the bottom. This prevents the formation of stagnant water near the bottom, and the estuary is flooded with high salinity water far inland. The neapspring cycle of tidal intrusion fronts on flood coupled with aspiration during ebb interacts with the sill to play an important role in the transport and retention of salt within the estuary.     

Nontidal circulation and mixing processes in the lower Potomac estuary

An analysis of the vertical structure of nontidal longitudinal currents and salinity in a reach of the lower Potomac River Estuary suggests that values for vertical eddy viscosity and eddy diffusivity scale with water depth H, tidal current amplitude U and bulk Richardson number according to conventional empirical formulas. However, the constant which relates the vertical eddy coefficients under conditions of neutral stability to UH is found to be an order of magnitude less than that expected for tidal conditions. Analyses also suggest that the degree of enhancement of longitudinal dispersion by the shear effect associated with the nontidal currents is a strong function of bulk Richardson number.     

曹娥江出口江道的演变与整治

揭示了曹娥江出口江道演变的影响因子:既有自然的,也有人为的,主要包括地转偏向力、径流量、来沙量和人类活动等.认为曹娥江出口江道的频繁摆动在很大程度上受制于钱塘江主槽的变迁和南股槽的变化.从兼顾防洪排涝和两岸围涂造地考虑,曹娥江出口江道以出东北方案为佳.提出了"因势利导,围涂治江"的整治原则.堵截南股槽可以稳定钱塘江主槽走向,进而稳定曹娥江出东北的走向,同时也可增加围涂面积.     

Impact of Channel Deepening on Tidal and Gravitational Circulation in a Highly Engineered Estuarine Basin

Deepening of estuarine channels is a common practice to ensure navigation. Here, we investigate whether such deepening impacts physical processes such as the strength of the estuarine exchange flow, the horizontal salinity gradient, and tidal dynamics. We analyze recent and historical hydrodynamic observations in Newark Bay, New Jersey, to assess the effect of channel deepening on tides, circulation, and salinity. The Bay’s navigational channel has undergone significant deepening, from 3 to 10 m in the nineteenth century to ~16 m today. Observations presented here include sea-level data from the nineteenth, twentieth, and twenty-first century, and moored Doppler current data and bottom salinity measurements made over the past 20 years. Results show a doubling of the estuarine exchange flow, a slight increase in salinity and in the horizontal salinity gradient, a decrease in tidal current amplitude, and a spatially variable change in the tidal range. The doubling of the exchange flow is consistent with the Hansen and Rattray scaling provided that the horizontal salinity gradient is unable to fully adjust landward because the dredging is limited to a short reach of the estuary. However, uncertainty in channel depth leaves open the possibility that the exchange flow is also augmented by an increase in the horizontal salinity gradient and/or a reduction in vertical mixing. Nevertheless, results demonstrate that a relatively small (15%) increase in depth appears to have doubled the exchange flow. We believe that this result is relevant to other systems where dredging is limited to a short reach of an estuary.     

Environmental control on diverse stromatolite morphologies in the 3000 Myr Pongola Supergroup, South Africa

A unique outcrop of partly silicified dolomite in the White Umfolozi section of the Pongola Supergroup, South Africa indicates that stromatolites were diverse and adapted to a range of shallow, tidal depositional settings 3000 Myr ago. Composite columnar stromatolitic bioherms 0.7-1.6m high and 0.4-1.0m in diameter formed along the margins of a tidal channel. They were flanked, away from the channel, by flat stratiform and small domical stromatolites growing in low energy tidal flat environments. Conical stromatolites, 0.05-0.30m high and 0.03-0.10m in diameter, accreted in high-energy coarse-grained carbonate sand along the bottom of the tidal channel. The stromatolites probably formed through the activities of filamentous, oxygen-producing, photoautotrophic cyanobacteria.     

Exchange Between an Estuary and an Intertidal Marsh and Slough

The hydrodynamic characteristics of small, intertidal perimeter habitats make flushing and residence times in these environments difficult to quantify using conventional approaches. The flooding and draining of intertidal shallows surrounding small perimeter sloughs result in large volume changes relative to total system volume during each tidal cycle. In such environments, an Eulerian framework of flushing and residence time may not be the best approach for quantifying tidal exchange; thus, alternative approaches should be considered in analyzing hydrodynamic exchange in small perimeter habitats. In this study, the results of applying such an approach to a small intertidal perimeter slough in South San Francisco Bay are presented. Previous work has shown that hydrodynamic exchange in an estuarine system can be analyzed by making Eulerian measurements of hydrodynamic fluxes and binning them according to salinity and temperature classes, thus providing a quasi-Lagrangian method of analyzing exchange and transport in an estuarine system. We apply a method which uses this approach to estimate the volumetric exchange ratio M, which is used to estimate the tidal exchange within an estuary during each tidal cycle. We find that the estimation of volumetric exchange ratios and the calculation of hydrodynamic residence times in estuarine systems can be complicated by mixing conditions associated with very strong tidal forcing, particularly in small-volume systems such as small perimeter sloughs, where the tidal prism can be on the scale of or greater than the total system volume.     

Estimation of runoff distribution within river systems: Application to the Lena Basin (Siberia)

A channel account approach is proposed to estimate longitudinal changes in runoff along large river systems. This new method provides a quantitative basis for describing the fluvial transport of suspended particulate material and dissolved substances. This method includes an evaluation of basic elements of water balance in separate sections of the river network and subsequent correction of channel accounting equations for the entire system using a maximum likelihood principle. To calculate water discharges of tributaries that have no hydrological information, structural analysis of river network is performed. This approach provides less error in comparison with traditional methods of estimating lateral inflow. The method is used to trace water discharge with increasing distance along the Lena river basin and to evaluate the contribution of geologically and lithologically uneven sub-basins in water discharge formation during a summer low water period.     

现代沉积构造的水动力解析尝试--潮流沉积一例

在美国西海岸的一个小河口湾中沉积动力观测系统(SDS)进行现场试验之际,采集了四个箱式柱样,经过X光透射照相和粒度分析、判读分析等工作,获得了该处沉积作用的动态分析成果:1.确定出潮流活动层厚及其影响的因素。2.鉴定出涨潮流层与退潮流层。3.推移质沉积与悬移质沉积。4.确定出底层潮流中存在着优势流,且定为涨潮流。5.确定出该潮流的流速大小。以上解析结果都为现场SDS实测资料所证实是正确的,进一步求得动态的具体数据,最后建立了一个潮汐通道的简单沉积模式。     

梯形断面明渠中纵向离散系数研究

基于最大信息熵原理,提出了一种确定梯形断面纵向流速分布的方法,研究了梯形断面明渠中流动横向不均匀和垂向不均匀对纵向离散的影响,建立了一个针对梯形断面明渠流动的纵向离散系数计算公式。公式将纵向离散系数与反映断面流速分布不均匀和壁面影响的参数建立联系,机理更加清楚,预测结果与其他学者相应的实验结果吻合良好,该方法理论推导过程严密,不依赖于特定的试验结果或实际测量资料,为梯形断面明渠的污染物混合输移过程中参数确定提供了有效的方法和途径。