Seasonal Variability and Estuary–Shelf Interactions in Circulation Dynamics of a River-dominated Estuary

The long-term response of circulation processes to external forcing has been quantified for the Columbia River estuary using in situ data from an existing coastal observatory. Circulation patterns were determined from four Acoustic Doppler Profilers (ADP) and several conductivity–temperature sensors placed in the two main channels. Because of the very strong river discharge, baroclinic processes play a crucial role in the circulation dynamics, and the interaction of the tidal and subtidal baroclinic pressure gradients plays a major role in structuring the velocity field. The input of river flow and the resulting low-frequency flow dynamics in the two channels are quite distinct. Current and salinity data were analyzed on two time scales—subtidal (or residual) and tidal (both diurnal and semidiurnal components). The residual currents in both channels usually showed a classical two-layer baroclinic circulation system with inflow at the bottom and outflow near the surface. However, this two-layer system is transient and breaks down under strong discharge and tidal conditions because of enhanced vertical mixing. Influence of shelf winds on estuarine processes was also observed via the interactions with upwelling and downwelling processes and coastal plume transport. The transient nature of residual inflow affects the long-term transport characteristics of the estuary. Effects of vertical mixing could also be seen at the tidal time scale. Tidal velocities were separated into their diurnal and semidiurnal components using continuous wavelet transforms to account for the nonstationary nature of velocity amplitudes. The vertical structure of velocity amplitudes were considerably altered by baroclinic gradients. This was particularly true for the diurnal components, where tidal asymmetry led to stronger tidal velocities near the bottom.     

The Effects of Wind,Runoff and Tides on Salinity in a Strongly Tidal Sub-estuary

Measurements over an annual cycle of longitudinal and vertical salinity distributions in a small sub-estuary, the Tavy Estuary, UK, are used to illustrate the dependence of salt intrusion and stratification on environmental variables. The interpretations are aided by vertical profiling and near-bed data recorded in the main channel and on the mudflats. Generally, high water (HW) salt intrusion at the bed is close to the tidal limit and is dominated by runoff and winds, with decreasing salt intrusion associated with increasing runoff and increasing up-estuary winds (or vice versa). Tidal effects are not statistically significant because of two compensating processes: the long tidal excursion, which is comparable with the sub-estuary length for all but the smallest neap tides, and the enhanced, near-bed, buoyancy-driven salt transport that occurs at small neap tides close to the limit of saline intrusion. The effect of wind on HW surface salt intrusion in the main channel is not statistically significant, partly because it is obscured by the opposing local and estuary-wide effects of an up-estuary or down-estuary wind stress. These processes are investigated using a simple tidal model that incorporates lateral, channel–mudflat bathymetry and reproduces, approximately, observed channel and mudflat velocities. Surface salinity at HW increases with tidal range because of enhanced spring-tide vertical mixing—a process that also reduces salinity stratification. Stratification increases with runoff because of increased buoyancy inputs and decreases with up-estuary winds because of reduced near-bed salt intrusion. Stratification and plume formation are interpreted in terms of the bulk and estuarine Richardson Numbers, and processes at the confluence of the sub-estuary and main estuary are described.     

Modeling the Effects of Tidal Energy Extraction on Estuarine Hydrodynamics in a Stratified Estuary

A 3-D coastal ocean model with a tidal turbine module was used in this paper to study the effects of tidal energy extraction on temperature and salinity stratification and density-driven two-layer estuarine circulation. Numerical experiments with various turbine array configurations were carried out to investigate the changes in tidally averaged temperature, salinity, and velocity profiles in an idealized stratified estuary that connects to coastal water through a narrow tidal channel. The model was driven by tides, river inflow, and sea surface heat flux. To represent the realistic size of commercial tidal farms, model simulations were conducted based on a small percentage (less than 10 %) of the total number of turbines that would generate the maximum extractable energy in the system. Model results show that extraction of tidal in-stream energy will increase the vertical mixing and decrease the stratification in the estuary. Installation of in-stream tidal farm will cause a phase lag in tidal wave, which leads to large differences in tidal currents between baseline and tidal farm conditions. Extraction of tidal energy in an estuarine system has stronger impact on the tidally averaged salinity, temperature, and velocity in the surface layer than the bottom layer even though the turbine hub height is close to the bottom. Finally, model results also indicate that extraction of tidal energy weakens the two-layer estuarine circulation, especially during neap tides when tidal mixing is weakest and energy extraction is smallest.     

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.     

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.     

A modeling study of a tidal intrusion front and its impact on larval dispersion in the James River estuary, Virginia

A tidally-induced frontal system regularly develops in a small area off Newport News Point in the lower James River, one of the tributaries of the Chesapeake Bay. In conjunction with the front, a strong counter-clockwise eddy develops on the shoals flanking the northern side of the channel as the result of tidal interaction with the local bathymetry and estuarine stratification. A three-dimensional hydrodynamic model was applied to simulate the eddy evolution and front development, and to investigate time-varying circulation and material transport over a spring-neap tidal cycle. The model results show that variation of tidal range, together with periodic stratification-destratification of the estuary, has a significant impact on the residual circulation of the lower James River. The net surface water circulation, which takes the form of a counterclockwise eddy on the Hampton Flats, is stronger during neap tide than during spring tide. Strong stratification and weak flood current during neap tide results in a dominant ebb flow at the surface, which delays flooding within the channel and advances the phase lead of flood tide on shoals adjacent to the channel, thus increasing both period and intensity of the eddy. Front development in the area off Newport News Point provides a linkage between shoal surface water and channel bottom water, producing a strong net upriver bottom transport. The existence of the vertical transport mechanism was independently demonstrated through tracer experiments. The impact of the dynamics on larval dispersion was investigated through a series of model simulations of the movement of shellfish larvae over multiple tidal cycles following their release at selected bottom sites. These results show that eddy-induced horizontal circulation and vertical transport associated with the frontal system are important mechanisms for the retention of larval organisms in the James River.     

On the advection of turdidity in the saint lawrence middle estuary

The suspension transport away from the extensive turbidity zone of the St. Lawrence estuary is largely determined by the channel topography. The suspended sediments are advected downstream by a 40 km long turbid plume which takes its source in a turbidity maximum at the head of the estuary and flows downstream partly confined by the South Channel. During the ebbing phase of tidal cycles, the turbid waters of the plume are forced downstream through narrow converging sections of the channel, and slowed down through more opened diverging regions, particularly down-stream of the St. Roch Traverse. These, large fluctuations in stratification modulate the vertical transport of suspended material from the bottom to the surface layer. Midway down the estuary, dispersion of the plume occurs along a frontal zone which seasonally migrates 30 km or more in response to changes in fresh water discharge. The plume is reinforced and the turbidity gradient is intensified by local injections of inshore waters from Ste. Anne Bay, a subtidal platform highly enriched in suspended material by intertidal exchanges with large mudllats. Lateral erosion of the plume and cross-channel transport of suspended matter from the South into the North Channel is made possible by large horizontal shears developing in the central part of the middle estuary during the early flood. These are created by a one-hour tidal phase difference between the North and the South Channel.     

Circulation changes in the Sheepscot River estuary,maine, following removal of a causeway

A causeway which had restricted tidal flow in a portion of the Sheepscot River estuary was removed late in 1974. Flowmeter data from moored plankton nets fished over full tidal cycles, and salinity observations made in conjunction with the net sets, were used to evaluate the effects of causeway removal on circulation in the estuary. Tidal flows in the main channel increased by almost 50%. This increase was accompanied by substantial decreases in salinity stratification and in the strength of the gravitational circulation.     

长江口-杭州湾毗连海区的现代沉积速率

运用沉积地层同位素210 Pb、137Cs测年技术,估算长江口 杭州湾毗连海区的现代沉积速率。研究结果表明,以泗礁水道为界,东侧的马鞍列岛区受外海高盐水的顶托,长江来沙少,淤积缓慢;西侧至长江口、杭州湾海岸交汇点 (南汇咀 )之间水域,是长江泥沙扩散南下的主要通道。其中,受长江冲淡水次级锋和杭州湾锋面输沙影响的湾口浅滩区,沉积速率约 3cm/a;洋山港区,水深大于 20m的峡道区,受强动力作用而处于侵蚀状态,峡道两端出口的浅水区,沉积速率为0.3～ 1.6cm/a;马迹山港区,位于典型的峡道沿岸淤泥质陡坡,其沉积过程不稳定,正常沉积速率为 0.7～ 3.4cm/a,但常有快速淤积和地层滑塌流失现象发生.     

国外河口表面锋机制研究概述

回顾了国外河口锋面研究的最新成果,阐述了河口羽状锋、河口潮汐混合锋和河口切变锋的动力机制.河口羽状锋的机制研究以Garvine等人的观点最为特出.河口潮汐混合锋是由河口垂向环流中水体密度梯度所引起.河口切变锋是由滩槽流速切变引起的.     

Modeling of the Cape Fear River Estuary plume

The Environmental Fluid Dynamic Code, an estuarine and coastal ocean circulation model, is used to simulate the distribution of the salinity plume in the vicinity of the mouth of the Cape Fear River Estuary, North Carolina. The individual and coupled effects of the astronomical tides, river discharge, and atmospheric winds on the spatial and temporal distributions of coastal water levels and the salinity plume were investigated. These modeled effects were compared with water level observations made by the National Oceanic and Atmospheric Administration and salinity surveys conducted by the Coastal Ocean Research and Monitoring Program. Model results and observations of salinity distributions and coastal water level showed good agreement. The simulations indicate that strong winds tend to reduce the surface plume size and distort the bulge shape near the estuary mouth due to enhanced wind-induced surface mixing. Under normal discharge conditions, tides, and light winds, the southward outwelling plume veers west. Relatively moderate winds can mechanically reverse the flow direction of the plume. Under conditions of weak to moderate winds the water column does not mix vertically to the bottom, while in strong wind cases the plume becomes vertically well mixed. Under conditions of high river discharge the plume increases in size and reaches the bottom. Vertical mixing induced by strong spring tides can also enable the plume to reach the bottom.     

Characteristics of a Shallow River Plume: Observations from the Saco River Coastal Observing System

Interest in the coastal dynamics of river plumes has mainly focused on large rivers, but plumes from the more numerous smaller rivers have important local consequences and may, in aggregate, be significant contributors to coastal circulation. We studied the dynamics of the plume from the Saco River in Saco Bay, Gulf of Maine, over a 3-year period. The transport and salinity in the region are governed by river discharge, tides, winds, and interaction with the Western Maine Coastal Current. The dynamics of the flow field vary with location within the plume and discharge. The far-field dynamics of the Saco River plume are dominated by inertial processes (hence qualifying it as a small-scale river plume), during times of low discharge, with low salinity water present both up and downstream of the river mouth, but are dominated by rotational processes during times of high discharge (thus qualifying it as a large-scale river plume), with buoyant water primarily advected downshelf. Near-field dynamics are governed by weak, subcritical flow during low discharge but strongly inertial, supercritical flow during high discharge. Offshore movement of the plume is not governed by Ekman dynamics but is instead a result of discharge, wind-induced vertical mixing, and the geography of the coastline and adjacent islands.     

Toward a unified theory of tidally-averaged estuarine salinity structure

Equations are developed for the tidally-averaged, width-averaged estuarine salinity and circulation in a rectangular estuary. Width and depth may vary along the length of the channel, as may coefficients of vertical turbulent mixing and along channel diffusion. The system is reduced to a single first-order, nonlinear, ordinary differential equation governing the section-averaged salinity. A technique for specifying the ocean boundary condition is given, and solutions are found by numerical integration. Under different assumptions for the diffusion it is possible to reproduce the few existing analytical solutions, in particular the Hansen and Rattray (1965) Central Regime solution, and Chatwin's (1976) solution. The mathematical framework allows easy comparison of the results of different channel geometries and mixing coefficients. Of particular interest is the along-channel distribution of the diffusive fraction of up-estuary salt flux. It is shown that the Hansen and Rattray solution is always diffusion-dominated near the mouth. A theory is presented for estimating the diffusion coefficient within a tidal excursion of the mouth. It is shown that the resulting rapid along-channel increase of diffusion may explain some observed patterns of salinity structure: a decrease in both stratification and along-channel salinity gradient near the mouth. The theory is applied to the Delaware Estuary and Northern San Francisco Bay, and shows reasonable agreement with observed sensitivities of salt intrusion distance to river flow.     

Transport and Dispersion of a Conservative Tracer in Coastal Waters with Large Intertidal Areas

In late December 1991, an accidental release of 5,700 CI of tritiated water (HTO) from the Savannah River Site was transported via site streams into the Savannah River where it was carried downstream to the coastal zone. HTO released into a semitropical Georgia estuary was forced into the tidal marshes surrounding the estuary as well as discharged directly into the Atlantic Ocean. The spreading of HTO was studied with a 3D hydrodynamic model (ALGE) that includes flooding and draining of intertidal areas. Comparisons of model simulations to measured HTO concentration showed that ALGE simulated well the general increase and decrease of HTO as its plume passed a given area. The “sheet flow” approximation for marsh and small tidal creek flow largely compensated for lack of model resolution and accurate bathymetry in areas with numerous small to medium-sized tidal creeks. The water volume of the unresolved tidal creeks had to be accounted for in the simulations by increasing the initial water depth over the marshes. ALGE and a simple box model both reproduced the trapping of HTO in intertidal areas. The time scale over which intertidal areas import and export HTO back to the tidal channels varies between 10 and 30 days.     

Distortion of tidal currents and the lateral transfer of salt in a shallow coastal plain estuary (O estuário do Mira, Portugal)

We describe the tidal circulation of a coastal plain estuary across a flood tide delta located at its entrance. The area connects the downstream portion of the main estuary extending 30–40 km inland to the more complex delta reach that consists of a shallow main channel and a series of smaller side channels. The delta acts as a frictionally dominated zone that modifies the tidal wave from a simple sinusoid to one with ebb currents that accelerate to maximum early in the tidal cycle and last more than one-half of the tidal cycle. Along smaller side channels, the tidal currents favor stronger flood or ebb currents, depending upon the local surrounding morphology. The phase difference between ebb currents in the small channels relative to those in the main channel cause some of the salt to be retained thus reducing the tendency of freshwater discharge to flush salt out of the system. This mechanism of retention differs from the selective withdrawal mechanism described for this estuary in Blanton et al. (2000).     

珠江口磨刀门枯季水文特征及河口动力过程

根据磨刀门2003年12月9~15日的大、中潮同步水文观测资料,分析了磨刀门枯季的潮汐、潮流、余流、悬移质含沙量、盐度等水文特征,并对枯季河口动力过程,如咸淡水混合、河口射流等进行了初步研究。在枯季由于径流较弱,潮流成为主要动力。表层由于受径流和风的影响余流基本上沿河道走向向下游,中层以下有稳定的向上的余流存在。枯季磨刀门含沙量较小(<1 kg/m3),盐度在平面上和垂向上均有一定变化。磨刀门枯季咸淡水混合类型为缓混合型,各站盐度分层参数均在0.01~1.0。从实测流速的分布情况来看,河口下层有反向的水流,存在明显的因密度差而形成的密度环流。根据枯季实测资料计算所得的密度弗劳德数,磨刀门枯季以浮力射流为主。     

Impact of vertical structure on water mass circulation in a tropical lagoon (Ebrié, Ivory Coast)

A one-dimensional vertical model has been developed to simulate the water mass circulation along the vertical structure in all deep coastal areas. The model has hydrodynamic and transport components solved using finite difference scheme. The one-dimensional vertical model results are coupled to the vertically averaged two-dimensional model results at each point of a horizontal grid. A theoretical salinity profile is introduced for each vertically integrated value obtained from the 2DH model results. A viscosity profile, simulating a viscosity value close to zero at the surface and with large viscosity gradients, is applied along the water column. The model is applied to the Vridi channel, connecting the Ebrié lagoon to the sea (Ivory Coast).The response of the Ebrié lagoon is studied in terms of inflow and outflow of water in the system through the Vridi channel. Due to the abrupt variation of the surface slope, vertical velocities along the water column show an anticlockwise spiral from bottom to surface during a tidal cycle. Due to the bottom friction and to the vertical viscosity profile, velocities decrease from surface to bottom. However, the freshwater inflow slows down the tidal propagation during the flood and causes the surface velocity to be smaller than the bottom velocity at mid-tide. Close to the bottom, velocities follow an anticlockwise movement due to the tidal propagation. At the water surface, velocities follow only an alternative movement of either ebb or flood, along the channel direction. No cross shore velocities can develop at the surface in the channel.     

Manganese and suspended matter in the Yaquina Estuary,Oregon

The longitudinal distribution of total suspended matter and total, dissolved, and particulate manganese in a small coastal plain estuary is described. The distribution of manganese is a consequence of estuarine circulation; a within-estuary maximum is inversely correlated with river flow, and is a function of residence time in the estuary, resuspension in the upper estuary, and desorption from particles introduced from within the estuary or from the river. The turbidity maximum is similarly most pronounced during low river flows. The upper estuary (salinity <15‰), comprising a small percentage of the total estuary volume during low flow, receives material from the river and along the bottom from the lower estuary; this material is returned to the water column by resuspension and desorption from estuarine and riverine particles. The lower estuary tends to damp out these processes because of the greater volume and (residence) time available for mixing.     

山溪性强潮河口最大浑浊带形成机制及其模拟

为阐明强潮河口最大浑浊带的形成机制及其运动规律,通过瓯江和椒（灵）江实测资料分析,系统分析了强潮河口最大浑浊带形成的影响因素及其与河口地貌的响应关系。考虑黏性细颗粒泥沙运动特性和盐度的影响,开发了强潮河口最大浑浊带数学模型,对椒（灵）江枯季大潮最大浑浊带运移过程进行了模拟。结果表明:①强潮河口最大浑浊带是潮波变形、咸淡水混合、泥沙再悬浮等复杂因素在一定河口边界和泥沙条件下相互作用的产物,潮波变形和泥沙供给是影响最大浑浊带形成的关键因素。②强潮河口最大浑浊带模拟必须充分考虑潮流、盐淡水混合、泥沙周期性起动、絮凝和沉积密实等因素,所建立的数学模型可用于强潮河口最大浑浊带研究。     

Modeling the Hydrodynamic and Morphologic Response of an Estuary Restoration

Estuary evolution is investigated using the hydrodynamic and sediment transport model, Delft3D, to study the response of a dammed tidal basin to restored tidal processes. The development of decadal (10-year) morphological simulations of the restored estuary required simplifying several data inputs and implementing a time-scale acceleration technique. An innovative river sediment discharge schematization was developed that connected sediment discharge to morphological change in the estuary. Mud erodibility parameters were determined from laboratory analysis of sediment cores from the modern lakebed and statistical refinement with a Bayes network of the probability of occurrence. The changing estuary morphology appears to have a dominant impact on the physical habitat (substrate, inundation frequency, mean salinity, and salinity range). The numerical model provides a tool to compare the functions of the historical estuary and possible future alternatives for a restored estuary. Sensitivity of the morphological model to sediment types and erodibility parameters was also examined. A conceptual model covering morphology and indicators of physical habitat for three phases of estuary evolution during restoration is presented that could be applied to estuarine systems that are severely out of equilibrium.