共查询到20条相似文献，搜索用时 15 毫秒
1.
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. 相似文献
2.
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. 相似文献
3.
Control of estuarine stratification and mixing by wind-induced straining of the estuarine density field 总被引：2，自引：0，他引：2
Observations from the York River Estuary, Virginia, demonstrate that the along-channel wind plays a dominant role in governing the estuarine exchange flow and the corresponding increase or decrease in vertical density stratification. Contrary to previous findings that suggest wind stress acts predominantly as a source of energy to mix away estuarine stratification, our results demonstrate that the wind can play a more important role in straining the along-channel estuarine density gradient. Down-estuary winds enhance the tidally-averaged vertical shear, which interacts with the along-channel density gradient to increase vertical stratification. Up-estuary winds tend to reduce, or even reverse the vertical shear, reducing vertical stratification. In two experiments each lasting approximately a month, the estuarine exchange flow was highly correlated with the along-channel component of the wind. The changes in stratification caused by the exchange flow appear to control the amount of vertical mixing as parameterized by the vertical eddy viscosity. The degree of stratification induced by wind straining also appears to play an important role in controlling the effectiveness of wind and tidal mixing. 相似文献
4.
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. 相似文献
5.
Periodic frontogenesis in a region of freshwater influence 总被引：1，自引：0，他引：1
Observations are presented from a series of three conductivity-temperature-depth (CTD) surveys of the salinity and temperature structure of Liverpool Bay, a region that is strongly influenced by the input of fresh water from the rivers of northwest England. The surveys demonstrate the development, seaward movement, and eventual decay of a haline front. The frontogenesis is driven by the relaxation of a freshwater-induced horizontal density gradient following the decrease in tidal range at neap tides. It results in the area of Liverpool Bay being stratified for a period of 8 d before the increase in tidal mixing as the spring tide approaches returns the region to its initial vertically mixed state. In Liverpool Bay this process usually repeats on the spring-neap cycle, though strong wind-mixing may prevent the frontogenesis and subsequent stratification. Analysis with a 1-dimensional numerical model suggests that relaxation of an initially nonlinear horizontal density field, creating the front, is triggered by the stability produced by tidal straining of the water column during the ebb half-cycle. The reduction in tidal mixing energy approaching neap tide does not lead to frontogenesis without this initial stability. Such a regular stratification signal will have a marked effect on the local environment. The periodic frontogenesis will act as a tidal pump, moving buoyant substances in the water column offshore, while the onshore residual currents lower in the water column will more deeper dissolved substances inshore. The cycling of stability on the springneap time scale is considerably faster than the seasonal cycle of thermal stratification in the shelf seas, but is similar in creating the conditions required for phytoplankton blooms. Conditions favorable for enhanced primary production may therefore occur frequently in such regions of freshwater influence. 相似文献
6.
To assess the effects of tidal energy extraction on water quality in a simplified estuarine system, which consists of a tidal bay connected to the coastal ocean through a narrow channel where energy is extracted using in-stream tidal turbines, a three-dimensional coastal ocean model with built-in tidal turbine and water quality modules was applied. The effects of tidal energy extraction on water quality were examined for two energy extraction scenarios as compared with the baseline condition. It was found, in general, that the environmental impacts associated with energy extraction depend highly on the amount of power extracted from the system. Model results indicate that, as a result of energy extraction from the channel, the competition between decreased flushing rates in the bay and increased vertical mixing in the channel directly affects water quality responses in the bay. The decreased flushing rates tend to cause a stronger but negative impact on water quality. On the other hand, the increased vertical mixing could lead to higher bottom dissolved oxygen at times. As the first modeling effort directly aimed at examining the impacts of tidal energy extraction on estuarine water quality, this study demonstrates that numerical models can serve as a very useful tool for this purpose. However, more careful efforts are warranted to address system-specific environmental issues in real-world, complex estuarine systems. 相似文献
7.
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。 相似文献
8.
Bo G. Gustafsson 《Estuaries and Coasts》2000,23(2):231-252
A prognostic model for the estuarine circulation in the Baltic entrance area is described. The model is based on the work by Stigebrandt (1983) and is built of a number of flow-regulating physical processes and forced by oceanic sea level, local meteorological conditions, and freshwater supply to the Baltic. It resolves the horizontal variations by dividing the entrance area into 7 sub-basins. The vertical stratification in each sub-basin evolves with time due to mixing, diffusion, and water exchange with adjacent basins. Instead of using a fixed vertical grid, the stratification is described by a variable number of layers created by inflowing water and by a pycnocline retreat near the surface. The model is a highly cost-effective tool compared to high-resolution 3D-models since computations are 105–106 times faster. Still, the model reproduces the stratification on time-scales longer than a few days as verified by comparison with observed time-series. The magnitude of the simulated average estuarine circulation conforms well to independent estimates of the water exchange. The model is used to quantify the temporal and horizontal variability of circulation, mixing, and diffusion. Long-term average rate of work against the buoyancy forces by entrainment and diffusion is calculated for each sub-basin. The highest rates of work against the buoyancy forces by diffusion are found in the northern Kattegat and in the Fehmarn Belt while the lowest rate is found in the Öresund. The total mixing in terms of transformation of water masses is also quantified. The average residence times for surface and deep water are estimated for each subbasin. Residence times longer than 1 mo are found in Fehmarn Belt and in the deep water of southern Kattegat. In other parts of Kattegat and the Samsø Belt the residence times are 1 to 2 wk for surface water and 2 to 3 wk for deep water. The age of the water is defined as the time spent since a particular water mass was in contact with either the sea surface or with the vertical model boundaries has been estimated. By comparing the age distribution in the Kattegat with observations of oxygen concentration, it is concluded that the variability of the ventilation of deeper layers is of less importance to the occurrence of low oxygen concentrations compared to other factors such as interannual variability in primary production. 相似文献
9.
Sarah N. Giddings Derek A. Fong Stephen G. Monismith C. Chris Chickadel Kathleen A. Edwards William J. Plant Bing Wang Oliver B. Fringer Alexander R. Horner-Devine Andrew T. Jessup 《Estuaries and Coasts》2012,35(2):665-681
Estuarine fronts are well known to influence transport of waterborne constituents such as phytoplankton and sediment, yet
due to their ephemeral nature, capturing the physical driving mechanisms and their influence on stratification and mixing
is difficult. We investigate a repetitive estuarine frontal feature in the Snohomish River Estuary that results from complex
bathymetric shoal/channel interactions. In particular, we highlight a trapping mechanism by which mid-density water trapped
over intertidal mudflats converges with dense water in the main channel forming a sharp front. The frontal density interface
is maintained via convergent transverse circulation driven by the competition of lateral baroclinic and centrifugal forcing.
The frontal presence and propagation give rise to spatial and temporal variations in stratification and vertical mixing. Importantly,
this front leads to enhanced stratification and suppressed vertical mixing at the end of the large flood tide, in contrast
to what is found in many estuarine systems. The observed mechanism fits within the broader context of frontogenesis mechanisms
in which varying bathymetry drives lateral convergence and baroclinic forcing. We expect similar trapping-generated fronts
may occur in a wide variety of estuaries with shoal/channel morphology and/or braided channels and will similarly influence
stratification, mixing, and transport. 相似文献
10.
磨刀门水道盐度混合层化机制 总被引：3，自引：0，他引：3
基于Simpson方法和磨刀门水道2009年枯季水文实测资料,选取上、下游两个站位的径流层化、潮汐混合、风致扰动3个影响河口水体分层的主要因素进行盐度混合的层化机制分析。研究表明:由于M1站位处上游,径流作用相对占优,分层不明显,只在涨潮急流时出现微弱的盐度分层;M2站则水体分层明显,小潮期间径流作用占主导,水体呈持续性分层,当由小潮转为中潮后,潮流作用增强,出现周期性分层现象,大潮以后,由于上游径流增加,潮流与径流作用相当,仍为周期性分层,但分层有所加强。层化的发育程度依赖径流致层化作用与潮汐、风致混合作用的博弈。 相似文献
11.
Arun Chawla David A. Jay António M. Baptista Michael Wilkin Charles Seaton 《Estuaries and Coasts》2008,31(2):269-288
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. 相似文献
12.
13.
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. 相似文献
14.
为研究磨刀门盐水混合层化特征,基于SCHISM模型,建立了三维盐度数值模型,根据实测资料对其进行验证。结合水体势能异常理论,对枯季磨刀门河口混合层化的时空变化特征及深槽与浅滩的层化机制差异进行分析。结果表明:磨刀门河口小潮时水体层化最强,中潮时水体层化最弱,且拦门沙至挂定角段水体层化始终较强。磨刀门深槽水体层化主要受纵向平流、纵向水深平均应变和垂向混合影响,而浅滩水体层化则受横向平流、横向水深平均应变和垂向混合影响;磨刀门河口表、底层水体湍动能耗散率较高,而中间水层存在低耗散区,且涨潮时湍动能耗散率比落潮时大。 相似文献
15.
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. 相似文献
16.
Three and a half years of hydrographic, velocity, and meteorological observations are used to examine the dynamics of upper Elkhorn Slough, a seasonally inverse, shallow, mesotidal estuary in central California. The long-term observations revealed that residual circulation in Elkhorn Slough is seasonally variable, with classic estuarine circulation in the winter and inverse estuarine circulation in the summer. The strength of this exchange flow varied both within years and between years, driven by the annual cycle of dry summers and wet winters. Subtidal circulation is a combination of both tidal and density-driven mechanisms. The subtidal magnitude and reversal of the exchange flows is controlled primarily by the density gradient despite the significant tidal energy. As the density gradient weakens, the underlying tidal processes generate vertically sheared exchange flows with the same sign as that expected for an inverse density gradient. The inverse density gradient may then further strengthen this inverse circulation. These data were collected as part of the Land/Ocean Biogeochemical Observatory and demonstrate the utility of long-term in situ measurements in a coastal system, as consideration of such a wide range of forcing conditions would not have been possible with a less comprehensive data set. 相似文献
17.
Ryan E. Morse Jian Shen Jose L. Blanco-Garcia William S. Hunley Scott Fentress Mike Wiggins Margaret R. Mulholland 《Estuaries and Coasts》2011,34(5):1006-1025
Massive blooms of the harmful alga Cochlodinium polykrikoides Margalef occurred in the lower Chesapeake Bay and its tributaries during the summers of 2007 and 2008. The Lafayette and
Elizabeth Rivers appeared to act as initiation grounds for these blooms during both years. However, in 2008 there were also
localized sites of initiation and growth of populations within the mesohaline portion of the James River. Bloom initiation
appeared to be correlated with intense, highly localized rainfall events during neap tides. Subsequent spring tides increased
tidal flushing and transport of C. polykrikoides from the Lafayette and Elizabeth Rivers into the lower James River where it was transported upriver by local estuarine circulation.
Blooms dissipated in response to increased wind-driven mixing associated with frontal systems moving through the region. A
combination of physical factors including, seasonal rainfall patterns, increased stratification, nutrient loading, spring-neap
tidal modulation, and complex estuarine mixing and circulation allowed C. polykrikoides to spread and form massive blooms over large portions of the tidal James River and the lower Chesapeake Bay. 相似文献
18.
19.
A local, one-dimensional, depth-dependent model is used in conjunction with a one-dimensional, longitudinal, hydrodynamical model to examine the mechanisms affecting yertical profiles of longitudinal residual current in the macrotidal (tidal range typically exceeds 4 m during spring tides), partly-mixed Tamar Estuary. Residual currents are simulated at a deep (15m) station in the lower reaches, which possesses a small tidal amplitude to depth ratio and a nonzero salinity throughout the tidal cycle, as well as at a shallow station in the upper reaches, which varies in depth from 1 m at low water, when salinity is zero, to 5 m at high water. A slow, up-estuary current dominates the residual circulation just beneath the high-water level at the deeper station. Further down the water column a down-estuary residual current develops which is the near-surface component of a two-layer gravitational circulation. The up-estuary component of this gravitational circulation occurs deeper in the column and extends to the bed at the deep station, whereas at the shallow station it is eventually dominated by a down-estuary current in the bottom 1 m. Simulated residual currents are fairly insensitive to estuary-bed slope and to observed depth variations in longitudinal density gradient. Residual current profiles of the observed form can only be generated by a longitudinal density gradient. The reduction in vertical eddy viscosity by water column stability due to stratification is an essential requirement for producing a strong gravitational circulation of the observed magnitude. Stratification at the shallow station is much higher during the ebb than during the flood and this asymmetry enhances the gravitational circulation in the upper reaches. The formation of residual flows at both stations is illustrated by showing time-series data over a tidal cycle for the simulated current profiles. 相似文献
20.
Bo G. Gustafsson 《Estuaries and Coasts》2000,23(2):253-266
A time-dependent model for stratification and circulation within the Baltic entrance area (Gustafsson 2000) is tested against observed salinities for the period 1961–1993. Although the Baltic Sea is one of the largest estuarine systems on earth, this model could be applicable to smaller estuarine systems and embayments with tidal exchange. The seasonal cycle of freshwater flux across the sill area does not follow the seasonal cycle of freshwater supply to the Baltic Sea. The seasonal variation of the flux is a combined effect of the seasonal variation in freshwater supply, in Baltic mean sea level, and in dispersion of salt across the sills. The seasonal variation in dispersion of salt is due to the seasonal cycle of sea level variability. The model is used to predict the inflow of high saline water to the Baltic Sea. The resulting inflow time-series is consistent with variations in the deep-water salinity and temperature in the deeper parts of the Baltic Sea. A comparison with previous estimates of the magnitude of major Baltic inflows shows that the model is able to reproduce the characteristics fairly well although the magnitude of the flows of water and salt appears lower than other estimates. It is shown that a climatic change that increases the wind mixing does not significantly change the major inflows. Both increased amplitudes of sea level variations in the Kattegat and decreased freshwater supply to the Baltic Sea substantially increase the magnitude of the inflows. It is shown that deep-water renewal in the Baltic Sea is obstructed during years with high freshwater supply even if the sea level forcing is favorable to a major inflow. 相似文献