Tidal straining,density currents,and stirring in the control of estuarine stratification

Buoyancy input as fresh water exerts a stratifying influence in estuaries and adjacent coastal waters. Predicting the development and breakdown of such stratification is an inherently more difficult problem than that involved in the analogous case of stratification induced by surface heating because the buoyancy input originates at the lateral boundaries. In the approach adopted here, we have adapted the energy considerations used in the surface heating problem to describe the competition between the stabilizing effect of fresh water and the vertical mixing brought about by tidal and wind stirring. Freshwater input induces horizontal gradients which drive the estuarine circulation in which lighter fluid at the surface is moved seaward over heavier fluid moving landward below. This contribution to stratification is expected to vary in time as the level of turbulence varies over the tidal cycle. The density gradient also interacts directly with the vertical shear in the tidal current to induce a periodic input to stratification which is positive on the ebb phase of the tide. Comparison of this input with the available stirring energy leads to a simple criterion for the existence of strain-induced stratification. Observations in a region of Liverpool Bay satisfying this criterion confirm the occurrence of a strong semidiurnal variation in stratification with complete vertical mixing apparent around high water except at neap tides when more permanent stratification may develop. A simulation of the monthly cycle based on a model including straining, stirring, and the estuarine circulation is in qualitative agreement with the main features of the observations.     

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.     

磨刀门河口环流与咸淡水混合层化机制

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。     

磨刀门水道盐度混合层化机制

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。     

Frontogenesis and Frontal Progression of a Trapping-Generated Estuarine Convergence Front and Its Influence on Mixing and Stratification

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.     

磨刀门水道咸潮上溯动力特性分析

为研究磨刀门水道咸潮上溯的动力特性,基于非结构网格海洋模型（Finite Volume Coastal Ocean Model,FVCOM）,构建了覆盖珠江河口及其上游网河区的高分辨率三维斜压数值模型,采用实测资料对其进行率定和验证,并开展了咸潮上溯的数值模拟计算。根据计算结果和实测资料,对磨刀门水道大、中、小潮期间的盐淡水分层与混合特征、盐分物质的分层输移机制进行分析,探讨其咸潮上溯强度时空分布差异的原因。结果表明:小潮期,底层累积盐通量明显大于表层,净输移方向为陆向;大潮期,表层累积盐通量明显大于底层,净输移方向为海向;而平衡点一般出现于中潮期,这就是磨刀门水道咸潮上溯最强和最弱时刻分别出现于小潮和大潮后的中潮期的原因所在。     

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.     

Variations and net transport of dissolved inorganic nutrients in the South Passage of the Changjiang （Yangtze River） Estuary

Dissolved inorganic nutrient elements were analyzed from the samples collected in the South Passage of the Changjiang (Yangtze River) Estuary in March 2003, including NH4 , NO3-, NO2- and PO43-. The water samples were collected with a Niskin sampler hourly at the near-surface, middle and near-bottom depths at the three stations -A1, A2 and A3-during two complete tidal cycles of neap tide and spring tide. Results showed that 1) the concentrations of NH4 , NO3- and NO2- were a little higher respectively during the neap tide than those during the spring tide, while PO43- showed an opposite trend, and each was higher in the ebb tide than in the flood tide, either for the neap tidal cycle or the spring tidal cycle; 2) higher stratification of the nutrients existed obviously in this area, with the concentrations of which increased from the bottom to the surface, especially for NH4 and NO3-; 3) the coefficient of variation (C.V.) values of all dissolved inorganic nutrients varied from 4.06% to 36.8% beyond different influences of the tidal current and Changjiang runoff; 4) with increasing suspended matter in the water column, the concentrations of PO43- became lower in the filtered water; and 5) the total transport of each tidal cycle was much more in the spring tide than in the neap tide, and the positive values indicated that the nutrients had been exported to the East China Sea. Studies on the variations and net transport of dissolved inorganic nutrients in the South Passage of the Changjiang Estuary will provide the scientific basis for the study of the mechanism of red tide in the East China Sea.     

On the hydrographic distribution in the source region of the Delaware coastal current

The buoyant discharge from Delaware Bay forms two separate branches of residual outflow near the bay mouth, one along each shore. Upon exiting the bay, the branch along the Delaware shore turns right to form the southward flowing Delaware coastal current along the inner continental shelf off the Delaware, Maryland, and Virginia coasts. CTD and thermosalinograph, data collected at the mouth of Delaware Bay over two semidiurnal tidal cycles are used to examine the hydrographic distribution at the source region of the Delaware coastal current. In this region the buoyant source water of the coastal current, is largely detached from the shoreline and confined to the top 15 m of the water column over much of the tidal cycles. The core of the coastal current's source water, as defined by the point of salinity minimum, is located over the deep channel well offshore of the Delaware coast. The separation between this buoyant water and the more saline waters right along the Delaware coast and that in the central part of the bay mouth are marked by regions of high horizontal salinity gradients. The horizontal salinity gradients around the inshore and offshore boundaries of the source water of the coastal current are intensified during the flood tide, and clearly defined fronts (with a change of 3‰ over a distance of 150 m) are present at the offshore boundary near the end of the flood tide. The structure of the mean flow and the distribution of the brackish coastal current on the inner continental shelf contribute to the persistence of stratification in the source region off the Delaware shore throughout the ebb and flood tides. In contrast, the ebb-induced stratification in the region off the New Jersey shore is quickly destroyed with the onset of the flood current.     

Changes in beach water table elevation during neap and spring tides on a sandy estuarine beach, Delaware Bay, New Jersey

A field investigation of temporal and spatial changes in wind and wave characteristics, runup and beach water table elevation was conducted on the foreshore of an estuarine beach in Delaware Bay during neap (April 9, 1995) and spring (April 16, 1995) tides under low wave-energy conditions. The beach has a relatively steep, sandy foreshore and semi-diurnal tides with a mean range of 1.6 m and a mean spring range of 1.9 m. Data from a pressure transducer placed on the low tide terrace reveal a rate of rise and fall of the water level on April 16 of 0.09 mm s⁻¹ resulting in a steeper tidal curve than the neap tide on April 9. Data from three pressure transducers placed in wells in the intertidal foreshore reveal that the landward slope of the water table during the rising neap tide was lower than the slope during spring tide, and there was a slower rate of fall of the beach water table relative to the fall of the tide. Wave heights were lower on April 9 (significant height from 17.1 min records <0.16 m). The water table elevation was 0.08 m higher than the water in the bay at the time of high water, when maximum runup elevation was 0.29 m above high water and maximum runup width was 2.0 m. The elevation of the water table was 0.13 m higher than the maximum elevation of water level in the bay 74 min after high water, when wave height was 0.12 m and wave period was 2.7 s. The use of mean bay water level at high tide will underpredict the elevation of the water table in the beach, and demarcation of biological sampling stations across the intertidal profile based on mean tide conditions will not accurately reflect the water content of the sandy beach matrix.     

Tidal intrusion fronts

A tidal intrusion front forms as a dense seawater inflow plunges (subducts) beneath ambient estuarine water during flood tide. The associated foam lines and color changes have been observed on many smaller estuaries with constricted mouths. Internal hydraulic theory and laboratory experiments are reviewed and expressions are obtained for the position of plunging and the amount of associated mixing. The existence of a tidal intrusion front and its structure are discussed in terms of densimetric Froude numbers. These fronts are particularly important in smaller estuaries in which the intrusion process may dominate wind and tidal mixing and thus determine the overall stratification of the estuary. Three classes of three-dimensional plunging flow are identified and discussed. In particular, it is suggested that the peculiar, cursive V-shape plunge line is characteristic of strongly plunging flow.     

A radioisotope tracer investigation to determine the direction of groundwater movement adjacent to a tidal creek during spring and neap tides

A radioisotope tracer (⁸²Br) was injected into a sand aquifer adjacent to a tidal creek at Hat Head, New South Wales, Australia. The injection was timed to coincide with the falling limb of a spring tide in August 2001 and was repeated in July 2002 during a period of neap tides. The tracer movement was detected using gamma logging and fixed gamma detectors in a bore 0.9 m from an injection bore and in a line approximately perpendicular to the creek. Movement of the tracer was detected by comparing measured gamma activity with calibrated activities determined under laboratory conditions. Net movement of the tracer indicated approximately 0.001 m/day laterally towards the creek and 0.07 m/day vertically upward during spring tide conditions. This pattern is reversed during neap tide conditions with little net vertical movement but horizontal movement of 0.15 m/day. The measurements indicated an oscillatory motion coinciding with the tide but lagging the tide by approximately 4.5–6.5 h. The vertical flow during spring tides indicates that vertical movement of the saline interface occurs and results in mixing of different water types beneath the banks of the creek.     

Temporal and spatial variations in density and velocity fields of the waters of Gareloch,Scotland

 Spatial variations in the density and velocity fields have been observed in the Gareloch (Scotland) during surveys in 1987–1988 and 1993–1994. The variation of the density field has been analyzed on a variety of time scales from semidiurnal to seasonal in order to quantify effects caused by the forcing factors of tidal mixing, freshwater input, and wind. Initial results indicate that water density in the loch is controlled (to a major degree) by the freshwater input from runoff from the local catchment area and from freshwater entering on the flood tide from the Clyde Estuary. It is estimated that during winter periods the high freshwater flows from the rivers Leven and Clyde into the Clyde Estuary account for up to 75% of the freshwater creating the density structure in the loch. Analysis of long-term dissolved oxygen data reveals that major bottom water renewals occurred between July and January in the years 1987–1994. Major bottom water dissolved oxygen renewals have a general trend but during the year sporadic renewals can take place due to abnormal dry spells increasing the density of the water entering from the Clyde, or consistently strong winds from the north reducing stratification in the loch and producing better mixed conditions. Velocities vary spatially, with the highest velocities of up to 0.6 m s^–1 being associated with the velocity jet effect at the constriction at the sill of the loch. Observed near-surface mid-loch velocities increased as the vertical density gradients in the upper layers increased. This indicates for the observed conditions that increased stratification in the upper layers inhibits the entrainment rate and hence rate of gain of thickness of the wind-driven surface layer, resulting in increased surface velocities for a given wind speed and direction. The main flow is concentrated in the upper 10 m and velocities below 10 m are low. Observed mean spring tide surface velocities are on average 30% greater than mean neap tide surface velocities. Received: 22 May 1995 · Accepted: 23 August 1995     

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.     

珠江河口夏季缺氧现象的模拟

夏季底层水体缺氧现象是珠江河口存在的环境问题之一。使用三维水动力-生态耦合模型来分析珠江口缺氧现象的分布状况和产生原因。模拟结果表明:模型能很好的再现珠江口的缺氧敏感性区域和强度。珠江口存在的底层水体缺氧现象是水体强烈层化和生化耗氧过程共同作用的结果。缺氧现象的发展与减退受潮汐涨落的影响。珠江口盐度-潮汐混合锋面和层化作用控制低氧水团的范围和强度。     

大凌河口水动力数值模拟分析

采用大凌河实测长系列河道径流及水下地形等数据, 通过建立水动力数值模型, 综合考虑潮流和河道径流的影响, 对大凌河口的潮流进行模拟, 探讨大凌河口及辽东湾北部海域水动力过程的主要特征. 结果表明: 大凌河口附近海域的海流以潮流为主, 具有明显的往复性质. 潮流总的运动趋势是, 涨潮主流向为北东, 落潮主流向为南西. 大潮流速大于小潮流速, 涨潮流历时与落潮流历时几乎相当. 最大涨潮流速约为0.52 m/s, 最大落潮流速约为0.4 m/s, 潮流涨落平均潮流强度的分布大体和该海域等深线相适应.     

Fortnightly Changes in Water Transport Direction Across the Mouth of a Narrow Estuary

This research investigates the dynamics of the axial tidal flow and residual circulation at the lower Guadiana Estuary, south Portugal, a narrow mesotidal estuary with low freshwater inputs. Current data were collected near the deepest part of the channel for 21 months and across the channel during two (spring and neap) tidal cycles. Results indicate that at the deep channel, depth-averaged currents are stronger and longer during the ebb at spring and during the flood at neap, resulting in opposite water transport directions at a fortnightly time scale. The net water transport across the entire channel is up-estuary at spring and down-estuary at neap, i.e., opposite to the one at the deep channel. At spring tide, when the estuary is considered to be well mixed, the observed pattern of circulation (outflow in the deep channel, inflow over the shoals) results from the combination of the Stokes transport and compensating return flow, which varies laterally with the bathymetry. At neap tide (in particular for those of lowest amplitude each month), inflows at the deep channel are consistently associated with the development of gravitational circulation. Comparisons with previous studies suggest that the baroclinic pressure gradient (rather than internal tidal asymmetries) is the main driver of the residual water transport. Our observations also indicate that the flushing out of the water accumulated up-estuary (at spring) may also produce strong unidirectional barotropic outflow across the entire channel around neap tide.     

Environmental and Physical Controls on the Formation and Transport of Blooms of the Dinoflagellate <Emphasis Type="Italic">Cochlodinium polykrikoides</Emphasis> Margalef in the Lower Chesapeake Bay and Its Tributaries

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.     

渤海层化结构及潮汐锋面季节变化的数值研究

采用三维斜压海流模式(POM)模拟了渤海海温的季节变化,以海表与海底温差ΔT作为判别依据,发现3月份前整个渤海的表、底温差小于0.5℃,说明渤海处于充分混合状态;进入4月份以后,莱州湾、渤海中部以及渤海海峡的局部水域出现超过2℃的表、底温差,意味着垂向层化结构开始形成;层化区域面积随着海表热通量增大的趋势可一直持续到8月,9月后由于日照量逐渐减小,季节性温越层逐渐消失,11月以后渤海又恢复到充分混合的状态.     

Larval Brachyuran Crab Timing and Distribution in Relation to Water Properties and Flow in a High-Latitude Estuary

Crab larval dispersal and, thus, adult population size can be influenced by oceanographic conditions, especially in highly dynamic and variable environments such as high-latitude estuaries. This study's primary goal was to understand larval timing, abundance, and distribution in relation to oceanographic effects between the more estuarine inner and more oceanic outer part of the subarctic estuary of Kachemak Bay (south-central Alaska, USA). Temperature and salinity measurements and plankton tows were taken along the boundary between the two bay parts from 31 March to 28 October 2008 on spring and neap tides. Summer water flow and stratification in Kachemak Bay were chiefly buoyancy driven, and predominant flow patterns were not affected by the large tidal range in the region. Larvae of seven brachyuran species were found over the sampling period, with seasonally different peak abundances. Larval spatial distribution patterns, presence of most larval stages for each species, and their seasonal timing and persistence within the water column suggest that most species are locally transported within or close to Kachemak Bay. The larval assemblage was correlated with seasonal changes in water temperature, and peak abundances of some species coincided with spring tides and/or frontal systems between the in- and outflow regions of the bay. The strong influence of freshwater on the local circulation pattern in the summer indicates that larval transport will likely be strongly affected by future changes in glacial melt and precipitation.