长江口盐水入侵自动化监测系统

根据盐水入侵路径,在长江口附近海域建立9个自动化监测站,对表层盐度、水温实施实时、连续的自动化监测。长江口盐水入侵自动监测系统的建成,可在远端获取实时数据,为上海水源地取水提供服务,有效地防止和减少盐水入侵灾害造成损失。     

特枯水文年长江口南槽盐水入侵分析

2006年6月至2007年5月长江入海径流量较多年平均减少1/3,为特枯水文年.根据长江口南槽江亚北水道实测盐度等资料,分析了该特枯水文年的盐水入侵状况.结果显示:研究区该时间段几乎均为盐水所占据,96.6% 的天数处于盐水入侵状态;与常年相比,盐度具有年变幅大、洪枯季差异小、高盐月份多的特点.研究表明:盐水入侵主要受径、潮流控制,多发生在小径流量-大潮期;大通径流量 30 000 m3/s 是盐水入侵与否的阈值.     

Semidiurnal Dynamics of Salinity, Nutrients and Suspended Particulate Matter in an Estuary in the Seto Inland Sea, Japan, during a Spring Tide Cycle

The physical and chemical variability of the water column at subtidal station of an estuary in the Seto Inland Sea, Japan, was studied over a 24-hour period during a spring tide (tidal range ca. 2 m) in May 1995. Surface water and several depths through the water column were monitored every one and two hours, respectively. At each occasion, water temperature, salinity and dissolved oxygen concentration were measured and water samples were collected for the determination of nutrients and suspended particulate matter (SPM). Disruptive changes in the physical and chemical characteristics of the water was produced by the tidal cycle and the mixing of water masses of different origin. These changes were highly significant both spatially and temporally, yet with varying effects on physical parameters, nutrients and the different components of SPM. Significant differences in nutrient concentrations were also observed when the data-set was divided into ebb and flood components, irrespective of the depth. Nitrate and nitrite rose to 1.8 times higher during the flood. Spatial differences of SPM were less marked than those of nutrients, only particulate organic carbon (POC) being significantly higher at the surface than in the intermediate and the lower layer. Both POC and pheopigment concentrations increased markedly through the water column, being highest shortly before the lower low tide. In contrast, suspended solid (SS) content increased sharply after the lower low tide (>40 mg l⁻¹) and this coincided with a marked decrease of the C/SS content (<20 mg g⁻¹). The lagtime between POC and SS tidal transport was caused by particle resuspension from the exposed intertidal sediments as the tidal level rose, and particle transport selection in relation to the tidal state. This revised version was published online in August 2006 with corrections to the Cover Date.     

Application of Two Turbulence Closure Schemes in the Modelling of Tidal Currents and Salinity in the Hooghly Estuary

This paper has two purposes. The first is to study the circulation and salinity in Hooghly Estuary, along the east coast of India and the second is to compare the performance of two turbulence closure schemes by modelling it. A breadth averaged numerical model using a sigma co-ordinate system in the vertical is briefly described. Vertical diffusion of momentum and salt are parameterized by a simple first-order turbulent closure or by a one equation model for turbulent kinetic energy (TKE) which uses a specified mixing length. The results are compared with the available neap and spring tide observations along the estuary for both low and high discharge periods.The computed elevations and currents are in reasonable agreement with the observations showing no major differences in vertical current profiles by both the turbulent schemes. However, there is a slight under-prediction of bottom currents. The salinity profiles predicted by TKE model show better matching with observations. Statistical tests are also conducted to study the comparative performance of the turbulent closure schemes. The maintenance of two layer structure in residual currents and salt variability are also studied by using the model.     

Studies on the behaviour of nutrients in the Mandovi Estuary during premonsoon

Behaviour of silicate, nitrate and phosphate in the Mandovi Estuary was studied during the premonsoon season. The study shows that silicate is removed from the water column while nitrate showed an internal source. This nitrate source has been identified as the wash water from an iron ore screening plant which discharges the muddy waste directly into the river. Applying mixing relations, the distribution of this nitrate both up- and downstream of the outfall has been studied. Phosphate showed very low concentrations in the low salinity region; however, its concentration increased slightly towards the higher salinity region.     

Dynamics of Turbidity in the Tweed Estuary

Results are presented of turbidity measurements made in the Tweed Estuary during 14–29 September 1993. The observations covered a spring-neap period of very strong and relatively weak tides, which included strong wave activity at the mouth of the estuary, the aftermath of a strong freshwater inflow event and a minor inflow event which coincided with neap tides. Turbidity levels between the mouth and the limit of saline intrusion during this period were observed to lie in the range 2–30 ppm. Temperature–salinity relationships, based on rapid sampling throughout the estuary, often indicated conservative mixing between riverine and coastal waters. Turbidity–salinity relationships throughout the estuary were approximately linear for most of the lower salinity range (<30) although the relationships varied throughout a tidal cycle. At the highest salinities, in the lower 2·5 km of estuary, significantly enhanced turbidities occurred during strong, onshore swell-wave conditions. Near-mouth turbidities were very low (<5 ppm) at high-water (HW) when the swell-wave height was small (<0·3 m). There was no correlation between near-mouth turbidity at HW and tidal range, whereas the correlation between near-mouth turbidity and swell-wave height explained over 90% of the variance in near-mouth turbidity. The temporal trends in freshwater turbidity and freshwater inflow during the fieldwork period were fairly similar. A statistically significant relationship existed between these variables when freshwater turbidity was correlated against inflow 30 h earlier. A ‘ model ’ of turbidity, based on 5 days of inflows and used to hindcast turbidity from 14 days of inflows between 16–29 September, explained over 80% of the variance in freshwater turbidity. Therefore, inflow exerted a significant control on the fluvial turbidity. It appears that very fine-grained particles were responsible for the observed turbidity in the central and upper reaches of the Tweed. During the flood, in the presence of strong wave activity in the coastal zone, larger sediment was rapidly winnowed in the lower reaches, close to the mouth, whereas fine silt particles remained in suspension and reached the limit of saline intrusion.     

Spatial and seasonal variations in the chemistry of sediment interstitial waters in the Tamar Estuary

Chemical concentration gradients in the interstitial waters of shallow (0–10 cm), reducing sediments are reported for four stations in the Tamar Estuary over a two-year period. The products of organic matter breakdown, sedimentary redox changes and diagenetic reactions have been determined and a multicomponent analytical approach has enabled the interactions of complementary processes to be examined. While many components show spatial and seasonal patterns related to the supply of organic matter and salinity associated sulphate distribution, the effects of temperature, sediment disturbance and river flow are also apparent.Sediment stored prior to the separation of interstitial water showed significant changes in chemical composition compared to samples processed within 24 h of collection.     

Modeling bed-load transport of coarse sediments in the Great Bay Estuary, New Hampshire

Current, sea level and bed-load transport are investigated in the Lower Piscataqua River section of the Great Bay Estuary, New Hampshire, USA—a well-mixed and geometrically complex system with low freshwater input, having main channel tidal currents ranging between 0.5 and 2 m s⁻¹. Current and sea level forced by the M₂M₄M₆ tides at the estuarine mouth are simulated by a vertically averaged, non-linear, time-stepping finite element model. The hydrodynamic model uses a fixed boundary computational domain and accounts for flooding–drying of tidal flats by making use of a groundwater component. Inertia terms are neglected in comparison with pressure gradient and bottom friction terms, which is consistent with the observed principal dynamic balance for this section of the system. The accuracy of hydrodynamic predictions in the study area is demonstrated by comparison with four tidal elevation stations and two cross-section averaged current measurements. Simulated current is then used to model bed-load transport in the vicinity of a rapidly growing shoal located in the main channel of the lower system. Consisting of coarse sand and gravel, the shoal must be dredged every five to eight years. Two approaches are taken—an Eulerian parametric method in which nodal bed-load flux vectors are averaged over the tidal cycle and a Lagrangian particle tracking approach in which a finite number of sediment particles are released and tracked. Both methods yield pathways and accumulations in agreement with the observed shoal formation and the long-term rate of sediment accumulation in the shoal area.     

On salinity regimes and the vertical structure of residual flows in narrow tidal estuaries

An examination is made of the circulation in narrow estuaries subject to a predominant tidal forcing. Velocity structures are derived separately for residual flow components associated with (a) river flow, (b) wind stress, (c) a well-mixed longitudinal density gradient and (d) a fully stratified saline wedge. Dimensionless parameters are introduced to indicate the magnitude of each component and these parameters are evaluated for 9 major estuaries, thereby revealing their sensitivity to each component.For a channel of constant breadth and depth, formulae are deduced for the length of saline intrusion, L. Comparisons with observed data show that such formulae may be used with confidence to predict changes in L arising from variations in river flow, tidal range or channel depths.The level of stratification is shown to be related to a product of two parameters, one associated with velocity structure and a second involving the square of the ‘flow ratio’ $\text{u}\text{u}\text{?}$ (i.e. residual velocity/amplitude of the tidal velocity). This relationship provides a simple classification system for estuarine stratification which can be used to indicate the sensitivity of any particular estuary to changing conditions.     

The principal factors contributing to the flux of salt in a narrow, partially stratified estuary

Observations of the velocity and salinity structure of the Tees estuary were made at eight stations along the estuary axis between Victoria Bridge and the sea during the summer of 1975. The measurements were made on ten separate tidal periods covering neap and spring tides.The data were collected over a period of relatively low freshwater flows and the residual current was found to have a strong dependence on the Stokes drift. At the upstream stations, the residuals were more than an order of magnitude greater than the currents anticipated from the freshwater discharge. Although the mean stratification decreased as the tidal range increased, the vertical circulation was stronger on spring tides than on neaps. Vertical variations in the amplitude and phase of the tidal current results in a current which strengthens the vertical circulation. However, this effect only made a relatively small contribution to the observed vertical circulation.The relative contribution of the individual salt flux terms to the net upstream transport of salt varies along the estuary. As the estuary narrows, the contribution by the oscillatory terms dominates that from the shear in the steady state flow. Of these oscillatory terms, the correlation of velocity and salinity fluctuations plays a key rôle in the salt transport. The depth mean values make a greater contribution than deviations from the depth mean and the flux due to phase variations over depth is smaller than either of these. Since the Stokes drift is compensated by a down-stream steady state flow, it does not contribute to the tidal mean transport of salt.At the seaward end of the estuary, the salt fluxes due to the steady state vertical shear and the convariance of the tidal fluctuations act in a complementary way to counter the seaward transport of salt by the freshwater flow. With the possible exceptions of the wide or narrow reaches of the Tees, the longitudinal fluxes of salt due to transverse variations in velocity, salinity and depth and turbulent fluctuations are of secondary importance as contributors to the estuary salt budget.On both neap and spring tides, the computed total salt transports at the Newport and Victoria bridges did not match the values required for a salt balance with the corresponding freshwater flows. These fluxes were probably the cause of the observed downstream displacement of the tidal mean salinity distribution between neap and spring tides.     

长江河口盐淡水混合的数值模拟计算

建立了河口三维非线性斜压潮流盐度数学模型，数学模型中的水平压强梯度分为水位梯度的正压项和密度变化的斜压项，以反映河口盐淡水混合中的斜压现象。采用欧拉－拉格朗日法离散潮流运动方程中的对流项和盐度扩散方程，以提高计算稳定性和数值模拟精度，提出盐度边界条件，初始条件以及盐度场计算稳定标准的设置方法，针对长江河口水域的盐淡水混合情况进行模拟计算，结果反映了部分混合型河口盐度分布特征，表明所建立的河口三维非线性斜压潮流盐度数学模型能够较好地模拟长江河口盐淡水混合的时空分布。     

Settleable and Non-Settleable Suspended Sediments in the Ogeechee River Estuary, Georgia, U.S.A.

Suspended particle dynamics were investigated in the Ogeechee River Estuary during neap tide in July 1996. Samples were operationally separated into ‘ truly suspended ’ (settling velocity <0·006 cm s⁻¹) and ‘ settleable ’ (settling velocity >0·006 cm s⁻¹) fractions over the course of a tidal cycle to determine whether these two fractions were comprised of particles with differing biological and chemical characteristics. Total suspended sediment, organic carbon and nitrogen, chlorophyll a and phaeopigment concentrations were measured in each fraction, as well as rates of bacterial hydrolytic enzyme activity [β-1,4-glucosidase (βGase) and β-xylosidase (βXase)]. The majority of the suspended sediment (by weight) was in the truly suspended fraction; all measured parameters were largely associated with this fraction as well. When compared to the settleable material, the truly suspended material was significantly higher in % POC (5·7±0·6 vs. 3·9±1·8), % chlorophyll (0·07±0·02 vs. 0·03±0·01), % phaeopigment (0·030±0·006 vs. 0·018±0·012), and weight-specific maximal uptake rates (V_maxper mg suspended sediment) of both enzymes (1·8±0·4 vs. 0·7± 0·2 nmol mg⁻¹ h⁻¹βGase and 1·1±0·3vs . 0·3±0·2 nmol mg⁻¹ h⁻¹βXase), providing clear evidence for a qualitative distinction between the two fractions. These results are interpreted to mean that the more organic-rich, biologically active material associated with the suspended fraction is likely to have a different fate in this Estuary, as ‘ truly suspended ’ sediments will be readily transported whereas ‘ settleable ’ sediments will settle and be resuspended with each tide. These types of qualitative differences should be incorporated into models of particle dynamics in estuaries.     

Three Dimensional Baroclinic Numerical Model for Simulating Fresh and Salt Water Mixing in the Yangtze Estuary

For simulating fresh and salt water mixing in estuaries, a three dimensional nonlinear baroclinic numerical model is developed, in which the gradients of horizontal pressure contain the gradient of barotropic pressure arising from the gradi-ent of tidal level and the gradient of baroclinic pressure due to the gradient of salinity. The Eulerian-Lagrangian method is employed to descretize both the momentum equations of tidal motion and the equation of salt water diffusion so as to im-prove the computational stability and accuracy. The methods to provide the boundary conditions and the initial conditions are proposed, and the criterion for computational stability of the salinity fields is presented. The present model is used for modeling fresh and salt water mixing in the Yangtze Estuary. Computations show that the salinity distribution has the characteristics of partial mixing pattern, and that the present model is suitable for simulalion of fresh and salt waler mixing in ihe Yanglze Esluary.     

Dissolved silicon in the Yaquina Estuary,Oregon

The longitudinal distribution of dissolved silicon in the Yaquina Estuary is described in surface and bottom waters. Non-conservative behaviour is a function of high insolation and low runoff; residence time is the primary controlling factor.     

Nutrient flux in the Rhode River: Tidal exchange of nutrients by brackish marshes

Tidal exchanges of nitrogen, phosphorus, and organic carbon by a high and a low elevation marsh in the Rhode River estuary were measured throughout the year. Both marshes tended to import particulate matter and export dissolved matter, although they differed in the fluxes of certain nutrients. Compared with tidal exchanges, bulk precipitation was a major source of ammonia and nitrate and a minor source of other nutrients. There was a net retention of nutrients by the portion of the Rhode River that included both marshes and a mudflat. However, the marshes accounted for only 10% of the phosphorus retention and 1% of the nitrogen retention while they released organic carbon amounting to 20% of the retention. This suggests that the mudflat acted as a sink for nutrients. The primary role of the marshes seems to be transformation of particulate to dissolved nutrients rather than nutrient retention or release.