Turbulence measurements in a small subtropical estuary under king tide conditions

In natural waterways and estuaries, the understanding of turbulent mixing is critical to the knowledge of sediment transport, stormwater runoff during flood events, and release of nutrient-rich wastewater into ecosystems. In the present study, some field measurements were conducted in a small subtropical estuary with micro-tidal range and semi-diurnal tides during king tide conditions: i.e., the tidal range was the largest for both 2009 and 2010. The turbulent velocity measurements were performed continuously at high-frequency (50Hz) for 60?h. Two acoustic Doppler velocimeters (ADVs) were sampled simultaneously in the middle estuarine zone, and a third ADV was deployed in the upper estuary for 12?h only. The results provided an unique characterisation of the turbulence in both middle and upper estuarine zones under the king tide conditions. The present observations showed some marked differences between king tide and neap tide conditions. During the king tide conditions, the tidal forcing was the dominant water exchange and circulation mechanism in the estuary. In contrast, the long-term oscillations linked with internal and external resonance played a major role in the turbulent mixing during neap tides. The data set showed further that the upper estuarine zone was drastically less affected by the spring tide range: the flow motion remained slow, but the turbulent velocity data were affected by the propagation of a transient front during the very early flood tide motion at the sampling site.     

Undular tidal bores: effect of channel constriction and bridge piers

A tidal bore may occur in a macro-tidal estuary when the tidal range exceeds 4.5–6 m and the estuary bathymetry amplifies the tidal wave. Its upstream propagation induces a strong mixing of the estuarine waters. The propagation of undular tidal bores was investigated herein to study the effect of bridge piers on the bore propagation and characteristics. Both the tidal bore profile and the turbulence generated by the bore were recorded. The free-surface undulation profiles exhibited a quasi-periodic shape, and the potential energy of the undulations represented up to 30% of the potential energy of the tidal bore. The presence of the channel constriction had a major impact on the free-surface properties. The undular tidal bore lost nearly one third of its potential energy per surface area as it propagated through the channel constriction. The detailed instantaneous velocity measurements showed a marked effect of the tidal bore passage suggesting the upstream advection of energetic events and vorticity “clouds” behind the bore front in both channel configurations: prismatic and with constriction. The turbulence patches were linked to some secondary motions and the proposed mechanisms were consistent with some field observations in the Daly River tidal bore. The findings emphasise the strong mixing induced by the tidal bore processes, and the impact of bridge structures on the phenomenon.     

Experimental investigation of bubbly flow and turbulence in hydraulic jumps

Many environmental problems are linked to multiphase flows encompassing ecological issues, chemical processes and mixing or diffusion, with applications in different engineering fields. The transition from a supercritical flow to a subcritical motion constitutes a hydraulic jump. This flow regime is characterised by strong interactions between turbulence, free surface and air–water mixing. Although a hydraulic jump contributes to some dissipation of the flow kinetic energy, it is also associated with increases of turbulent shear stresses and the development of turbulent eddies with implications in terms of scour, erosion and sediment transport. Despite a number of experimental, theoretical and numerical studies, there is a lack of knowledge concerning the physical mechanisms involved in the diffusion and air–water mixing processes within hydraulic jumps, as well as on the interaction between the free-surface and turbulence. New experimental investigations were undertaken in hydraulic jumps with Froude numbers up to Fr = 8.3. Two-phase flow measurements were performed with phase-detection conductivity probes. Basic results related to the distributions of void fraction, bubble frequency and mean bubble chord length are presented. New developments are discussed for the interfacial bubble velocities and their fluctuations, characterizing the turbulence level and integral time scales of turbulence representing a “lifetime” of the longitudinal bubbly flow structures. The analyses show good agreement with previous studies in terms of the vertical profiles of void fraction, bubble frequency and mean bubble chord length. The dimensionless distributions of interfacial velocities compared favourably with wall-jet equations. Measurements showed high turbulence levels. Turbulence time scales were found to be dependent on the distance downstream of the toe as well as on the distance to the bottom showing the importance of the lower (channel bed) and upper (free surface) boundary conditions on the turbulence structure.     

我国硝基苯的海水水质基准及生态风险评估研究

以一种重要的化工原料硝基苯为研究对象,通过收集、筛选我国本土物种的硝基苯海水生物毒性数据,同时针对我国海区生物特点补充8种典型海洋受试生物的毒理学实验,应用物种敏感度分布(SSD)方法推导了用于保护水生生物的我国硝基苯海水水质基准值。在此基础上,尝试应用2种概率生态风险评估方法初步评估了硝基苯在我国东海椒江口水体中的生态风险。研究结果表明,用于保护我国海水生物的硝基苯水质基准高值为1.42 mg·L-1,低值为0.037 mg·L-1,与应用SSD方法推导的硝基苯淡水水质基准差异不大。商值概率分布法和联合概率曲线法的风险表征结果表明,硝基苯对椒江口中的水生生物存在潜在的生态风险,需要管理部门采取一定的风险管控措施。研究结果有望为我国水质基准、生态风险研究及硝基苯的海水水质标准制定提供参考。     

Environmental Impact of Undular Tidal Bores in Tropical Rivers

A tidal bore impacts significantly on the estuarine ecosystem, although little is known on the flow field, mixing and sediment motion beneath tidal bores. In the absence of detailed systematic field measurements, a quasi-steady flow analogy was applied to investigate undular tidal bores with inflow Froude numbers between 1.25 and 1.6. Experimental results indicated that rapid flow redistributions occur beneath the free-surface undulations, with significant variations in bed shear stress between wave crests and troughs. Dynamic similarity was used to predict detailed flow characteristics of undular tidal bores. The effects of periodic loading on river sediments, scour of river bed and flow mixing behind the bore are discussed. A better understanding of these processes will contribute to better management practices in tidal bore affected rivers, including the Styx and Daly rivers in tropical Australia.     

A numerical (LES) investigation of a shallow-water, mid-latitude, tidally-driven boundary layer

We investigate turbulent mixing in a tidally driven, mid-latitude, shallow-water basin. The study is carried out numerically at a laboratory-scale, using large-eddy simulation. We compared the results of the simulation with those of a correspondent purely oscillatory flow (Stokes boundary layer). The effect of rotation on the flow dynamics is twofold. First, rotation gives rise to a mean spanwise flow that concurs to redistribute the turbulent energy among the Reynolds stresses, in particular between the horizontal directions, thus increasing the mixing across the water column and thickening the layer where developed turbulence is observable. Second, the presence of the horizontal component of the background vorticity (latitude effect) breaks the symmetry between the two semi-cycles of the oscillation, since turbulence results suppressed/enhanced during the first/second semi-cycle. These two effects significantly modify the turbulent characteristics with respect to the purely oscillating flow, although the mechanisms that generates turbulence present similar features. The qualitative agreement between our results and some measurements carried out in two sites with characteristics similar to the case analyzed suggests that the outcomes here provided may be of general use for the analysis of mid-latitude, neutrally stratified, shallow-water basins mainly driven by semi-diurnal tidal currents.     

Turbulence and turbulent flux events in a small estuary

Relatively little systematic research has been conducted on the turbulence characteristics of small estuaries. In the present study, detailed measurements were conducted in a small subtropical estuary with a focus on turbulent flux events. Acoustic Doppler velocimeters were installed in the mid-estuary at fixed locations and sampled simultaneously and continuously for 50 h. A turbulent flux event analysis was performed for the entire data sets extending the technique of Narasimha et al. (Phil Trans R Soc Ser A 365:841–858, 2007) to the unsteady open channel flow motion and to turbulent sub-events. Turbulent bursting events were defined in terms of the instantaneous turbulent flux. The data showed close results for all ADV units. The very-large majority of turbulent events lasted between 0.04 and 0.3 s with an average of 1 to 4 turbulent events observed per second. A number of turbulent bursting events consisted of consecutive turbulent sub-events, with between 1 and 3 sub-events per main event on average. For all ADV systems, the number of events, event duration and event amplitude showed some tidal trends, with basic differences between high- and low-water periods. A comparison between the present estuary data and the atmospheric boundary layer results of Narasimha et al. (Phil Trans R Soc Ser A 365:841–858, 2007) showed a number of similarities and demonstrated the significance of turbulent events in environmental flows. A burstiness index of 0.85 was found for the present data.     

Numerical simulation of water mixing and renewals in the Barcelona harbour area: the winter season

In the present paper, we use numerical simulation to investigate currents, mixing and water renewal in Barcelona harbour under typical conditions of wind forcing for the winter season. This site is of particular importance due to the interplay between touristic and commercial activities, requiring detailed and high-definition studies of water quality within the harbour. We use Large Eddy Simulation (LES) which directly resolves the anisotropic and energetic large scales of motion and parametrizes the small, dissipative, ones. Small-scale turbulence is modelled by the anisotropic Smagorinsky model (ASM) to be employed in presence of large cell anisotropy. The complexity of the harbour is modelled using a combination of curvilinear, structured, non-staggered grid and the immersed boundary method. Boundary conditions for wind and currents at the inlets of the port are obtained from in-situ measurements. Analysis of the numerical results is carried out based on both instantaneous and time-averaged velocity fields. First- and second-order statistics, such as turbulent kinetic energy and horizontal and vertical eddy viscosities, are calculated and their spatial distribution is discussed. The study shows the presence of intense current in the narrow and elongated part of the harbour together with sub-surface along-shore elongated rolling structures (with a time scale of a few hours), and they contribute to the vertical water mixing. Time-averaged velocity field reveals intense upwelling and downwelling zones along the walls of the harbour. The analysis of second-order statistics shows strong inhomogeneity of turbulent kinetic energy and horizontal and vertical eddy viscosities in the horizontal plane, with larger values in the regions characterized by stronger currents. The water renewal within the port is quantified for particular sub-domain regions, showing that the complexity of the harbour is such that certain in-harbour basins have a water renewal of over five days, including the yacht marina area. The LES solution compares favourably with available current-meter data. The LES solution is also compared with a RANS solution obtained in literature for the same site under the same forcing conditions, the comparison demonstrating a large sensitivity of properties to model resolution and frictional parametrization.     

广州城市行道树特征分析

本文运用生态学方法，探讨了广州市区行道树的生物群落，生态质量及空间分布特下。     

Simulation of exchange flow between open water and floating vegetation using a modified RNG <Emphasis Type="Italic">k</Emphasis>-<Emphasis Type="Italic">ε</Emphasis> turbulence model

The paper focuses on the numerical simulation of the exchange flow between open water and floating vegetation, which plays an important role in maintaining the ecological balance by transporting nutrient matter. The simulation was conducted using a new solver developed upon OpenFOAM. A modified RNG k-ε turbulence model, which is expected to model both the high- and low-Reynolds number flows correctly, was used to determine the eddy viscosity. Several particular terms were added into the momentum equations and turbulence model equations to model the effects of vegetation and buoyancy. Among these terms, the term for the effect of vegetation in the ε-equation was re-modelled. The model was validated by properly predicting the profiles of mean velocity and turbulent kinetic energy for flows through suspended canopies. The density flow between open and vegetated water was simulated with the same conditions as those of the experiment conducted by Zhang and Nepf. The predicted results agreed well with the experimental data and provided more detailed information of such exchange flow. The convection between the root layer and the layer beneath the roots, which was not observed in the experiment, was observed in the numerical simulation.     

Chemical exergy based evaluation of water quality

The thermodynamic concept of chemical exergy is introduced for water quality evaluation, to develop unified objective indicators in contrast to conventional indicators characteristic of subjectivity. While a quantity termed specific standard chemical exergy based on the global reference substances is used to evaluate the standard water quality, an indicator as specific relative chemical exergy with reference to a spectrum of substances associated with some specified water quality standard is developed for practical water quality evaluation, with related concepts of carrying deficit and carrying capacity well embodied in exergy terms. Based on the data collected in the GEMS/WATER project, water qualities of 72 rivers and 24 lakes over the world are evaluated, as a detailed case study to illustrate the adaptability of the chemical exergy based indicators for water quality evaluation.     

Population dynamics of vibrio species in the river Narmada at Jabalpur

Several studies on the presence and ecology of various Vibrio sp have been reported in coastal and estuarine waters throughout the world, but there is trifling information available on the distribution of this organism of colossal pathogenic potential in the fresh water riverine environment. Thus, we conducted a multiyearenvironmental study to scrutinize the occurrence of members of genus Vibrio in the largest west flowing river of the Indian subcontinent, which is also the largest river of central India, the Narmada. Statistical analysis was done to reveal major environmental factors controlling the presence of Vibrio sp in the river Narmada. Monthly field samplings were conducted between January 2002 and December 2003 at four different sites in Jabalpur (MP), India. At each site, water samples were taken and physicochemical and bacteriological parameters were measured. The identity of the isolates was confirmed by employing 16S rRNA analysis. The organisms were found to be widely distributed in the river with regular seasonal variations. The density of Vibrio was found to be correlated with temperature, coliforms and other heterotrophic bacteria. Water temperature accounted for most of the variability in the concentration of Vibrio spAs typical fecal pollution indicators may not access public health risk from potential pathogens such as vibrios, hence special monitoring programme for vibrios may adequately be included in the water quality management.     

Migration and stock studies on the Australian school prawn Metapenaeus macleayi

The results of tagging studies conducted to determine the relationship between estuarine (juvenile) populations and adults at sea suggest that maturing Metapenaeus macleayi leave the estuaries and move along the coast in a northerly direction; the longest migration recorded was 120 km, but most prawns appear to disperse in shallow water (<40 m) within about 70 km of their juvenile habitat. There are approximately 60 estuarine populations of M. macleayi in south-east Australia; the geographical range of prawns emigrating from most estuarine populations overlap (at sea) to some degree with that of adjacent or nearby populations, but there is little or no overlap between any two of the 6 major populations. The mixing of individuals from a major population and from nearby smaller populations is negligible for the purposes of fisheries management, hence the prawns in each of the major estuarine populations and the adults in the coastal area north for about 70 km may be regarded as a unit stock.     

Impact of drought on the ecological and chemical status of surface water and on the content of arsenic and fluoride pollutants of groundwater in the province of Salamanca (Western Spain)

The impact of drought on the ecological and chemical status of surface and groundwaters of the River Tormes (River Duero basin, northwestern Iberian Peninsula) was studied to evaluate the evolution of the quality of the river during its passage through the city of Salamanca (Spain). The Water Quality Index (WQI) of the river revealed that the drought period of 2005 did not significantly affect water chemical quality. However, during the study period differences were found in surface water ecological quality, using phytoplankton quality as AN indicator. These differences may be accentuated as a result of regulation of the River Tormes by the Santa Teresa reservoir. Arsenic and fluoride concentrations were measured in water wells, finding higher arsenic concentrations after the drought period and no correlation between the arsenic and fluoride contents. The results are useful for an overall understanding of potential impact of climate change on the ecological and chemical status of water in regional systems.     

南昌市生态空间和生态保护红线划定研究

生态保护红线作为生态空间的刚性约束区域,是保障和维护国家生态安全的底线和生命线。结合国土空间规划双评价技术,以南昌市为例,选取水源涵养功能、水土保持功能、水质保护功能、生物多样性保护功能4种重要性指标和土壤侵蚀、酸沉降、水污染3种敏感性指标,采用模型评估法和净初级生产力定量指标评估法,开展生态空间和生态保护红线划定研究。结果表明:南昌市生态保护红线面积为1648.43 km2,占总面积的22.27%,以水质保护与水污染敏感红线和生物多样性维护功能生态保护红线为主,面积分别为787.93 km2和628.96 km2,主要分布在南昌市梅岭森林公园及周边地区、中部赣江与瑶湖范围、东部军山湖与青岚湖地区、西部鄱阳湖湖域以及近鄱阳湖部分区域,以水域、农田、森林3种地类为主;生态空间面积为4271.43 km2,占总面积的57.70%,主要分布在南昌东部、南部以及西南方向,以农田、森林地类为主;其他空间面积为1482.67 km2,占总面积的20.03%,主要分布在南昌市中部以及市区周边部分地区,以建设用地为主。研究结果可为区域国土空间规划和生态安全提供参考。     

基于生态足迹法的区域水资源生态承载力计算与评价——以珠海市为例

研究了人类社会对水资源的消耗、对自然生态环境的影响以及该影响的量化方法和量化指标,将生态足迹法引入区域水资源承载力系统,构建珠海市水资源生态承载力计算模型,分析计算水资源供给对区域经济社会发展的生态承载力,并对该地区的水资源生态承载力进行生态盈亏和敏感性评价.结果表明,总体上珠海市水资源系统呈现生态亏损,亏损率为37%,水资源生态承载呈现超载现象,珠海市城市化进程中流动人口增长对区域水资源的生态承载压力较大.研究结果还表明,将生态足迹模型运用到地区水资源承载力中具有较高的可信度,对地方尺度具有适用性.     

不同环境场所夏季空气负离子浓度分布特征及其与环境因子的关系

大气离子是由许多自然和人为的原因产生,并且它们的浓度在不同环境场所下差别很大。以我国南部沿海某省份作为研究区域,挑选了近10处有代表性的环境场所进行为期一周的实测和研究,收集了空气正、负离子浓度、风速、空气温度、相对湿度等数据。研究显示,空气负离子浓度与风速、水、植物、相对湿度等有较为密切的关系,其中最主要的影响因素是水,其次是风,最小的是气温。为了改善城市生态环境,建议在城市规划建设中采取有效措施以提高空气负离子浓度。     

海水混合和层化对叶绿素a垂直分布的影响

根据长江口海区、近岸浅水区、黄海冷水团海区和水深超过200m的陆架区等四个区域的定点调查获得的数据，分析了温度、盐度与叶绿素a垂直分布的相互关系。结果表明，长江口海区，陆源径流与海水混合不充分，表层营养盐含量较高，表层叶绿素a含量高于中下水层。近岸浅水区的苏北近岸海水垂向没有温跃层和盐跃层，叶绿素a的垂向分布也均匀，东海西部沿岸出现逆温跃层和逆盐跃层，海水垂直混合不充分，叶绿素a含量(6．72mg／m^3)在10m深水域最高。黄海冷水团海区海水的垂直混合不充分，叶绿素a的垂直变化显著，高值区出现在温跃层下方(4．37mg／m^3)。水深超过200m的陆架区，温度的阶梯状结构、营养盐跃层、光照等因素共同导致整个水层叶绿素a含量普遍较低。同时，结合历史资料分析认为，海水的混合、温度和盐度的层化将影响营养盐的浓度和分布，从而影响海水中叶绿素a的垂直分布。     

Ecological risk assessment from the viewpoint of surface water pollution in Xiamen City,China

The ecological security of urban surface water is subject to significant risk due to rapid urbanization. Pollutant discharge and accumulation are among the most critical stressors endangering urban surface water and affecting the normal operation of urban aquatic ecosystem services. In this study, we assessed how pollutant accumulation stresses water purification systems, which perform important urban ecosystem services. First, we applied a water environmental capacity model to calculate thresholds of urban surface water environmental capacity under a given water quality target. Second, based on a stepwise regression method, an equation was used to describe the relationship between stressor factors (pollutant accumulation) and measurable socioeconomic indicators. Third, an ecological risk index was used as an assessment endpoint indicator to assess the negative ecological effect of pollutant accumulation. Finally, risk level was classified according to the risk quotient method. Taking Xiamen City as an example, we analyzed the contribution of different sources of pollutants and evaluated the urban ecological risk posed by two major contaminants present in the environment by measuring chemical oxygen demand (COD) and ammonium nitrogen (NH₄⁺-N). The results show that the ecological risk indexes of both COD and NH₄⁺-N are expected to decrease from 2020 to 2030; that of COD is expected to fall from medium to low, whereas that of NH₄⁺-N is expected to fall from high to medium. These findings demonstrate that the ecological risk posed to the surface water in Xiamen City can be reduced by controlling population growth, optimizing industrial structure, and promoting economic development.     

Eddy covariance CO2 flux measurements in nocturnal conditions: an analysis of the problem

A detailed analysis of the various processes at work in stable boundary layers was made. It pointed out that two main mechanisms may affect eddy covariance measurements in stable conditions and that their impacts were different. On one hand, intermittent turbulence produces strongly nonstationary events during which the validity of turbulent transport and storage measurements is uncertain. On the other hand, during breeze and drainage flow events, significant advection takes place and competes with turbulent flux and storage. Intermittent turbulence questions both the ability of eddy covariance systems to adequately capture turbulent flux and storage and the representativeness of the measurements. Ability of the systems to capture the fluxes could be improved by adapting the averaging time period or the high pass filtering characteristics. However, beyond this, the question of representativeness of the flux remains open as the flux measured during an intermittent turbulence event represents not only the source term, but also the removal of CO2 that built up in the control volume and that cannot be simply related to the source term. In these conditions, the u* discrimination is likely to be insufficient and should be completed with a stationarity criterion. Further research should allow determining better selection criteria. Advection occurs mainly in presence of flows associated with topographical slopes (drainage flows) or with land use changes (breezes). Direct advection measurements were performed at several sites, but the results were shown to be strongly site dependent. A classification based on the general flow pattern and on the source intensity evolution along streamlines was proposed here. Five different patterns were identified that helped to classify the different observations. The classification was found to be a fairly good fit for the observations. This could serve as a tool to better understand and quantify the fluxes at sites subjected to repeatable patterns.