Estimation of far-field horizontal and vertical turbulent diffusion coefficients from the concentration field of a wastewater plume near the Akashi Strait

We carried out a field study of the plume discharged by a near-shore wastewater outfall near the Akashi Strait, Japan. Using an Acoustic Doppler Current Profiler and a tow-body CTD, we measured the near-surface salinity and temperature fields in the region throughout an M₂ tidal cycle. We filtered the data in T–S space to remove water masses other than the wastewater, and then used the adiabatic mixing assumption to calculate the concentration of wastewater in the far field of this plume. Averaging the T–S fields of repeated surveys over a time period during which the tidal regime did not change substantially, allowed comparison of the time-averaged plume with the analytical solution for a plume diffusing in both the horizontal and vertical dimensions. The resulting vertical turbulent diffusion coefficients agreed well with those resulting from Thorpe scales determined via a vertically-profiling CTD, as well as with the canonical value for open channel flow of D _z = 0.067hu _*. The corresponding horizontal turbulent diffusion coefficients, however, were two orders of magnitude larger than those typically observed in straight channels, and an order of magnitude larger than those observed in meandering rivers. This is likely a result of enhanced horizontal mixing due to barotropic eddies generated by the interaction of strong tidal flow with headlands and levees, as well as due to the time-varying nature of tidal flow, and baroclinic spreading of the buoyant wastewater plume.     

A kinematic model of the city of Miami ocean outfall plume behavior

A kinematic model is developed to predict the movement of effluent from the City of Miami ocean outfall. The outfall terminates at the 5 m isobath in the near vicinity of three tidal inlets opening into Biscayne Bay. Due to the shallow, uncomplicated topography and oscillatory coastal currents a linear bottom friction law is valid, which justifies the use of a potential flow model. The inlets are modeled as sinks and the outfall as a source discharging into a uniform coastal current. The model predicts that effluent will enter Biscayne Bay in approximately 3 h during flood tide for coastal current headings within an arc of 210° to 40°. Dye-tracking experiments were in good agreement with the model results. Local wind records suggest that sewage effluent enters Biscayne Bay on about 80% of the flood tides, or roughly 11 times per week.     

Dynamics of the buoyant plume off the Pearl River Estuary in summer

Field measurements of salinity, wind and river discharge and numerical simulations of hydrodynamics from 1978 to 1984 are used to investigate the dynamics of the buoyant plume off the Pearl River Estuary (PRE), China during summer. The studies have shown that there are four major horizontal buoyant plume types in summer: Offshore Bulge Spreading (Type I), West Alongshore Spreading (Type II), East Offshore Spreading (Type III), and Symmetrical Alongshore Spreading (Type IV). River mouth conditions, winds and ambient coastal currents have inter-influences to the transport processes of the buoyant plume. It is found that all of the four types are surface-advected plumes by analysing the vertical characteristic of the plumes, and the monthly variations of the river discharge affect the plume size dominantly. The correlation coefficient between the PRE plume size and the river discharge reaches 0.85 during the high river discharge season. A wind strength index has been introduced to examine the wind effect. It is confirmed that winds play a significant role in forming the plume morphology. The alongshore wind stress and the coastal currents determine the alongshore plume spreading. The impact of the ambient currents such as Dongsha Current and South China Sea (SCS) Warm Current on the plume off the shelf has also assessed. The present study has demonstrated that both the river discharge and wind conditions affect the plume evolution.     

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.     

Numerical analysis on the Brisbane River plume in Moreton Bay due to Queensland floods 2010–2011

During the Queensland floods in the summer of 2010–2011, a flood-driven Brisbane River plume extended into Moreton Bay, Queensland, Australia, and then seaward, travelling in a northward direction. It covered approximately 500 km $^{2}$ . This paper presents a three- dimensional hydrodynamic numerical model investigation into the behaviour of the Brisbane River plume. The model was verified by using satellite observations and field measurement data. The present study concludes that the high river discharge was the primary factor determining the plume size and its seaward extensions. A notable finding was that the plume was a bottom-trapped type rather than a buoyant type. Further, the southerly winds were found to have moderately confined the alongshore extension of the plume, and had caused the plume to mix thoroughly with the ocean water.     

Numerical modeling of aquaculture dissolved waste transport in a coastal embayment

Marine aquaculture is expanding rapidly without reliable quantification of effluents. The present study focuses on understanding the transport of dissolved wastes from aquaculture pens in near-coastal environments using the hydrodynamics code SUNTANS (Stanford Unstructured Nonhydrostatic Terrain-following Adaptive Navier–Stokes Simulator), which employs unstructured grids to compute flows in the coastal ocean at very high resolution. Simulations of a pollutant concentration field (in time and space) as a function of the local environment (bathymetry), flow conditions (tides and wind-induced currents), and the location of the pens were performed to study their effects on the evolution of the waste plume. The presence of the fish farm pens cause partial blockage of the flow, leading to the deceleration of the approaching flow and formation of downstream wakes. Results of both the near-field area (area within 10 to 20 pen diameters of the fish-pen site) as well as far-field behavior of the pollutant field are presented. These detailed results highlight for the first time the importance of the wake vortex dynamics on the evolution of the near-field plume as well as the rotation of the earth on the far-field plume. The results provide an understanding of the impact of aquaculture fish-pens on coastal water quality.     

Structure of turbulent plumes from a momentumless source in a smooth bed

The spatial development of a passive scalar plume is studied within the inhomogeneous turbulence of a boundary layer flow in a recirculating laboratory flume with a smooth bed. The source of the scalar is located flush with the bed, and the low-momentum source design is intended to simulate a diffusive-type scalar release. A weakly diffusive fluorescent dye is used as the scalar. Planar laser-induced fluorescence (PLIF) techniques were used to record the structure of the plume at a spatial resolution of 150 μm. The measured structure of the mean concentration field is compared to an analytical solution for shear-free, homogeneous turbulence. The laboratory plume exhibits spatial development in the mean concentration field that deviates from the self-similar behavior predicted by the analytical solution; this deviation is due to the mean shear and inhomogeneity of the turbulence. In particular, the influence of the viscous sublayer on the plume development is seen to be significant. Nonetheless, the analytical solution replicates some of the features seen in the laboratory plume, and the solution suggests methods of reducing the laboratory data even for cases where the results deviate from the analysis. We also examine the spatial development of the root-mean-square (rms) fluctuating concentration field, and use scalar probability density functions to examine the relationship between the mean and fluctuating concentrations.     

Seasonal and diurnal presence of finless porpoises at a corridor to the ocean from their habitat

A number of local populations of finless porpoises (Neophocaena phocaenoides) are widely distributed throughout the warm coastal waters of Asia. The Omura Bay population, consisting of approximately 300 individuals, is the smallest of five populations inhabiting Japanese waters. It is a relatively new population that established after the global warming that took place approximately 9000 years ago. To observe whether these porpoises appear in the major corridor to the ocean from Omura Bay, we used acoustic monitoring to record occurrences of finless porpoises from November 2007 to May 2009. A stereo acoustic event recorder recorded the intensity and the sound source direction of biosonar signals, providing independent traces of sound sources corresponding to each detected animal. A total of 226 individuals were detected over the 1.5-year monitoring period, of which 76% occurred at night and 73% occurred during March and April. We compared the presence of porpoises to the Japanese anchovy catch in Omura Bay and the Hario Strait over the same period. Results suggested that possible reductions in anchovy resources in the bay could attract porpoises to the outside of their normal habitat. In total, 70% of the porpoise recordings took place when the tidal current was moving out of Omura Bay. Porpoises might follow the prey that are transported out of the bay due to the strong outbound current. The finless porpoises confined to the bay might extend their swimming area if prey is available.     

苯酚在滩涂沉积物上的吸附特性

研究了苯酚在泉州湾滩涂沉积物上的吸附行为，尤其是pH值和温度对吸附作用的影响，探讨了其吸附机理，同时绘制了吸附等温线。实验结果表明，常温下苯酚在泉州湾滩涂沉积物上的吸附平衡时间约需6h；吸附作用随pH值和温度的升高而减弱；苯酚在泉州湾滩涂沉积物上的吸附行为可以较好地用Henry、Langmuir和Freundlich型吸附等温式来描述，相比而言，Freundlich型拟合效果最好，这说明其吸附机理实际上非常复杂。     

Rookery Bay and Naples Bay circulation simulations: Applications to tides and fresh water inflow regulation

A three-dimensional, baroclinic numerical circulation model is applied to the combined Rookery Bay and Naples Bay estuary complex. We focus on two aspects of the model simulations: the tidal circulation, for which we have observations of sea level and currents at several stations spanning Rookery Bay, and the salinity variations, for which we have observations at one station. After establishing model veracity through quantitative comparisons with observations, we use the model to address salinity changes that could arise due to regulatory fresh water flow rate modifications. Two different sets of experiments are performed. The first considers the salinity changes in Rookery Bay by increasing the fresh water flow rates into Henderson Creek through the addition of either 1.4 m³/s or 2.8 m³/s to the ambient flow rates. The second considers the salinity changes in Naples Bay by decreasing the fresh water flow rates through the subtraction of 9.9 m³/s from the Golden Gate Canals’ ambient flow rates. All of these scenarios result in significant, quantifiable salinity changes within the Rookery Bay and Naples Bay estuary complexes, demonstrating the environmental management utility of using high resolution, three-dimensional, numerical circulation models, as applied herein, for assessing the water properties of complex, multi-connected estuarine systems.     

Large Eddy Simulation of turbulence generated by a weak breaking tidal bore

A tidal bore is a natural and fragile phenomenon, which is of great importance for the ecology of an estuary. The bore development is closely linked with the tidal range and the river mouth shape, and its existence is sensitive to any small change in boundary conditions. Despite their ecological and cultural value, little is known on the flow field, turbulent mixing and sediment motion beneath tidal bores. Indeed, some striking features can be highlighted in two-dimensional simulations, such as large velocity fluctuations and flow recirculation structures. Using Large Eddy Simulation method, the numerical results emphasised the complicated turbulent structures and their unsteadiness under a tidal bore.     

Methodological approaches for delimitating mixing zones in rivers: establishing admissibility criteria and flow regime representation

The present paper addresses defining the extent of the mixing zones of effluents discharged into rivers, which is a problem that should be analyzed based on probabilistic terms, as it is governed by several random processes. A river’s flow regime is one of the main variables, and it has a high dependence on hydrological processes. Additionally, after calculating the extent of the mixing zone, it is necessary to determine if the resulting dimensions are admissible or not. Common practice includes the admissibility criteria associated with the geometry of the river. However, this practice does not consider the environmental characteristics of the river that make it capable of absorbing the impact of the effluent (the biological and hydromorphological status of the river and the presence of structures that can change the river flow conditions, ecologically sensitive area or threatened species). This paper presents work on two important topics: on the one hand, the development of a methodology to establish the admissible extent of the mixing zone as a function of the environmental vulnerability of the river to the discharged effluent and, on the other hand, the proposal of a procedure to perform the calculations of the effluent mixing considering the variability of the river’s flow regime. The proposed methodological approaches are illustrated with an application to a real case, including a numerical simulation of the hydrodynamic and effluent evolution of the river during a year, to test the proposed methodology’s suitability and demonstrate the important savings in computational effort that can be achieved.     

Simulations of the flow in the Mahakam river–lake–delta system,Indonesia

Large rivers often present a river–lake–delta system, with a wide range of temporal and spatial scales of the flow due to the combined effects of human activities and various natural factors, e.g., river discharge, tides, climatic variability, droughts, floods. Numerical models that allow for simulating the flow in these river–lake–delta systems are essential to study them and predict their evolution under the impact of various forcings. This is because they provide information that cannot be easily measured with sufficient temporal and spatial detail. In this study, we combine one-dimensional sectional-averaged (1D) and two-dimensional depth-averaged (2D) models, in the framework of the finite element model SLIM, to simulate the flow in the Mahakam river–lake–delta system (Indonesia). The 1D model representing the Mahakam River and four tributaries is coupled to the 2D unstructured mesh model implemented on the Mahakam Delta, the adjacent Makassar Strait, and three lakes in the central part of the river catchment. Using observations of water elevation at five stations, the bottom friction for river and tributaries, lakes, delta, and adjacent coastal zone is calibrated. Next, the model is validated using another period of observations of water elevation, flow velocity, and water discharge at various stations. Several criteria are implemented to assess the quality of the simulations, and a good agreement between simulations and observations is achieved in both calibration and validation stages. Different aspects of the flow, i.e., the division of water at two bifurcations in the delta, the effects of the lakes on the flow in the lower part of the system, the area of tidal propagation, are also quantified and discussed.     

Heavy metals and marine diatoms: Influence of dissolved organic compounds on toxicity and selection for metal tolerance among four species

Experiments were conducted to determine whether diatom clones isolated from waters (Corio Bay or Hobson's Bay, Australia) chronically contaminated with heavy metals were more metal-resistant than clones of the same species isolated from cleaner waters (Bass Strait). Four species (2 clones per species) and 3 metals (Cu, Zn, Cd) were examined. The growth response of each clone to metal additions was assessed in both Corio Bay water and Bass Strait water. Generally, Cu was readily chelated and the most toxic metal and Cd the least chelated and least toxic. Toxicity of Cu to growth was found to be directly related to cellular Cu levels, which in turn were mediated by dissolved organic compounds in seawater. Corio Bay water had a greater metal-complexing capacity than Bass Strait water, and metal additions generally proved more toxic to each clone when in Bass Strait water. Ultraviolet irradiation eliminated differences between the water types. Relative to Bass Strait clones, no trend of enhanced metal tolerance was noted among the Corio Bay and Hobson's Bay clones; on the contrary the Corio Bay and Hobson's Bay cells were generally less to lerant of metals than were their Bass Strait counterparts, being particularly affected when in Bass Strait water. The results suggest that metal additions to waters rich in dissolved organic compounds would likely have less effect (in terms of direct toxicity and as a selective agent for metal tolerance) on resident phytoplankters than similar additions to waters low in organics.Publication No. 274 in the Ministry for Conservation, Environmental Studies Series     

Vertical migration strategies with respect to advection and stratification in a semi-enclosed lough: a comparison of mero- and holozooplankton

Patterns of zooplankton vertical movement are often difficult to interpret because of multiple, complex and confounding environmental factors. Behavioural adaptations to these environmental variables are compared within and between the holo- and meroplankton constituents of a community. We used a nested design to analyse patterns at several scales in time; (semi-diel, diel, spring-neap tidal cycle and season) and two in space; (depth and site). To reduce complexity and aid interpretation we studied a semi-isolated community in a semi-enclosed, seasonally stratified sea lough (Lough Hyne Marine Nature Reserve, Ireland). In this, the main environmental gradient was water flow rate (or water residence time) caused by tidal currents. Vertical profiles of abundance showed that populations of the most abundant species of holo- and meroplankton in the lough have considerable behavioural plasticity, enabling them to switch between sedentary and migratory behaviour and patterns of migration. Some species migrate vertically in synchrony with diel cycles and others in response to semi-diel tidal currents; a few do both, but the majority did neither. It is suggested that water column structure and hydrographic discontinuities caused by flow rate and pycnocline dynamics are responsible for the variable patterns of vertical migration and distribution.Communicated by J.P. Thorpe, Port Erin     

A model diagnostic study of age of river-borne sediment transport in the tidal York River Estuary

As nutrients and organic matters are transported preferentially in an adsorbed state and tend to bind to the sediments, sediment transport plays an important role on eutrophication processes in the estuaries. The timescale of sediment transport is of significance for studying the retention of pollutants and eutrophication processes in the estuaries. Unlike transport of dissolved substances that is mainly controlled by advection and diffusion processes, the sediment transport is significantly affected by the intermittent settling and resuspension processes. A three-dimensional model with suspended sediment transport was utilized to investigate the transport timescale of river-borne sediment in the tidal York River Estuary. The results indicate that river discharge dominantly determines the age of river-borne sediment in the estuary. High river discharge results in a low sediment age compared to that under mean flow. The intermittent effects of settling and resuspension events greatly affect the river-borne sediment age. Both settling velocity and critical shear stress are shown to be key parameters in determining the sediment transport timescale. The sediment age decreases as settling velocity and/or critical shear stress decrease, while it increases with the increase of settling velocity that prevents the sediment to be transported out of the estuary.     

Sun-compass migration by Aurelia aurita (Scyphozoa): population retention and reproduction in Saanich Inlet,British Columbia

The scyphomedusa Aurelia aurita in Saanich Inlet, a north-south oriented fjord in British Columbia, uses a sun compass to migrate in a southeasterly direction during the day. When the sky is overcast and at night, A. aurita orients randomly and is dispersed passively by gentle tidal currents. The net result is daily reaggregation of medusae into enormous swarms along the southeastern shore of the fjord. Observations of spawning A. aurita in these swarms suggest that sun-compass migration and aggregative behavior may have evolved to facilitate reproduction and to maintain the population within this fjord throughout the year.     

Shallow wake behind exposed wood-induced bar in a gravel-bed river

Recent theoretical research indicates that dynamics of shallow flows can be strongly affected by waves developing on the free surface. In this study a shallow wake with an oblique pressure wave behind a model of a tree-centered emergent bar is investigated in a gravel-bed river. A bar was constructed in a shallow river reach with nearly uniform flow. The structure of flow was assessed with an array of velocimeters. Flow visualization with a solute of fluorescent dye complemented the measurements and provided qualitative information on the wake behavior. This study indicates that quantitative criteria for shallow wakes classification developed in laboratory setups are not straightforward in explaining the field results. According to the wake stability criteria, the expected dynamics for examined wake flow is a vortex street (VS) type. Contrary to this expectation, measurements and visualizations in this study show that mean momentum differential and turbulent vortices in the wake decrease stronger than expected in VS type and therefore the wake should be classified as unsteady bubble type with a weak downstream instability. Analysis of velocity differential dynamics in the examined shallow wake suggests that the bed friction alone is insufficient to explain the inconsistency of VS criterion whereas accounting for advective fluxes driven by inhomogeneous pressure field leads to a correct prediction of the wake behavior.     

A simplified approach to address turbulence modulation in turbidity currents as a response to slope breaks and loss of lateral confinement

Turbidity currents traversing canyon-fan systems flow over bed slopes that decrease in the downstream direction. This slope decrease eventually causes turbidity currents to decelerate and enter a net-depositional mode. When the slope decrease is relatively rapid in the downstream direction, the turbidity current undergoes a concomitantly rapid and substantial transition. Similar conditions are found when turbidity currents debouch to fan systems with loss of lateral confinement. In this work a simplified approach to perform direct numerical simulation of continuous turbidity currents undergoing slope breaks and loss of lateral confinement is presented and applied to study turbulence modulation in the flow. The presence of settling sediment particles breaks the top–bottom symmetry of the flow, with a tendency to self-stratify. This self-stratification damps turbulence, particularly near the bottom wall, affecting substantially the flow’s ability to transport sediment in suspension. This work reports results on two different situations: turbidity currents driven by fine and coarser sediment flowing through a decreasing slope. In the case of fine sediment, after the reduction in the slope of the channel, the flow remains turbulent with only a modest influence on turbulence statistics. In the case of coarse sediments, after the change in slope, turbulence is totally suppressed.     

Numerical simulation of benzene transport in shoreline groundwater affected by tides under different conditions

● An approach for assessing the transport of benzene on the beach was proposed. ● The behavior of benzene in the subsurface of the beach was impacted by tide. ● Tidal amplitude influenced the travel speed and the benzene biodegradation. ● Hydraulic conductivity had the impact on plume residence time and biodegradation. ● Plume dispersed and concentration decreased due to high longitudinal dispersivity. The release and transport of benzene in coastal aquifers were investigated in the present study. Numerical simulations were implemented using the SEAM3D, coupled with GMS, to study the behavior of benzene in the subsurface of tidally influenced beaches. The transport and fate of the benzene plume were simulated, considering advection, dispersion, sorption, biodegradation, and dissolution on the beach. Different tide amplitudes, aquifer characteristics, and pollutant release locations were studied. It was found that the tide amplitude, hydraulic conductivity, and longitudinal dispersivity were the primary factors affecting the fate and transport of benzene. The tidal amplitude influenced the transport speed and percentage of biodegradation of benzene plume in the beach. A high tidal range reduced the spreading area and enhanced the rate of benzene biodegradation. Hydraulic conductivity had an impact on plume residence time and the percentage of contaminant biodegradation. Lower hydraulic conductivity induced longer residence time in each beach portion and a higher percentage of biodegradation on the beach. The plume dispersed and the concentration decreased due to high longitudinal dispersivity. The results can be used to support future risk assessment and management for the shorelines impacted by spill and leaking accidents. Modeling the heterogeneous beach aquifer subjected to tides can also be further explored in the future study.