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Abstract
The flow patterns in semi-enclosed water bodies are generally complicated and have significant effect on the water quality and fluid exchange there. The exchange of of fluid between the semi-enclosed water body and outside is due to two physical mechanisms. The first is the flushing due to tidal effect. The second is the shear induced by the velocity difference between the flow within the water body and outside. To predict the flow and solute transport in these water bodies a three-dimensional layer-integrated numerical model has been developed. To account for the turbulence which consists of the free shear component and the bottom friction component the k-e model is employed. The numerical model has been applied to the cases of tidal flow as well as steady river flow outside a rectangular harbour with or without breakwater. Compared with the available experimental results, the gross mass exchange both due to tidal effect and shear can be estimated satisfactorily by the numerical model.
... Regarding studies of mass exchange in open-channel, single lateral cavities, about half of them exhibit specific geometrical or flow details that prohibit a direct comparison: [16,15] respectively measured and computed the mass exchange coefficient k in a lateral cavity with an interface section S ex (see Eq. 1) limited by a partial wall; [13] considered an oscillating inflow; [25] included a step at the entrance of the lateral cavity; [8] considered cavities with complex rough bathymetries and [9] considered a semi-circular and two conic-shaped cavities. These studies are then excluded from the data used for comparison in Table 2. ...
Lateral cavities are major storage zones in riverine environments for which the mass exchanges with the main stream strongly impact the characteristics of the habitat in these dead zones. An experimental work is presented here with a controlled main stream and a connected open-channel lateral cavity to assess the processes responsible for these exchanges and to quantify the exchange capacities. In a first step, the measurements of passive scalar transport allow us to identify the physical processes involved in the exchange of mass from the main stream and its spreading within the cavity. In a second step, the quantitative mass exchange coefficient, representative of the exchange capacity, is measured for 28 flow and cavity configurations. The sensibility analysis to the governing parameters proposed by the dimensional analysis then reveals that changing the geometric aspect ratio of the cavity does not affect the exchange coefficient while increasing the normalized water depth or decreasing the Reynolds number of the main stream tend to increase this coefficient. Indeed, these parameters modify both the growth rate of the mixing layer width at the interface and the amplitude of the alternating transverse velocity across the interface, thus affecting the exchange capacities from the main stream to the cavity.
... Since field measurements are usually costly and some times impossible due to physical inabilities, application of numerical models becomes more and more important in the simulations of coastal water bodies. Use of three-dimensional models is unavoidable in all cases where the influence of density distribution, or the vertical velocity variations can not be neglected and in the simulation of wind induced circulation [1], [2], [3]. The detailed knowledge of the water motion is very crucial for a reliable prediction of suspended sediment transport [4], [5]. ...
A numerical model HYDROTAM-3, is developed that simulates turbulent coastal transport processes due to tidal or nontidal forcing. Model includes four sub model components; hydrodynamic, turbulence, salinity and temperature transport, suspended sediment transport. The only simplifying assumption used in the model is Boussinesq approximation, i.e. the density differences are neglected unless the differences are multiplied by the gravity. It is a composite finite difference finite element model. Model has been applied to Fethiye Bay located at the Mediterrenean Sea. At three Stations in the Bay, continuous measurements of velocity throughout the water depth and water level, were taken for 27 days. Model well simulates the measurements.
... Water exchange in a river-harbour system has been recently investigated by Li & Gu (2001). A sophisticated quasi-3D layered-integrated model served to calculate the flow and a technique similar to the one applied by Signell (1992) cited above was used to quantify the flushing of the harbour both in steady-state and tidal conditions. ...
The aim of the present study is to measure the passive-scalar exchange coefficients in simplified open-channel lateral cavities and to investigate the influence of the flow and cavity parameters on this coefficient. Two methodologies proposed in the literature are applied herein: i) velocity measurement in the mixing interface and ii) dye concentration measurement in the cavity, applying a mass conservation analysis. Both estimates appear to be in fair agreement. Present results along with data from the literature reveal that: i) increasing only the cavity width does not impact the obtained exchange coefficient, ii) increasing the dimensions (while keeping the same aspect ratio) of the cavity or increasing only the water depth tends to increase this coefficient and iii) when increasing U (keeping all other parameters equal) no clear tendency is retrieved and iv) fitting trends valid for groyne fields do not apply for lateral cavity configurations.
This study presents a numerical solution to the three-dimensional solute transport in heterogeneous media by using a layer-integrated approach. Omitting vertical spatial variation of soil and hydraulic properties within each layer, a three-dimensional solute transport can be simplified as a quasi-three-dimensional solute transport which couples a horizontal two-dimensional simulation and a vertical one-dimensional computation. The finite analytic numerical method was used to discretize the derived two-dimensional governing equation. A quadratic function was used to approximate the vertical one-dimensional concentration distribution in the layer to ensure the continuity of concentration and flux at the interface between the adjacent layers. By integration over each layer, a set of system of equations can be generated for a single column of vertical cells and solved numerically to give the vertical solute concentration profile. The solute concentration field was then obtained by solving all columns of vertical cells to achieve convergence with the iterative solution procedure. The proposed model was verified through examples from the published literatures including four verifications in terms of analytical and experimental cases. Comparison of simulation results indicates that the proposed model satisfies the solute concentration profiles obtained from experiments in time and space.
A procedure to correlate extreme wave heights and extreme water levels in coastal waters using numerical models together with joint probability analysis has been proposed. A third-generation wave model for wave simulation and a three-dimensional flow model for water level simulation are coupled through the surface atmospheric boundary layer. The model has been calibrated and validated against wind, wave and water level data collected in the coastal waters of Hong Kong. The annual maximum wave height and the concomitant water level have been obtained by simulating the annual extreme typhoon event for 50 consecutive years. The results from bivariate extreme value analysis of the simulated data show that the commonly used empirical method may lead to underestimation of the design water level.
A three-dimensional multi-level hydrodynamic model has recently been developed and applied to tidal motion in Singapore’s coastal waters. This paper describes a series of numerical experiments to evaluate the sensitivity of the tidal currents and elevations to model parameters. The results show that the predicted tidal elevations are insensitive to three model parameters: horizontal eddy viscosity coefficient (Smagorinsky constant, ch), bottom friction coefficient (cb) and internal friction coefficient (cv), whereas the effects of these parameters are quite different for tidal current velocities. The velocities are slightly reduced with an increase in ch and cb. The bottom friction effects on velocity profiles increase with water depth. The effect of cv might be significant for the tidal velocities at all levels. The velocities at upper layers of the water column decrease with the increase in cv, whereas the velocities at the bottom layer show the reverse trend. The effects of three model parameters on the magnitude and phase of the simulated currents are in the order (from strong to weak) of cv, cb and ch.
A novel semi-three-dimensional (semi-3D) layer-integrated approach was proposed in this study for the shallow water free-surface flow computation. A quadratic shape function proposed to approximate the velocity distribution in the layer ensures the continuity of velocities and shear stresses at interfaces. To discretize the governing equations, the finite volume formulation with staggered grid is used. As the two-dimensional (2D) sub-model for locating the free surface is not needed in this approach, the computational time consumption has been dramatically reduced. The model was verified through the wind-induced circulation in a closed basin by comparing the velocity profiles to the analytical solution. The formation of the velocity profiles due to change of viscosity distribution and the evolution process of water surface slope were also investigated. Further, the eddy viscosity is estimated by a function related to discharge, water depth, and Manning's n under the assumption of parabolic distribution of longitudinal velocity along the vertical direction for open channel flows. Two designed hypothetical cases were conducted to examine the proposed eddy viscosity relation and to demonstrate the capabilities of the proposed model. Copyright © 2007 John Wiley & Sons, Ltd.
A comparison of two three-dimensional numerical modeling systems for tidal elevations and velocities in the coastal waters is presented. The two modeling systems are: (1) the Princeton Ocean Model (POM) and (2) the MIKE 3 flow model. The model performance results for Singapore's coastal waters show that the predicted tidal elevations from the two hydrodynamic modeling systems are almost identical and are in very good agreement with field measurement data. The simulated tidal current velocities match well with field measurement data at the selected stations, but it seems that the POM provides the slightly better simulation, compared to the MIKE 3 flow model. The depth profiles of the velocities obtained from the two modeling systems may be greatly different at some time, due to the vertical diffusion coefficient calculated from different turbulent sub-models in the two modeling systems. The POM generally predicts larger peak tidal velocities. The maximum speed differences for the model results from the two modeling systems occur in the top and differ from time to time and from location to location, reaching up to 20%.
The city of Suva is home to nearly a quarter of the population of the Fiji thereby placing a lot of anthropogenic pressure on its lagoon. The Suva lagoon (comprising of Suva Harbour and Laucala Bay) has been subject to substantial sediment inputs generated by erosion and human activities. Freshwater input into the lagoon comes predominantly from the Rewa River, the largest fluvial system in the country. The high sedimentary load from the Rewa River, especially during the wet-warm period (November-April), has a strong impact on the lagoon. In addition, there are local sources of pollutant input into the lagoon via the Vatuwaqa River and the Kinoya Sewage Treatment Plant, which discharges effluents into the lagoon and degrades the marine environment. The salinity, temperature and turbidity in the Suva lagoon are some important parameters for water quality which are continuously changing with the seasons and need to be studied because they are efficient indicators of variations in the lagoon and can transform the marine ecosystem. Several field trips were undertaken in the Suva lagoon to collect hydrographical data to study the water properties in the lagoon. Results obtained showed that the salinity near the head of Laucala Bay during the wet-warm period was below 24.8 psu and was 33.7 psu during the dry-cool period (May - October). The temperature range during the wet-warm period was between 28.0-30.5°C and between 24.5-25.5°C during the dry-cool period. The turbidity was always above 3.0 FTU near the river mouths. The field results show that the variations of CTD measurements are dependent predominantly on the river discharge, while the dominant wind regime is the southeast trade winds.
In this note it is suggested that the three-tube yaw probe be placed on the boundary and used as a Preston tube to measure the boundary shear stress when both its magnitude and direction are unknown. The calibration results necessary for this method are also presented herein.
UV disinfection is now widely used for the treatment of water for consumption and wastewater in many countries. It offers advantages over other techniques in specific circumstances. Analysis of these systems has been carried out using a three-dimensional Computational Fluid Dynamics (CFD) procedure. This allows for efficient testing of prototypes. Sensitivity tests are shown for grid size, discretisation and turbulence model.
Four different configurations of the apparatus are evaluated in terms of maximum dosage, flow patterns, particle tracks and transient dosage. This leads to conclusions about the most efficient design and shows that significant improvements can be achieved with minor changes to the design. Further conclusions are drawn about the CFD procedure itself. This work opens up the possibility of an internet-based design tool for small- and medium-sized enterprises.
This study is divided into three parts centred around modelling the complex turbulent fluxes across strong shear layers, such as exist between the channel and floodplain flow in an over bank flood flow. The three stages utilize Adaptive Neuro-Fuzzy Inference Systems (ANFIS) to make fuzzy mappings between the fluxes and different data types. The de-fuzzification stage commonly used in Fuzzy Inference Systems is adapted to avoid the generation of crisp outputs, a process which tends to hide the underlying uncertainty implicit in the fuzzy relationship.
Each stage of the study utilizes conditioning data that makes the fuzzy mappings more tenuously linked with what would normally be considered physically based relationships. The need to make such mappings in distributed models of complex systems, such as flood models, stems from the sparsity of available distributed information (e.g. roughness) with which to condition the models. If patterns in distributed observables which clearly affect, or are affected by, the river hydraulics can be linked to the local fluxes, then the conditioning of the model would improve. Mappings such as these often suffer from scaling effects, an issue addressed here through training the fuzzy rules on the basis of both laboratory and field collected data.
In the late 1970s. the adverse effects of man-made eutrophication became manifest in many countries. which explains. perhaps. why there was such a broad interest when the former Resources and Environment Area of the International Institute for Applied Systems Analysis (IIASA) organized a workshop on the subject. There was such an enthusiasm among the partici pants that two further workshops were quickly organized. one on deep and the other on shallow lake eutrophication problems. The organization of these meetings was extremely stimulating. and the round table discussions among scientists from both West and East remain thought provoking for those who took part. The general feeling emerged that the complexity and multifaceted nature of the problem, even though perhaps not fully recognized at that time. clearly demanded a systems analysis approach. No wonder. then. that the request made by the Hungarian Member Organization of nASA to adopt Lake Balaton as a "real life laboratory" for an nASA case study fell on fertile ground. the more so since it appeared that shallow lake eutrophication had received less attention and was less well understood than that of deep lakes. And so the nASA Lake Balaton Case Study began. with the appointment of Gerrit van Straten as the first leader of the project.