Geophysical modeling for groundwater and soil contamination risk assessment

This PhD thesis is focused on the study of environmental problems linked to contaminant detection and transport in soil and groundwater. The research has two main objectives: development, testing and application of geophysical data inversion methods for identifying and characterizing possible anomaly sources of contamination and development and application of numerical models for simulating contaminant propagation in saturated and unsaturated conditions. Initially, three different approaches for self-potential (SP) data inversion, based on spectral, tomographical and global optimization methods, respectively, are proposed to characterize the SP anomalous sources and to study their time evolution. The developed approaches are first tested on synthetic SP data generated by simple polarized structures, (like sphere, vertical cylinder, horizontal cylinder and inclined sheet) and, then, applied to SP field data taken from literature. In particular, the comparison of the results with those coming from other numerical approaches strengthens their usefulness. As it concerns the modelling of groundwater flow and contaminant transport, two cellular automata (CA) models have been developed to simulate diffusion-dispersion processes in unsaturated and saturated conditions, respectively, and to delineate the most dangerous scenarios in terms of maximum distances travelled by the contaminant. The developed CA models have been applied to two study areas affected by a different phenomenon of contamination. The first area is located in the western basin of the Crete island (Greece), which is affected by organic contaminant due to olive oil mills wastes (OOMWs). The numerical simulations provided by the CA model predict contaminant infiltration in the saturated zone and such results are in very good agreement with the high phenol concentrations provided by geochemical analyses on soil samples collected in the survey area at different depths and times. The second case study refers to an area located in the western basin of Solofrana river valley (southern Italy), which is often affected by heavy flooding and contamination from agricultural and industrial activities in the surroundings. The application of a multidisciplinary approach, which integrates geophysical data with hydrogeological and geochemical studies, and the development of a CA model for contaminant propagation in saturated conditions, have permitted to identify a possible phenomenon of contamination and the delineation of the most dangerous scenarios in terms of infiltration rates are currently in progress

The geomorphology of coarse clastic surfaces in arid environments

This study explores the linkages between slope form and slope process in arid environments. In doing so, questions of the development of slopes in arid environments are examined. The age of many arid environment surfaces, combined with the sporadic nature of formative events, means that long-term surface and slope development remains an elusive question in geomorphology. Deserts have inspired many of the most enduring theories of landscape evolution and continue to provide a test-bed for new and emerging ideas in geomorphology. The clast-mantled surface of the northeast Jordan Badia presents an ideal opportunity to study the links between surface character and slope processes in arid environments. The northeast Badia also provides an opportunity to explore theories of slope development and the behaviour of earth surface systems. The nature of the clast covered ground surface has been assessed using a new digital aerial photography and image analysis technique. A field study of surface processes has been used to explore links between surface form and slope process. Additionally, a computer based simulation of long-term modification of the spatial distribution of surface clast has been undertaken. Given the subtle variation in earth surface form between disparate locations, a new semi-quantitative method of locating sample sites has been developed. The characterization of surface form has identified statistically significant relationships between ground surface character and two-dimensional slope form. Systematic variations in ground surface configuration, both within and between basalt flows, are found to be indicative of the action of slope processes. The first study of ground surface hydrology in the north eastern Badia has been undertaken. The results from a series of rain-storm simulation experiments show subtle but significant links between the action of surface processes and variations in ground surface form. The controls on surface process are diverse and vary in significance with position in the landscape. A combination of ground surface characterization and process studies has identified several interesting geomorphological phenomena The surfaces exhibit systematic variations in structure and organization. Homeostatic links between form and process are clearly apparent, which suggests that surface form influences and is influenced by process action via a process of positive feedbacks. Given the sporadic and infrequent recurrence of formative events in arid environments, a modelling approach has been developed to understand the long-term, spatial dynamics of the ground surface. The model has been used to simulate structure in the surface clast arrangement and the sensitivity of surface organization to physically constrained variations in model parameters. The model also allows the surfaces to be considered as self-organizing earth surface systems. The model results provide new insights into the process-form linkages in operation on clast-mantled arid surfaces. The model results provide new ways of examining and understanding the dynamics of clast mantled arid surfaces and have implications for the application of self-organization in geomorphology

NUMERICAL SIMULATION OF AERODYNAMIC AND ELECTROSTATIC EFFECTS IN A POWDER COATING FLUIDIZED SYSTEM

The work described in this thesis was carried out for a better understanding of the phenomena in an electrostatic fluidized bed for powder coating. The publications in different areas of science (Computer Science, Chemical Engineering and Electrical Engineering) have been of great help to obtain a final written computer code that combines novel computational methods which aim at simulating the physical effects that occur in any electrostatic fluidized system. One of the main purposes of existing industrial fluidized beds is the coating of metallic pieces. The process to achieve this includes using insulating powder of a particle size appropriate for fluidization. The metallic piece to be coated has to be pre-heated substantially above the melting point of the powder coating material in order to obtain enough enthalpy to cure the required mass of coating insulation. Once this appropriate temperature is reached, the piece is dipped in the fluidized bed for the length of time necessary to acquire a uniform coating. This way of coating in industrial fluidized beds is very inefficient since energy is spent pre-heating the metallic piece and because usually once the dipped piece has been coated, it is placed in an oven again for a new heating period, to finally obtain a uniform surface coating. All these procedures involve substantial increases in energy costs. A new method to obtain uniform powder coated pieces with minimal energy costs was thought of. This idea implies the electrification of the insulating powder particles. In this case if a cold metallic electrically connected to ground piece is dipped in a fluidized bed, the electric field generated by the space charge in the bed will propel the charged particles towards the piece to be coated and remain attached to it. Once taken out of the fluidized bed, the charged particles remain attached (due to the Coulomb attraction force) while the piece is taken to the oven for curing. In 2004, experiments were carried out at the Applied Electrostatic Research Centre (A.E.R.C.) at the University of Western Ontario. An experimental fluidized bed with powder particles was used which included a suction pump system. Through this suction system fluidized powder particles were sucked from the system, and forced to travel around a Teflon tube. Due to the difference of work functions the particles acquired a high positive charge. Furthermore, the particles were injected back into the system, increasing the total net charge of the system. Thus, so when a metallic electrically earthed piece was dipped, due to the Coulomb attraction forces between the charged powder particles in the system and the electrically earthed piece a good surface coating would be obtained. Results from these experiments showed that during injection the total net charge of the system increased for a certain period of time and then gradually decreased. It was concluded that the electrification (injection of highly positive charged particles) of the bed powder had to be carried out only during the time that the piece to be coated was dipped in the bed; otherwise a good coating was not possible. These unexplained findings are the basis for this thesis. In this project, a numerical complex simulation of a fluidized system was performed, for a system with three different size particles (60 pm, 80 pm and 100 pm). Throughout this thesis snapshots of these particles’ positions are shown to better visualize their trajectory paths. After a certain period of time the system acquired a total net charge due to the tribocharging process, at this point a new set of highly positively charged particles (120 pm) was injected. It was expected that the total net charge of the system increases accordingly to the total charge of these new injected particles, however, the total net charge increased up to a certain level and then it seemed to fade away as first stated by the results of the A.E.R.C.’s experiments. The simulations performed in this work tried to reproduce the conditions set in the experiments done in the A.E.R.C. (2004). One of the main achievements was the visualization of the trajectories of all the particles in the system. Appendix 1 presents snapshots of this visualization. Additionally, Chapter 6 shows other insightful results. As mentioned earlier, one of the main objectives of this thesis was to find out why the most efficient coating occurred only when the metallic piece and the positive charged particles were dipped and injected simultaneously. Analyzing the video and graphs obtained the conclusion was that if the injection of charged particles had been done prior to the dipping of the metallic piece, part of the total net charge would have gradually discharged into the electrically earthed walls. Therefore, by the time the earthed metallic piece was dipped the total net charge of the system was not going to be as high as expected which consequently would have ended up in a poor coating

Depth-averaged and 3D Finite Volume numerical models for viscous fluids, with application to the simulation of lava flows

This Ph.D. project was initially born from the motivation to contribute to the depth-averaged and 3D modeling of lava flows. Still, we can frame the work done in a broader and more generalist vision. We developed two models that may be used for generic viscous fluids, and we applied efficient numerical schemes for both cases, as explained in the following. The new solvers simulate free-surface viscous fluids whose temperature changes are due to radiative, convective, and conductive heat exchanges. A temperature-dependent viscoplastic model is used for the final application to lava flows. Both the models behind the solvers were derived from mass, momentum, and energy conservation laws. Still, one was obtained by following the depth-averaged model approach and the other by the 3D model approach. The numerical schemes adopted in both our models belong to the family of finite volume methods, based on the integral form of the conservation laws. This choice of methods family is fundamental because it allows the creation and propagation of discontinuities in the solutions and enforces the conservation properties of the equations. We propose a depth-averaged model for a viscous fluid in an incompressible and laminar regime with an additional transport equation for a scalar quantity varying horizontally and a variable density that depends on such transported quantity. Viscosity and non-constant vertical profiles for the velocity and the transported quantity are assumed, overtaking the classic shallow-water formulation. The classic formulation bases on several assumptions, such as the fact that the vertical pressure distribution is hydrostatic, that the vertical component of the velocity can be neglected, and that the horizontal velocity field can be considered constant with depth because the classic formulation accounts for non-viscous fluids. When the vertical shear is essential, the last assumption is too restrictive, so it must relax, producing a modified momentum equation in which a coefficient, known as the Boussinesq factor, appears in the advective term. The spatial discretization method we employed is a modified version of the central-upwind scheme introduced by Kurganov and Petrova in 2007 for the classical shallow water equations. This method is based on a semi-discretization of the computational domain, is stable, and, being a high-order method, has a low numerical diffusion. For the temporal discretization, we used an implicit-explicit Runge-Kutta technique discussed by Russo in 2005 that permits an implicit treatment of the stiff terms. The whole scheme is proved to preserve the positivity of flow thickness and the stationary steady-states. Several numerical experiments validate the proposed method, show the incidence on the numerical solutions of shape coefficients introduced in the model and present the effects of the viscosity-related parameters on the final emplacement of a lava flow. Our 3D model describes the dynamics of two incompressible, viscous, and immiscible fluids, possibly belonging to different phases. Being interested in the final application of lava flows, we also have an equation for energy that models the thermal exchanges between the fluid and the environment. We implemented this model in OpenFOAM, which employs a segregated strategy and the Finite Volume Methods to solve the equations. The Volume of Fluid (VoF) technique introduced by Hirt and Nichols in 1981 is used to deal with the multiphase dynamics (based on the Interphase Capturing strategy), and hence a new transport equation for the volume fraction of one phase is added. The challenging effort of maintaining an accurate description of the interphase between fluids is solved by using the Multidimensional Universal Limiter for Explicit Solution (MULES) method (described by Marquez Damian in 2013) that implements the Flux-Corrected Transport (FCT) technique introduced by Boris and Book in 1973, proposing a mix of high and low order schemes. The choice of the framework to use for any new numerical code is crucial. Our contribution consists of creating a new solver called interThermalRadConvFoam in the OpenFOAM framework by modifying the already existing solver interFoam (described by Deshpande et al. in 2012). Finally, we compared the results of our simulations with some benchmarks to evaluate the performances of our model

Forecasting long-term sediment yield from the upper North Fork Toutle River, Mount St. Helens, USA

The Toutle-Cowlitz River system experienced dramatic landscape disturbance during the catastrophic eruption of Mount St Helens on May 18, 1980. The eruption was triggered by a 2.5 km3 debris avalanche which buried the upper 60 km2 of the North Fork Toutle River catchment to an average depth of 45 m and obliterated the surface drainage network. Subsequent channel response on the debris avalanche, dominated by incision and widening, has delivered significant quantities of sediment to downstream reaches where resultant deposition has reduced channel capacity and heightened flood risk. Estimates of future sediment yield from the upper North Fork Toutle River are therefore required to inform development of sustainable options for long-term flood risk mitigation. Previous estimates have been based on extrapolation of post-eruption trends in sediment yield and channel network evolution, but the divergent predictions reported in a number of studies have clouded effective decision-making regarding long-term sediment management. This study therefore uses a numerical, landscape evolution model (CAESAR-Lisflood) to make long-term forecasts of sediment yield based on process simulation rather than extrapolation. A suite of forecasts of cumulative catchment sediment yields up to 2100 are produced using scenario-based model runs designed to account for uncertainty associated with the hydrological impacts of climate change and the model coefficient for lateral mobility. The forecasts fall in a narrow band +/-20% of the mean that lies between two previous estimates derived from the extrapolation of post-eruption trends. Importantly, predicted trends in future annual sediment yield are predominantly linear, although some limited decay is evident for runs in which modelled channel lateral mobility is lower. Sustained sediment production in the upper North Fork Toutle River is found to result from persistent bank erosion and channel widening. These findings cast doubt on the applicability of negative exponential decay functions based on the rate law to characterise post-disturbance sediment yield when lateral rather than vertical adjustments dominate channel evolution. Moreover, forecast trends in future sediment yield suggest that it may not be possible to manage future sediment-related flood risk along the lower Cowlitz solely by retaining sediment in the upper North Fork Toutle River catchment

Coupled modelling of turbidity currents over erodible beds

Turbidity currents are significant due to their role in dictating reservoir sedimentation, the safety of deep sea facilities and the formation of submarine morphological features and turbidites. Interactions exist between turbidity current, sediment transport, bed topography and deformation. However, existing mathematical models have ignored these interactions either partly or completely. Therefore these models can be referred to as decoupled or partially coupled models. Uncertainties arising from these simplifications remain unclear. To help address this, the present study advances modelling capability and understanding of turbidity currents in three areas. First, the significance of the interactions is analysed theoretically. Second, a fully coupled mathematical model, which incorporates explicitly the interactions between turbidity current, sediment transport, bed topography and deformation, is developed and tested. Third, the model is applied to submarine turbidity currents and reservoir turbidity currents. It is demonstrated that the model is a viable tool for effective reservoir sediment management and facilitates an improved understanding of the formation of submarine morphological features. Three issues need to be carefully dealt with in turbidity current modelling: 1) the internal hydraulic jumps, 2) the moving current front, -and 3) the irregular topographies in the field. These necessitate a mathematical model being well-balanced and capable of automatically capturing shock waves and tracking the wet/dry front. But to the writer’s knowledge, these aspects have so far not been simultaneously implemented in existing models of turbidity currents. In this study, the finite volume method is used to solve the governing equations and the slope limited centred scheme (SLIC) is employed to estimate the numerical fluxes, rendering the model capable of automatically capturing shock waves. The weighted surface depth gradient method (WSDGM) is implemented in the SLIC scheme, making the model well-balanced and thus applicable to both regular and irregular topographies. The wellbalanced property is demonstrated by successful reproduction of an initially subaqueous static turbidity volume over an irregular hump, as well as the successful application of the model to a real reservoir. The experimentally observed internal hydraulic jump is satisfactorily reproduced by the model, suggesting the ability of the model to accurately capture shock-waves. The accuracy of the model in reproducing key current variables is also demonstrated as against experimental data. The significance of fully coupled modelling is investigated theoretically using the multipletime- scale theory. This is complemented by numerical simulations of self-accelerating turbidity currents. Fully coupled modelling is shown to be critical for refined quality of turbidity current modelling, especially for those cases featuring rapid bed deformation. Decoupled and partially coupled models may be approximately applicable only to turbidity currents with mild bed deformation. Existing understanding of the formation of submarine morphological features is based mainly on indirect back-estimations, which cannot resolve the physical process. Applying the fully coupled model, the formation processes of canyons, channel-levees and lobes are numerically resolved. It is demonstrated that appropriate bed slope and sediment particle size may favour the formation of channel-levee morphology over submarine fans, as larger Richardson number does. Turbidity currents have been generated in a series of water-sediment regulation experiments in the Yellow River, China, aiming to get as much sediment as possible transported to the downstream and therefore reduce reservoir sedimentation. However, post-experiment analyses are mainly in the form of observed data comparisons. Two events of turbidity currents in the Xiaolangdi reservoir are investigated numerically. The advance of the current front and the sediment transport rate are reproduced by the model fairly well. These suggest the present model as a viable tool for determining the timing for operating the bottom outlets, which is critical for effective reservoir sediment management.UK Overseas Research Students Awards Scheme (ORSAS)National Science Foundation of China. Grant No. 10672126Doctoral Program Foundation, Ministry of Education, China. Grant No. 2006048601