Effects of tectonic structures, salt solution mining, and density-driven groundwater hydraulics on evaporite dissolution (Switzerland)

Subsurface dissolution (subrosion) of evaporites such as halite and gypsum can lead to extensive land subsidence. Recent land subsidences have been surveyed at six separate locations in northwestern Switzerland. The diameters of the affected surface areas range from 100 to 1,500 m, and corresponding subsidence rates reached more than 100 mm/year. Based on a geometrical model, three sites could be outlined where land subsidence can likely be attributed to salt solution mining. The effects of increased hydrostatic gradient due to both groundwater withdrawal and fluid density contrasts were evaluated in more detail for the remaining sites with a series of 2D density-coupled solute-transport simulations along an approximately 1,000-m-long and 150-m-deep 2D cross section. Simulation results indicate that the upconing process of saline groundwater into the main aquifer occurs under different distributions of subsurface parameters and hydraulic boundary conditions. For the presented setup, the simulations also revealed that the most sensitive factor for the dissolution rate is the structure or dip of the halite formation, which leads to an increase of dissolution rate with increasing dip. Due to the increased density of the brine, an intrinsic flow dynamic develops which follows the direction of the di

Comparison of light transmission and reflection techniques to determine concentrations in flow tank experiments

Transmissive and reflective intensity measurements for visual concentration determinations in 2D flow tank experiments were compared and evaluated for their applicability in the study of flow and transport phenomena. A density-dependent heterogeneous flow experiment was conducted and transmission and reflection images of the dyed saltwater plume were analyzed. A single light source and dark curtains forced the light to pass through the porous media only, thus facilitating the transmission measurements. The reflection images delivered a more homogeneous spatial illumination than the transmission images. Major perturbations of the transmission images were lens flare effects and light dispersion within the bead-water-Plexiglas system which smear the front of the plume. Based on the conducted evaluation of transmissive and reflective intensity measurements, the reflection data delivered more reliable intensity values to derive solute concentrations in intermediate scale flow tank experiment

2D benchmark experiments and simulations of density coupled flow problems

Variations of fluid densities can alter flow patterns and transport processes, if solute concentration differences are high enough to cause relevant density contrasts. Since numerous environmental problems are related to these phenomena, the need for accurate process description and modeling continues to increase. The numerical simulation of such processes is challenging due to the strong non-linear coupling of flow and transport processes. Therefore, experimental studies are required to elaborate the basic principles and to test numerical codes in order to provide reliable tools for water resources management and planning. In this thesis, density-coupled flow processes under the influence of geometrical boundary conditions are studied and numerical codes are tested against high resolution experimental data. Photometric methods were further developed to increase the accuracy of measurements in flow tank experiments. They directly related digitally measured intensities of a tracer dye to solute concentrations. This enabled an effective processing of a large number of images in order to compute concentration time series at various points of the flow tank and concentration contour lines. Perturbations of the measurements were lens flare effects and the image resolution. Transmissive and reflective intensity measurements were compared. The reflection images were more homogeneous in spatial illumination than the transmission images. Major perturbations of the transmissive images were lens flare effects and light dispersion within the bead-water-Plexiglas system which smeared the front of the plume. Based on the conducted evaluation of transmissive and reflective intensity measurements, the reflection data delivered more reliable intensity values to derive solute concentrations in intermediate scale flow tank experiments. The newly developed resistivity measurement system used two different input voltages at gilded electrode sticks to enable the measurement of salt concentrations from 0 to 300 g/l. The method was highly precise and the major perturbations were caused by temperature changes, which can be controlled in the laboratory. The two measurement approaches, photometric and resistivity methods, were compared with regard to their usefulness in providing data for benchmark experiments. Due to the unknown measurement volume of the electrodes, the photometric method was better to determine experiments in a series of laboratory-scale 2D porous medium tank experiments. Various density-driven flow problems were investigated using well-defined experimental parameters and boundary conditions. The experiments were carried out both in a rectangular flow tank (158×100×4 cm3) and in a more complex geometrical setup aiming to study variable density flow in geological formations of aquifers and aquicludes connected via fault zones. An impermeable layer within the porous medium tank forced the solutes to pass through a channel to reach the outlet of the tank. The porous medium was homogeneous in both cases. The image analysis technique deliverd 2, 10, 50 and 80% salt concentration isolines at distinct times and breakthrough curves of the dyed saltwater. The experimental data were presented as benchmark problems to evaluate numerical codes. A numerical model based on Mixed Finite Elements for the fluid flow problem and a combination of Discontinuous Galerkin Finite Element and Multi-Point Flux Approximation methods for the transport turned out to be adequate for the simulation of the physical experiments. The high data availability made the proposed benchmark experiments a valuable tool for assessing the performance of density-coupled flow models. Heterogeneous porous medium experiments were conducted with a low permeability zone in the centre of the tank. Three different boundary conditions, corresponding to different localizations of the inflow and the outflow openings at the opposite edges of the tank, were applied and different flow scenarios are observed in the heterogeneous tank. The numerical model used for the simulations was based on efficient advanced approximations for both spatial and temporal discretizations. The Method Of Lines (MOL) was used to allow higher-order temporal discretization and the model adapted in both the order of approximation and time step to provide the necessary accuracy. The model was able to reproduce the experiments. The numerical results were improved by assuming a non-Fickian dispersivity for high density experiments

Market liquidity and funding liquidity

We provide a model that links an asset's market liquidity; i.e., the ease with which it is traded; and traders' funding liquidity, i.e. the ease with which they can obtain funding. Traders provide market liquidity, and their ability to do so depends on their availability of funding. Conversely, traders' funding, i.e., their capital and the margins they are charged, depend on the assets' market liquidity. We show that, under certain conditions, margins are destabilizing and market liquidity and funding liquidity are mutually reinforcing, leading to liquidity spirals. The model explains the empirically documented features that market liquidity (i) can suddenly dry up, (ii) has commonality across securities, (iii) is related to volatility, (iv) is subject to "flight to quality", and (v) comoves with the market, and it provides new testable predictions

On the value of glacier mass balances for hydrological model calibration

Hydrological modelling of glacierized catchments is challenging because internal inconsistencies might be hidden due to ice melt which represents an additional source of water. This is even more significant if there are no data available to evaluate model simulations, as is often the case in remote areas. On the other hand, these glacierized catchments are important source regions for water, and detailed knowledge of water availability is a prerequisite for good resource management strategies. An important question is how useful a limited amount of data might be for model applications. Therefore, in this study the predictive power of limited discharge measurements, mass balance observations and the combination of both was analyzed by means of Monte Carlo analyses with multi-criteria model performance evaluation. Ensembles of 100 parameter sets were selected by evaluating the simulations based on a limited number of discharge measurements, glacier mass balance, and the combination of discharge and mass balance observations. Then the ensemble simulation of runoff was evaluated for the entire runoff series. The result indicated that a single annual glacier mass balance observation contained useful information to constrain hydrological models. Combining mass balance observations with a few discharge data improved the internal consistency and significantly reduced the uncertainties compared to parameter set selections based on discharge measurements alone. To obtain good ensemble predictions, information on discharge was required for at least 3 days during the melting season. This demonstrated that the timing of runoff measurements is important for the information contained in these data

Comparison of light transmission and reflection techniques to determine concentrations in flow tank experiments

Transmissive and reflective intensity measurements for visual concentration determinations in 2D flow tank experiments were compared and evaluated for their applicability in the study of flow and transport phenomena. A density-dependent heterogeneous flow experiment was conducted and transmission and reflection images of the dyed saltwater plume were analyzed. A single light source and dark curtains forced the light to pass through the porous media only, thus facilitating the transmission measurements. The reflection images delivered a more homogeneous spatial illumination than the transmission images. Major perturbations of the transmission images were lens flare effects and light dispersion within the bead-water-Plexiglas system which smear the front of the plume. Based on the conducted evaluation of transmissive and reflective intensity measurements, the reflection data delivered more reliable intensity values to derive solute concentrations in intermediate scale flow tank experiments

Sediment transport modelling in a distributed physically based hydrological catchment model

Bedload sediment transport and erosion processes in channels are important components of water induced natural hazards in alpine environments. A raster based distributed hydrological model, TOPKAPI, has been further developed to support continuous simulations of river bed erosion and deposition processes. The hydrological model simulates all relevant components of the water cycle and non-linear reservoir methods are applied for water fluxes in the soil, on the ground surface and in the channel. The sediment transport simulations are performed on a sub-grid level, which allows for a better discretization of the channel geometry, whereas water fluxes are calculated on the grid level in order to be CPU efficient. Several transport equations as well as the effects of an armour layer on the transport threshold discharge are considered. Flow resistance due to macro roughness is also considered. The advantage of this approach is the integrated simulation of the entire basin runoff response combined with hillslope-channel coupled erosion and transport simulation. The comparison with the modelling tool SETRAC demonstrates the reliability of the modelling concept. The devised technique is very fast and of comparable accuracy to the more specialised sediment transport model SETRAC.ISSN:1027-5606ISSN:1607-793