02 Fakultät Bau- und Umweltingenieurwissenschaften
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Item Open Access Application of a non-parametric classification scheme to catchment hydrology(2008) He, Yi; Bárdossy, András (Prof. Dr. rer. nat. Dr.-Ing. habil.)Classification has been considered a fundamental step towards improved catchment hydrology science. Catchments classification has been traditionally carried out via Linnaeus-type cluster analysis, mainly represented by hierarchical approaches and methods based on partitioning of hydrological data set. This paper proposes a new scheme where the classification procedure is based on similarity interpreted as distances between catchments. The similarity or distance is defined under the following premises: 1. similar catchments behave similarly; 2. similarity can be described with catchments' characteristics; and 3. hydrological models are able to capture catchments' similarity. If many sets of model parameters lead to similar model performance for two catchments, they are considered as similar catchments. To implement the proposed scheme, two procedures, namely multidimensional scaling (MDS) and local variance reduction (LVR), are undertaken to construct a configuration of n catchments' characteristics in Euclidean space using information about similar performance between the catchments. The MDS is used to determine the appropriate dimension of the Euclidean space and the LVR is used to obtain the transformation matrix and the coordinates in the transformed Euclidean space. This scheme avoids the idea of parametric regression-based regionalization approaches where a regression function is pre-defined between model parameters and catchment descriptors. In the aforementioned approach, the function that is selected is usually subjective and arbitrary and one can also argue that a priori function is neither able to represent the highly complex hydrological processes nor consider the interdependences amongst model parameters. The proposed scheme is initially tested with a research version of the HBV-IWS model on a number of catchments within the Rhine Basin. Additionally a modified Xinanjiang model is applied to the same catchments to check if the assumption of invariant catchment similarity holds true. Invariant catchment similarity here assumes the catchments genuinely carry their similarities independent of the model used for simulation. This test is also a backstop measure to determine if the models under consideration are capturing the underlying simplified hydrological processes in a rational manner. The scheme will be extended to regional calibration of rainfall runoff models as well as regional drought or flood studies once similarity within catchments has been established. The proposed scheme will eventually contribute to the PUB (Predictions in Ungauged Basins) initiative.Item Open Access Comparison study of phase-field and level-set method for three-phase systems including two minerals(2022) Kelm, Mathis; Gärttner, Stephan; Bringedal, Carina; Flemisch, Bernd; Knabner, Peter; Ray, NadjaWe investigate reactive flow and transport in evolving porous media. Solute species that are transported within the fluid phase are taking part in mineral precipitation and dissolution reactions for two competing mineral phases. The evolution of the three phases is not known a-priori but depends on the concentration of the dissolved solute species. To model the coupled behavior, phase-field and level-set models are formulated. These formulations are compared in three increasingly challenging setups including significant mineral overgrowth. Simulation outcomes are examined with respect to mineral volumes and surface areas as well as derived effective quantities such as diffusion and permeability tensors. In doing so, we extend the results of current benchmarks for mineral dissolution/precipitation at the pore-scale to the multiphasic solid case. Both approaches are found to be able to simulate the evolution of the three-phase system, but the phase-field model is influenced by curvature-driven motion.Item Open Access Parallele Numerische Simulation instationärer Probleme mit adaptiven Methoden auf unstrukturierten Gittern(2001) Lang, Stefan; Helmig, Rainer (Prof. Dr. Ing.)Ziel dieser Arbeit ist die Numerische Simulation von partiellen Differentialgleichungen mit den aktuellen Methoden: Mehrgitterverfahren, lokale Gitteradaption und Parallelität. Besonders Aspekte des Softwareengineerings und der Implementierung werden im Detail betrachtet. Diese Methoden werden durch paralleles I/0 und parallele Graphik zu einer skalierbaren Toolkette erweitert. Anhand von drei Gleichungstypen: Dichtegetriebene Grundwasserströmung, Zweiphasen-Strömung und Elastoplastizität wird die Leistungsfähigkeit der realisierten Simulationsplattform untersucht. Beispielsweise konnte bei einer Simulation auf 512 Prozessoren eine Beschleungiung von mehr als 300 erzielt werden.Item Open Access Numerical investigations of the role of hysteresis in heterogeneous two-phase flow systems(2008) Papafotiou, Alexandros; Helmig, Rainer (Prof. Dr.-Ing.)Various groundwater applications often involve the flow of two immiscible fluids in heterogeneous porous media. In problems that involve the assessment of travel times of hazardous substances in the unsaturated zone or monitoring and predicting the fate of groundwater contaminations, efficient tools and approaches need to be developed to achieve accurate predictions of two-phase flow behavior in heterogeneous porous media. However, this is not an easy task, as heterogeneities -observed on different spatial scales- have a strong influence on the distribution of the fluid phases and therefore form a significant source of uncertainty. Moreover, the prediction of two-phase flow in heterogeneous porous media becomes complicated through alternating drainage and imbibition conditions taking place in the complex heterogeneous pore structure that lead to hysteresis effects in the capillary pressure-saturation relationship. Numerical simulations are widely used to predict hysteretic two-phase flow in heterogeneous porous media in lab or field applications. This approach, however, demands good knowledge on the hydraulic properties of the materials that form the heterogeneous structures involved in the application. Traditionally, the hydraulic properties and the hysteretic behavior of porous media are empirically determined on the local scale with lab experiments conducted on material samples. On the other hand, alternative methods suggest the direct determination of hydraulic properties, including hysteretic capillary pressure-saturation relationships, from a pore-scale consideration. This is done using available information on the pore structure of a material. Nevertheless, it remains unclear how accurate predictions can be in problems of hysteretic two-phase flow in porous media, even when advanced state-of-the-art methods are used on different scales for the determination of the hydraulic properties. The first part of this thesis deals with the implementation of two hysteresis concepts in a numerical model for the simulation of two-phase flow in heterogeneous porous media. Special attention is given on the combination of the hysteresis concepts with a capillary interface condition for the treatment of material interfaces and the approximation of saturation discontinuities due to heterogeneities. This provides an efficient and consistent approach for the prediction of hysteretic two-phase flow in heterogeneous porous media. In the second part, predictions made with the numerical implementations of the hysteresis concepts are compared to measurements from a 1-D monitored transient experiment, that involves successive alternating drainage and imbibition conditions. Conclusions related to the importance of hysteresis and the possibilities of the applied hysteresis concepts are drawn. Furthermore, the comparative study presents remarks on the beneficial combination of different approaches -from the modeling and the experimental viewpoint- that lead to reliable predictions on hysteretic two-phase flow. The last part of this work focuses on predictions of hysteretic two-phase flow made with hydraulic properties determined on different spatial scales. In this case, numerical simulations of drainage and imbibition are compared to experimental measurements in a 3-D heterogeneous structure. The hydraulic properties that are used as input for the numerical simulations are determined with two approaches: -On the local scale with multistep outflow/inflow experiments. -On the pore scale with advanced image analysis and lattice Boltzmann flow simulations in mapped sand geometries. The comparative study in this case reveals the possibilities for predictions of hysteretic two-phase flow made with hydraulic properties determined on different scales (local and pore scale), indicates sensitivities in such hydraulic properties, reveals the significant influence of material interfaces in heterogeneous structures and finally detects the apparent temporal- and spatial-scale dependency of non-wetting phase trapping effects during imbibition processes. Conclusions related to the observed hysteresis are drawn, considering the assumptions and the conceptual differences involved in the different approaches. Finally the comparison between simulations and experiment triggers a discussion on the potentials of our modeling approaches in the case of heterogeneous structures, shows how one needs to approach applications of hysteretic two-phase flow in heterogeneous porous media and what aspects must be taken into account when dealing with different scales.Item Open Access Simulation von Mehrphasenvorgängen in porösen Medien unter Einbeziehung von Hysterese-Effekten(1999) Sheta, Hussam; Helmig, Rainer (Prof. Dr.)Die vorliegende Arbeit entstand im Rahmen des Forschungsprogramms der Versuchseinrichtung zur Grundwasser- und Altlastensanierung VEGAS. Sie beschäftigt sich mit einem zentralen Thema zur Beschreibung von Transport und Schadstoffausbreitung im natürtlichen Untergrund.Item Open Access Numerical simulation of CO2 sequestration in geological formations(2007) Bielinski, Andreas; Helmig, Rainer (Prof. Dr.-Ing.)CO2 sequestration in geological formations is regarded as a measure for reducing the emissions of this greenhouse gas into the atmosphere. With this method, carbon dioxide is captured at large point sources, e.g. coal-fired power plants, and injected into subsurface reservoirs for long-term storage. Such formations can be exploited gas or oil reservoirs or saline aquifers. The latter have the advantage that they are abundant in many places and provide large storage volumes. In this study, the topic of CO2 sequestration in saline aquifers is addressed using numerical methods. A model concept is developed, taking into account a CO2-rich phase, a brine phase and the rock matrix as a rigid porous medium. The focus is on the correct description of the fluid properties including super- and subcritical properties of the CO2-rich phase. Mutual dissolution of the components in the phases and the effect of the salt content in the aqueous phase on dissolution behavior and fluid properties are considered. Non-isothermal effects are taken into account. Advective and diffusive flow processes are included into the model concept. Multi-phase behavior is considered using relative permeability and capillary pressure functions. The set-up of the mathematical model and its numerical implementation in the numerical simulator MUFTE-UG are described in detail. Various simulations show the capability of the developed model and give insight into principle processes that occur during CO2 sequestration in geological formations. The short-term carbon dioxide plume propagation during the injection process is investigated. The long-term behavior is studied in another example considering the dissolution of a CO2 plume in the subsurface over a period of 100 years. Furthermore, non-isothermal processes are investigated in a simulation example. This work provides a tool which is capable of reproducing and predicting many of the processes that occur during CO2 sequestration in geological formations. It can be used for principle studies as well as for the field cases that this technology is going to be applied to. It provides the foundation for further extensive research into many open questions to be addressed in the future.Item Open Access Modernization criteria assessment for water resources planning; Klamath Irrigation Project, U.S.(2008) Freeman, Beau J.; Bárdossy, András (Prof. Dr. rer. nat. Dr.-Ing.)Agricultural irrigation is the largest consumer of diverted surface water and groundwater resources in the world, with major regions becoming critically water deficit. Agriculture in the western United States (US) and elsewhere has reached the point where the demands from irrigators, domestic users, and various commercial interests for allocated quantities and qualities are beyond acceptable levels for environmental needs in many river basins. Despite decades of investment in irrigation projects by governments, foreign lending agencies, and development banks in numerous countries, irrigation performance remains unsatisfactorily low and in many places progress is being reversed due to water logging, salinization, over-drafting of aquifers, environmental degradation, and infrastructure deterioration. Maintaining current irrigation practices will lead to worsening environmental and economic consequences. To restore healthy ecosystems and sustain irrigated agriculture, irrigation modernization should be promoted as a key component of basin-level water management to effectively balance competing water needs. Improvements in the technical and economic efficiency of irrigation water use through modernization increase the quantity and quality of freshwater available in a river basin. Significant public and private investments in modernization will be required to facilitate the precise control and monitoring of reallocated flows at different levels of irrigation systems, especially on a real-time basis, and thus provide excellent water delivery service to water districts, end-users, and other commercial and environmental stakeholders. This doctoral study investigates a specific problem that many irrigation professionals and water resources planners will face in the future: how to effectively analyze and make an assessment of irrigation modernization project-alternatives. Selecting the best modernization strategy to pursue from potential project-alternatives in water resources planning is a complex decision-making process. Irrigation modernization alternatives and their impacts involve a variety of diverse stakeholders in the selection of preferred engineering solutions based on subjectively defined criteria (quantitative and qualitative). As a consequence, technical feasibility, environmental, social/community, institutional, political, and economic factors have to be properly assessed as part of water resources planning. This research introduces a strategic decision analysis methodology for the definition, evaluation, ranking, and selection of appropriate modernization strategies in an engineering case study of the Klamath Irrigation Project (89,000 ha). In 2001 a combination of events occurred there that led to one of the most prominent conflicts over water supplies in the U.S. Due to stricter flow requirements put in place to protect fish species and a critical drought, irrigation water was unexpectedly withheld from the majority of farms in the Project, resulting in major economic losses, calling the basis for environmental restrictions into question, and generating intense political controversy. The composite programming approach is applied to develop a project ranking index based on standardized indicators – effective for analyzing the trade-offs associated with balancing technical and water conservation considerations with eco-system health, economics, and risk. This modernization criteria assessment requires defining the management objectives according to the nature of the internal processes and agro-hydrological features of the system, selection of alternative engineering solutions, selection of appropriate decision criteria relevant to the specific water-related problems, and the assignment of desirable and critical threshold values pertinent to each criterion. Input data consist of hydrologic, agronomic, engineering, economic, and political/policy information.Item Open Access Stochastische Generierung räumlich zusammenhängender Niederschlagszeitreihen(2008) Brommundt, Jürgen; Bárdossy, András (Prof. Dr. rer. nat. Dr.-Ing.)Die Arbeit beschreibt die Entwicklung eines Simulators zur Erzeugung simultaner stochastischer Niederschlagszeitreihen, seine Implementierung und erfolgreiche Anwendung. Sie wurde im Rahmen des BWPLUS-Projekts „Erzeugung simultan-synthetischer Niederschlagsreihen in hoher zeitlicher und räumlicher Auflösung für Baden-Württemberg“ durchgeführt. In Baden-Württemberg besteht mit dem stochastischen Niederschlagszeitreihengenerator NiedSim ein System zur Erzeugung 30 Jahre langer Niederschlagszeitreihen in Fünfminutenauflösung zur Verfügung, das am Institut für Wasserbau von Professor Bárdossy entwickelt wurde. Die generierten Reihen werden hauptsächlich zur Dimensionierung und Auslegung von städtischen Kanalnetzen verwendet. Die erzeugten Punktniederschlagszeitreihen sind repräsentativ für das Niederschlagsverhalten an dem Ort, für den sie generiert werden, und die unmittelbare Umgebung. Für größere Gebiete werden daher zwei oder mehr simultane Niederschlagszeitreihen benötigt, um die räumliche Variabilität des Niederschlags zu berücksichtigen und so eine korrekte Nachbildung bei der Generierung zu ermöglichen. Der Generator NiedSim basiert auf einem nicht-parametrischen Generierungsansatz, bei dem aus regionalisierten, ortsspezifischen statistischen Niederschlagseigenschaften zufällig eine Reihe von Stundenwerten erzeugt wird, die in einer Optimierung solange rearrangiert werden, bis die Zeitreihe die gewünschten - langfristig beobachteten - statistischen Eigenschaften besitzt. Anschließend kann die erzeugte Reihe zu Fünfminutenwerten disaggregiert werden, was nach einem ähnlichen Schema abläuft. Mit dieser Methodik können bei der Generierung beliebige Eigenschaften, also auch räumliche Eigenschaften, berücksichtigt werden. Für die flächendeckende Generierung zusammenhängender Niederschlagszeitreihen in ganz Baden-Württemberg müssen diese räumlichen Eigenschaften ebenfalls regionalisiert werden, damit sie für beliebige Stationspaare zur Verfügung stehen. Insgesamt wurden drei Eigenschaften definiert und untersucht. Der allgemeine Zusammenhang der simultanen Niederschlagszeitreihen wurde durch die Korrelation auf Tages- und Stundenskala erfasst. Zur Regionalisierung der Korrelation wurde ein neuer Kriging-Ansatz entwickelt. In dem vierdimensionalen Raum, den die vier Ortskoordinaten der Stationspaare aufspannen, wird die Korrelation direkt aus den Messdaten aller beobachteten Stationspaare mit Ordinary und External Drift Kriging berechnet. Beide Techniken lieferten sehr gute Interpolationsergebnisse. Umfangreiche Versuche für die Regionalisierung auch Radardaten zu verwenden führten nicht zu befriedigenden Ergebnisse. Aus der späteren Anwendung der erzeugten Niederschlagszeitreihen in hydrologischen Modellen ergibt sich, dass die Nachbildung von Extrema und größeren Niederschlägen besondere Bedeutung haben muss, da diese die schlimmsten Auswirkungen haben. Hierzu wurde aus Radardaten die räumliche Abminderung der Spitzen von Extremereignissen in Abhängigkeit von Fläche, Dauerstufe und Jährlichkeit berechnet. Des Weiteren hat die Zugrichtung und -geschwindigkeit von Niederschlagsfeldern maßgeblichen Einfluss auf die Abflussreaktion eines Einzugsgebiets. Das Zugverhalten wurde ebenfalls aus Radardaten ermittelt und in Abhängigkeit von der herrschenden Großwetterlage definiert, wodurch es auch in Zeiträumen ohne Radarbeobachtung beschrieben werden kann, da die herrschende Großwetterlage lückenlos als Aufzeichnung vorliegt. Das Generierungs- und Disaggregationsschema wurde anschließend so erweitert, dass es diese drei zeitlich-räumlichen Informationen bei der simultanen Generierung berücksichtigt. An Hand von drei Fallstudien wurde der entwickelte Zeitreihengenerator abschließend getestet. Zunächst wurden Aufzeichnungen dreier Messstationen mit drei dazu generierten simultanen Zeitreihen verglichen. Anschließend wurde im Stadtgebiet von Karlsruhe der Einfluss einer ungleichmäßigen Überregnung auf die Stadtentwässerung untersucht. Abschließend wurde der Einsatz des simultanen Generators bei der hydrologischen Modellierung kleiner Einzugsgebiete erprobt. In allen drei Fallstudien erwies sich der erstellte Generator als nützliches Werkzeug zur genaueren Nachbildung der im natürlichen Niederschlag beobachteten Variabilität. Mit Hilfe der räumlich differenzierten Eingangsdaten kann eine Schwäche bei der möglichst realitätsnahen Modellierung des Niederschlag-Abfluss-Verhaltens abgemildert werden. Zum Anderen können unbekannte Reserven existierender Systeme quantifiziert und das allgemeine Systemverständnis verbessert werden.Item Open Access Numerical investigation on flow and transport characteristics to improve long-term simulation of reservoir sedimentation(2007) Tesfaye Kebede Gurmessa; Westrich, Bernhard (Prof. Dr.-Ing. habil.)Long-term prediction of the quantity and spatial distribution of sedimentation is required in the planning and management of reservoirs. Numerical models, conceptual models, empirical models, scale models, or a combination of them can be used in order to predict long-term sedimentation of reservoirs. Some approaches are complex while others tend to oversimplify addressing practical questions related to sediment management. This work has attempted to address simplified methodologies to predict the amount and spatial distribution of reservoir sedimentation. Two complementary approaches were specifically assessed: the numerical and the data-driven modeling approaches. Numerical modeling of long-term sedimentation processes involve model, parameter, and data uncertainties and often require very high computational costs. Model conceptualization and solution procedures involved in long-term simulation of sedimentation processes of riverine systems are challenging. The first part of the study deals with the investigation of simplification of numerical simulations. The study begins with investigations made on the basic system response of reservoirs to unsteady inputs. By making use of hypothetical domains and periodic inputs, efforts were made to try to specify the theoretical concepts. The response of the reservoirs was investigated by using aggregate parameters such as theoretical residence time, critical erosion discharge, amplitude and the frequency of inputs. The analytical solutions of ideal continuously stirred reactors and/or plug flow reactors as compared with the real two-dimensional advection-dispersion numerical solutions of the TELEMAC modeling system were evaluated. Criteria for the refinement of quasisteady steps were investigated revealing the importance of reservoir shape, range of discharges, and residence time. The work then continues by investigating into long-term simulation and simplification strategies. A thorough evaluation of hydrological, topographical, and sediment data was conducted on the Lautrach reservoir of River Iller. Using the two-dimensional depth-integrated TELEMAC modeling system, sensitivity studies were conducted with relevance to data aggregation, temporal and spatial discretization, coupling methods, turbulence models, and sediment gradation. Based on the simplifications from the preliminary studies, long-term reservoir sedimentation was calibrated and validated with fully unsteady simulation. The validated morphological simulations were tested for the extent of the applicability of quasisteady approximation considering large steady time steps. It was found out that except for extreme discharges and low flows with high sediment concentrations, the quasisteady approximation with large time steps can be successfully applied without major discrepancy from the fully unsteady simulation. Comparisons of the prediction of bed evolution were made using complete unsteady, mixed-unsteady-quasisteady, and steady approximations. In the second part of the work, an approach on the use of principal component regression in modeling the spatio-temporal bed evolution processes of riverine system was developed. The principal component analysis made on long-term bed evolution simulation indicated that only the first four principal components represent some 95 percent of the variance. This indicated that a significant simplification and representation of the spatio-temporal simulated data can be made in a condensed form. Multiple regression models were then investigated between the first four principal component scores and the flow and the sediment inputs, resulting in a very good correlation. The reconstruction of reservoir bed evolution resulted in an excellent agreement when multiple regression was used between the principal component score and the time series of discharge, change in discharge, and suspended sediment concentration. The model was also reasonably validated with acceptable uncertainty for ranges outside the period of reconstruction. It was found that the regression parameters are dynamic due to the dynamic nature of reservoir bed as well as flow and sediment parameters. The work is a step forward towards simplifying the complex and computationally demanding task of modeling of long-term reservoir sedimentation by assimilating dynamic and data modeling techniques.Item Open Access Effects of enzymatically induced carbonate precipitation on capillary pressure : saturation relations(2022) Hommel, Johannes; Gehring, Luca; Weinhardt, Felix; Ruf, Matthias; Steeb, HolgerLeakage mitigation methods are an important part of reservoir engineering and subsurface fluid storage, in particular. In the context of multi-phase systems of subsurface storage, e.g., subsurface CO2 storage, a reduction in the intrinsic permeability is not the only parameter to influence the potential flow or leakage; multi-phase flow parameters, such as relative permeability and capillary pressure, are key parameters that are likely to be influenced by pore-space reduction due to leakage mitigation methods, such as induced precipitation. In this study, we investigate the effects of enzymatically induced carbonate precipitation on capillary pressure-saturation relations as the first step in accounting for the effects of induced precipitation on multi-phase flow parameters. This is, to our knowledge, the first exploration of the effect of enzymatically induced carbonate precipitation on capillary pressure-saturation relations thus far. First, pore-scale resolved microfluidic experiments in 2D glass cells and 3D sintered glass-bead columns were conducted, and the change in the pore geometry was observed by light microscopy and micro X-ray computed tomography, respectively. Second, the effects of the geometric change on the capillary pressure-saturation curves were evaluated by numerical drainage experiments using pore-network modeling on the pore networks extracted from the observed geometries. Finally, parameters of both the Brooks-Corey and Van Genuchten relations were fitted to the capillary pressure-saturation curves determined by pore-network modeling and compared with the reduction in porosity as an average measure of the pore geometry’s change due to induced precipitation. The capillary pressures increased with increasing precipitation and reduced porosity. For the 2D setups, the change in the parameters of the capillary pressure-saturation relation was parameterized. However, for more realistic initial geometries of the 3D samples, while the general patterns of increasing capillary pressure may be observed, such a parameterization was not possible using only porosity or porosity reduction, likely due to the much higher variability in the pore-scale distribution of the precipitates between the experiments. Likely, additional parameters other than porosity will need to be considered to accurately describe the effects of induced carbonate precipitation on the capillary pressure-saturation relation of porous media.