Huhn, Katrin
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Huhn, Katrin
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Huhn, Katrin
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Item-typ:Veröffentlichung, Shelf architecture and submarine landsliding at Gela Basin (Strait of Sicily)(2014-10-10); ; ; The investigation of processes and mechanisms involved in the destabilisation of submarine slopes is a subject of intense research. However, key parameters controlling sediment instabilities are largely dependent on a multitude of environmental parameters and have to be assessed individually for each case study. In this context, it is not only important to answer questions about the spatial distribution of submarine landsliding, but as well about their temporal evolution and response to changes in environmental conditions - a task that is indispensable not only for the understanding of sedimentary processes but also for landslide-related risk assessment. This thesis aims to address this gap by studying frequent failure on a shallow-shelf setting at Gela Basin on the southern continental shelf of Sicily. It integrates high-resolution acoustic datasets and drilled core material in order to address (1) the role of climatic fluctuations in shaping the sedimentary architecture and associated depositional processes (2) the timing and recurrence of landslide events, especially with regard to the governing environmental conditions, and (3) the mechanisms of failure including the evaluation of predisposing factors and trigger mechanisms.Dissertation1318 294 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Numerical Simulation of Sediment Transport at the Agulhas Drift on the South African Gateway in relation to its Geodynamic Development(2012-02-09); ; ; The South African Gateway is a critical region for the water mass exchange between the Atlantic and the Indian Ocean. The variability in the interoceanic exchange is relevant to global climate change. High resolution seismic reflection data were collected in the Transkei Basin during R/V SONNE cruise SO 182 in 2005. These seismic observations have indicated that a large elongate mounded contourite drift, the Agulhas Drift, has been built up by eastward flows of Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW) since about 3 Ma. For a better understanding of the influences of the various currents on sediment transport patterns in the vicinity of the Agulhas Drift, the Regional Oceanic Modeling System (ROMS) has been used to simulate the current system and suspended sediment transport processes. The results from current system simulation showed that a recurrent individual cyclonic eddy event is the key hydrodynamic feature in the Transkei Basin. These eddies are almost fixed in the center of the Transkei Basin and spread over the entire water column. Flow in their northern segments is eastward across the southern flank of the Agulhas Drift. These eddies are supposedly the result of perturbations in the Subtropical Convergence (STC) in the region of south of Africa, where planetary waves on the STC deform. The perturbations enhance the surface eddy kinetic energy (EKE) in the Transkei Basin. The high EKE is then transferred from the surface to the deep ocean, where it causes the formation of eddies at abyssal depth. Based on the ocean circulation simulations, grain size experiments were carried out to test the transport behaviors for 4 different silt grain size classes. Our simulations reveal that sediment transport patterns in the Transkei Basin are strongly influenced by the mesoscale eddies. During eddy events, silts at all tested grain sizes can be resuspended by the AABW along the northeastern flank of the Agulhas Plateau that borders the Transkei Bsin to the east, whereas not all of them can be further transported into the region of the Agulhas Drift. Coarse silt cannot be transported over long distances, but it is deposited in the vicinity of the resuspension area. Medium silt can be transported into this region of the Agulhas Drift, only when the eddy current is strong enough. Fine silt and very fine silt can easily be carried into the drift area. The simulations show that medium and fine silt can be deposited on the Agulhas Drift, whereas very fine silt is further transported out of the area into the southern Transkei Basin.Dissertation1966 204 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Micro Scale Sediment - Fluid Interactions(2013-07-23); ; ; This thesis was inspired by the inherent limitations in the quantification of the physical parameters (e.g. flow speed, grain size and density) controlling sediment-fluid interactions in the direct vicinity of the sediment water interface of a seabed. For this, the transport behaviour of layered - and mixed sediment beds consisting of a simplified two-grain fraction distribution (silt and sand) was tested in an analogue laboratory-based annular flume. The results showed that the sediment bed stabilized with increasing silt composition. To investigate this effect in more detail, a high resolution 3D numerical model was designed for the simulation of sediment transport by a fluid by coupling two numerical techniques i.e., Finite Difference Method (FDM) and the Distinct Element Method (DEM). It verified that in the case of the deposition of fine particles on top of, or mixed into, a coarse matrix, the bed stability increased significantly. The model was also used to examine the role of grain density variations on the threshold conditions and erosion behavior of sediment beds. The experiments showed that bed stability increased when the heavy mineral concentrations were increased, which validated the process of heavy mineral enrichment.Dissertation276 271 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, The mechanics of an accretionary wedge affected by a layer with varying rheology, viscosity, and thickness(2014-02-13); ; ; An accretionary wedge evolves wide extended, flat and with a characteristic thrust system, if the wedge décollement is located in a viscous evaporite. The deformation processes of such wedges are complex and until now not fully understood. Therefore, in this present work 2D numerical sandbox experiments are developed in which an incoming sediment sequence is accreted against a fixed vertical wall due to a moving box bottom. The experiments contain a brittle incoming sequence in which either a brittle Mohr Coulomb layer or a viscoelastic-plastic layer (Burger s rheology) is embedded. The first study focuses on the differences which occur in the mechanics of two wedge experiments if either a weak viscoelastic-plastic or a low-friction brittle layer is embedded between two high friction brittle layers. To find out the viscosity under which the wedge deforms most likely as it would grow on top of an evaporite, the second study analyzes the effects of different viscous materials on wedge mechanics. Finally, the calibrated model is used to study the effects of different incoming sediment thicknesses on the accretion process.Dissertation267 210 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Deformation and micromechanics of granular materials in shear zones - investigated with the Discrete Element Method(2007-05-11); ; ; This thesis was inspired by the lack of detailed (i.e. particle scale) knowledge concerning deformation processes of mechanically weak sediments, especially intrinsically weak layers on the basal shear plane of submarine landslides. It has been known for some time that many different parameters influence shear strength and localization features. This is true not only for sediments, but also for other kinds of weak layers, such as fault gouge. These parameters include for example mineralogy (e.g. smectite, illite, quartz), sediment composition (clay, silt), sediment structure and texture (microfabric), grain size distribution, excess pore pressure, magnitude of effective stress, and deformation history. However, to date it has not been possible to rank or to quantify the influence of each of these parameters. The main goal of this study is to analyze the influence of some of these parameters and, if possible, rank and quantify them.Standard methods to examine shear strength of sediments and fault gouge are various geotechnical shear experiments. In these, a sample is sheared under defined conditions and resulting coefficient of friction, void ratio change, and other meaningful parameters are analyzed. Unfortunately, it is not possible to 'look' inside a shear box during a test and to analyze grain deformation behaviour on a microscopic scale. Therefore, this study employs a different approach to specifically address the problem of microscopic deformation processes. Here, a numerical modelling technique, the Discrete Element Method (DEM), is used.The DEM is a numerical tool based on the behaviour of granular materials. Within some limitations, soils and fault gouge can be considered as granular. Thus, the DEM allows simulating deformation behaviour of weak layers. The DEM principle is based upon simple physical contact and motion laws and can reproduce a wide range of grain features and behaviour. The technique has already been used to model other kinds of granular deformation processes such as large and small scale deformation processes.Utilizing the DEM a numerical shear box, very similar to geotechnical ring shear tests, was designed. Inside this box, a variety of different numerical 'samples' were generated. These 'sediments', or 'fault gouges', were designed with close specifications in each study (manuscript), respectively. During the numerical experiments a multitude of micromechanical parameters (particle displacement and rotation, microfabric evolution, coordination number, sliding fraction, contact force distribution and orientation) were measured. These also encompass classical geotechnical measurements, such as coefficient of friction, void ratio or volume change.At the end of this thesis a ranking order of tested parameters is presented. In this ranking, it is important to distinguish between purely numerically derived conclusions and their implication for natural materials. Hence, within model limitations, grain roughness and clay size fraction exhibit maximum influence on frictional strength and localization of sediments and fault gouge. The impact of boundary surface roughness is smaller; followed by grain sphericity. The influence of stratigraphic layering is difficult to place into this ranking as it has a different impact in different settings.Dissertation472 174 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Micromechanical Analysis of Sheared granular Materials Using the Discrete Element Method(2013-12-05); ; ; This PhD thesis was written at the Department of Geosciences at the University of Bremen in Germany and funded through MARUM Center for Marine Environmental Sciences . The work was undertaken from May 2009 to September 2013. The data analyzed in this thesis were produced by the author through the application of numerical Discrete Element simulations. Based on this approach, different methods of analyzing the microfabric of granular materials were applied and developed. This thesis contains five chapters including three research papers that improve the understanding of the effect of the microfabric on the shear strength of granular matter. The introduction gives a general overview of granular matter and its relevance in nature and industry. After that, the focus is on the empirical concepts of friction, strength, and failure as controlling factors for the granular stability. This is followed by a description of the typical physical and mechanical properties of granular matter, such as the composition, the grain size and shape, and the packing and porosity. The introduction also includes a description of the motivation and the main objectives of this research and provides general open scientific questions that will be addressed in the subsequent chapters. The first chapter closes with a detailed description of the different types of model setups and lists all relevant numerical methods applied in this research. The body of the thesis is represented by three manuscripts (Chapters 2 - 4). Each of them focuses on the coefficient of friction whose behavior is equivalent to the bulk shear stress according to the Mohr-Coulomb criterion. Within the first study, the influence of the model dimensionality, 2D vs. 3D, on shear behavior was investigated. The focus of the second study was the contact formation and evolution of complex particle shape arrangements. Therein, the microfabric of three different basic particle shapes was compared. The third study was designed to examine pore volume changes caused by grain shape variation. The final chapter of the thesis synthesizes the main conclusions from the three manuscripts and presents some future perspectives.Dissertation306 115 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Submarine landslides in active margin environments - Slope stability vs. neotectonic activity on the northeastern margin of Crete, eastern Mediterranean(2009-09-04); ; ; This thesis is a study of the processes and factors that may govern submarine slope destabilization and mass movement as well as a slope's resistance to failure in submarine active margin environments. Prior studies on this topic in active margin settings (e.g., western North America) have shown that their slopes can be peculiar, poorly understood systems with a low recurrence of slope collapse despite the paradigm of extensive, diffuse, and widespread mass movement generally associated with the tectonic activity. The focus of this study is the active margin present along the northeast margin of Crete in the Hellenic subduction zone of the eastern Mediterranean. The previously unknown distribution and recurrence rate of landslides, their failure and transport kinematics, as well as the trigger mechanisms required for sediment instability along the northeast Cretan margin are thus investigated and reconstructed. Such data help answer the open questions related to such active margin settings.Dissertation500 390