Huhn-Frehers, Katrin
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Preferred name
Huhn-Frehers, Katrin
Official Name
Huhn-Frehers, Katrin
Alternative Name
Huhn, Katrin
Huhn, K.
Huhn-Frehers, K.
Email
khuhn@marum.de
khuhn@uni-bremen.de
ORCID
GND
Scopus Author ID
16024630300
57211054663
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Item-typ:Veröffentlichung, Micro-mechanics of weak layers: key role of sediment structure and composition(2021-09-20); ; ; Submarine landslides are gravity-driven mass movements that occur in underwater slope settings worldwide. They are one of the volumetrically most important processes for transporting sediments from the continental margin into the deep ocean. Despite the hazard they pose to coastal communities and critical seafloor infrastructure, many aspects of submarine landslides remain poorly understood. Our understanding of submarine landslides is often based on hypotheses that are hard to test, and we tend to infer landslide behaviour rather than understand the reason behind their formation. Sufficient information regarding the internal structure and composition, i.e. from sediment cores and in-situ measurements is often missing. Therefore, some key questions still remain unanswered, which include why some areas fail while adjacent slopes do not, or how submarine landslides can fail on low angle slops (<2°). Many studies proposed that these phenomena and the large areal extend of submarine landslides may be explained by laterally-extensive weak layers within the slope stratigraphy. Our knowledge regarding weak layers, in particular their compositional and structural characteristics, as well as the processes that control and form them, however, is still very limited. This thesis makes use of a variety of datasets at different scales and resolution in order to both qualitatively and quantitatively investigate the role of sediment structure and composition on weak layer and submarine landslide formation. Furthermore, the role of the environmental setting on the formation of weak layers, and their control on the triggering mechanism are investigated. Establishing such a relation is crucial to identify conditions (i.e. failure mechanism) under which slope failure may occur. Part of this thesis is a comprehensive literature review of published submarine landslide studies that examine the failure planes and apparent weak layers of historic and ancient submarine landslides, to evaluate what types of sediment are capable of forming weak layers and to understand their global distribution. The results show that failure planes usually form in the vicinity of an interface between distinct lithologies that together comprise a weak layer. The review further demonstrates that different types of weak layers show an affinity to specific geographical and physiographical locations. These include contourite or turbidite systems that can create siliciclastic sediment sequences, areas of high productivity or upwelling where biogenic sediments may dominate, or regions that experience repeated ash deposition from proximal or distal volcanic sources. Weak layers are further investivated by means of two selected case studies, a cohesive submarine landslide that occurred in a low angle sheeted contourite drift (namely the AFEN Slide) and a coastal retrogressive submarine landside that initiated along a regional turbidite event bed (namely the Finneidfjord Slide). The AFEN Slide is investigated using a combination of geophysical, sedimentological, geochemical, and geotechnical data. These data reveal abrupt lithological contrasts characterised by distinct changes in physical, geochemical and geotechnical properties. The findings indicate that failure likely initiated along this distinct climatically-controlled lithological contrast, which marks the boundary between a sandy contourite and underlying softer mud-rich sediments. Whether climate change played a role in triggering slope failure remains unclear, however, the data demonstrate its role in dictating the location of the failure plane. Furthermore, the results highlight the necessity to integrate high-resolution sediment core analyses and information about the regional setting to identify potential weak layers over the depth range of stratigraphy. The second case study, the Finneidfjord Slide offshore Norway, is investigated by means of high-resolution 3D micro-Computed Tomography imaging. The results reveal clear compositional and structural differences between individual sub-units of the weak layer, as well as the background sediment. The pore space distribution is highly spatially variable. Such high variability may be masked by bulk porosity measurements. Bulk-porosity measurements work on a centimetre-scale, while the observed changes are found on a millimetre-scale. Such differences, however, may be crucial for the formation of weak layers as they appear to dictate the location of the failure plane. These findings have important implications for understanding how weak layers are formed and their influence on failure plane formation. The results further enable a better constraint on the relation between environmental setting and weak layer distribution, as well as triggering and failure mechanisms.Dissertation328 313 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, The influence of sediment texture on the mobility of mixed beds : Annular flume experiments and numerical modelling(2016-06-24); ; ; The research objectives in this dissertation were inspired by studies investigating the influence of fine sediment on the mobility of a mixed sediment bed on different grain scales and in different flow environments. While studies investigating fluvial sediment transport have concluded that the addition of fine material mobilizes the riverbed, other studies investigating mainly estuarine and marine sediment transport have found that the addition of fine material leads to bed stabilization. Two series of laboratory flume experiments (with a) spherical glass beads and b) sand and silt) and a numerical model were used to analyse the influences of the sediment texture and the particle shape on the near-bed flow field and the mobility of a mixed bed, and to find a possible transition between the different modes of behaviour. The sediment texture was characterized by the fine-grained fraction and the grain-size ratio RD = Dcoarse/Dfine between the diameters of the coarse and the fine particles. It could be shown that the bed texture influences flow processes at the bed surface that subsequently control particle entrainment.Dissertation334 271 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Alteration of flow and sediment dynamics by coastal biogenic structures: case studies of crab burrows and mangrove roots(2020-02-26); ; In the coastal environment, a large number of structures may be observed. Apart from anthropogenic structures (e.g. dikes, piers), a great diversity of natural structures (e.g. seagrass meadows, oyster reefs) are found along the coast; these structures generated by living organisms are referred to as biogenic structures. This thesis was motivated to understand the interplay between natural biogenic structures, hydro- and sediment dynamics. In aquatic ecosystems, the flow controls the transport of particles in-cluding nutrients, organic matter and sediment; hydrodynamics are, consequently, of great relevance to the ecosystems. Flow alterations by the presence of biogenic structures have been extensively studied within arrays of aboveground structures. However, the modification of hydrodynamics by belowground structures such as cavities dug by animals is poorly known. Furthermore, the connection between flow change and sediment transport due to the presence of biogenic structures is still lacking knowledge since single biogenic structures exhibit many different geometries (e.g. shape, dimensions) and arrays of structures have various properties (e.g. density, arrangement). In this thesis, the role of structure dimensions and array properties was investigated by considering two case studies: crab burrows as a representative for belowground structures and mangrove roots as a representative for aboveground structures. The options to investigate these structures are various; however, in the present thesis, a twofold approach was chosen: on one hand numerical modelling and on the other hand field observations. A combined field-numerical study provided measurements on flow and sediment dynamics around crab burrows, whereas numerical modelling appeared as a suitable tool to investigate hydro- and sediment dynamics around mangrove roots. Numerical simulations were run by using an existing Computational Fluid Dynamics (CFD) code available in the C++ toolbox OpenFOAM that employs the Finite Volume Method (Chapter 2). In Chapter 3, a literature review was conducted to determine the state of the art in hydro- and sediment dynamics in mangrove ecosystems with a focus on the methods employed and the spatial scales of experiments. Results emphasized the missing knowledge of hydro- and sediment dynamics at rather small spatial scales (below 10 m) and the under-use of numerical modelling to investigate abiotic pro-cesses in mangrove ecosystems in comparison with field and flume experiments. In Chapter 4, to elucidate the impact of mangrove structures on flow and sediment transport at small spatial scales, a three-dimensional model was implemented and successfully validated. A first set of numerical simulations was carried out to determine how a simple mangrove structure (Rhizophora man-grove seedling) modifies the flow and the sediment transport in its close vicinity under five current ve-locities. A similar flow structure was reported for all tested velocities: (a) a downward flow, (b) a horse-shoe vortex, (c) a flow separation and (d) vortices that shed from the rear of the seedling, while flow conditions clearly controlled the magnitude of flow alteration and, consequently, sediment transport. This flow structure enhanced scouring around the foot of the seedling and sediment resuspension in its trail. In Chapter 5, a second set of numerical simulations was conducted to unravel the flow dynamics and sediment transport at a larger scale than a single mangrove seedling: around rows of pneumatophores (mangrove pencil-like roots) subject to a unidirectional current. Three flow directions were simulated in combination with two or four spacing values. Modelled results revealed that the properties of the pneu-matophore arrays (orientation and spacing) controlled the hydrodynamic mechanisms that were gener-ated around the pneumatophores as well as the magnitude of such effects. For a flow parallel to the row of pneumatophores, a “sheltering effect” reduced approaching velocity and enhanced turbulence that caused “global scour” around the array of pneumatophores regardless the spacing. For a flow perpen-dicular to the line of pneumatophores, a “blockage effect” led to flow constriction in-between pneumato-phores. The magnitude of this “blockage effect” was sufficiently high to bring about “global scour” around pneumatophores for small spacing values due to a high-flow blockage. In the case of oblique flow, no interaction between flow features stemming from adjacent pneumatophores was reported for large spac-ing value suggesting that pneumatophores acted as isolated obstructions that in turn caused “local scour”. A similar sedimentation pattern to the perpendicular case was observed for a small spacing due to a “blockage effect” and an increase in approaching velocity. In Chapter 6, a field study supported by a two-dimensional numerical model was carried out to inves-tigate the flow around crab burrows with different aspect ratios (depth/height) and, thus, to elucidate how these belowground biogenic structures affect the particle trapping. Field observations revealed that the material capture (sediment, organic matter) was larger in burrows with a large opening than in burrows with a small opening but same depth. This difference can be explained by the fact that the burrow aspect ratio control the flushing rate, as shown by the modelled results. In addition, an increase in sediment capture was observed, during the field campaign, in arrays that contained more burrows with a small opening, which likely resulted from a change in turbulence level. Numerical simulations revealed that turbulence was higher in burrows with a large opening; however, turbulence persisted for a longer dis-tance downstream of the burrows with a small opening. This turbulence pattern associated to burrows with a small opening may have increased sediment fluxes and, therefore, promoted the sediment cap-ture into neighbouring burrows. This thesis provides valuable insights into the abiotic processes (non-living factors) of flow alteration and sediment transport in response to coastal biogenic structures. The hydrodynamic mechanisms that control the sediment transport around biogenic structures with various geometries and spatial properties were identified and quantified through the large set of physical parameters mainly obtained via numerical simulations. While the study on crab burrows is one of the first applications of CFD to investigate bio-genic belowground structures, the model used in the studies on mangrove seedling and pneumatophores is, to my knowledge, one of the first attempts to simulate small-scale hydro- and sediment dy-namics within mangrove structures using CFD. Therefore, this innovative work lays the foundations for future CFD simulations of hydro- and sediment dynamics around biogenic structures and, this thesis emphasises the potential of the numerical approach for future research in the field of ecohydraulics.Dissertation394 740