245 research outputs found

    Anisotropy characteristics of exposed gravel beds revealed in high-point-density airborne laser scanning data

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    The aim of this study was to examine the relationship between the anisotropy direction of exposed gravel bed and flow direction. Previous studies have shown that the anisotropy direction of a gravel bed surface can be visually determined in the elliptical contours of 2-D variogram surface (2DVS). In this letter, airborne laser scanning (ALS) point clouds were acquired at a gravel bed, and the whole data set was divided into a series of 6 m × 6 m subsets. To estimate the direction of anisotropy, we proposed an ellipse-fitting-based automatic procedure with consideration given to the grain size characteristic d50 to estimate the primary axis of anisotropy [hereafter referred to as the primary continuity direction (PCD)] in the 2DVS. The ALS-derived PCDs were compared to the flow directions (for both high and low flow) derived from hydrodynamic model simulation. Comparison of ALS-derived PCDs and simulated flow directions suggested that ALS-derived PCDs could be used to infer flow direction at different flow rates. Furthermore, we found that the ALS-derived PCDs estimated from any elliptical contour of the 2DVS exhibited a similar orientation when the contours of the 2DVS reveal the clear anisotropic structure, demonstrating the robustness of the technique

    Quantifying the scales of spatial variation in gravel beds using terrestrial and airborne laser scanning data

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    Previous studies measured gravel bed surfaces by terrestrial laser scanning (TLS) and close-range photogrammetry suggested the presence of at least two different scales of spatial variation in gravel bed surfaces. This study investigated the spatial variation of airborne laser scanning (ALS) point clouds acquired in gravel bed. Due to the large footprint of ALS systems, a smoother surface is expected, but there exists some uncertainty over the precise scale of ALS measurement (hereafter referred to as the spatial support). As a result, we applied the regularization method, which is a variogram upscaling approach, to investigate the true support of ALS data. The regularization results suggested that the gravel bed surface described by the ALS is much smoother than expected in terms of the ALS reported measurement scale. Moreover, we applied the factorial kriging (FK) method, which allows mapping of different scales of variation present in the data separately (different from ordinary kriging which produces a single map), to obtain the river bed topography at each scale of spatial variation. We found that the short-range and long-range FK maps of the TLS-derived DSMs were able to highlight the edges of gravels and clusters of gravels, respectively. The long-range FK maps of the ALS data shows a pattern of gravel-bed clusters and aggregations of gravels. However, the short-range FK maps of the ALS data produced noisy maps, due to the smoothing effect. This analysis, thus, shows clearly that ALS data may be insufficient for geomorphological and hydraulic engineering applications that require the resolution of individual gravels. © 2018 G.-H. Huang et al. published by De Gruyte

    Quantification of landscape evolution on multiple time-scales

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    Essential information about the activity or even the mechanics of tectonic and erosional processes can be extracted from their surface expression. For this purpose, it is necessary to appropriately constrain the temporal as well as the spatial framework, in which to consider a specific process. While recently developed dating techniques, such as thermochronology or radiocarbon dating, allow to assess the age of landforms and therefore rates of tectonic and erosional processes, detailed spatial information is also required to assess these rates correctly. Due to a lack of appropriate topographic data in the past it was sometimes challenging to reliably approximate the spatial framework, because the size of a particular landform can often cover a wide range of spatial scales. Recently available, conventional topographic data, such as those of the Shuttle Radar Topography Mission, substantially improved the definition of an appropriate spatial framework due to their spatial coverage and resolution of down to less than 1 m. However, to constrain this framework at a detail beyond the resolution of several decimeter terrestrial laser scanning provides a highly efficient approach. This technique permits the rapid acquisition (within minutes) of tremendous amounts of topographic data with both, a high resolution of a few centimeters and a high accuracy of a few millimeters. High-resolution topographic maps of a certain area of the surface of the Earth are derived from individual laser-scanner measurements, that in turn allow to characterize the in-situ geomorphic setting at great detail. Moreover, repeated measurements of this area allow to quantify morphological changes thereby supporting the survey of surface processes on short-term scales ranging from days up to several years. The former approach is best suited for tectono- and the latter one for fluvial-geomorphic studies, and we present results from two case studies that are either based on single or repeated laser-scanner measurements. In the first case, we combined field mapping and high-resolution digital elevation model (DEM) analysis to evaluate the detailed meter- to hundred meter-scale structure and surface expression of one flank of the Rex Hills pressure ridge in the western United States. Based on terrestrial laser scanning (Riegl LMS-Z420i) we derived a DEM with cm-scale resolution and extracted high-resolution topographic cross-sections. This enabled us to identify fault scarps and determine their relative ages and geometry. In the second case, we carried out a detailed field mapping of erosion and sedimentation patterns in the Alp Valley, central Switzerland, to assess its Holocene evolution. Simultaneously, we conducted repeated high-resolution (less than 1 cm locally) laser-scanning surveys (Topcon TLS-1000) along two tributaries, the Erlenbach and Vogelbach, to determine channel-morphology changes and the nature of shortest-term sediment transport by comparing the individual DEMs derived from these measurements, as well as to evaluate the context to the longer-term evolution of the Alp Valley. Both case studies, however, highlight the potential of medium-range laser scanners with measurement distances of up to hundreds of meters. Such scanners are most appropriate to efficiently analyze closely-spaced fault scarps across a broad range of spatial scales, and to document complex morphologic changes in small mountainous torrents due to sediment transport. Moreover, terrestrial laser scanning is a key tool to monitor surface processes, but the insights gained from this method are generally evaluated best in the context of further data sets including geochronological, structural, subsurface, or climate data. Surface processes, in particular erosion, sediment transport, and deposition in sedimentary basins are intermittent in space and time challenging both, the appropriate definition of a spatiotemporal framework addressed above and a comprehensive process understanding. A major objective of this thesis is to contribute to a better understanding of scale linkage concerning these processes. We therefore first carried out a comprehensive comparison of short- to long-term erosion measurements from the Alps based on an approach originally established to evaluate the significance of geologic and geodetic measurements along intra-continental faults on time scales of millions to tens of years. In a second step, we re-assessed the sediment budget of the Alps, a data set that is usually considered to be an appropriate measure of long-term erosion in the Alps. The two major results of both studies indicate that: short- and medium-term erosion in the Alps over years to ten thousands of years is dominantly influenced by climate and weather variability, e.g., due to seasonal differences in the amount of precipitation; whereas long-term erosion over millions of years is controlled by tectonic processes

    Engineering geology of British rocks and soils : Lias Group

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    The report begins with an introduction and a detailed modern assessment of the geology of the Lias Group in terms of both stratigraphy and lithology. The modern lithostratigraphy is placed in the context of the old, and sometimes more familiar, usage. The next two chapters deal with the mineralogy of a suite of samples collected for the project, and an assessment of the nature and influence of weathering based on a detailed analysis of the Lias dataset held in the BGS National Geotechnical Properties Database. The following chapters cover geohazards associated with the Lias Group, and a brief overview of the wide variety of industrial applications for which the Lias is well known. The geotechnical database forms the basis of the penultimate chapter, geotechnical properties. The contents of the database are analysed, interpreted, presented in graphical form, and discussed in terms of statistical variation and in the light of likely engineering behaviour. The engineering geology of the Lias Group is discussed in the final chapter, borrowing from the preceding chapters. A comprehensive cited reference list and a bibliography are provided. In addition to the large number of technical data provided to BGS, a small data set has been generated by BGS laboratories, particularly in areas where the main database was deficient, and also in connection with associated BGS studies of the swelling and shrinkage properties of the Lias Group. The individual items of data making up the database are not attributed. However, the contribution of a wide range of consultancies, contractors, authorities, and individuals is acknowledged. It is hoped that this report will provide a source of useful information to a wide range of engineers, planners, scientists, and other interested parties concerned with Lias Group materials. It should be noted that whilst quantitative technical data are included in this report, these should not be used as a substitute for proper site investigation

    TLS supported volumetric and DFN modeling of a fault zone in the Lower Buntsandstein, SW Germany

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    Fluid flow is governed by primary and secondary porosity of rocks but also by their permeability. Often the values of primary porosities and permeabilities are not sufficient to allow fluids to flow from potential geothermal or hydrocarbon reservoirs. To ensure an efficient productivity, fractured reservoirs come into focus as they might provide an economically viable fluid flow. Subsurface fractured reservoirs are difficult to investigate, outcrop analogues like the one investigated help in a better understanding. The studied outcrop represents a Lower Triassic braided river succession within an arid alluvial plain, affected by the main fault of the western Rhine Graben (southwestern Germany). The research thesis was carried out with the help of terrestrial laser scanning (TLS) to generate a digital outcrop model (DOM), used to digitize data and serve as basis for the subsequent modeling in two steps. These are (i) the volumetric modeling of the investigated fault zone within the Triassic Lower Buntsandstein, and (ii) subsequent modeling of the discrete fracture network (DFN). Volumetric modeling comprises three main points: (i) the application of a fault zone facies concept, (ii) stair-stepped fault gridding, and (iii) splitting the fault zone into two geobodies, well established in structural terminology, the damage zone ‘DZ’ and the fault core ‘FC’. For the subsequent DFN calculations a thorough fracture data parametrization was carried out providing six defined fracture sets, the fracture shape, the log-normal aperture distribution, the log-normal length distribution, the P32 intensity, and fracture truncation percentages at bed boundaries (DZ only). DFN upscaling was then conducted with the “Oda” and “Oda Corrected” methods for the fracture permeability calculations. The resulting volumetric model comprises 13 fault zone facies types. Their distribution within the DZ follows the encountered beds’ morphology. Within the FC three facies distribution cases were modeled. Seven different DFN configurations were calculated, consisting of 162 fracture sets in total. Fracture permeability amounts between 190 and 720 D within the DZ and 14,130 to 55,189 D within the FC, while the fracture porosity shows values of about 0.4 % for the DZ and 2.38 % for the FC. The study shows that volumetric fault zone modeling requires a simultaneous fault facies analysis and grid construction. Because stair-stepped fault grids facilitate a high complexity but lack cell size flexibility, a thoroughly considered choice of the cell size, dependent on the smallest geological objects present, is crucial. Characterization and processing of fracture aperture constitutes the most important part of the parametrization, as different methods can lead to distinct differences in the modeled final fracture permeabilities, spanning multiple orders of magnitude, even for exactly the same values of mechanical aperture. Inclusion of fracture connectivity lowers the resultant horizontal fracture permeability by 26 to 38 %, while truncation of fractures on bed boundaries can overestimate permeability values. Although the FC shows a significantly higher fracture permeability than the DZ it is affected by extreme fracture permeability cutoffs due to the fault cores’ specific architecture, resulting in a conduitbarrier system. Fracture porosities are more insensitive to parameter changes, because of its dependence on the mechanical aperture only. The presented multi-approach thesis highlights the challenges, limitations, and great possibilities of fault zone models, to help in a better understanding of the impact fault zones might have on geothermal and hydrocarbon reservoirs, and thereby support exploration

    MORPHODYNAMICS AND SEDIMENT TRANSFER IN A HUMAN-IMPACTED ALPINE RIVER

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    In the Anthropocene, river basins have been affected by human impact both indirectly, e.g. through changes in climate and land-use, and directly, e.g. through river engineering and flow management. The Swiss RhĂŽne basin is a notable example, where its Alpine tributaries are heavily impacted upon by climate warming and hydro-electric power exploitation (hydropower). Upstream, the RhĂŽne’s catchment is responding to the accelerated release of unconsolidated sediment associated with a history of glacial recession and ongoing climate change. In parallel, there is extensive abstraction of flow for hydropower, a common practice throughout the European Alps, which strongly reduces river sediment transport capacity whilst, contrary to classical reservoir dams, maintains sediment supply to streams. Combined, these impacts affect the morphodynamics and sediment storage of Alpine streams and hence the downstream transfer of sediment to the main rivers such as the RhĂŽne.In this thesis, the morphodynamics and sediment transfer of a left bank tributary of the RhĂŽne, the Borgne d’Arolla, are studied in response to the combined effects of climate change and flow abstraction. The river has a strongly regulated flow with c. 90% of water abstracted at intakes. The intakes have to be flushed of sediment and most flow then occurs as a result of this short duration (typically 30 minutes to 2 hours) flushing. Additional flow can occur if the upstream water transfer tunnels are full and the intake has to be opened for longer duration. The result is that the stream bed is dry for most of the time, but perturbed by flushing events. This makes the field site well adapted for the quantification of morphological change, through the remote sensing of the dry braided river bed at timescales from the daily to the decadal. It also allows provides high quality flow records at the intakes, which also record flushing, and so allow reconstruction of sediment supply rates from the 1970s. The river bed level evolution since the onset of flow abstraction in the early 1960s was analysed through the development of Structure from Motion (SfM) photogrammetric techniques for application to archival imagery. This partly automated methodology allows for the generation of detailed and accurate historical Digital Elevation Models (DEMs), although it requires the careful consideration of photogrammetric principles, particularly in low-relief environments such as braided rivers. Over shorter timescales (daily) the river morphodynamics were monitored using a long-range terrestrial laser scanner. The reaches just below the main intake show considerable aggradation (up to 5 meters) since the onset of flow abstraction. Widespread aggradation however did not commence until the onset of glacier retreat in the late 1980s and the dry and notably warm years of the early 1990s which intake records suggest led to an increase in upstream sediment supply. Surprisingly, most of the supplied sediment (c. 75%) was transferred through the studied reaches despite flow abstraction reducing transport capacity by an order of magnitude. This was because the natural transport capacity was substantially greater than sediment supply rates such that the reduction in capacity due to abstraction still allowed significant sediment flux. However, abstraction rendered the system more sensitive to internal and external forcing, whether ‘natural’ or human-induced. The spatial and temporal distribution of sediment transport and morphological change within the reaches varies strongly between events with similar levels of forcing (imposed flow and sediment supply). This stresses the importance of antecedent conditions, i.e. river bed topography and sediment stored, and internal morphological feedbacks on sediment transport rates and challenges simplistic notions regarding the equilibrium morphology in these systems. The system sensitivity also leads to the rapid response of the river to climate-driven hydrological variability and climate induced changes in sediment delivery rates and intake system functioning. The flow abstraction itself was designed under different climatic conditions such that the transfer tunnels are no longer sufficient to transport all glacial melt under extreme summer temperatures. This led to a strong increase in non- regulated flood events when the intake is opened for longer durations of time. The onset of these events has had a major impact on the downstream export of sediment from the reaches. In the wider river basin, the climate driven increases in sediment supply are conveyed downstream and reflected in: (1) temporal trends in sediment mining in the tributary basin; (2) the abundance of headwater sediments in the river bed sediment composition near the tributary outlet; and (3) increasing suspended load at the outlet of the RhĂŽne in Lake Geneva. -- Au cours de l’AnthropocĂšne, les bassins versants alpins ont Ă©tĂ© affectĂ©s par les activitĂ©s humaines de maniĂšre indirecte, via les changements climatiques et l’évolution des usages du sol, et directement Ă  travers l’ingĂ©nierie des cours d’eau et la rĂ©gulation des dĂ©bits. Le bassin du RhĂŽne suisse est un exemple notoire, ses tributaires alpins Ă©tant significativement impactĂ© par le rĂ©chauffement climatique et l’exploitation hydroĂ©lectrique. Les environnements alpins s’adaptent en rĂ©ponse Ă  l’accĂ©lĂ©ration du taux de livraison de sĂ©diments non-consolidĂ©s depuis l’amont en lien avec la rĂ©cession glaciaire rĂ©cente et les changements climatiques en cours. ParallĂšlement, beaucoup de bassins versants alpins sont affectĂ©s par des prĂ©lĂšvements d’eau pour l’exploitation hydroĂ©lectrique. Cette pratique, commune Ă  travers la chaine alpine, rĂ©duit drastiquement la capacitĂ© de transport des cours d’eau tout en laissant dans le lit l’entier de la masse sĂ©dimentaire, contrairement aux lacs de barrage qui tendent Ă  stocker les matĂ©riaux et produire des dĂ©ficits sĂ©dimentaires Ă  l’aval. CombinĂ©s, les impacts des changements climatiques et des prĂ©lĂšvements d’eau affectent ainsi la dynamique morphologique, la capacitĂ© de stockage et la capacitĂ© de transfert des tributaires alpins vers les Ă©missaires principaux, le RhĂŽne dans ce cas prĂ©cis. Dans ce travail de thĂšse, ce sont la dynamique morphologique et les transferts sĂ©dimentaires de la Borgne d’Arolla, tributaire du RhĂŽne, qui ont Ă©tĂ© Ă©tudiĂ©s en rĂ©ponse aux effets combinĂ©s des changements climatiques et des prĂ©lĂšvements d’eau pour la production hydroĂ©lectrique. Le caractĂšre intermittent et hautement rĂ©gulĂ© des Ă©coulements permet la quantification prĂ©cise des changements morphologiques Ă  travers la tĂ©lĂ©dĂ©tection du lit pendant les pĂ©riodes sĂšches oĂč le dĂ©bit est nul, et la reconstruction du taux de livraison sĂ©dimentaire depuis les annĂ©es 1970 est rendue possible par les donnĂ©es de purge des captages. L’évolution du lit de la Borgne depuis le dĂ©but de l’exploitation hydroĂ©lectrique au dĂ©but des annĂ©es 1960 a Ă©tĂ© Ă©tudiĂ© Ă  travers l’application de mĂ©thodes photogrammĂ©triques ‘Structure from Motion’ Ă  des images aĂ©riennes historiques. Cette mĂ©thode semi-automatique permet la production de modĂšles numĂ©riques de terrain historiques Ă  haute rĂ©solution (MNTs), oĂč la faible amplitude altitudinale des plaines alluviales alpines requiĂšre un contrĂŽle consciencieux des rĂ©sultats photogrammĂ©triques. Les rĂ©sultats de l’étude montrent que le tronçon situĂ© directement en aval du captage principal a subi une aggradation considĂ©rable (jusqu’à 5 mĂštres) depuis le dĂ©but des prĂ©lĂšvements d’eau.L’aggradation du lit Ă  plus large Ă©chelle n’a toutefois pas dĂ©butĂ© jusqu’à l’initiation du retrait glaciaire Ă  la fin des annĂ©es 1980 et au cours des annĂ©es particuliĂšrement chaudes du dĂ©but des annĂ©es 1990 qui ont conduit Ă  une augmentation dans le taux de livraison sĂ©dimentaire depuis l’amont. MalgrĂ© cela, les donnĂ©es montrent que la majeure partie des sĂ©diments (environ 75%) ont pu ĂȘtre transfĂ©rĂ©s Ă  travers le tronçon d’étude. En effet, si les prĂ©lĂšvements d’eau ont significativement rĂ©duit la capacitĂ© de transport de la Borgne, il apparait que la capacitĂ© de transport rĂ©siduelle reste proche du taux de livraison depuis l’amont. Cet Ă©quilibre rend le systĂšme hautement sensible aux forçages internes et externes, qu’ils soient ‘naturels’ ou anthropiques. La distribution spatiale et temporelle du transport sĂ©dimentaire et les changements morphologiques au sein des tronçons Ă©tudiĂ©s varient significativement entre des sĂ©quences intermittentes d’écoulement qui correspondent Ă  des ‘purges sĂ©dimentaires’ des captages d’eau. Ceci souligne l’importance des conditions antĂ©rieures Ă  la purge Ă  l’aval du captage, notamment la morphologie des chenaux de la plaine alluviale et le stockage des sĂ©diments en leur sein, ainsi que leurs rĂ©troactions sur les taux de transport, pour la comprĂ©hension de l’équilibre morphologique de ces systĂšmes. La sensibilitĂ© du systĂšme conduit Ă©galement Ă  des rĂ©ponses rapides face aux forçages externes liĂ©s aux changements climatiques et hydrologiques dans le bassin versant, notamment l’évolution des taux de livraison sĂ©dimentaire et la frĂ©quence des purges des captages d’eau qui avait Ă©tĂ© dimensionnĂ©s par le passĂ© sur la base de conditions climatiques plus froides. Ceci conduit Ă  une forte augmentation de la frĂ©quence des crues non rĂ©gulĂ©es en raison de la surcharge du systĂšme de captage et de transfert des eaux, ce qui impacte significativement l’export de sĂ©diment vers l’aval. Dans le bassin versant Ă  plus large Ă©chelle, l’augmentation du taux de livraison sĂ©dimentaire dĂ» aux changements climatiques et sa propagation peuvent ĂȘtre perçus: (1) dans la variabilitĂ© temporelle des prĂ©lĂšvements de matĂ©riaux dans les carriĂšres du bassin versant; (2) dans le volume des cĂŽnes de dĂ©jection Ă  la confluence des tributaires avec les Ă©missaires principaux; et (3) dans l’augmentation de la charge en suspension dans le delta du RhĂŽne sur le lac LĂ©man

    Flow-vegetation interactions at the plant-scale: the importance of volumetric canopy morphology on flow field dynamics

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    Vegetation is abundant in rivers, and has a significant influence on their hydraulic, geomorphological, and ecological functioning. However, past modelling of the influence of vegetation has generally neglected the complexity of natural plants. This thesis develops a novel numerical representation of flow through and around floodplain and riparian vegetation, focusing on flow-vegetation interactions at the plant-scale. The plant volumetric canopy morphology, which comprises the distribution of vegetal elements over the three-dimensional plant structure, is accurately captured at the millimetre scale spatial resolution using Terrestrial Laser Scanning (TLS), and incorporated into a Computational Fluid Dynamics (CFD) model used to predict flow. Numerical modelling, with vegetation conceptualised as a porous blockage, is used to improve the process-understanding of flow-vegetation interactions. Model predictions are validated against flume experiments, with plant motion dynamics investigated, and analysis extended to consider turbulent flow structures and the plant drag response. Results demonstrate the spatially heterogeneous velocity fields associated with plant volumetric canopy morphology. The presence of leaves, in addition to the posture and aspect of the plant, significantly modifies flow field dynamics. New insights into flow-vegetation interactions include the control of plant porosity, influencing ‘bleed-flow’ through the plant body. As the porosity of the plant reduces, and bleed-flow is prevented, the volume of flow acceleration increases by up to ~150%, with more sub-canopy flow diverted beneath the impermeable plant blockage. Species-dependent drag coefficients are quantified; these are shown to be dynamic as the plant reconfigures, differing from the commonly assigned value of unity, and for the species’ investigated in this thesis range between 0.95 and 2.92. The newly quantified drag coefficients are used to re-evaluate vegetative flow resistance, and the physically-determined Manning’s n values calculated are highly applicable to conveyance estimators and industry standard hydraulic models used in the management of the river corridor

    Studies of sediments in a tidal environment

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