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  • 11
    Publication Date: 2018-03-09
    Description: Initially observed in the semi-arid basins of southwestern USA, earth fissures due to aquifer over-exploitation are presently threatening a large number of subsiding basins in various countries worldwide. Different mechanics have been proposed to explain this process, such as differential compaction, horizontal movements, and fault reactivation. Numerical modeling and prediction of this major geo-hazard caused by overuse of groundwater resources are challenging because of two main requirements: shifting from the classical continuous to discontinuous geomechanics and incorporating 2-dimensional features (the earth fissures) into large 3-dimensional (3D) modeling domain (the subsiding basin). In this work, we proposed a novel modeling approach to simulate earth fissure generation and propagation in 3D complex geological settings. A nested two-scale approach associated with an original non-linear elasto-plastic finite element/interface element simulator allows modeling the mechanics of earth discontinuities, in terms of both sliding and opening. The model is applied on a case study in Wuxi, China, where groundwater pumping between 1985 and 2004 has caused land subsidence larger than 2 m. The model outcomes highlight that the presence of a shallow (∼80 m deep) bedrock ridge crossing the Yangtze River delta is the key factor triggering the earth fissure development in this area. Bending of the alluvial deposits around the ridge tip and shear stress due to the uneven piezometric change and asymmetrical shape of the bedrock have caused the earth fissure to onset at the land surface and propagate downward to a maximum depth of about 20-30 m. Maximum sliding and opening are computed in the range of 10 to 40 cm, in agreement with the order of magnitude estimated in the field.
    Print ISSN: 0043-1397
    Electronic ISSN: 1944-7973
    Topics: Architecture, Civil Engineering, Surveying , Geography
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  • 12
    Publication Date: 2018-03-09
    Description: Delivery in water markets is generally operated by agencies through channel systems, which imposes physical and institutional market constraints. Many water markets allow water users to post selling and buying requests on a board. However, water users may not be able to choose efficiently when the information (including the constraints) becomes complex. This study proposes an innovative two-phase model to address this problem based on practical experience in China. The first phase seeks and determines the optimal assignment that maximizes the incremental improvement of the system's social welfare according to the bids and asks in the water market. The second phase sets appropriate prices under constraints. Applying this model to China's Xiying Irrigation District shows that it can improve social welfare more than the current “pool exchange” method can. Within the second phase, we evaluate three objective functions (minimum variance, threshold-based balance, and two-sided balance), which represent different managerial goals. The threshold-based balance function should be preferred by most users, while the two-sided balance should be preferred by players who post extreme prices.
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    Electronic ISSN: 1944-7973
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  • 13
    Publication Date: 2018-03-09
    Description: Space-borne instruments can measure river water surface elevation, slope and width. Remote sensing of river discharge in ungauged basins is far more challenging, however. This work investigates the estimation of river discharge from simulated observations of the forthcoming Surface Water and Ocean Topography (SWOT) satellite mission using a variant of the classical variational data assimilation method “4D-Var”. The variational assimilation scheme simultaneously estimates discharge, river bathymetry and bed roughness in the context of a 1.5D full Saint Venant hydraulic model. Algorithms and procedures are developed to apply the method to fully ungauged basins. The method was tested on the Po and Sacramento Rivers. The SWOT hydrology simulator was used to produce synthetic SWOT observations at each overpass time by simulating the interaction of SWOT radar measurements with the river water surface and nearby land surface topography at a scale of approximately 1 m, thus accounting for layover, thermal noise and other effects. SWOT data products were synthesized by vectorizing the simulated radar returns, leading to height and width estimates at 200 m increments along the river centerlines. The ingestion of simulated SWOT data generally led to local improvements on prior bathymetry and roughness estimates which allowed the prediction of river discharge at the overpass times with relative root-mean-squared errors of 12.1% and 11.2% for the Po and Sacramento rivers respectively. Nevertheless, equifinality issues that arise from the simultaneous estimation of bed elevation and roughness may prevent their use for different applications, other than discharge estimation through the presented framework.
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  • 14
    Publication Date: 2018-03-09
    Description: A new measurement method for continuous detection of bed forms in movable bed laboratory experiments is presented and tested. The device consists of a line laser coupled to a 3D-camera, which makes use of triangulation. This allows to measure bed forms during morphodynamic experiments, without removing the water from the flume. A correction is applied for the effect of laser refraction at the air-water interface. We conclude that the absolute measurement error increases with increasing flow velocity, its standard deviation increases with water depth and flow velocity, and the percentage of missing values increases with water depth. Although 71% of the data is lost in a pilot moving bed experiment with sand, still high agreement between flowing water and dry bed measurements is found when a robust LOcally weighted regrESSion (LOESS) procedure is applied. This is promising for bed form tracking applications in laboratory experiments, especially when lightweight sediments like polystyrene are used, which require smaller flow velocities to achieve dynamic similarity to the prototype. This is confirmed in a moving bed experiment with polystyrene.
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  • 15
    Publication Date: 2018-03-09
    Description: Dissolved oxygen (DO) concentrations and consumption rates are primary indicators of heterotrophic respiration and redox conditions in the hyporheic zone (HZ). Due to the complexity of hyporheic flow and interactions between hyporheic hydraulics and the biogeochemical processes, a detailed, mechanistic and predictive understanding of the biogeochemical activity in the HZ has not yet been developed. Previous studies of microbial activity in the HZ have treated the metabolic DO consumption rate constant (K DO ) as a temporally fixed and spatially homogeneous property that is determined primarily by the concentration of bioavailable carbon. These studies have generally treated bioactivity as temporally steady state, failing to capture the temporal dynamics of a changeable system. We demonstrate that hyporheic hydraulics controls rate constants in a hyporheic system that is relatively abundant in bioavailable carbon, such that K DO is a linear function of the local downwelling flux. We further demonstrate that, for triangular dunes, the downwelling velocities are lognormally distributed, as are the K DO values. By comparing measured and modeled DO profiles, we demonstrate that treating K DO as a function of the downwelling flux yields a significant improvement in the accuracy of predicted DO profiles. Additionally, our results demonstrate the temporal effect of carbon consumption on microbial respiration rates.
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  • 16
    Publication Date: 2018-03-09
    Description: Cumulative impacts of multiple stressors on freshwater biodiversity and ecosystem function likely increase with elevation, thereby possibly placing alpine communities at greatest risk. Here, consideration of species traits enables stressor effects on taxonomic composition to be translated into potential functional impacts. We analyzed data for 47 taxa across 137 mountain lakes and ponds spanning large latitudinal (491 km) and elevational (1399 m) gradients in western Canada, to assess regional and local factors of the taxonomic composition and functional structure of zooplankton communities. Multivariate community analyses revealed that small body size, clonal reproduction via parthenogenesis, and lack of pigmentation were species traits associated with both introduced non-native sportfish and also environmental conditions reflecting a warmer and drier climate – namely higher water temperatures, shallower water depths, and more chemically concentrated water. Thus, historical introductions of sportfish appear to have potentially induced greater tolerance in zooplankton communities of future climatic warming, especially in previously fishless alpine lakes. Although alpine lake communities occupied a relatively small functional space (i.e. low functional diversity), they were contained within the broader regional functional structure. Therefore, our findings point to the importance of dispersal by lower montane species to the future functional stability of alpine communities.
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  • 17
    Publication Date: 2018-03-09
    Description: Collaboration between academics and practitioners promotes knowledge transfer between research and industry, with both sides benefiting greatly. However, academic approaches are often not feasible given real-world limits on time, cost and data availability, especially for risk and uncertainty analyses. Although the need for uncertainty quantification and risk assessment are clear, there are few published studies examining how scientific methods can be used in practice. In this work, we introduce possible strategies for transferring and communicating academic approaches to real-world applications, countering the current disconnect between increasingly sophisticated academic methods and methods that work and are accepted in practice. We analyze a collaboration between academics and water suppliers in Germany who wanted to design optimal groundwater monitoring networks for drinking-water well-catchments. Our key conclusions are: to prefer multi-objective over single-objective optimization; to replace Monte-Carlo analyses by scenario methods; and to replace data-hungry quantitative risk assessment by easy-to-communicate qualitative methods. For improved communication, it is critical to set up common glossaries of terms to avoid misunderstandings, use striking visualization to communicate key concepts, and jointly and continually revisit the project objectives. Ultimately, these approaches and recommendations are simple and utilitarian enough to be transferred directly to other practical water-resource related problems.
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  • 18
    Publication Date: 2018-03-09
    Description: The Levenberg-Marquardt (LM) method is commonly used for inverting models used to describe geothermal, groundwater, or oil and gas reservoirs. In previous studies LM parameter updates have been made tractable for highly parameterized inverse problems with large data sets by applying matrix factorization methods or iterative linear solvers to approximately solve the update equations. Some studies have shown that basing model updates on the truncated singular value decomposition (TSVD) of a dimensionless sensitivity matrix achieved using Lanczos iteration can speed up the inversion of reservoir models. Lanczos iterations only require the sensitivity matrix times a vector and its transpose times a vector, which are found efficiently using adjoint and direct simulations without the expense of forming a large sensitivity matrix. Nevertheless, Lanczos iteration has the drawback of being a serial process, requiring a separate adjoint solve and direct solve every Lanczos iteration. Randomized methods, developed for low-rank matrix approximation of large matrices, are more efficient alternatives to the standard Lanczos method. Here we develop LM variants which use randomized methods to find a TSVD of a dimensionless sensitivity matrix when updating parameters. The randomized approach offers improved efficiency by enabling simultaneous solution of all adjoint and direct problems for a parameter update.
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  • 19
    Publication Date: 2018-03-09
    Description: Liang et al. (2017) presented an analysis of the impacts of unsaturated zone processes on streamflow recession using methodology from Brutsaert and Nieber (1977) with a constant time step in computation of the time derivative of flow. Over the past 10 years many authors have demonstrated that this method may produce artifacts that lead to incorrect interpretations. To demonstrate the impact of the choice of analysis methods, this comment presents an estimation of recession parameters using the Liang et al. (2017) discharge data that eliminates artifacts introduced through the constant time-step. Here we use the exponential time step method, which revealed recession coefficient b greater than 1 which are inconsistent with the fitting framework used in Liang et al. (2017).
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  • 20
    Publication Date: 2018-03-09
    Description: Determining the probability of a flood event in a catchment given that another flood has occurred in a nearby catchment is useful in the design of infrastructure such as road networks that have multiple crossings of a river. These conditional probabilities of flood can be estimated by calculating conditional probabilities of extreme rainfall and then transforming rainfall to runoff through a hydrologic model. Each catchment's hydrological response times are unlikely to be the same, so in order to estimate these conditional probabilities one must consider the dependence of extreme rainfall both across space and across critical storm durations. To represent these types of dependence, this study proposes a new approach for combining extreme rainfall across different durations within a spatial extreme value model using max-stable process theory. This is achieved in a stepwise manner. The first step defines a set of common parameters for the marginal distributions across multiple durations. The parameters are then spatially interpolated to develop a spatial field. Storm-level dependence is represented through the max-stable process for rainfall extremes across different durations. The dependence model shows a reasonable fit between the observed pairwise extremal coefficients and the theoretical pairwise extremal coefficient function across all durations. The study demonstrates how the approach can be applied to develop conditional maps of the return period and return level across different durations.
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