82 research outputs found
Distribution-aware estimation of the minimum achievable uncertainty in diffusion-tensor imaging (DTI)
No full textDiffusion tensor imaging (DTI) provides exquisite sensitivity to structural and microstructural characteristics of brain tissue, and is routinely employed in advanced neu-roimaging applications. DTI is commonly performed using intrinsically noisy echo-planar imaging techniques and poses high demands both on scanner performance and on in-scanner subject time, which in turn is directly related to the number of diffusion-weighting direction one requires. While DTI-derived indices such as fractional anisotropy (FA), diffusion tensor trace and anisotropy mode have proven extremely useful in characterizing disease-related aberrations, their estimation is commonly performed using fitting routines that do not properly take into account MRI noise distribution. In this paper, we present a distribution-aware maximum likelihood tensor estimation framework which also allows, for the first time, separate local noise estimation in both diffusion weighted and reference images. We validate our framework using multiple water phantom diffusion weighted acquisitions, and demonstrate its feasibility in human data. We then employ our framework within Monte Carlo simulations to show how the minimum achievable uncertainty attainable in DTI depends on signal-to-noise ratio (SNR) and number of diffusion gradient directions, demonstrating that these dependencies could be recast into simple power laws which may serve as guidelines for application-specific DTI protocol design
Epidermal growth factor receptor and caveolin-1 coexpression identifies adult supratentorial ependymomas with rapid unfavorable outcomes
No full textSupratentorial ependymomas account for a minority of intracranial ependymomas, which still have uncertain prognostic markers. Among them, epidermal growth factor receptor (EGFR) overexpression correlates with a poor prognosis. In glioblastoma cells, EGFR function has been reported to be regulated by its migration from cell membrane infoldings called caveolae and by its colo-calization with the caveolae-associated protein caveolin-1 (cav-1). Therefore, we decided to investigate cav-1 expression and coexpression with EGFR in a series of adult intracranial ependymomas. We analyzed 22 adult supratentorial ependymomas and compared tumor grades as determined by the WHO classification and patient survival rates with the expression of EGFR, cav-1, and p53 and the values of the proliferation marker Ki-67, all tested by immunohistochemistry; in addition, we investigated the mutational profile of cav-1. The results demonstrate that the tumor grade is directly correlated with EGFR, Ki-67, and cav-1 expression only, whereas (by univariate analysis) the expression of all the studied markers, as well as the tumor histological grade, significantly correlated with the patient's overall survival (OS). By multivariate analysis using the Cox proportional hazards model, among all variables considered, cav-1 was the only independent prognostic marker related to OS (relative risk = 13.92; P =.013). Among grade II epen-dymomas, only cav-1 correlated with poor OS (P =.011), distinguishing 2 distinct subgroups of tumors with different outcomes despite sharing identical grading. All the patients studied carried wild-type cav-1 sequences, demonstrating that cav-1 overexpression is not driven by activating mutations, as previously reported in other tumor types. Interestingly, after stratifying all cases into 4 distinct groups according to cav-1 and EGFR expression (cav-1+/EGFR+, cav-1+/EGFR+, cav-1+/EGFR+, and cav-1+/EGFR+), the coexpres-sion of cav-1 and EGFR identified a subset of patients with definitively poor prognoses. Further studies are needed to support this evidence on a larger scale and to clarify how cav-1 and EGFR interaction can influence tumor aggressiveness. © The Author(s) 2010
Human Stability On Slopes Under Overtopping Waves
No full textIn a world affected by climate change and sea-level rise, intense storms are expected to become more frequent in the future. This implies that our coastal protections will be more often and more intensely affected by overtopping waves, potentially endangering the safety of our coastal communities. The objective of the present study is to investigate the hazard to people/pedestrians by postwave overtopping flows over an inclined surface, simulating a coastal dike.Hydraulic Structures and Flood RiskCoastal Engineerin
Final Report Levee Challenge: Team Hans Brinker
No full textIn this assignment we simulate a two meters wide dike stretch which is subjected to overflow. Three failure mechanisms could occur due to overflow: full erosion of the soil of the dike, macro instability of the levee if too much soil is eroded away, leading to insufficient counterweight on the inside of the levee, and surface slip failure due to reduction of strength of the dike by flow of water in the dike core. Prevention of failure can be achieved by either keeping the water away from the berm surface or holding the soil particles in place while the water flows alongside it. The desired repair is an emergency repair that can placed quickly during a storm, or between two storms to prevent a damaged levee from failing. This is a temporary repair that will later be removed for full reconstruction of the levee. This provides constraints to the installation and repair procedure.To decide which solution is best, 10 concept solutions are compared based on their score in the Multi Criteria Analysis with 8 weighted criteria. The solution with the highest score is the best solution according to the Multi Criteria Analysis. The selected repair option is to locally cover the damaged area with flexible overlapping sheets of Tyvek®. This is a light and thin material that can easily be transported and cut to size at the location of the repair. These sheets can be easily secured against flow loads using Gripple anchors, while pins are used to keep the Tyvek® in place during windy conditions. An installation plan and cost breakdown was made. Using 3 m wide Tyvek® rolls, the costs would be around 19.19 euros per meter height.The damaged areas of the levee are covered with Tyvek® which is a waterproof, damp open material. This prevents the penetration of water into the core through the damaged areas. The Tyvek® also prevents the exposed soil from being eroded. Moreover, Tyvek® is a very strong material that won’t be torn off. As there is no strength reduction due to water penetration or further loss of surface material the chance for macro instability is minimized.First, a design was created, with components dimensioned for a dike stretch in the Hedwigepolder. Subsequently, the design was validated with tests. Including two types of damage. The first was removal of a 2 by 2 m grass layer (0.1 m thick) at the top of the dike and the second was a dug-out step-section across the entire width of the dike with a height difference of 0.5 meters at the toe of the dike. Only the second damage went through the clay layer and into the sand core of dike.During and after the tests some important observations were made. During all test volumes the plastic sheets were kept in place due to the anchors and pins. Some pins however, came loose due to vibration of the sheets (we presumed the cause of vibration is flow of water between two sheets or turbulence of water). The trench of the lower damage was partly washed away by the water. The erosion of soil in the trench did not result in changing the stability of the top sheet. At the lower damage the plastic sheet was torn off after a tear of 1 m was made with a knife.Our solution is designed to protect any dike from macro-instability by keeping the water flow away from the berm. This is done by layering multiple sheets of plastic foil over the inner berm, fastened with ground anchors and pins. The solution is a durable and reliable design due to proven waterproof, damp open and UV resistant properties, together with Gripple's demonstrated anchoring capabilities. Another major advantage of our solution is that the repair of the damage is selective, i.e. the solution is applied only to the parts of the levee where damage has occurred. In case of only localized and small damage to the levee, the costs and workload are very minimal compared to other solutions that are applied to the entire surface area of the levee.Polder2CCivil Engineerin
Improving the sea defense of Central Termoeléctrica Antonio Guiteras
No full textThe central Thermo Electrico Antonio Guiteras (CTE Antonio Guiteras) is a thermoelectric power plant located in the bay of Matanzas. In 2017, hurricane Irma passed the north coast of Cuba and destroyed the primary sea defense in front of the CTE, causing major damage to the plant. The power plant is renovated, and a new and improved sea defense is currently being constructed. The goal of this report is to answer the following question: to what extend is the power plant protected during extreme weather conditions and what improvements are needed to ensure that the power plant can remain operational during these extreme weather conditions? To determine what the hydrodynamic and meteorological effects are of a extreme weather event such as a tropical cyclone, a synthetic tropical cyclone is created. This synthetic hurricane must generate large significant waves in combination with a big storm surge, to have severe impact on the CTE. It must also have a significant probability of occurrence. To determine this normative synthetic hurricane, multiple synthetic hurricanes are simulated in Delft3D and XBeach and their corresponding return period is determined. As Irma significantly damaged the CTE, this hurricane is taken as the basis for all synthetic hurricane combinations. The hurricanes each vary from Irma in maximum wind velocities, forward speeds and their tracks. To simulate the physics of hurricane Irma, a spiderweb grid is created at the locations of the hourly best track of Irma. This is then used in the Delft3D model as input for the pressure and wind fields of the hurricane. The output of the Delft3D model is validated with recorded data of observations stations in the Gulf of Mexico. Recorded water levels and wind speeds of buoys near Key West are used for validation. XBeach is used to simulate the nearshore physical processes. XBeach can more accurately predict wave propagation and includes higher order processes in its simulation. As input for the XBeach model, the output of the Delft3D model is used.After running all the synthetic hurricanes in Delft3D, the five resulting normative hurricanes are run in XBeach. The synthetic hurricane that creates the largest significant wave heights at the project area is taken as a basis for the final design. This normative hurricane gives a maximum significant wave height of 8.8 m with a corresponding storm surge of 1.61 m at the location of the CTE. With these values a research on the current defense wall is done. Ultimately for a part of the sea defense an adjustment on the existing defense wall is proposed. A second but lower vertical wall with a bigger bullnose is placed in front of the existing one. This creates a triangular shaped stilling basin, from which the water can flow out at the seaside of the wall. For the other part of the sea defense no adjustments on the wall are proposed but an improvement of the existing drainage capacity is proposed. The existing drainage channel, which lies behind this section, is widened and deepened. Additionally, a drainage wall is built around the powerplant, which diverts the overland flow caused by intense rainfall into the drainage channel. MP304Civil Engineering | Hydraulic Engineerin
The investigation of animal burrows in levees: Using experimental data to develop a probabilistic model that aims to improve the efficiency of manual inspection of animal burrows on levees
No full textAnimal-induced anomalies on a levee surface negatively affect the erosion-resistance of a levee, leading to rapid failure of the inner slope in case of overtopping and/or overflow during high water. Where active prevention techniques of burrows from large animals, such as muskrats, are applied in and around levees in the Netherlands, the identification of smaller burrow holes made by voles and moles and the evaluation of the severity of the damage rely solely on the assessment and experience of the levee inspector. The Polder2C’s project aims to quantitively assess the impact of animal burrows on the levee surface by conducting experiments at the Living Lab Hedwige and Prosper Polder (LHPP) and this thesis aims to contribute in this by analyzing the experiment results and developing a model for levee inspection that includes uncertainties of the geometrical characteristics of vole and mole burrow systems. This thesis distinguishes two phases, the first of which focuses on the question: ‘How do animal burrows influence levee performance?’, and it aims to form a foundation for the second phase of the report by focusing on the behavior of burrowing animals and the geometrical characteristics of their burrows by conducting a literature review on existing research on this topic. Guidelines were set up that contribute to the identification of mole and vole burrow system by investigating the main differences between them, with the aim to easily identify separate burrow hole groups in the experiment data. These conclusions could then be used to identify different vole and mole burrow hole groups in data that was collected from several experiments at the Living Lab Hedwige and Prosper Polder (LLHPP) between October 2021 and February 2022 and answer the research question for the second phase, which was: ‘Based on data from levee inspection experiments, which probabilistic model can assess the inspection success rate of animal burrows?’.Civil Engineering | Hydraulic Engineerin
Grand Bahama after Hurricane Dorian: Interdisciplinary approach to Build Back Better
No full textIn collaboration with the University of the Bahamas, we used the Charette model, the Three-point Approach and the conceptual framework: Build Back Better to propose a solution that involves several aspects: Reduced risk overall, increased social cohesion on the island, improved life quality and reduced costs during reconstruction in the aftermath of a hurricane event. Based on the analysis of the island, the hurricane and the important physical processes, a division between protection method is proposed based on the location on the island: collective andindividual protection.The main vision for the reconstruction of Grand Bahama is to Build Back Better. This is done by taking an interdisciplinary approach and connecting engineering to spatial planning and design. The proposed strategy reduces the risk by taking into account exposure and vulnerability of the general risk approach. The main point of the strategy is to create a resilient urban environment in which vital infrastructure like the airport remains functional. This is done by making a collective protection zone of the economic and social city centre of Freeport, a zone that also offers shelter. Individual protection and evacuation shelters will be given to residents, buildings and facilities in the less densely built areas, east and west of the city.MP31
Interaction between a sustainable water management strategy and Build Back Better: Sustainable drinking water management strategy for Grand Bahama Island
No full textThis thesis examines the role of the concepts of “Build Back Better”, “Disaster Cycle”, and “Sustainability” in water management and the possibility of the interactions between these concepts when designing water management infrastructure. Especially in cases where the disaster cycle is relatively short: 10 to 15 years between disasters. The case study is on the island of Grand Bahama. The island of Grand Bahama depends on the freshwater groundwater lens for its drinking water. Flooding during hurricanes introduces saline water into that groundwater lens. It is therefore highly probable that the current drinking water production method will not be able to meet the demand in the future. An alternative drinking water supply system should therefore be examined. To investigate the interactions between the concepts, they are put into a conceptual design process, which uses the methodology “RenewIsland”, and used to set up some preliminary designs. Three Alternatives were made with the methodology “RenewIsland”. These Alternative designs were then subjected to a Multi-Criteria Analysis (MCA). This thesis substantiates from the process that the interaction between the concepts is possible and that the inclusion of Build Back Better in the Water management decisions gives more possibilities to make balanced choices. A blueprint has been made for a method to analyze swiftly if an alternative is compatible with the different cycles and phases of a location regarding a disaster and normal circumstances. This thesis also states that the study has to be extended with more research to make it generally applicable.Civil Engineerin
Reliability Analysis of Foundation Pile Designs from Eurocode 7
No full textRisk-based and probabilistic reliability design methods are starting to be integrated into the daily geotechnical engineering practice, as shown by the current draft of EN1997 (Eurocode 7), which explicitly allows Reliability-based verification of limit states. The semi-probabilistic design methods in EN1997 are a compromise between ease of use and accuracy of the achieved reliability levels. In contrast, EN1990 (Eurocode 0) requires accurate full probabilistic design targets. An assessment to investigate if the semi probabilistic methods from EN1997 comply with the EN1990 reliability targets is therefore necessary since the link between the norms is not explicitly established. In this study, pile foundations serve as an example to assess compatibility between the EN1990 and EN1997. The deterministic Dutch pile design approach by van Mierlo & Koppejan (1956), CPT based design method, is adapted to the semi probabilistic model pile recipe in the EN1997 draft. This thesis assesses what reliability levels are achieved by the model pile design method in draft EN1997 considering the axial bearing capacity and how these compare to the reliability targets in EN1990.Methodology: To assess the performance of the model pile method from EN1997, the design outcome from the semi-probabilistic model pile method is compared to fully probabilistic quantification methods of the resistances and loads. Two probabilistic quantifications are used in the assessment, a Bayesian and a Student-T model. The achieved reliabilities are assessed with both the First Order Reliability Method (FORM) and Monte Carlo Simulation (MCS). Additional insight into the influence of different parameters is provided with sensitivity analysis and the calibration of partial factors. The assessment was applied in two case studies located in the Netherlands. In both case studies, the EN1997 designs resulted in reliability levels in good agreement with the reliability targets stated in EN 1990. This suggests an agreement between the semi-probabilistic and the full probabilistic models if sufficient CPTs are used. In both case studies, EN1997 designs accounted for more model uncertainty than the probabilistic models suggest, thus partial model factor (γRd) in EN1997 may be conservative. EN1997 covers spatial variability and uncertainty due to limited observations with the correlation factor (ζ, a factor that transforms calculated to representative values, and the partial resistance factor (γRc). The coverage of uncertainty with these two factors seems to be rather low, especially for situations with few CPTs available (less than 10), although the results seem to depend strongly on the degree of spatial variability in the CPT field. In the case of homogeneous soil conditions between observations, EN1997 design methods led to over-conservative designs for low numbers of observations. If the variability between limited observations (less than 10 CPTs) was high, designs are assessed to be less reliable than EN1990 requires.Overall, the results suggest good agreement of the semi-probabilistic design methods from EN1997 with the reliability targets defined in EN1990, for designs based on sufficient observations. The sensitivity analysis and isolation of the resistance uncertainty showed for the two cases that the uncertainty of the resistance has the dominating influence on the reliability of the piles, suggesting a low impact of complex stochastic load models. The latter finding suggests that probabilistic treatment of the resistance may be sufficient for assessing pile reliability in practice, while design values could be used for the loads.Civil Engineerin
Evaluating the stability of terps in the Netherlands
No full textTerps are artificial dwelling mounds mostly found in the northern regions of the Netherlands, built to provide safe ground against water in such areas affected by flooding, storm surges, and high tides, before the development of dikes. In this thesis, the stability of these terps was evaluated via FEM models and analyses performed on PLAXIS. Results highlight considerable stability risks when the terp slopes are subjected to considerable external loading, such as those exerted by heavy agricultural vehicles being operated in proximity of these.Geo-Engineerin
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