1,720,957 research outputs found
The longshore dimension in dune overwash modelling: Development, verification and validation of XBeach
No full textThe primary objective of this thesis is to generate a 2DH-numerical model to simulate dune overwash. The first stage in this is achieved by modifying the program code of an existing overwash model in development, XBeach, to enable 2DH calculations. In the second stage the hydrodynamics of the model are verified using theoretical and laboratory and field tests. In the third stage the model is validated by simulating overwash and washover on Santa Rosa Island, Florida, during Hurricane Ivan. The secondary objective of this thesis is to evaluate the effect of longshore bathymetry variations on the patterns and amount of overwash using the newly developed XBeach model. A numerical area model of part of Santa Rosa Island, Florida, is developed. The model is forced using Hurricane Ivan wave and surge conditions. The XBeach model shows five phases of morphology on the barrier island, leading from foredune erosion to breaching of the island. The model results are compared to high resolution post-storm altimetry data. It is shown that although the XBeach model produces morphological features common to overwash conditions, the amount of erosion is an order greater than the measured erosion. Sensitivity studies are carried out to determine the influence of the hydraulic boundary conditions on the final erosion profile. It is found that the model is sensitive to total surge levels and surge level gradients across the island, but insensitive to wave heights. It is shown that under inundation overwash conditions the amount of erosion and patterns of deposition are almost entirely determined by longshore bathymetry features with length scales in the order of kilometres. The primary recommendation given in this thesis is to develop and implement better sediment transport relations in XBeach and to account for effects of vegetation on the hydrodynamics and morphodynamics of the subaerial barrier island.Civil Engineering and Geoscience
Incipient motion of sand and oil agglomerates: A numerical study on the mobility of particles under wave-induced forcing
No full textAfter the Deepwater Horizon oil spill in 2010, oil was released into the Gulf of Mexico and partly reached the shorelines. Here, it mixed with sand in the surf zone to form sand and oil agglomerates (SOAs). These cm-sized particles are heavier than water and are therefore found on the seabed. Years after the initial formation of these particles, they keep appearing on the beaches, despite earlier clean-up. Several studies to the formation and transport of these particles have been conducted, but traditional shear-stress based formula, which are generally used for sediment transport, do not predict the mobility of SOAs well. This study focuses on the incipient motion of SOAs and aims to propose an improved method of predicting SOA mobility under waves. Sliding and rotation are found to be the two governing mechanisms of incipient motion of SOAs. For both mechanisms and two particle shapes, spherical and disc-shaped, a mobility parameter is defined. A critical value of this parameter is derived from a force balance and moment of force balance and is a function of several parameters: the drag coefficient CD, the inertia coefficient CM, the lift coefficient CL, the location of the resulting forces relative to the point of rotation, the Keulegan-Carpenter number (KC), the friction coefficient and the relative distance of the point of rotation to the point of gravity. When the mobility number exceeds the critical value, initiation of motion of SOAs is expected. The forces on the object are simulated in 2DV with the CFD package OpenFOAM. The results of the numerical simulations indicate that the critical mobility parameter strongly correlates with the KC-number. For low KC-numbers, the inertia force has a significant contribution to the horizontal force and reduces the stability of the particles, so less hydrodynamic forcing is required for incipient motion. Sliding is the governing mechanism of incipient motion for disc-shaped particles, whereas rotation is expected for spherical particles. Burial reduces the mobility of the particle, as a smaller part of the particle disturbs the flow and soil pressures act as an additional stabilizing force. The results of this study can be applied to predict incipient motion of SOAs on a fixed bed. Due to the sensitivity of the mobility to SOA shape, density and diameter, a probabilistic method is recommended for predicting mobility of a collection of SOAs instead of a single particle.Civil Engineering and GeosciencesHydraulic Engineerin
Long-term modelling with XBeach: combining stationary and surfbeat mode in an integrated approach
No full textXBeach is a process-based morphological model that has been used for modelling short term behaviour of beaches and dunes. In the past years interest has been shown by researchers and engineers to use XBeach for longer simulation periods (multiple years). XBeach has multiple modules: stationary mode is often used during calm conditions and surfbeat mode is often used during storm conditions. In this study was investigated whether the coupling of the two modes can increase the performance of long-term models compared to the separate models, while focusing on the cross-shore processes. Also the added value of (quasi-)2D models was studied. The high frequency bathymetryic data of Vlugtenburg, an alongshore uniform beach in The Netherlands, was used for creating 1D models for a period of 1 year. To compare the difference between singular and coupled models, first the potential performance of the stationary and surfbeat modules was investigated separately by optimising the settings for asymmetry, skewness and groundwater flow. It was found that the skill of the stationary and surfbeat models with default settings was negative. Erosion was overestimated (especially in surfbeat mode) and the models showed no seasonal effects. Settings for asymmetry and skewness proved to be effective measures to improve the model performance and were able to introduce seasonal effects. The optimised stationary model was able to predict the profile development reasonably well. The groundwater flow module did not affect the model stability as much as was found in the studies of Zimmermann et al. (2015) and Pender (2013). In order to validate whether a coupling between stationary and surfbeat mode would benefit the model performance, it was investigated whether stationary mode was able to restore the dune and beach erosion of the surfbeat model. Therefore surfbeat was run during the first 100 days of the year (during more extreme conditions) and stationary mode during the remainder of the year (more mild conditions). Stationary mode appeared not to be able to restore the eroded surfbeat profile. The performance of the coupled model was better than the surfbeat model, but the stationary mode was found to perform better than the coupled model for long-term modelling on the most criteria. It was found that a deficiency in XBeach is the ability to transport sediment from the intertidal zone (and just above) further up the beach and dunes. The erosion volumes in the dunes in stationary mode were in the same order of magnitude as the onshore directed aeolian transport. Therefore is expected that the results of long-term models can be improved by coupling XBeach with a wind model. The quasi-2D model was found to have a better performance than 1D models due to the spreading of infra-gravity waves in 2D; in both the separate and coupled models the erosion volumes were not overestimated as much as in the 1D models.Civil Engineering and GeosciencesHydraulic EngineeringCoastal Engineerin
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Opening and closure of intermittently open natural inlets
No full textTidal inlet systems are found all over the world. These highly dynamic coastal features can be classified into three main types wave-dominated, tide-dominated or river-dominated tidal inlet systems. A tidal inlet is described as an opening in the shoreline that provides a connection between the ocean and a bay and is maintained by tidal currents. Tidal inlet systems are the whole area of the near shore ocean, inlet and bay with river catchment area. Recently, Thuy (2013) developed a dimensionless hydraulic parameter tool to classify tidal inlet systems. The study focusses on wave-dominated tidal inlet systems and specifically the sub-class intermittently open natural inlets. The question arises if the intermittently open natural inlets can be distinct by use of the parameter. This type of sub-class has an alternately open or closed connection to the ocean, which is indicated by the word intermittently. A natural inlet is defined as an inlet with no other human interventions at the inlet entrance other than artificial breaching of the entrance berm when the lagoon water level exceeds a critical value. The distinction for the sub-class can be integrated into the wave-dominated classification parameter, therefore the overall objective is to make a contribution to the wave-dominated classification parameter of Thuy (2013) to make a more accurate classification for the sub-class intermittently open natural inlets. The overall objective is too extensive to be answered completely by only this study, therefore a specific objective is formulated. The specific objective is to predict the lagoon water level of intermittently open natural inlets forced by waves, tide and river discharge. The prediction of the lagoon water level will indicate the important forcing working on the inlet, while the entrance is in an open state. In total nine opening/closure events are found in the data and analysed. Data included, offshore wave climate, tidal water levels, lagoon water level and rainfall. To get a more accurate representation of the wave climate near the research sites, the offshore wave climate is adjusted to get the near shore wave climate. Based on the analysis the following main conclusion is found. The lagoon water level is a crucial parameter for prediction of the entrance state open or closed. Therefore, it is important to understand in which way the high waves will affect the elevation of the lagoon water level. Two analytical models are introduces to understand the forces that work on the open inlet. A basic energy equation, after Nielsen (2009), is the starting point of both models. The basic energy equation describes the river discharge and flood tide as inflow parameters, but the effect of waves is not taken into account. The first introduced model is the extended energy equation. The data analysis indicated that shore-normal waves are dominant during the elevation of the lagoon water level. The shore-normal wave forcing is introduced by means of a wave overwash discharge. The outcome of this model gives an underestimation of the lagoon water level, although the model performs better than the basic energy equation. The second introduced model is an alternative approach of the basic energy equation. The data analysis indicates that the near shore wave heights are very important for the elevation of the lagoon water level. Accordingly the elevation of the lagoon water level is assumed to be a function of the near shore wave height. The alternative model gives a better performance of the lagoon water level than the basic and the extended model. Based on this study it is recommended to further develop the alternative model for further research of different tidal inlet systems. A meaningful addition to the model could be the introduction of an entrance width that varies in time. During closure the inlet entrance gets narrower, which will result in different forcing and a different response of the lagoon water level.Hydraulic EngineeringCivil Engineering and Geoscience
Infra-gravity wave transformation across macro-tidal rocky shore platforms.
No full textThis thesis investigates the governing generating mechanisms, shoaling and reflection behaviour of infra-gravity waves on a mild-sloping, macro-tidal, rocky shore platform. The methods comprise among others, field data analysis of the Lilstock platform, numerical modelling using a calibrated XBeach model and a cross-correlation analysis to determine the evolution of the infra-gravity waves across the platform.Civil Engineering and GeosciencesHydraulic Engineerin
Modeling the contribution of infragravity and incident band swash on wave run-up on East coast South-Korean beaches
No full textThe maximum wave run-up on a beach is an important factor in the design of coastal protection measures. Therefore, it is desired to predict this run-up based on numerical models. In this thesis, this is done with the use of the model XBeach. Wave run-up consists of multiple components: a swash height, S, and a wave set-up, . The swash height can be divided in a high frequency (incident) and low frequency (infragravity) part. Which of these two frequencies prevail depends on multiple factors such as offshore wave energy and local bathymetry. In general, it is expected that on low sloping beaches or dissipative coasts, low frequency swash is dominant. Steep beaches or reflective coasts experience a dominance of high frequency swash motions. Intermediate beaches experience both high and low frequency motions, and thus lie somewhere between these two extremes. This report uses a case study, which is such an intermediate beach, with an alongshore varying bathymetry and topography: Anmok beach in South-Korea. This report also shows that the prevailing frequency band determines what type of (simplified) model can be used. This is done with the use of both 1D and 2DH models. As no data are available, all conclusions are made based on the model results only. The main part of this thesis consists of 1D results, as this required less computational time. Therefore, many synthetic storms could be analysed. Their analysis showed that low frequency motions prevail in the wave run-up and that the use of a simplified model is allowed. Even though a 1D model requires less computational demand, the question can arise whether it is valid to use it, since the case study showed a strongly alongshore varying coast. To research this, a 2DH model of the case study site has been analysed as well. The analysis is limited to one storm condition, which was also used in the 1D model, due to time constraints. The results showed differences between the 1D and 2DH model in the generation of low frequency motions, which lead to differences in the maximum wave run-up as well. The low frequency motions do not prevail as was seen in the 1D model. Contrary to the 1D model, very low frequency motions could also be found in the wave run-up of the 2DH model. As the 2DH model results are assumed to be closest to reality, the results suggest that a more simplified approach in the determination of wave run-up is not allowed for this particular case study. However, since the results are based on a single stormcondition, further research is necessary to provide more insight and better conclusions.Civil Engineering and GeosciencesHydraulic engineeringCoastal Engineerin
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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