Publicaties

2973 resultaten

  • A continuation approach to the optimization of hydropower operations

    Auteurs: Baayen, J.; Rauw, J.; Piovesan, T. (2018)
    Gepubliceerd in: arXiv : Mathematics - Optimization and Control (2018), pagina 1-13

    The instantaneous power generation from a hydroelectric turbine is proportional to the product of head difference and turbine flow. The equation relating power to hydraulic variables is therefore nonlinear. Hence, optimization problems subject to this relation, such as release schedule optimization, are nonconvex and may admit multiple local isolated minima. This renders such problems problematic for use in operational model predictive control. This paper shows that release schedule optimization problems subject to the nonlinear turbine generation equation may be set up using a continuation approach to be both zero-convex and path stable. In this way such optimization problems become suitable for decision support systems based on model predictive control. An example problem is studied, and it is shown that significant productivity gains may be realized using the presented methodology.

  • A continuation approach to nonlinear model predictive control of open channel systems

    Auteurs: Baayen, J.; Piovesan, T. (2018)
    Gepubliceerd in: arXiv : Mathematics - Optimization and Control (2018), pagina 1-19

    The equations governing open channel flow are strongly nonlinear when considering the full range of possible flows. Hence, optimization problems that aim to select optimal flow regimes for hydraulic structures such as weirs and pumps are subject to nonlinear relations between optimization variables. Such optimization problems are nonconvex, and may admit multiple isolated local minima, rendering them problematic for use in operational model predictive control. This paper introduces the notions of zero-convexity and path stability, i.e., the property of a parametric optimization problem that I) is convex at the starting parameter value and II) that when computing a path of solutions as a function of the parameter, such a path exists and no bifurcations arise. Path stability ensures that the parametric optimization problem can be solved in a deterministic and numerically stable fashion. It is shown that a class of numerical optimal control problems subject to simplified 1D shallow water hydraulics is zero-convex and path stable, and hence suitable for deployment in decision support systems based on model predictive control.

  • The state of the world’s beaches

    Auteurs: Luijendijk, A.P.; Hagenaars, G.; Ranasinghe, R.; Baart, F.; Donchyts, G.; Aarninkhof, S.G.J. (2018)

    Coastal zones constitute one of the most heavily populated and developed land zones in the world. Despite the utility and economic benefits that coasts provide, there is no reliable global-scale assessment of historical shoreline change trends. Here, via the use of freely available optical satellite images captured since 1984, in conjunction with sophisticated image interrogation and analysis methods, we present a global-scale assessment of the occurrence of sandy beaches and rates of shoreline change therein. Applying pixel-based supervised classification, we found that 31% of the world’s ice-free shorelines are sandy. The application of an automated shoreline detection method to the sandy shorelines thus identified resulted in a global dataset of shoreline change rates for the 33 year period 1984–2016. Analysis of the satellite derived shoreline data indicates that 24% of the world’s sandy beaches are eroding at rates exceeding 0.5 m/yr, while 28% are accreting and 48% are stable. The majority of the sandy shorelines in marine protected areas are eroding, raising cause for serious concern.

  • Beyond equilibrium : re-evaluating physical modelling of fluvial systems to represent climate changes

    Auteurs: Baynes, E.R.C.; Dijkstra, J. (2018)
    Gepubliceerd in: Earth-science reviews, volume 181 (2018), pagina 82-97

    The interactions between water, sediment and biology in fluvial systems are complex and driven by multiple forcing mechanisms across a range of spatial and temporal scales. In a changing climate, some meteorological drivers are expected to become more extreme with, for example, more prolonged droughts or more frequent flooding. Such environmental changes will potentially have significant consequences for the human populations and ecosystems that are dependent on riverscapes, but our understanding of fluvial system response to external drivers remains incomplete. As a consequence, many of the predictions of the effects of climate change have a large uncertainty that hampers effective management of fluvial environments. Amongst the array of methodological approaches available to scientists and engineers charged with improving that understanding, is physical modelling. Here, we review the role of physical modelling for understanding both biotic and abiotic processes and their interactions in fluvial systems. The approaches currently employed for scaling and representing fluvial processes in physical models are explored, from 1:1 experiments that reproduce processes at real-time or time scales of 10.-1 to 10.0 years, to analogue models that compress spatial scales to simulate processes over time scales exceeding 10.2 to 10.3 years. An important gap in existing capabilities identified in this study is the representation of fluvial systems over time scales relevant for managing the immediate impacts of global climatic change; 10.1 to 10.2 years, the representation of variable forcing (e.g. storms), and the representation of biological processes. Research to fill this knowledge gap is proposed, including examples of how the time scale of study in directly scaled models could be extended and the time scale of landscape models could be compressed in the future, through the use of lightweight sediments, and innovative approaches for representing vegetation and biostabilisation in fluvial environments at condensed time scales, such as small-scale vegetation, plastic plants and polymers. It is argued that by improving physical modelling capabilities and coupling physical and numerical models, it should be possible to improve understanding of the complex interactions and processes induced by variable forcing within fluvial systems over a broader range of time scales. This will enable policymakers and environmental managers to help reduce and mitigate the risks associated with the impacts of climate change in rivers.

  • The relative contribution of peat compaction and oxidation to subsidence in built-up areas in the Rhine-Meuse delta, The Netherlands

    Auteurs: Asselen, S. van; Erkens, G.; Stouthamer, E.; Woolderink, H.A.G.; Geeraert, R.E.E.; Hefting, M.M. (2018)
    Gepubliceerd in: Science of the total environment, volume 636 (2018), pagina 177-191

    An increasing number of people lives in coastal zones with a subsurface consisting of heterogenic soft-soil sequences. Many of these sequences contain substantial amounts of peat. While population growth and urbanization continues in coastal zones, they are threatened by global sea-level rise and land subsidence. Peat compaction and oxidation, caused by loading and drainage, are important contributors to land subsidence, and hence relative sea-level rise, in peat-rich coastal zones. Especially built-up areas, having densely-spaced urban assets, are heavily impacted by land subsidence, in terms of livelihoods and damage-related costs. Yet, built-up areas have been largely avoided in peat compaction and oxidation field studies. Consequently, essential information on the relative contributions of both processes to total subsidence and underlying mechanisms,which is required for developing effective land use planning strategies, is lacking. Therefore, we quantified subsidence due to peat compaction and oxidation in built-up areas in the Rhine-Meuse delta, The Netherlands, using lithological borehole data and measurements of dry bulk density, organic matter, and CO2 respiration. We reconstructed subsidence over the last 1000 years of up to ~4 m, and recent subsidence rates of up to ~140 mm·yr−1 averaged over an 11-year time span. The amount and rate of subsidence due to peat compaction and oxidation is variable in time and space, depending on the Holocene sequence composition, overburden thickness, loading time, organic-matter content, and groundwater-table depth. In our study area, the potential for future subsidence due to peat compaction and oxidation is substantial, especially where the peat layer occurs at shallow depth and is relatively uncompacted. We expect this is the case for many peat-rich coastal zones worldwide. We propose to use subsurface-based spatial planning, using specific subsurface information mentioned above, to inform land use planners about the most optimal building sites in organo-clastic coastal zones.

  • The challenges of assessing the effectiveness of biodiversity-related development aid

    Auteurs: Stepping, K.M.K.; Meijer, K.S. (2018)
    Gepubliceerd in: Tropical conservation science, volume 11 (2018), pagina 1-11

    Official Development Assistance is a major funding source for biodiversity conservation in developing countries, and it is therefore important to understand the effectiveness of biodiversity aid. However, three challenges hamper the analysis of how effectively biodiversity-related development aid (biodiversity aid) contributes to the conservation of biodiversity and its sustainable use. First, few indicators measure biodiversity aspects at country level in a consistent and comparable way. Second, biodiversity aid reporting methods do not reveal the exact funding amount for projects’ biodiversity component. Third, changes in biodiversity status are empirically and conceptually difficult to attribute to aid activities. Based on a theoretical elaboration of these challenges, we argue that for a better assessment of how biodiversity aid contributes to conserving biodiversity and to reducing biodiversity loss, three improvements are required: a more frequent and more consistent assessment of the biodiversity status across countries, more exact quantification of biodiversity aid, and a more detailed understanding about biodiversity loss and the role biodiversity aid plays. These improvements will allow for more reliable aid-effectiveness analyses, which will, in turn, enable better informed aid-allocation decisions to be made.

  • Paste 2018 : proceedings of the 21st International seminar on paste and thickened tailings (11-13 April 2018, Perth)

    Auteurs: Jewell, R.J.; Fourie, A.B. (2018)

  • Shear settling in laminar open channel flow : analytical solution, measurements and numerical simulation

    Auteurs: Talmon, A.M.; Sittoni, L.; Meshkati Shahmirzadi, E.; Hanssen, J.L.J. (2018)
    Gepubliceerd in: Paste 2018 : proceedings of the 21st International seminar on paste and thickened tailings (11-13 April 2018, Perth) (2018), pagina 181-194

    In beaching of tailings, sand and clays may segregate. In laminar flow this is due to shear settling. First implementations of shear settling in numerical flow models are seen, offering unprecedented potential to conduct tailings management studies. In order to validate numerical codes, reference materials are necessary. For laminar flow, there is a small set of flume tests available from an earlier study. An analytical solution for transient sand concentration profile development with distance in laminar open channel flow appeared recently. This analytical method is more complete than an analytical model developed earlier at the author’s institute. Data and analytical solutions are analysed and applied to serve for the validation of numerical flow simulation of beaching in tailings storage facilities. Fair agreement is observed between measurements and the analytical method. Moreover, fair agreement is obtained between an earlier produced computational outcome of the numerical model Delft3D-slurry and analytical solution. This contributes to building confidence in this model as an aid in supporting tailings deposition management.

  • Sediment geochemistry of the urban Lake Paulo Gorski

    Auteurs: Remor, M.B.; Sampaio, S.C.; Rijk, S. de; Vilas Boas, M.A.; Gotardo, J.T.; Pinto, E.T.; Schardong, F.A. (2018)
    Gepubliceerd in: International journal of sediment research (2018), pagina 1-34

    The objective of this study was to evaluate the concentration and distribution of heavy metals in the sediments of Paulo Gorski Lake, as well as the metals’ bioavailability and potential ecological risk, and to define the anthropogenic and natural heavy metal contributions to the lake. The chemical elements calcium (Ca), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), potassium (K), magnesium (Mg), manganese (Mn), sodium (Na), nickel (Ni), lead (Pb), and zinc (Zn) were quantified by flame atomic absorption spectrophotometry with two extraction methods to quantify the bioavailable and non-bioavailable fractions. The data were evaluated using multivariate statistics and sediment quality indices. All sediment collection points (S1, S2, S3, S4 and S5) are different in terms of the concentration of heavy metals, except for S4 and S5, which were statistically equal. The bioavailable fraction of the elements in the sediment follows the sequence Pb>Cu>Mn>Zn>Ni>Cr>phosphorus (P) for all points. The elements Co, Cr, Pb, and Zn showed moderate to considerable contamination at all points. Only points S3 and S5 had moderate ecological risk. Urbanization has been affecting Paulo Gorski Lake via the input of chemical elements, especially Co and Pb. The points most affected by heavy metal contamination are S3 and S5 when the sedimentological sensitivity factor is considered. The lake has high hydrodynamics, causing some of the contaminants that enter the system to leave it, leading to potential negative impacts downstream.

  • Methods and tools supporting urban resilience planning : experiences from Cork, Ireland

    Auteurs: Bruijn, K.M. de; Most, H. van der; Cumiskey, L.; Hounjet, M.; Mens, M.J.P. (2018)
    Gepubliceerd in: Journal of geoscience and environment protection, volume 6 (2018) nummer 4, pagina 290-309

    To prevent flood disasters, policymakers call for resilient cities which are better able to cope with flood hazards. However, actual adoption of resilience measures in urban planning is still limited, partly because it is not sufficiently clear how and to what extent resilience should and can be enhanced. To develop resilience strategies, information on the current resilience and on the effects of measures should be available. Since cities are complex systems, an assessment of resilience requires the input of different actors. To obtain and combine this input, a comprehensive approach which brings together many actors is required. Furthermore, resilience must be integrated in planning frameworks in order to enhance adoption by city policy makers. Tools which support and structure the contribution of different disciplines and actors will help to obtain information on the current resilience and to develop a shared vision on measures to enhance urban resilience. We illustrate our view with an example on Cork, Ireland.

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