Publications

3240 results

  • A multispectral Bayesian classification method for increased acoustic discrimination of seabed sediments using multi-frequency multibeam backscatter data

    Authors: Gaida, T.C.; Tengku Ali, T.A.; Snellen, M.; Amiri-Simkooei, A.; Dijk, T.A.G.P. van; Simons, D.G. (2018)
    Published in: Geosciences, volume 8 (2018) issue 12, page 1-25

    Multi-frequency backscatter data collected from multibeam echosounders (MBESs) is increasingly becoming available. The ability to collect data at multiple frequencies at the same time is expected to allow for better discrimination between seabed sediments. We propose an extension of the Bayesian method for seabed classification to multi-frequency backscatter. By combining the information retrieved at single frequencies we produce a multispectral acoustic classification map, which allows us to distinguish more seabed environments. In this study we use three triple-frequency (100, 200, and 400 kHz) backscatter datasets acquired with an R2Sonic 2026 in the Bedford Basin, Canada in 2016 and 2017 and in the Patricia Bay, Canada in 2016. The results are threefold: (1) combining 100 and 400 kHz, in general, reveals the most additional information about the seabed; (2) the use of multiple frequencies allows for a better acoustic discrimination of seabed sediments than single-frequency data; and (3) the optimal frequency selection for acoustic sediment classification depends on the local seabed. However, a quantification of the benefit using multiple frequencies cannot clearly be determined based on the existing ground-truth data. Still, a qualitative comparison and a geological interpretation indicate an improved discrimination between different seabed environments using multi-frequency backscatter.

  • A hybrid surrogate modelling strategy for simplification of detailed urban drainage simulators

    Authors: Mahmoodian, M.; Carbajal, J.P.; Bellos, V.; Leopold, U.; Schutz, G.; Clemens, F.H.L.R. (2018)
    Published in: Water resources management : an international journal (2018), page 1-16

    Urban drainage modelling typically requires development of highly detailed simulators due to the nature of various underlying surface and drainage processes, which makes them computationally too expensive. Application of such simulators is still challenging in activities such as real-time control (RTC), uncertainty quantification analysis or model calibration in which numerous simulations are required. The focus of this paper is to present a rather simple hybrid surrogate modelling (or emulation) strategy to simplify and accelerate a detailed urban drainage simulator (UDS). The proposed surrogate modelling strategy includes: a) identification of the variables to be emulated; b) development of a simplified conceptual model in which every component contributing to the variables identified in step (a) is replaced by a function; c) definition of these functions, either based on knowledge about the mechanisms of the simulator, or based on the data produced by the simulator; and finally, d) validation of the results produced by the surrogate model in comparison with the original detailed simulator. Herein, a detailed InfoWorks ICM simulator was selected for surrogate modelling. The case study area was a small urban drainage network in Luxembourg. An emulator was developed to map the rainfall time series, as input, to a storage tank volume and combined sewer overflow (CSO) in the case study network. The results showed that the introduced strategy provides a reliable method to simplify the simulator and reduce its run time significantly. For the specific case study, the emulator was approximately 1300 times faster than the original detailed simulator. For quantification of the emulation error, an ensemble of 500 rainfall scenarios with 1 month duration was generated by application of a multivariate autoregressive model for conditional simulation of rainfall time series. The results produced by the emulator were compared to the ones produced by the simulator. Finally, as an indicator of the emulation error, distributions of Nash-Sutcliffe efficiency (NSE) between the emulator and simulator results for prediction of storage tank volume and CSO flow time series were presented.

  • Wave overtopping over coastal structures with oblique wind and swell waves

    Authors: Werf, I.M. van der; Gent, M.R.A. van (2018)
    Published in: Journal of marine science and engineering, volume 6 (2018) issue 4, page 1-13

    Most guidelines on wave overtopping over coastal structures are based on conditions with waves from one direction only. Here, wave basin tests with oblique wave attack are presented where waves from one direction are combined with waves from another direction. This is especially important for locations where wind waves approach a coastal structure under a specific direction while swell waves approach the coastal structure under another direction. The tested structure was a dike with a smooth and impermeable 1:4 slope. The test programme consisted of four types of wave loading: 1. Wind waves only: “sea” (approaching the structure with an angle of 45o); 2. Wind waves and swell waves from the same direction (45o); 3. Wind waves and swell waves, simultaneously from two different directions (45o and -45o, thus perpendicular to each other); 4. Wind waves, simultaneously from two different directions (45o and -45o, thus perpendicular to each other). Existing guidelines on wave overtopping have been extended to predict wave overtopping discharges under the mentioned types of wave loading (oblique sea and swell conditions).

  • Climate change, coasts and coastal risk

    Authors: Ranasinghe, R.; Jongejan, R. (2018)
    Published in: Journal of marine science and engineering, volume 6 (2018) issue 4, page 1-4

    Editorial to a special issue. To avoid unacceptable future risks, due to coastal hazards and/or sub-optimal land use, it is imperative that risk informed and sustainable coastal planning/management strategies are implemented sooner rather than later. This requires comprehensive coastal risk assessments which combine state-of-the-art consequence (or damage) modelling and coastal hazard modelling. Apart from being of crucial importance to coastal managers/planners, this type of risk quantification will also be invaluable to the insurance and re-insurance industries for insurance pricing, which may have a follow-on effect on coastal property values. However, the present level of knowledge on generally applicable coastal hazard and risk assessment approaches, especially at local scale (~10 km) is rather limited. This special issue contains 13 papers that aim to address this knowledge gap.

  • Deltascenario's voor de 21e eeuw - Actualisering 2017 : achtergrondrapport over gebruiksfuncties en sectoren

    Authors: Wolters, H.A.; Hunink, J.C.; Delsman, J.R.; Lange, G.J. de; Born, G.J. van den; Reinhard, S. (2018)

    Dit rapport bevat de achtergrondinformatie bij het rapport ‘Deltascenario’s voor de 21e eeuw - Actualisering 2017’. Per sector of gebruiksfunctie (landbouw, natuur, stedelijk gebied, drinkwater, koelwater, koel- en proceswater industrie, scheepvaart, doorspoeling, peilbeheer veenweidegebieden) is in tabelvorm de informatie weergegeven die gebruikt is bij de opbouw van de verhaallijnen van de scenario’s en bij de doorvertaling ervan naar de modellering. Daarbij komen de uitgangspunten aan de orde over eventuele veranderingen in ruimtegebruik, wat als staand beleid is aangenomen, welke autonome maatregelen zijn verondersteld, welke economische parameters van belang zijn, en op welke manier de waterkwaliteit van belang kan zijn. Voor het opstellen van dit rapport heeft overleg plaats gevonden met externe betrokkenen en deskundigen van KNMI, WEnR, Schuttevaer, TUDelft, de Bakelse Stroom, CEDelft, NederlandICT en het project ‘Adaptieve aanpak drinkwatervoorziening op de lange termijn’.

  • Numerical prediction of integrated wave loads on crest walls on top of rubble mound structures

    Authors: Jacobsen, N.G.; Gent, M.R.A. van; Capel, A.; Borsboom, M. (2018)
    Published in: Coastal engineering : an international journal for coastal, harbour and offshore engineers, volume 142 (2018), page 110-124

    Wave loads on crest walls on top of rubble mound structures determine the size of these crest walls. For the design of crest walls some design guidelines exist, but their validity is limited to particular designs of the cross section (berm, no berm, toe, armour layout, protruding crest element, etc). The present work addresses the validation of OpenFoam/waves2Foam for the prediction of integrated forces on crest wall elements against a new set of experimental data in order to obtain a numerical model that can be applied for a wider field of application than the existing empirical guidelines. One key concern for the accurate modelling of wave loads is the spurious entrapment of air between the water surface and structural elements. The solution developed for this problem is a boundary condition that allows for air ventilation, while enforcing a predefined head loss characteristic. Compared to the existing technique of introducing small meshed tubes through the structure, the new method does not lead to excessive time-step limitations and is therefore more efficient (a practical case was accelerated by a factor 20). The new boundary condition is validated against experimental data of forces on bridge decks with girders. Subsequently, the numerical model is validated against experimental data for loads on crest wall elements from new experiments conducted in a wave flume. The comparison between numerical and experimental data is made both in the time domain and as probability of exceedance. Special emphasis is given to the openness of the faces of the crest wall to mimic the effect of mixing of water and air during the wave impact. Finally, the validated model is applied to evaluate the forces on crest walls as a function of the elevation of the crest wall with respect to the still-water level. This effect is of interest, since the level of the crest wall element is only tested to a limited extent in laboratory experiments and the bottom face was mainly wetted or submerged during these tests (existing empirical formulations). The numerical results are compared to an empirical design formulation [Pedersen, 1996] and conclusions on the general applicability of this particular empirical design formulation are presented. The effect of the shape of the wave spectrum on the resulting forces is investigated in a preliminary fashion.

  • Species and river specific effects of river fragmentation on European anadromous fish species

    Authors: Puijenbroek, P.J.T.M.; Buijse, A.D.; Kraak, M.H.S.; Verdonschot, P.F.M. (2018)
    Published in: River research and applications (2018), page 1-10

    Fragmentation is one of the major threats to riverine ecosystems and this is most explicitly expressed by the decline in numbers of migratory fish species. Yet each species has different migration requirements and their natural distribution can include several catchments with multiple dams. Hence, to prioritize candidate rivers for improving accessibility, differences between species and between catchments have to be taken into account. The aim of this study was therefore to analyse the species and river specific effects of river fragmentation on migratory fish on a European scale. The effect of river damming on migratory fish was quantified for all 16 European long- and mid‐distance anadromous species and for 33 large European rivers. The historical distribution was compared with the current upstream accessibility of the main river and the current distribution and population status of each species. The observed effects of reduced connectivity were further quantified using the Dendritic Connectivity Index for species and the Fragmentation Index for rivers. Our results showed that only very few rivers are still unaffected by dams in the main stem and that the few remaining viable migratory fish populations in Europe occur in these accessible rivers. Barriers were prioritized for making passable based on the potential accessibility gain and the number of benefitting species, showing that the main stems of the rivers Shannon and Nemunas are the best candidates. It was concluded that evaluating species and river specific effects of fragmentation strongly aids in prioritizing rivers for improving upstream accessibility.

  • Estimating the thickness of unconsolidated coastal aquifers along the global coastline.

    Authors: Zamrsky, D.; Oude Essink, G.H.P.; Bierkens, M.F.P. (2018)

    Knowledge of aquifer thickness is crucial for setting up numerical groundwater flow models to support groundwater resource management and control. Fresh groundwater reserves in coastal aquifers are particularly under threat of salinization and depletion as a result of climate change, sea-level rise, and excessive groundwater withdrawal under urbanization. To correctly assess the possible impacts of these pressures we need better information about subsurface conditions in coastal zones. Here, we propose a method that combines available global datasets to estimate, along the global coastline, the aquifer thickness in areas formed by unconsolidated sediments. To validate our final estimation results, we collected both borehole and literature data. Additionally, we performed a numerical modelling study to evaluate the effects of varying aquifer thickness and geological complexity on simulated saltwater intrusion. The results show that our aquifer thickness estimates can indeed be used for regional-scale groundwater flow modelling but that for local assessments additional geological information should be included. The final dataset has been made publicly available (https://doi.pangaea.de/10.1594/PANGAEA.880771).

  • Basisdocumentatie probleemstoffen KRW : basisdocument stap 3 - maatregelenanalyse

    Authors: Osté, L.A.; Postma, J.; Roskam, G.D.; Keijzers, R.; Duijnhoven, N. van (2018)

    De basisdocumenten bestaan uit 3 Excelbestanden: stap 1-diagnose ; stap 2- bronnenanalyse ; stap 3-maatregelenanalyse. Het rapport beschrijft de gevolgde methodiek en geeft in tekstboxen generieke achtergrondinformatie bij onderwerpen zoals alomtegenwoordige stoffen, toetsingssystematiek en gewasbeschermingsmiddelen. Deze documentatie is opgesteld om waterbeheerders te ondersteunen in het ontwerpen van aangepaste monitoring- en maatregelenprogramma's voor probleemstoffen en nieuwe prioritaire stoffen. Het resultaat omvat informatie voor alle KRW-probleemstoffen, die waterbeheerders kunnen gebruiken als die betreffende stof overschrijdt in een bepaald waterlichaam. In drie stappen -de diagnose, de bronnenanalyse en de maatregelenanalyse- wordt op een gestandaardiseerde, beknopte en overzichtelijke wijze weergegeven hoe het probleem in elkaar zit (diagnose), wat de oorzaak is (bronnenanalyse) en welke maatregelen genomen zouden kunnen worden en door wie (maatregelenanalyse).

  • Basisdocumentatie probleemstoffen KRW : basisdocument stap 2 - bronnenanalyse

    Authors: Osté, L.A.; Postma, J.; Roskam, G.D.; Keijzers, R.; Duijnhoven, N. van (2018)

    De basisdocumenten bestaan uit 3 Excelbestanden: stap 1-diagnose ; stap 2- bronnenanalyse ; stap 3-maatregelenanalyse. Het rapport beschrijft de gevolgde methodiek en geeft in tekstboxen generieke achtergrondinformatie bij onderwerpen zoals alomtegenwoordige stoffen, toetsingssystematiek en gewasbeschermingsmiddelen. Deze documentatie is opgesteld om waterbeheerders te ondersteunen in het ontwerpen van aangepaste monitoring- en maatregelenprogramma's voor probleemstoffen en nieuwe prioritaire stoffen. Het resultaat omvat informatie voor alle KRW-probleemstoffen, die waterbeheerders kunnen gebruiken als die betreffende stof overschrijdt in een bepaald waterlichaam. In drie stappen -de diagnose, de bronnenanalyse en de maatregelenanalyse- wordt op een gestandaardiseerde, beknopte en overzichtelijke wijze weergegeven hoe het probleem in elkaar zit (diagnose), wat de oorzaak is (bronnenanalyse) en welke maatregelen genomen zouden kunnen worden en door wie (maatregelenanalyse).

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