LTV O&M thema veiligheid: deelproject 1. Verbetering hydrodynamisch NEVLA model ten behoeve van scenario-analyse

In the framework of different projects, among which “LTV O&M thema Veiligheid - Deelproject 1” and “LTV O&M thema Toegankelijkheid - Ontwikkeling van een slibtransportmodel”, a calibration of the existing hydrodynamic NEVLA model of the Scheldt estuary is executed. This report gives a summary of all the steps which are undertaken to improve the model.The objective of the study “LTV O&M thema Veiligheid -- Deelproject 1” is to analyze the effect of different changes and their influence on the hydrodynamics of the estuary. Based on a literature review and data analysis some hypotheses concerning the change in tidal penetration in the estuary will be made. Afterwards these hypotheses will be verified with numerical models. In order to obtain reliable results, it is necessary that the numerical model performs well. Therefore a sensitivity analysis, a calibration and a validation of the NEVLA model, which will be used for 2D-3D hydrodynamic simulations, were carried out.The objective of the sensitivity analysis was to understand the impact of different model parameters on the tidal penetration. The results of this analysis are described in (Ides et al., 2008) and (Vanlede et al., 2008a). In (Vanlede et al., 2008b) the calibration was performed for the calculated water levels and discharges, based on the phase and magnitude of the most important harmonic tidal components of these parameters. In (Maximova et al., 2009) the calibration of the NEVLA model was extended. The resulting model parameters from (Vanlede et al., 2008b) were used as the reference simulation. The methodology was based on the comparison of phase and magnitude of the calculated and measured high and low water levels. This extended calibration was mainly focused on the Upper Sea Scheldt and the tributaries Zenne and Dijle because in these regions the differences between calculated and measured water levels were the largest. As a result of the calibration, the accuracy of the model for high water levels and low water levels was improved for most stations along the Scheldt estuary.The calibrated model was validated for a period with normal tide and for a period with an extreme high water. The calibrated model performs rather well for a period with normal tide. However, it does not simulate accurately a period with an extreme high water level. The reason for this and the possibilities to improve the model accuracy for such circumstances should be studied more in the future. The roughness field used for the calibrated model is strongly related to the bathymetry that was used during calibration of the model. If we use a different bathymetry for the scenario analysis the roughness field might change as well. Therefore, a simple roughness field without variation in the transversal direction was found, which gives rather similar results as the calibrated model with the space varying roughness field

Vervolgstudie inventarisatie en historische analyse van slikken en schorren langs de Zeeschelde: scenario analyse 2D model

The research described in this report is made in the framework of the project Vervolgstudie inventarisatie en historische analyse slikken en schorren langs de Zeeschelde. The main purpose of the project is to investigate why some of the evolutions of the slikke and schorre area in the past – described in the report Van Braeckel et al. (2006) – did occur. The main tools to investigate these evolutions are measured data from the past as well as numerical models.With the numerical models different scenarios were studied in order to see how each of them influenced the tidal penetration in the Scheldt estuary. In fact it is change in tidal penetration that will affect the slikke and schorre area. In each scenario one possible cause of tidal change was implemented. A distinction was made between natural evolutions (sea level rise, changes in fresh water discharge) as well as human interventions in the estuary (poldering, straightening of the river, deepening of the navigation channel, …) in the scenarios. In Coen et al (2009) the results of the 1D model are described. In Ides et al. (2008) a sensitivity analysis of the 2D model is carried out, in order to have an idea about the uncertainty interval of the results of the different scenarios. In this report the results of the scenarios will be given

Ultraviolet Signposts of Resonant Dynamics in the Starburst-Ringed Sab Galaxy, M94 (NGC 4736)

M94 (NGC 4736) is investigated using images from the Ultraviolet Imaging Telescope (FUV-band), Hubble Space Telescope (NUV-band), Kitt Peak 0.9-m telescope (H-alpha, R, and I bands), and Palomar 5-m telescope (B-band), along with spectra from the International Ultraviolet Explorer and Lick 1-m telescopes. The wide-field UIT image shows FUV emission from (a) an elongated nucleus, (b) a diffuse inner disk, where H-alpha is observed in absorption, (c) a bright inner ring of H II regions at the perimeter of the inner disk (R = 48 arcsec. = 1.1 kpc), and (d) two 500-pc size knots of hot stars exterior to the ring on diametrically opposite sides of the nucleus (R= 130 arcsec. = 2.9 kpc). The HST/FOC image resolves the NUV emission from the nuclear region into a bright core and a faint 20 arcsec. long ``mini-bar'' at a position angle of 30 deg. Optical and IUE spectroscopy of the nucleus and diffuse inner disk indicates an approximately 10^7 or 10^8 yr-old stellar population from low-level starbirth activity blended with some LINER activity. Analysis of the H-alpha, FUV, NUV, B, R, and I-band emission along with other observed tracers of stars and gas in M94 indicates that most of the star formation is being orchestrated via ring-bar dynamics involving the nuclear mini-bar, inner ring, oval disk, and outer ring. The inner starburst ring and bi-symmetric knots at intermediate radius, in particular, argue for bar-mediated resonances as the primary drivers of evolution in M94 at the present epoch. Similar processes may be governing the evolution of the ``core-dominated'' galaxies that have been observed at high redshift. The gravitationally-lensed ``Pretzel Galaxy'' (0024+1654) at a redshift of approximately 1.5 provides an important precedent in this regard.Comment: revised figure 1 (corrected coordinate labels on declination axis); 19 pages of text + 19 figures (jpg files); accepted for publication in A

Odour nuisance in Scheldt branch Gentbrugge-Melle [POSTER]

The tidal branch of the Sea Scheldt between the lock of Gentbrugge and Melle - part of the complex Ringvaart system around Ghent - has not received any upstream discharge since 1981. Consequently, ebb velocities were reduced, while flood velocities were left unaffected, causing sedimentation in the branch. At some locations along the branch, odour nuisance was regularly reported. In order to address the problem, Division Sea Scheldt of the Flemish Government proposed to dredge the associated muddy material. Flanders Hydraulics Research (FHR) was asked to conduct a study about the necessary upstream discharge at the lock of Gentbrugge needed to ensure the river branch’s self-erosiveness. To answer this question, monitored data were combined with modelling know-how. A 2D numerical morphological model was constructed, for which hydrological and sedimentological parameters, such as discharge, gauge height and sediment concentration, were used as input and for validation. All of these data were provided by FHR. After numerous numerical scenarios, it was concluded that the discharge required, lies between 20 and 25 m3/s. However, as a mean discharge of about 70 m3/s is discharged into the entire water system around Ghent, and as other waterways within this system require sufficient discharges (of which the Canal Ghent-Terneuzen has been determined in a Belgian-Dutch treaty) this discharge is not available in normal circumstances in the tidal branch. Currently, Flanders Hydraulics Research is investigating the possibilities of a lower discharge at the weir of Gentbrugge, that would allow to maintain the branch with a limited dredging

Symbiotic Ocean Modeling Using Physics-Controlled Echo State Networks

We introduce a “symbiotic” ocean modeling strategy that leverages data-driven and machine learning methods to allow high- and low-resolution dynamical models to mutually benefit from each other. In this work we mainly focus on how a low-resolution model can be enhanced within a symbiotic model configuration. The broader aim is to enhance the representation of unresolved processes in low-resolution models, while simultaneously improving the efficiency of high-resolution models. To achieve this, we use a grid-switching approach together with hybrid modeling techniques that combine linear regression-based methods with nonlinear echo state networks. The approach is applied to both the Kuramoto–Sivashinsky equation and a single-layer quasi-geostrophic ocean model, and shown to simulate short-term and long-term behavior better than either purely data-based methods or low-resolution models. By maintaining key flow characteristics, the hybrid modeling techniques are also able to provide higher quality initial conditions for high-resolution models, thereby improving their efficiency.</p

Symbiotic Ocean Modeling Using Physics-Controlled Echo State Networks

We introduce a “symbiotic” ocean modeling strategy that leverages data-driven and machine learning methods to allow high- and low-resolution dynamical models to mutually benefit from each other. In this work we mainly focus on how a low-resolution model can be enhanced within a symbiotic model configuration. The broader aim is to enhance the representation of unresolved processes in low-resolution models, while simultaneously improving the efficiency of high-resolution models. To achieve this, we use a grid-switching approach together with hybrid modeling techniques that combine linear regression-based methods with nonlinear echo state networks. The approach is applied to both the Kuramoto–Sivashinsky equation and a single-layer quasi-geostrophic ocean model, and shown to simulate short-term and long-term behavior better than either purely data-based methods or low-resolution models. By maintaining key flow characteristics, the hybrid modeling techniques are also able to provide higher quality initial conditions for high-resolution models, thereby improving their efficiency.</p