Antwerpen, Belgium
Antwerpen, Belgium

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Van Der Wal D.,Netherlands Institute of Ecology | Van Kessel T.,Deltares | Eleveld M.A.,VU University Amsterdam | Vanlede J.,Flanders Hydraulics Research
Ocean Dynamics | Year: 2010

The fate of mud in an estuary over an entire year was unravelled using complementary, independent, spatially explicit techniques. Sequential ERS-2 SAR and Envisat MERIS-FR data were used to derive synoptic changes in intertidal bottom mud and suspended particulate matter (SPM) in the top of the water column, respectively. These satellite data were combined with in situ measurements and with a high resolution three-dimensional cohesive sediment model, simulating mud transport, resuspension, settling and deposition under the influence of tides, wind, waves and freshwater discharge. The spatial distribution of both bottom mud and SPM as observed by in situ and satellite techniques was largely explained by modelled estuarine circulation, tidal and wind-induced variations in vertical mixing and horizontal advection. The three data sources also showed similar spring-neap and seasonal variations in SPM (all factor 1.5 to 2), but semi-diurnal tidal variations were underestimated by the model. Satellite data revealed that changes in intertidal bottom mud were spatially heterogeneous, but on average mud content doubled during summer, which was confirmed by in situ data. The model did not show such seasonal variation in bed sediment, suggesting that seasonal dynamics are not well explained by the physical factors presently implemented in the model, but may be largely attributed to other (internal) factors, including increased floc size in summer, temporal stabilisation of the sediment by microphytobenthos and a substantially lower roughness of the intertidal bed in summer as observed by the satellite. The effects of such factors on estuarine mud dynamics were evaluated. © Springer-Verlag 2010.

Lataire E.,Ghent University | Vantorre M.,Ghent University | Delefortrie G.,Flanders Hydraulics Research
Ocean Engineering | Year: 2012

The hydrodynamic behaviour of a vessel changes when sailing in shallow and/or confined water. The restricted space underneath and alongside a vessel has a noticeable influence on both the sinkage and trim of a vessel, also known as squat. To assess these influences an extensive model test program has been carried out in the Towing Tank for Manoeuvres in Shallow Water (cooperation Flanders Hydraulics Research - Ghent University) in Antwerp, Belgium with a scale model of the KVLCC2 Moeri tanker. This benchmark vessel was selected for its full hull form, to maximize the effects of the blockage. To thoroughly investigate the influences of the blockage on the squat of the vessel, tests have been carried out at different water depths, widths of the canal section and forward speeds (2 up to 16 knots full scale whenever possible). The squat observed during the model tests is compared with the squat predicted with a mathematical model based on mass conservation and the Bernoulli principle. The correlation between measured and modelled squat for each canal width for all tested speeds and water depths is very good, but shows a constant slope deviation. An improved model for the squat is proposed and takes into account the forward speed, propeller action, lateral position in the fairway, total width of the fairway and water depth. © 2012 Elsevier Ltd.

Lataire E.,Ghent University | Vantorre M.,Ghent University | Delefortrie G.,Flanders Hydraulics Research | Candries M.,Ghent University
Ocean Engineering | Year: 2012

The transfer of liquid cargo (crude oil and LNG) from a larger ship (the ship to be lightered, STBL) to a smaller vessel (service ship) when both ships are moored to each other and sail at a (slow) constant forward speed is known as lightering. These ship to ship operations are expected to increase in the near future and are expected to take place in harsher environmental conditions (polar regions). In order to better understand the hydrodynamic phenomena involved in this specific manoeuvre, a knowledge-building project with user involvement entitled Investigating Hydrodynamic Aspects and Control Strategies for Ship-to-Ship Operations was carried out in 2007-2011. As a part of this project, more than two thousand captive model tests were carried out at the towing tank for manoeuvres in shallow water (co-operation Flanders Hydraulics Research - Ghent University) in Antwerp, Belgium. A model of a very large crude oil carrier (VLCC) was attached to the main frame of the towing carriage and a model of an Aframax tanker was attached to the computer controlled planar motion carriage. Forces, moments and vertical positions were measured on both models. This paper covers the analysis of the extensive model test data reported by Lataire et al. (2009a) and the influence of different parameters on the manoeuvre. Particular attention is paid to the forces and moments induced on the service ship by the proximity of the ship to be lightered. A mathematical model of lightering manoeuvres for both the service ship and the ship to be lightered is derived. This model can be implemented in a ship manoeuvring simulator for training purposes. © 2012 Elsevier Ltd.

Kolokythas G.A.,Flanders Hydraulics Research | Dimas A.A.,University of Patras
Proceedings of the International Offshore and Polar Engineering Conference | Year: 2016

In the present study, the three-dimensional, turbulent, free-surface flow, developing by the oblique propagation of breaking waves over a constant slope bed is numerically simulated. Then, two-dimensional simulations of suspended sediment transport, induced by normal to the shore wave breaking over the same beach, are performed. The main objective is to investigate in depth the flow structure in the surf zone, as well as the behavior of sediment suspension. Predicted profiles of the undertow and the longshore current at several positions in the surf zone, are presented. For the sediment transport simulations, cases of sediments of various grain sizes are investigated. During wave breaking and also in the surf zone, strong uplift of bed sediment is observed. © Copyright 2016 by the International Society of Offshore and Polar Engineers (ISOPE).

Vandenbruwaene W.,University of Antwerp | Vandenbruwaene W.,Flanders Hydraulics Research | Bouma T.J.,Netherlands Institute for Sea Research | Meire P.,University of Antwerp | Temmerman S.,University of Antwerp
Earth Surface Processes and Landforms | Year: 2013

The long-term (10-100years) evolution of tidal channels is generally considered to interact with the bio-geomorphic evolution of the surrounding intertidal platform. Here we studied how the geometric properties of tidal channels (channel drainage density and channel width) change as (1) vegetation establishes on an initially bare intertidal platform and (2) sediment accretion on the intertidal platform leads to a reduction in the tidal prism (i.e. water volume that during a tidal cycle floods to and drains back from the intertidal platform). Based on a time series of aerial photographs and digital elevation models, we derived the channel geometric properties at different time steps during the evolution from an initially low-elevated bare tidal flat towards a high-elevated vegetated marsh. We found that vegetation establishment causes a marked increase in channel drainage density. This is explained as the friction exerted by patches of pioneer vegetation concentrates the flow in between the vegetation patches and promotes there the erosion of channels. Once vegetation has established, continued sediment accretion and tidal prism reduction do not result in significant further changes in channel drainage density and in channel widths. We hypothesize that this is explained by a partitioning of the tidal flow between concentrated channel flow, as long as the vegetation is not submerged, and more homogeneous sheet flow as the vegetation is deeply submerged. Hence, a reduction of the tidal prism due to sediment accretion on the intertidal platform, reduces especially the volume of sheet flow (which does not affect channel geometry), while the concentrated channel flow (i.e. the landscape forming volume of water) is not much affected by the tidal prism reduction. © 2012 John Wiley & Sons, Ltd.

Van Steenbergen N.,Catholic University of Leuven | Van Steenbergen N.,Flanders Hydraulics Research | Ronsyn J.,Koningin Elisabethlei 22 | Willems P.,Catholic University of Leuven
Environmental Modelling and Software | Year: 2012

In addition to structural measures, governmental authorities have set up flood forecasting systems to be used as early warning systems, to minimize the damage of future floods. These flood forecasting systems make use of hydrological and hydrodynamic models and input time series (measured and predicted rainfall, evapotranspiration, water levels and discharges). The uncertainty of these models and time series, certainly the predicted rainfall, is high and not always known. Consequently the prediction power of the flood forecasting systems is often unclear. To calculate the predictive uncertainty in the forecasts, a method has been set up, which involves computation of the exceedance probability of alert and alarm levels. The uncertainty results allow far more complete information to be provided to decision makers (in comparison with deterministic model-based forecasts).The uncertainty estimation is based on the statistical analysis of historical flood forecasting results. The forecast residuals (differences between predictions and measurements at river gauging stations) have been analysed using a non parametric technique. Because the residuals are correlated with the value of the simulated water level and time horizon, the residuals are split up into discrete classes of simulated water levels and time horizons. For each class, percentile values of the residuals are calculated and stored in a so called 'three dimensional error matrix'. Based on 3D interpolation in the error matrix, confidence intervals on forecasted water levels are calculated and visualised. The method is implemented in software for post processing of the forecast results, and is connected to the database of a river flood forecasting system in Belgium. Hereby it is possible to update the error matrix in real time, based on new simulations. © 2012 Elsevier Ltd.

Wei X.,Technical University of Delft | Schramkowski G.P.,Flanders Hydraulics Research | Schuttelaars H.M.,Technical University of Delft
Journal of Physical Oceanography | Year: 2016

Understanding salt dynamics is important to adequately model salt intrusion, baroclinic forcing, and sediment transport. In this paper, the importance of the residual salt transport due to tidal advection in well-mixed tidal estuaries is studied. The water motion is resolved in a consistent way with a width-averaged analytical model, coupled to an advection-diffusion equation describing the salt dynamics. The residual salt balance obtained from the coupled model shows that the seaward salt transport driven by river discharge is balanced by the landward salt transport due to tidal advection and horizontal diffusion. It is found that the tidal advection behaves as a diffusion process, and this contribution is named tidal advective diffusion. The horizontal diffusion parameterizes processes not explicitly resolved in the model and is called the prescribed diffusion. The tidal advective diffusion results from the correlation between the tidal velocity and salinity and can be explicitly calculated with the dominant semidiurnal water motion. The sensitivity analysis shows that tidal advective diffusivity increases with increasing bed roughness and decreasing vertical eddy viscosity. Furthermore, tidal advective diffusivity reaches its maximum for moderate water depth and moderate convergence length. The relative importance of tidal advective diffusion is investigated using the residual salt balance, with the prescribed diffusion coefficient obtained from the measured salinity field. The tidal advective diffusion dominates the residual salt transport in the Scheldt estuary, and other processes significantly contribute to the residual salt transport in the Delaware estuary and the Columbia estuary. © 2016 American Meteorological Society.

Van Steenbergen N.,Catholic University of Leuven | Van Steenbergen N.,Flanders Hydraulics Research | Willems P.,Catholic University of Leuven
Journal of Hydrology | Year: 2013

High wetness state levels can be considered as a primary indicator for potential river flooding. Therefore it is advisable to visualise real-time soil moisture information in flood forecasting or warning systems. Monitoring of soil moisture, however, is not an easy task due to its variable nature in time, space and depth. This paper presents and compares methods to assess the severity of the soil moisture state of hydrological catchments considered in a typical operational flood forecasting system. The severity of the relative soil moisture state is obtained and mapped by comparing the actual simulation result with the historical simulation results of a lumped conceptual hydrological model, directly by making use of the soil moisture component of the model or indirectly considering the baseflow component. Another approach uses rainfall, evapotranspiration and river flow observations. By applying a baseflow filter to the river flow observations and an advanced method for empirical catchment water balance computation, two indirect soil moisture indicators were defined, namely the filtered baseflow and the water balance based relative soil moisture content. It is shown that each of the methods allows to obtain useful estimates of the soil moisture state of a catchment in real time. The severity level of the soil moisture state is computed after comparison with long term statistics derived from a long term simulation. The severity level moreover is used to calculate the probability of exceedance of a predefined riverflow threshold, e.g. flood threshold, at the outlet or a given location in the catchment. This is done by means of a logit relation of the river flow probability of exceedance with the soil moisture indicator. The different soil moisture indicators are compared in their predicting capabilities by calculating and comparing the Brier score. Interestingly, the application of the logit relation or the use of a simple water balance computation for the catchment, based on real-time rainfall, evapotranspiration and river flow observations, leads to more reliable probability of exceedance estimates than the common direct use of total runoff results from a state-of-the art rainfall-runoff model. Mapping the probability of exceedance for the different hydrological catchments together with the width of the confidence interval on this probability is proposed as a useful tool to increase the preparedness for potential floods. © 2013 Elsevier B.V.

Van Steenbergen N.,Catholic University of Leuven | Van Steenbergen N.,Flanders Hydraulics Research | Willems P.,Catholic University of Leuven
Journal of Hydrologic Engineering | Year: 2014

Rainfall forecast errors are the key sources of uncertainty in flood forecasting. To quantify this uncertainty operational flood forecasting centers make use of rainfall forecasts obtained by ensemble predicting systems (EPS). The EPS forecasts are generated by perturbing the initial conditions of numerical weather prediction models. Question, however, remains whether these EPS cover the real forecast uncertainty range and whether the EPS-based uncertainty estimates are similar to the ones obtained by statistical methods. Both questions are addressed in this research based on data of a flood forecasting system in Belgium. Comparison is made between the uncertainty bounds generated by EPS and by a Monte Carlo-based statistical method after historical forecasted rainfall uncertainty analysis. The latter analysis calculates the error between forecasted and observed catchment rainfall, taking into account the dependency on lead time and rainfall depth. The forecasted rainfall errors are described by truncated normal distributions, which allow to calculate the full uncertainty distribution on a deterministic rainfall forecast. It is concluded that the EPS may underestimate the influence of the total forecasted rainfall uncertainty. For the Belgian case study of the Rivierbeek, the forecasted rainfall uncertainty explains 30% of the total river flow forecast uncertainty. © 2014 American Society of Civil Engineers.

Hu Z.,Technical University of Delft | Suzuki T.,Technical University of Delft | Suzuki T.,Flanders Hydraulics Research | Zitman T.,Technical University of Delft | And 2 more authors.
Coastal Engineering | Year: 2014

Coastal wetlands such as salt marshes and mangroves provide valuable ecosystem services including coastal protection. Many studies have assessed the influence of plant traits and wave conditions on vegetation-induced wave dissipation, whereas the effect of tidal currents is often ignored. To our knowledge, only two studies investigated wave dissipation by vegetation with the presence of following currents (current velocity is in the same direction as wave propagation) (Li and Yan, 2007; Paul et al., 2012). However, based on independent experiments, they have drawn contradictive conclusions whether steady currents increase or decrease wave attenuation. We show in this paper that this inconsistency may be caused by a difference in ratio of imposed current velocity to amplitude of the horizontal wave orbital velocity. We found that following currents can either increase or decrease wave dissipation depending on the velocity ratio, which explains the seeming inconsistency in the two previous studies. Wave dissipation in plant canopies is closely related to vegetation drag coefficients. We apply a new approach to obtain the drag coefficients. This new method eliminates the potential errors that are often introduced by the commonly used method. More importantly, it is capable of obtaining the vegetation drag coefficient in combined current-wave flows, which is not possible for the commonly used calibration method. Based on laboratory data, we propose an empirical relation between drag coefficient and Reynolds number, which can be useful for numerical modeling. The characteristics of drag coefficient variation and in-canopy velocity dynamics are incorporated into an analytical model to help understand the effect of following currents on vegetation-induced wave dissipation. © 2014 Elsevier B.V.

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