Finite element modelling of the Scheldt estuary and the adjacent Belgian/Dutch coastal zone with application to the transport of fecal bacteria

A fundamental problem in coastal modelling is the need to simultaneously consider large- and small-scale processes, especially when local dynamics or local environmental issues are of interest. The approach widely resorted to is based on a nesting strategy by which coarse grid large scale model provide boundary conditions to force fine resolution local models. This is probably the best solution for finite difference methods, needing structured grids. However, the use of structured grids leads to a marked lack of flexibility in the spatial resolution. Another solution is to take advantage of the potential of the more modern finite element methods, which allow the use of unstructured grids in which the mesh size may vary over a wide spectrum. With these methods only one model is required to describe both the larger and the smaller scales.Such a model is use herein, namely the Second-generation Louvain-la-Neuve Ice-ocean Model (SLIM, http://www.climate.be/SLIM). For one of its first realistic applications, the Scheldt Estuary area is studied. The hydrodynamics is primarily forced by the tide and the neatest way to take it into account is to fix it at the shelf break. This results in a multi-scale problem since the domain boundary lies at the shelf break, and covers about 1000km of the North Sea and 60km of the actual estuary, and ends with a 100km long section of the Scheldt River until Ghent where the river is not more than 50 m wide.Two-dimensional elements are used to simulate the hydrodynamics from the shelf break to Antwerp (80km upstream of the mouth) and one-dimensional elements for the riverine part between Antwerp and Ghent.For first application we consider the transport of faecal bacteria (Escherichia coli) which is an important water quality indicator.The model will be described in detail and the simulation results will be discussed. This modelling exercise actually falls within the framework of the interdisciplinary project TIMOTHY (http://www.climate.be/TIMOTHY) dedicated to the modelling of ecological indicators in the Scheldt area

Timing and placing samplings to optimally calibrate a reactive transport model: exploring the potential for <i>Escherichia coli</i> in the Scheldt estuary

For the calibration of any model, measurements are necessary. As measurements are expensive, it is of interest to determine beforehand which kind of samples will provide the maximum of information. Using a criterion related to the Fisher information matrix, it is possible to design a sampling scheme that will enable the most precise model parameter estimates. This approach was applied to a reactive transport model (based on SLIM) of Escherichia coli in the Scheldt Estuary. As this estuary is highly influenced by the tide, it is expected that careful timing of the samples with respect to the tidal cycle will have an effect on the quality of the data. The timing and also the positioning of samples were optimised according to the proposed criterion. In the investigated case studies the precision of the estimated parameters could be improved by up to a factor of ten, confirming the usefulness of this approach to maximize the amount of information that can be retrieved from a fixed number of samples

Modelling fecal bacteria in the Scheldt river and estuary

With its population density of over 500 inhabitants per km2, its active industrial developmentand its intensive agriculture and animal farming, the Scheldt watershed representsan extreme case of surface water and groundwater pollution which in turn hasan impact on eutrophication and the ecological functioning of the receiving coastalwaters. A Belgian interuniversity collaboration (http://www.climate.be/TIMOTHY)has recently started, aiming to better understand past, present and future changes inthe quality of surface, ground and marine waters and to relate them to changing humanactivities on the watershed. Part of the originality of the new network resides inthe coupling of existing hydrodynamical and biogeochemical models to describe thetransport and transformation of nutrients and contaminants.One of these couplings consists of connecting an ecological module to the SecondgenerationLouvain-la-Neuve Ice-oceanModel (SLIM, http://www.climate.be/SLIM).The results of a first application will be shown, where the ecological module considersthe dynamics of one fecal bacteria indicator (Escherichia coli). The power of SLIMis that it solves the governing hydrodynamical equations using finite elements on anunstructured mesh. As such it is able to accurately model the different scales in thedomain, going from the Scheldt river, over the estuary (including the special featureof sand banks being periodically submerged), to the North Sea.This modelling exercise illustrates the combined effect of hydrodynamics, mortalityand sedimentation on the abundance of E. coli in the study domain - with a resolutionthat is impossible to achieve by sampling alone. However, in order to have a reliableand accurate tool, much effort was put on data gathering and the optimal incorporationof this information (e.g. for the initial and boundary conditions, for the estimation ofmodel parameters, or for validation). In addition, the first modelling results helpedto guide future sampling campaigns such that data and modelling can be optimallyadjusted and a maximum of information can be retrieved.Although the hydrodynamical model and its coupling to an ecological module may beof scientific interest on their own, it is even more attractive that their output can beinterpreted in terms of practical needs, i.e. the abundance of fecal indicators which aredirectly related to sanitary risk and standards for water quality. In this framework, themodel is also intended for assessing the effect of different scenarios for the future, andadditional pollution indicators will also be included in the ecological module.info:eu-repo/semantics/publishe

Integrated modelling of faecal contamination in a densely populated river-sea continuum (Scheldt River and Estuary).

In order to simulate the long-term (months-years) median Escherichia coli distributions and variations in the tidal Scheldt River and Estuary, a dedicated module was developed for the Second-generation Louvain-la-Neuve Ice-ocean Model (SLIM, www.climate.be/slim). The resulting model (SLIM-EC2) presents two specific and new features compared to the older SLIM-EC model version. The first is that the E. coli concentrations in the river are split in three fractions: the free E. coli in the water column, the ones attached to suspended solids and those present in the bottom sediments, each with their own transport, decay and settling-resuspension dynamics. The bacteria attached to particles can settle and survive on the bottom, where they can be brought back in the water column during resuspension events. The second new feature of the model is that it is coupled to the catchment model SENEQUE-EC, which thus provides upstream boundary conditions to SLIM-EC2. The result is an integrated and multi-scale model of the whole Scheldt drainage network from its source down to the Belgian/Dutch coastal zone. This new model reproduces the long-term median E. coli concentration along the Scheldt River and Estuary. An extensive sensitivity study is performed demonstrating the relative robustness of the model with respect to the chosen parameterisations. In addition to reproducing the observed E. coli concentrations in 2007-2008 at various stations, two extreme wastewater management scenarios were considered. Overall, there is no doubt that the Scheldt Estuary acts as a cleaning filter of faecal contamination originating from large Belgian cities. As a result, at the mouth of the Scheldt Estuary E. coli concentration is negligible in all investigated conditions.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Preliminary results of a finite element model of the ecohydrodynamics of the Scheldt estuary and the adjacent Belgian/Dutch coastal zone

info:eu-repo/semantics/nonPublishe

Modelling metal speciation in the Scheldt Estuary: combining a flexible-resolution transport model with empirical functions

Predicting metal concentrations in surface waters is an important step in the understanding and ultimately the assessment of the ecological risk associated with metal contamination. In terms of risk an essential piece of information is the accurate knowledge of the partitioning of the metals between the dissolved and particulate phases, as the former species are generally regarded as the most bioavailable and thus harmful form. As a first step towards the understanding and prediction of metal speciation in the Scheldt Estuary (Belgium, the Netherlands), we carried out a detailed analysis of a historical dataset covering the period 1982-2011. This study reports on the results for two selected metals: Cu and Cd. Data analysis revealed that both the total metal concentration and the metal partitioning coefficient (Kd) could be predicted using relatively simple empirical functions of environmental variables such as salinity and suspended particulate matter concentration (SPM). The validity of these functions has been assessed by their application to salinity and SPM fields simulated by the hydro-environmental model SLIM. The high-resolution total and dissolved metal concentrations reconstructed using this approach, compared surprisingly well with an independent set of validation measurements. These first results from the combined mechanistic-empirical model approach suggest that it may be an interesting tool for risk assessment studies, e.g. to help identify conditions associated with elevated (dissolved) metal concentrations. © 2013 Elsevier B.V.SCOPUS: ar.jinfo:eu-repo/semantics/publishe