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22 nd Salt Water Intrusion Meeting: Salt Water Intrusion in Aquifers: Challenges and PerspectivesMODELLING THE HISTORICAL EVOLUTION OF THE FRESH-SALTWATER DISTRIBUTION IN A DUTCH-FLEMISH TRANSBOUNDARYAQUIFERVANDEVELDE, D. 1 ; KALAND, L. 2 ; LERMYTTE, J. 1; LEBBE, L. 3 ; OUDE ESSINK,G. H. P. 4 ; VANDENBOHEDE, A. 3 ; JANSSEN, G. 4 ; CLAUS, J. 3 ; D’HONT, D. 1 ;THOMAS, P. 1di.vandevelde@vmm.be1. Flemish Environment Agency2. Province of Zeeland3. Ghent University4. DeltaresVMM, Koning Albert II-laan 20, 1000 Brussels, BelgiumAbstract. Within ScaldWIN, a European project for a better quality of surface andgroundwater bodies in the Scheldt International River Basin District, a salinisation study isperformed along the Dutch-Flemish border. General aim is to predict the distribution of freshand saline groundwater in the phreatic aquifer under changing climate conditions and sealevel rise. Putting together a good and consistent data set underlying the transboundary studyis complicated by the two regions’ specific coordinate systems, typologies and densities ofdata acquisition. The transboundary nature of the study thus necessitates trade-off choicesbetween model consistency and model detail. To understand the present fresh-salt waterdistribution and to predict changes in coming decades, it is essential to investigate thebehavior of the groundwater system in the past. Modelling the historical evolution (viz. in thiscase the last 1000 years) of the fresh-salt water distribution is therefore an important aspectof the project. This historical simulation using the 3D density-dependent code MOCDENS3Dis discussed in the paper. The past 1000 years were characterized by a highly dynamiccoastal environment and by human intervention such as land reclamation. Starting with anatural coastal environment, characterized by tidal mud-flats, marshes and dunes, land wasgradually reclaimed by building dykes and developing a drainage system. This reclaimedland was regularly affected by floods and coastal erosion. Taking into account the number offloods and reclamations per century, a time discretisation is chosen. Considering differenttypes of historical data (such as medieval paintings), a landscape is reconstructed for eachstress period. A distinction is made between tidal channels, tidal mud-flats, dune belts, tidalmarshes, polders and the hinterland. For each geomorphological unit, hydraulic parametersand boundary conditions are set. The full paper will go into detail about the method for modelconceptualisation, landscape reconstruction and boundary conditions, and will present someresults of the historical modelling.Keywords: variable-density groundwater flow, historical evolution fresh-salt waterdistribution, Dutch-Flemish transboundary aquifer207