Evaluating the impact of river restoration on the local groundwater and ecological system: a case study in NE Flanders

River restoration changes the interaction between groundwater and surface water. Therefore, it is expected to have an impact on ecosystems at the interface between groundwater and surface water. Quantifying and generalizing the level of change of this interaction for different hydrogeological environments is scientifically and practically challenging. In this paper we investigated the impact of different restoration measures and the effect on the interaction of the temporal resolution of the groundwater modeling methodology. The interaction is analysed in the water bodies and wetlands in the valley of the Zwarte Beek, one of the most valuable nature reserves of Flanders. In the past, several changes have been made to the river and drainage system. These adaptations are now considered to be bottlenecks in maintaining a good ecological and hydrological status of its water dependent biotopes. Hence, in the context of the EU Water Framework Directive, it is necessary to (at least partly) restore the initial natural situation. The measures proposed include the reinstatement of old meanders and the removal of a weir. By removing the weir, fish migration is again possible. Reconnecting old meanders increases the habitat diversity. We used transient groundwater modeling to evaluate the impact on the groundwater system of the wetlands. Results indicate that a peat layer, present in most of the wetland, minimizes the effects of the restoration on the groundwater table. The largest changes are confined to the areas near the old meanders and the weir. Steady-state situations do not allow a calculation of average lowest and highest groundwater levels, which are essential for simulating ecological site conditions. Hence, transient simulations with 14 days time steps are required to detect a considerably greater range of groundwater fluctuation than indicated by the seasonal simulation. It is shown that the river restoration project thus resulted in an improvement of the structure of the watercourse rather than the rewetting of the valley. We concluded also that high resolution transient groundwater modeling is an essential step towards river restoration and ecohydrological predictions