Plans are made to enlarge the tidal flat area around the Zwin estuary which is silting up and situated around the Dutch-Belgian border. First, the evolution of the fresh-salt water flow and fresh-salt water distribution is simulated in this area and the surrounding polder area for a period of 50 years taking into account the forecasted sealevel rise. The initial fresh-salt water distribution is based on an existing map of the depth of the fresh-salt water interface with additional electromagnetic borehole loggings. MOCDENS3D is used to simulate the 3D groundwater flow. After this first simulation the evolution of the fresh-salt water distribution and flow is simulated for a period of 50 years taking into account the expansion of the estuary and its surrounding tidal flats with a surface of 120 hectare. This action results in an increase of the salt water seepage area in the adjacent polder area. The model shows that this increase can be mitigated by the application of deep drainage instead of the traditional shallow drainage. In the long run this mitigating measure allows even to reduce the present salt water seepage area.