In 2023 a large agricultural area called ‘Hedwige-Prosperpolder’ (~5 km2) on the Dutch-Belgium border was given back to the sea and transformed into a tidal area. The realization of the open connection to the sea can be seen as a local sea level rise for the adjacent agricultural area. The tidal area is incised by (man-made) tidal creeks through which filling of the tidal area with seawater occurs and deflating during low tide, twice a day. The question arises, how will the morphology of the tidal area and tidal creeks affect the aquifer heads in and outside the tidal area resulting from filling and deflating of the area and subsequently affecting the groundwater system in the agricultural area. Besides the morphology also the hydrogeological composition of the tidal area will affect the propagation of tides to the aquifer. A detailed monitoring network was installed to determine the reference situation and monitor the effects of the new tidal area on the groundwater system. At these locations heads as well as the fresh-saline interface are being monitored. Time series analysis was performed using Pastas to filter out effects of precipitation, evapotranspiration, and drainage in the time series to isolate effects from the opening of the connection to the sea. A groundwater model (MODFLOW) was built to reproduce the monitoring results and to simulate effects of measures to mitigate effects of this local sea level rise. The main conclusion from the measurements is that the water table in the tidal area will stay at the surface during low tide which is due to the short time available for groundwater to be drained. This has large consequences for the effect on hydraulic heads in the tidal area and head propagation to the agricultural area caused by sea level rise. In fact, the effect of the sea level is not the average of high and low tide (which is ~0 m MSL) but the space and time-weighted average of high tide and the water table at surface level when low tide levels drop below surface. So, the water pressure in the tidal area caused by this local sea level rise is not 0 m MSL but in this case >2.0 m MSL which is a huge difference. The impact of a sea level rise on groundwater systems is therefore highly dependent on the surface elevation of the tidal area and the drainage capacity of the tidal creeks. And this should always be taken into account when calculating effects of sea level rise on groundwater systems when the coastal zone contains tidal flats.