In the southwestern part of The Netherlands, a 75-ha-agricultural area called Perkpolder was transformed into a tidal area to restore a salt march ecosystem. In June 2015 the open connection to the sea was realized. This transformation created an average water level increase of about 1.5m and can be seen as a local sea level rise of 1.5m and may have significant effects on the groundwater system of the adjacent lying agricultural area. Below this agricultural area, a 20-meter-thick freshwater lens surrounded by saline groundwater provides the farmers fresh groundwater for irrigating their crops. It is expected that due to this local sea level rise, hydraulic heads increase resulting in shrinkage and eventually disappearance of this precious freshwater lens.
To protect this lens, a self-flowing seepage system called SeepCat was designed and installed to compensate effects resulting from this local sea level rise. The freshwater lens is one of the Badon-Ghyben-Herzberg type and will shrink when hydraulic heads below the lens increase. The task of the SeepCat-system is to release the increased pressure in the aquifer. The SeepCat-system consists of 61 vertical seepage wells with 5-10m long screens, installed in the aquifer at 15m depth, over a total length of 1100 m. Since the hydraulic heads in the aquifer are higher than the surface water level, the seepage wells are artesian and self-flowing. The extracted saline seepage water is discharged into a canal and pumped into the sea.
Since 2010, hydraulic heads (30 locations) and the fresh-salt interface (15 location) are being monitored to capture the reference situation, to determine effects resulting from the development of the new tidal area and to monitor the functioning of the SeepCat-system. Time series analysis was applied to quantify the effects. From the head measurements it is concluded that the seepage system is functioning well and fully compensate the effects of the local sea level rise. No changes in the fresh-salt interface have been detected yet. However, these processes occur at a much larger time scale since salt transport is involved. To determine these long-term effects, the numerical groundwater model SEAWAT was applied. The model results show that SeepCat is successful in mitigating the effects of this local sea level rise and without SeepCat, half of the precious freshwater lens would disappear after 100 years.
The Perkpolder case serves as a field laboratory where effects of sea level rise are measured and mitigation measures for sea level rise are tested. Since the system is technically and geohydrologically working well, it could also be applied in other coastal areas, like small ocean islands. The SeepCat-system could help to protect their vulnerable freshwater resources which are seriously threatened by the expected future sea level rise