The Northern European Enclosure Dam (NEED) is a conceptual idea for protecting Northern Europe from future sea-level rise. While it might be economically and technologically feasible to do, and even attractive compared to some other options (e.g. migration), it serves more as a warning of the threat posed by sea-level rise.
Computer simulations of the North Sea tides and ocean currents shows three things:
1) With NEED, the sea-levels would rise in the North Sea due to large water input from rivers. The excess water must therefore be pumped out.
2) The North Sea would gradually become less salty as the only source of water would be rivers and rain, which is fresh water. The North Sea water would likely become more like the Baltic Sea water.
3) There would be almost no tides in the North Sea. Tidal waves from the Atlantic Ocean would be deflected towards Norway instead. See animation here: https://www.youtube.com/watch?v=WHUj2StdG68
Groeskamp, S. and Kjellsson, J. (2020) NEED The Northern European Enclosure Dam for if climate change mitigation fails. Open Access Bulletin of the American Meteorological Society, 101 (7). E1174-E1189. DOI 10.1175/BAMS-D-19-0145.1.