It is a completely bonkers idea without any regard for local topography. First, the coastlines marked in red have been endangered by flooding for thousands of years, they have shifted with the storms over the eons, and in modern times, become fortified with an existing system of dams and additional protections such as river sluices, wave breakers and land-generating projects that extend the lands before the dams to protect the dams better.
The black line in Germany would cut off all of the shipping lanes and make gates and sluices necessary out there between the island chain. Where ground is totally unsuitable for that: The Wattenmeer is more of a tidal salt water swamp which goes out for tens of kilometers from the cost. Interrupted of course by deep cuts where the major rivers go out and shipping goes in. On the outer rim of the island chain, there is also a current along the coast that will make any major construction difficult, to say the least. Many islands struggle with that current reshaping them, for the most well-known example, see Sylt.
And before anyone says "well, they did it in the Netherlands", no, they didn't. The Ijsselmeer was a bay they cut off and drained, not an exposed coastline.
The realistic option (that is getting implemented over time) is just fortifying the existing dams. A few minor islands might cease to exist, but saving them won't be cost-effective.
The black line in Germany would cut off all of the shipping lanes and make gates and sluices necessary out there between the island chain. Where ground is totally unsuitable for that: The Wattenmeer is more of a tidal salt water swamp which goes out for tens of kilometers from the cost. Interrupted of course by deep cuts where the major rivers go out and shipping goes in. On the outer rim of the island chain, there is also a current along the coast that will make any major construction difficult, to say the least. Many islands struggle with that current reshaping them, for the most well-known example, see Sylt.
And before anyone says "well, they did it in the Netherlands", no, they didn't. The Ijsselmeer was a bay they cut off and drained, not an exposed coastline.
The realistic option (that is getting implemented over time) is just fortifying the existing dams. A few minor islands might cease to exist, but saving them won't be cost-effective.