Two dam ideas

 Against the sea

1: Giant long sand dam, poles deep in the ground and then finish the dam with a layer of concrete. The poles should be there to make sure the concrete stays where it is.

This concept still has to be tested.

2 : The same as the above idea, but this time with a long depth, long outstretched crater which is formed from making the sand dam. The long deep crater should get a concrete layer as well and pumps to pump the water away to somewhere else is optional. If this works.
1 hight 2 depth, first depth but the main is hight
One problem, how to keep sand out of the depth length
Update: First hight than depth is also possible and might be better

This concept still has to be tested

By Jeffrey Koelewijn
From Holland