My first thought for a tidal wave blocker was for an island. Its a finite space as opposed to the entire coastline of a country. You would need a super strong material, it would need a good foundation, and it would need to be not only moveable, but able to move quickly.

Material Candidates

Our list of candidates include steel, diamond, carbon nano tubes based on current known technology. Steel is strong but too heavy. Diamond is the hardest substance we know of and not impossible to create but it would take some doing to set that up, so lets put that on the shelf for a moment until we increase in production technology. Carbon nano tubes are stronger than steel but they are currently constructed like thread - so this is probably our best bet but some R & D will have to be performed in order to make a carbon nano tube alloy.

Creating an Island in 3D

This took a bit of thought - an island is not a simple 3D object to create. This island was made from a skew-able stacked cylinder. Its ok, but I want to try some other techniques to see if we can get better results.