I don't know the facts of the tsunami whether it was unprecedented or not, but considering the magnitude of the quake I'm guessing it was one of the largest tsunamis to ever hit Japan. This is speculation on my part.

They didn't ignore the risk of tsunamis, they had precautions against them but they weren't up to par. It was a series of malfunctioning safeties that caused the accident. The backup power generators conked out, and the backup to the backup was washed away by the floods. And the floods only managed to get that far because the protective walls weren't enough.

"Build better walls" seems like a trivial problem to solve, don't you think?

International regulatory bodies could also be more proactive in finding these flaws prior to accidents.

It's not a hard problem to solve in the long run. It'll be easier and quicker than finding a viable non-nuclear energy option anyway.

The earthquake itself killed over 15000 people.

Imo it's a massively overblown disaster. Yes, it's bad, especially the environmental effects, but it's absolutely nothing compared to the earthquake and tsunami itself.