The sea, which had been beating against the shores, suddenly broke the boundary that was imposed on it by nature. The sea rushed into the city. It coursed through the streets of the beautiful city. The sea covered up everything in the city. Even as they were all looking, Arjuna saw the beautiful buildings becoming submerged one by one. Arjuna took a last look at the mansion of Krishna. It was soon covered by the sea. In a matter of a few moments it was all over. The sea had now become as placid as a lake. There was no trace of the beautiful city which had been the favourite haunt of all the Pandavas. Dwaraka was just a name; just a memory.

This quote above came from the Mahabharata, a sanskirt text orginally quoted in the article. So far, it's the only refrence as to how Dwaraka was flooded that I saw in the article.

So what really happend to cause the sea to swallow up Dwaraka?

Was it a levy or dam that broke and caused the sea to rush in? Could it have been a tidal wave or earthquake? In any case, I doubt that the sea "broke the boundry" of the shore and came rushing in.