>tidal wave is a perfectly good word for a big wave,
>just as sunrise is a perfectly good word for the earth turning to reveal the sun.
> so i wonder why tidal wave was discarded and who chose tsunami, which literally means harbour
>wave, in its place?

Perhaps the reason is the habit of the English language to pick words from where it can. (without much thought as to why). I mean, forest was a perfectly good word why did they have to choose the Indian word Jungle?