Akallabêth = As in the Days of Noah

Tolkien may have believed in a limited, though not just local, Flood.

Or he may simply have seen the "sinking of Atlantis" (whether pre- or post-Noah's-Flood) as a parallel to it, and therefore Atlantis (on which Akallebêth is his fan fiction) just before the sinking as a parellel to the pre-Flood world just before the Flood.

Either way, it seems he knew a thing or two on how things behave at sea. He researched phases of the Moon in painstaking accuracy for Lord of the Rings, we can be sure he researched waves and giant waves and tsunamis in detail before writing Akallabêth.

Here is a quote or paraphrase from it, in Girl Next Gondor's summarising of Númenor's fall:

although The 19:35 Great Wave should have claimed their 19:36 ships along with everything else they 19:38 are instead Swept Away by a terrific 19:41 wind that snaps their masts and blows 19:43 them all the way to Middle Earth

Now, this is actually impossible, even this. Hence, it is impossible if the ships had been sailing on the sea, that they would not have been buried in the Flood. This also goes for the Ark.

Instead we must imagine that the Ark only started floating once the waters were far higher and already very much calmer. In fact, the Ark must have been very high up. I would say Noah knew where the single highest mountain on the pre-Flood world was, he built the Ark there, and he knew that the water line of the Ark would be half the way up. 15 cubits of the 30 cubits height.

Now read the appropriate verses of Genesis:

And the waters prevailed beyond measure upon the earth: and all the high mountains under the whole heaven were covered The water was fifteen cubits higher than the mountains which it covered
[Genesis 7:19-20]

The Ark never was near one of those big waves from when vast land masses suddenly came under water. The seas around the mountain were as wide as the world and as deep as that mountain was high, and when that "tip of the iceberg" sank and the Ark floated, it suffered no great commotion.

But, oh boy, what a fool Noah must have looked to people when they realised he was building a ship on the back then equivalent of the top of Mount Everest. Meanwhile, as God does not break His promise, we can rule out the theory that the Flood looked just like Valencian province some time ago, with lots of houses and hilly parts of Valencia still well above the water surface.
/Hans Georg Lundahl