There are other flood stories from a number of parts of the world. They do not always speak of all the land being totally covered. However, we have seen one and two day storms recently that dropped 40 and 50 inches of rain. What if because of ancient vulcanism or meteor event(s) there were 40 days of such powerful rain. Calculating 40” per two days for 40 days gives over 65 vertical feet of water. That would sure drown a lot of people and animals and leave a very strong impression; which, as people do, would be exaggerated some more in the retelling.
Do you remember where or which cultures Velikovsky described when he made his hypotensis about the hydrogen stream/bath? It has been many years since I read his books, but I seem to recall he would list a number of odd facts occurring around the same time to serve as a source for his conjectures. I specifically remember him describing great masses of tangled together bones in the far north of our continent which I have also read described elsewhere, and which figures in the Firestone theory of north American meteor strike(s). This is one reason why I have found Firestone’s work so believable.
"Earth In Upheaval" was the book where he gathered the evidence for large death assemblages in the Arctic, particularly Siberia; some of the Arctic islands off Siberia are stuffed with bones (and tusks) of now-extinct animals, clearly they were deposited in one fell swoop.
40 days of worldwide rain would not be within the capacity of the known hydrologic cycle, IOW, it would have to be an unusual event, and that's what is recorded here and there in ancient written sources, and oral traditions -- not great flood stories, although that does happen, but a massive worldwide rain.