Charles Ellicott Commentary Genesis 7:20

"Fifteen cubits upward did the waters prevail; and the mountains were covered." — Genesis 7:20 (ASV)

Fifteen cubits upward. — This apparently was the draft of the ark, computed after it had settled in the region of Ararat. Fifteen cubits would be about twenty-two feet, and as the ark floated onward without interruption until it finally grounded, there must have been this depth of water even on the highest summit in its course. Continuous rains for forty days and nights would scarcely produce so vast a mass of water, unless we suppose that the adâmâh was some low-lying spot of ground where the waters from many regions flowed together; but this is negated by the ark having travelled into Armenia. In England, the whole average mean rainfall in a year is not more than twenty-eight or thirty inches in depth. If we suppose this amount to have fallen in every twenty-four hours, the total quantity would be about 100 feet.

Such a rain would denude the mountains of all soil, uproot all trees, sweep away all buildings, dig out new courses for the rivers, completely alter the whole surface of the ground, and cover the lower lands with debris. Wherever there was any obstacle in their way, the waters would deepen in volume and quickly burst a passage through it. But as the waters would be seeking lower ground during the entire forty days, it is difficult to understand how they could cover any of the heights to a depth of twenty-two feet, unless there were some cosmic convulsion (see Note on Genesis 7:11). Through such a convulsion, waters from the equator could have been carried towards the poles, and in this way, there would be no difficulty in the ark being carried against the current of the Tigris and Euphrates up to the high lands of Armenia.