Biblical Commentary on the Old Testament, by Carl Friedrich Keil and Franz Delitzsh, [1857-78], at sacred-texts.com
When the ark was built, and the period of grace (Gen 6:3) had passed, Noah received instructions from Jehovah to enter the ark with his family, and with the animals, viz., seven of every kind of clean animals, and two of the unclean; and was informed that within seven days God would cause it to rain upon the earth forty days and forty nights. The date of the flood is then given (Gen 7:6): "Noah was six hundred years old, and the flood was (namely) water upon the earth;" and the execution of the divine command is recorded in Gen 7:7-9. There follows next the account of the bursting forth of the flood, the date being given with still greater minuteness; and the entrance of the men and animals into the ark is again described as being fully accomplished (Gen 7:10-16). - The fact that in the command to enter the ark a distinction is now made between clean and unclean animals, seven of the former being ordered to be taken, - i.e., three pair and a single one, probably a male for sacrifice-is no more a proof of different authorship, or of the fusion of two accounts, than the interchange of the names Jehovah and Elohim. For the distinction between clean and unclean animals did not originate with Moses, but was confirmed by him as a long established custom, in harmony with the law. It reached back to the very earliest times, and arose from a certain innate feeling of the human mind, when undisturbed by unnatural and ungodly influences, which detects types of sin and corruption in many animals, and instinctively recoils from them (see my biblische Archeologie ii. p. 20). That the variations in the names of God furnish no criterion by which to detect different documents, is evident enough from the fact, that in Gen 7:1 it is Jehovah who commands Noah to enter the ark, and in Gen 7:4 Noah does as Elohim had commanded, whilst in Gen 7:16, in two successive clauses, Elohim alternates with Jehovah-the animals entering the ark at the command of Elohim, and Jehovah shutting Noah in. With regard to the entrance of the animals into the ark, it is worthy of notice, that in Gen 7:9 and Gen 7:15 it is stated that "they came two and two," and in Gen 7:16 that "the coming ones came male and female of all flesh." In this expression "they came" it is clearly intimated, that the animals collected about Noah and were taken into the ark, without his having to exert himself to collect them, and that they did so in consequence of an instinct produced by God, like that which frequently leads animals to scent and try to flee from dangers, of which man has no presentiment. The time when the flood commenced is said to have been the 600th year of Noah's life, on the 17th day of the second month (Gen 7:11). The months must be reckoned, not according to the Mosaic ecclesiastical year, which commenced in the spring, but according to the natural of civil year, which commenced in the autumn at the beginning of sowing time, or the autumnal equinox; so that the flood would be pouring upon the earth in October and November. "The same day were all the fountains of the great deep (תּהום the unfathomable ocean) broken up, and the sluices (windows, lattices) of heaven opened, and there was (happened, came) pouring rain (גּשׁם in distinction from טטר) upon the earth 40 days and 40 nights." Thus the flood was produced by the bursting forth of fountains hidden within the earth, which drove seas and rivers above their banks, and by rain which continued incessantly for 40 days and 40 nights.
"In the self-same day had Noah...entered into the ark:" בּא, pluperfect "had come," not came, which would require יבא. The idea is not that Noah, with his family and all the animals, entered the ark on the very day on which the rain began, but that on that day he had entered, had completed the entering, which occupied the seven days between the giving of the command (Gen 7:4) and the commencement of the flood (Gen 7:10).
Gen 7:17-24 contain a description of the flood: how the water increased more and more, till it was 15 cubits above all the lofty mountains of the earth, and how, on the one hand, it raised the ark above the earth and above the mountains, and, on the other, destroyed every living being upon the dry land, from man to cattle, creeping things, and birds. "The description is simple and majestic; the almighty judgment of God, and the love manifest in the midst of the wrath, hold the historian fast. The tautologies depict the fearful monotony of the immeasurable expanse of water: omnia pontus erant et deerant litera ponto." The words of Gen 7:17, "and the flood was (came) upon the earth for forty days," relate to the 40 days' rain combined with the bursting forth of the foundations beneath the earth. By these the water was eventually raised to the height given, at which it remained 150 days (Gen 7:24). But if the water covered "all the high hills under the whole heaven," this clearly indicates the universality of the flood. The statement, indeed, that it rose 15 cubits above the mountains, is probably founded upon the fact, that the ark drew 15 feet of water, and that when the waters subsided, it rested upon the top of Ararat, from which the conclusion would very naturally be drawn as to the greatest height attained. Now as Ararat, according to the measurement of Perrot, is only 16,254 feet high, whereas the loftiest peaks of the Himalaya and Cordilleras are as much as 26,843, the submersion of these mountains has been thought impossible, and the statement in Gen 7:19 has been regarded as a rhetorical expression, like Deu 2:25 and Deu 4:19, which is not of universal application. But even if those peaks, which are higher than Ararat, were not covered by water, we cannot therefore pronounce the flood merely partial in its extent, but must regard it as universal, as extending over every part of the world, since the few peaks uncovered would not only sink into vanishing points in comparison with the surface covered, but would form an exception not worth mentioning, for the simple reason that no living beings could exist upon these mountains, covered with perpetual snow and ice; so that everything that lived upon the dry land, in whose nostrils there was a breath of life, would inevitably die, and, with the exception of those shut up in the ark, neither man nor beast would be able to rescue itself, and escape destruction. A flood which rose 15 cubits above the top of Ararat could not remain partial, if it only continued a few days, to say nothing of the fact that the water was rising for 40 days, and remained at the highest elevation for 150 days. To speak of such a flood as partial is absurd, even if it broke out at only one spot, it would spread over the earth from one end to the other, and reach everywhere to the same elevation. However impossible, therefore, scientific men may declare it to be for them to conceive of a universal flood of such a height and duration in accordance with the known laws of nature, this inability on their part does not justify any one in questioning the possibility of such an event being produced by the omnipotence of God. It has been justly remarked, too, that the proportion of such a quantity of water to the entire mass of the earth, in relation to which the mountains are but like the scratches of a needle on a globe, is no greater than that of a profuse perspiration to the body of a man. And to this must be added, that, apart from the legend of a flood, which is found in nearly every nation, the earth presents unquestionable traces of submersion in the fossil remains of animals and plants, which are found upon the Cordilleras and Himalaya even beyond the limit of perpetual snow.
(Note: The geological facts which testify to the submersion of the entire globe are collected in Buckland's reliquiae diluv., Schubert's Gesch. der Natur, and C. v. Raumer's Geography, and are of such importance that even Cuvier acknowledged "Je pense donc, avec MM. Deluc et Dolomieu, que s'il y a quelque chose de constat en gologie; c'est que la surface de notre globe a t victime d'une grande et subite rvolution, dont la date ne peut remonter beaucoup au del de cinq ou six mille ans" (Discours sur les rvol. de la surface du globe, p. 190, ed. 6). The latest phase of geology, however, denies that these facts furnish any testimony to the historical character of the flood, and substitutes the hypothesis of a submersion of the entire globe before the creation of man: 1. because the animals found are very different from those at present in existence; and 2. because no certain traces have hitherto been found of fossil human bones. We have already shown that there is no force in these arguments. Vid., Keerl, pp. 489ff.)
In Gen 7:23, instead of ויּמּח (imperf. Niphal) read ויּמח (imperf. Kal): "and He (Jehovah) destroyed every existing thing," as He had said in Gen 7:4.