The flood narrative is a masterful blend of two separate flood accounts. It takes the form of what Wenham calls a palistrophe or to use a more recognizable term, a chiastic-like structure (A, B, C, D, C', B', A'). Naturally, any flood will rise and abate, if you get on a boat you then later get off of it. We would expect some unintentional literary symmentry any such story but these parallels, which are not perfect, go far beyond random coincidence into specific literary form. The image below is from Gordon Wenham's Rethinking Genesis 1-11, pg. 39, and highlights the chiastic structure of the flood account. What is most remarkable is that the final editor did this with two separate flood accounts!
Wenham writes, "Some of them are so contrived they must be deliberate." He then zooms in on the following sequence which shows the numbers (7, 7, 40 150) are reversed (150, 40, 7, 7). They correpond to the letters labeled letter pattern written below:
H, I, L, O, --> O', L', I'. H'
Wenham (ibid, pg. 39) writes,
"But this is contrived because the first two mentions of seven days actually refer to just one week, the week between the command to enter the ark and the flood's onset. Whereas the last two mentions of seven days cover three weeks of the dove's reconnaissance flights."
Why does the author do this? As Wenham correctly suggests, the author is eager to make a point and its found in the center of the entire structure in line P where "God remembers Noah." The point of all this literary show as Wenham writes (ibid pg. 40):
"It is God who saved Noah, not, as in other oriental accounts, the hero's own energy and good fortune."
The Genesis flood most certainly rearranges a lot of the furniture when it comes to surrounding flood accounts of which it shares many parallels and details. This is a clear example along with a statement of Jewish monotheism and the sovereignty of God. It is also significant that the chiasm is broken into two halves: un-creation and re-creation. God's created order in Genesis is clearly being undone during the flood and creation is restored afterwards as the flood abates. G. V. Smith (Structure and Purpose in Genesis 1-11, JETS 20 : 310-11) came up with the following points of contact between creation and the flood (chapters 1-2 with 8-9). I have put the relationship from Smith in list format:
(a) Since man could not live on the earth when it was covered with water in chaps. 1 and 8, a subsiding of the water and separation of the land from the water took place, allowing the dry land to appear (1:9-10; 8:1-13);
(b) "birds and animals and every creeping thing that creeps on the earth" are brought forth to "swarm upon the earth" in 1:20-21, 24-25 and 8:17-19;
(c) God establishes the days and seasons in 1:14-18 and 8:22;
(d) God's blessing rests upon the animals as he commands them to "be fruitful and multiply on the earth" in both 1:22 and 8:17;
(e) man is brought forth and he receives the blessing of God: "Be fruitful and multiply and fill the earth" in 1:28 and 9:1, 7;
(f) Man is given dominion over the animal kingdom in 1:28 and 9:2;
(g) God provides food for man in 1:29-30 and 9:3 (this latter regulation makes a direct reference back to the previous passage when it includes the statement, "As I have given the green plant");
(h) in 9:6 the writer quotes from 1:26-27 concerning the image of God in man. The author repeatedly emphasizes the fact that the world is beginning again with a fresh start. But Noah does not return to the paradise of Adam, for the significant difference is that "the intent of man's heart is evil" (Gen. 8:21)"
The importance of this for understanding the universal nature of the Genesis flood is discussed in another article on this website. Also, such an extensive literary device in a work would generally caution me from looking too closely at the details of the story as if they were all meant to be factually true. Even though in this case some elements of a chiastic structure are inherent to the story itself, the chiasm is very elaborate and historical reality is not usually this neat and structured. You can certainly narrate a historical event in a chiastic structure if you are allowed a degree of latitude but when the construct for the entire scene is a fancy literary device that leads to known
contrived elements with the aim of telling a theological truth, caution is warranted on the historical front
. The most important part of the flood account is how its central feature portrays God's salvific role in contrast to what happens in other Mesopotamian flood mythology.