|A general synopsis of how the annual 150-day Nile River flood became, in Egyptian mythology, the myth of the flood of Nun and Nu's solar barque (3100BC), which became, in Christian mythology, the Biblical story of Noah's flood and Noah's ark (500BC). |
The Nile River, prior to the construction of the Aswan dam (1970), "flooded", annually, for 150-days, beginning Jun 25 to Nov 21, start marked by the helical rising of Sirius, ending late Dec (near Dec 25, the darkest day of the year), during which time the waters, owing to the melting of the snow in the Ethiopian highlands, would rise to heights of 30 feet (9 meters); the details of which are shown below: 
The following is a view of the "flooded Nile" river, from the point of view of a person in 1830 standing near the Pyramids at Giza, looking towards Cairo, according to which, from the point of view of a person in a Heliopolis creation myth mindset (geocentric; flat earth), it looks to the "senses" (see: common sense), as though the waters "rise" to great heights, thereby "covering" the mountains: 
In 500BC, during the redaction of the Egyptian Book of the Dead into the monotheistic format of the Old Testament, of the Bible, the "real" 150-day Nile River flood became the "mythical" 150-days of Noah (Genesis 7:24).
Religio-mythology | Scholars
In c.1500, Leonardo da Vinci rejected Biblical flood theory as being responsible for depositing fossils many miles from their origin. (Ѻ)
In 1736, Marquis of Argens, in his The Philosophy of Good Sense, which was peer-reviewed by Voltaire, wherein he argues that we must continually revise our knowledge about things, and attacks scriptural authority, arguing, e.g., that there is not enough water on the globe to make possible a deluge that would cover all the highest mountain tops of the world. 
In 2000, Gary Greenberg, in his 101 Myths of the Bible, section Myth #32: God Sent a Flood to Destroy Mankind, recounted the mythical origins of Noah's flood as follows: 
The Myth: The earth also was corrupt before god, and the earth was filled with violence. And god looked upon the earth, and, behold, it was corrupt; for all flesh had corrupted his way upon the earth. And god said unto Noah, The end of all flesh is come before me; for the earth is filled with violence through them; and, behold, I will destroy them with the earth. (Gen. 6:11-13)
The Reality: The story of Noah and the flood originated as a monotheistic version of the Hermopolitan Creation myth and presented an expanded account of events on the first day of creation.
In the Hermopolitan Creation myth, four males and four females emerged from the primeval flood and crawled onto the first land. These four males and four females, known as the Ogdoad (i.e., group of eight) collectively gave birth to Re, the Hermopolitan Creator deity, who floated on a lotus while the benben bird flew above.
The four male deities were Nun, Huh, Kuk, and Amen, who represented the four primary elements of the universe before creation, but in some texts other deities were substituted. Nun signified the primeval flood and Egyptians usually portrayed him in anthropomorphic form, standing waist-high in the primeval waters and holding aloft the solar boat that carried other deities.
In the story of Noah, to four males and four females emerged from a worldwide flood after a mountain arose out of the waters, during which time a sole child may have been born (see Myth #33) with some interesting questions about who his parents were. It also includes the appearance of birds, one of which behaves differently than the others (see Myth #34). In addition, the names of Noah and his three sons closely resemble names associated with the Egyptian creation cycle.
In old biblical Hebrew, the name "Noah" (which should be transliterated as "Noach") consists of only two letters "Nun" and "Ched”. We don't know what the original vowels were because old Hebrew text did not use vowels. The present assignment of vowels is speculative. It is interesting that “Nun”, the Hebrew name for the first letter of Noah’s name, is the same word the Egyptians use to name the primeval flood. The name of the biblical flood here, therefore, corresponds to the name of the Egyptian deity who represents the great flood of creation and guides the solar boat across the waters.”
According to 2004 polls, 60% of Americans believe in the story of Noah's ark and a global flood. (Ѻ)
1. Thims, Libb. (2016). Smart Atheism: For Kids (pdf | 309-pgs). Publisher.
2. (b) Argens, Marquis. (1736). The Philosophy of Good Sense. Publisher.
(b) Gasper, Julia. (2013). The Marquis d’Argens: a Philosophical Life (pg. 100). Lexington Books.
3. Greenberg, Gary. (2000). 101 Myths of the Bible: How Ancient Scribes Invented Biblical History (Myth #32: God Sent a Flood to Destroy Mankind, pgs. 73-75). Source Books.
● List of flood myths – Wikipedia.
● Flood myth – Wikipedia.
● Genesis flood narrative – Wikipedia.