Parallel accounts of the flood, some written hundreds of years before Genesis was written, support its historical basis. In his book, Noahs Ark and the Ziusudra Epic, Robert Best states: "Distinctive story elements and phrases that are common to two or more of these six stories indicate a common origin. Parallel quotations make it obvious that these six flood stories did not originate independently."

Certain words and phrases used in Genesis and parallel accounts such as heart, roof, cubits, seed, pitch, animals, fountain(s) of the deep, raven, dove, sweet savor, etc, make a compelling case for a common source for all the narratives and help corroborate a historical flood.

The flood texts are from Ziusudra (in Sumerian), Atrahasis (in Accadian), the eleventh tablet of Gilgamesh (written in four languages), Genesis 6-9, Berossus, and a version from Moses of Khoren.
.
"Side-wall ... pay attention" Ziusudra iv, 155
"Wall, listen to me." Atrahasis III,i,20
"Wall, pay attention" Gilgamesh XI,22

"when their heart led the great gods to produce the flood." Gilgamesh
"every imagination of the thoughts of his heart was only evil." Genesis 6:5

"the decision that mankind is to be destroyed" Ziusudra iv,157-158
"The gods commanded total destruction" Atrahasis Il,viii,34

"The great gods decided to make a flood" Gilgamesh XI,14
"God ... decided to make an end of all flesh" Genesis 6:13

"Destroy your house, spurn property, save life" Atrahasis III,i,22
"Tear down house, abandon property, save life" Gilgamesh XI,24-26

"Enki...over the capitals the storm will sweep" Ziusudra iv, 156
"He [Enki] told him of the coming of the flood" Atrahasis III,i,37
"God said to Noah ...I will bring a flood" Genesis 6:13,17
"Kronos...said ...mankind would be destroyed by a flood" Berossus

"...the huge boat" Ziusudra v,207
"Build a ship" Atrahasis III,i,22
"Build a ship" Gilgamesh XI,24
"Make yourself an ark" Genesis 6:14
"build a boat" Berossus

"who protected the seed of mankind" Ziusudra vi,259
"Bring into the ship the seed of all life" Gilgamesh XI,27
"to keep their seed alive" Genesis 7:3

"Draw a design of it on the ground." Atrahasis
"The ship that thou shalt build" Gilgamesh XI
"And this is the fashion which thou shalt make it of ..." Genesis 6:15

"Ten dozen cubits the height of each of her walls" Gilgamesh XI
"... three hundred cubits, the breadth of it fifty cubits" Genesis 6:15

"Like the apsu you shall roof it" Atrahasis 111j,29
"Like the apsu you shall roof it" Gilgamesh XI,31
"Make a roof for the ark" Genesis 6:16

"coming of the flood on the seventh night" Atrahasis,lll,i,37
"after seven days the waters of the flood came" Genesis 7:10

"...and addressed the elders" Atrahasis 111,01
"I answer the city people and the elders" Gilgamesh XI,35

"I cannot live in (your city)" Atrahasis III,I,47
"I cannot live in (your city)" Gilgamesh XI, 44

"pitch I poured into the inside" Gilgamesh XI,66
"cover it inside and out with pitch" Genesis 6:14
"some people scrape pitch off the boat" Berossus

"your family, your relatives" Atrahasis DT,42(w),8
"he sent his family on board" Atrahasis III,ii,42
"into the ship all my family and relatives" Gilgamesh XI,84
"Go into the ark, you and all your household" Genesis 7:1
"he sent his wife and children and friends on board" Berossus

"animals which emerge from the earth" Ziusudra vi,253
"all the wild creatures of the steppe" Atrahasis DT,42(w),9
"The cattle of the field, the beast of the plain" Gilgamesh XI,85
"clean animals and of animals that are not clean" Genesis 7:8
"and put both birds and animals on board" Berossus

"Enter the boat and close the boat's door" Atrahasis DT,42(w),6
"Pitch was brought for him to close his door" Atrahasis III,ii,51
"I entered the boat and closed the door' Gilgamesh XI,93
"And they that entered ... and the Lord shut him in" Genesis 7:16

"Ninurta went forth making the dikes [overflow]" Atrahasis U rev, 14
"Ninurta went forth making the dikes overflow" Gilgamesh XI,102

"One person could [not] see another" Atrahasis III,iii,13
"One person could not see another" Gilgamesh XI,111

"the storm had swept ... for seven days and seven nights" Ziusudra
"For seven days and seven nights came the storm" Atrahasis III,iv,24
"Six days and seven nights the wind and storm" Gilgamesh XI,127
"rain fell upon the earth forty days and forty nights" Genesis 7:12

"consigned the peoples to destruction" Atrahasis III,iii,54
"All mankind was turned to clay" Gilgamesh XI,133
"And all flesh died ... and every man" Genesis 7:21

"Below the fountain of the deep was stopped" Atrahasis
"The fountains also of the deep and the windows of heaven were stopped"
Genesis 8:2

"Ziusudra made an opening in the large boat" Ziusudra vi,207
"I opened the window" Gilgamesh XI,135
"Noah opened the window of the ark" Genesis 8:6
"he pried open a portion of the boat" Berossus

"On Mount Nisir the boat grounded" Gilgamesh XI,140
"the ark came to rest upon the mountains" Genesis 8:4
"the boat had grounded upon a mountain" Berossus
"After Khsisuthros...landed ...a long mountain" Moses of Khoren

"The dove went out and returned" Gilgamesh XI,147
"sent forth the dove and the dove came back to him" Genesis 8:10
"let out the birds and they again returned to the ship" Berossus

"Then I sent forth and set free a raven" Gilgamesh XI
"And he sent forth a raven" Genesis 8:7

"The king slaughtered ... bulls and sheep" Ziusudra vi,211
"He offered [a sacrifice]" Atrahasis III,v,31
"And offered a sacrifice" Gilgamesh XI,155
"offered burnt offerings on the altar" Genesis 8:20
"built an altar and sacrificed to the gods" Berossus

"[The gods smelled] the savor' Atrahasis III,v,34
"The gods smelled the sweet savor" Gilgamesh XI,160
"And the Lord smelled the sweet savor..." Genesis 8:21

"the lapis around my neck" Atrahasis Ill,vi,2
"the lapis lazuli on my neck" Gilgamesh X1,164

"That I may remember it [every] day" Atrahasis III,vi,4
"I shall remember these days and never forget" Gilgamesh XI,165
"I shall remember my covenant ...I may remember' Genesis 9:15-16

"How did man survive the destruction?" Atrahasis III,vi,10
"No man was to survive the destruction" Gilgamesh XI,173

"[on the criminal] impose your penalty" Atrahasis III,vi,25
"On the criminal impose his crimes" Gilgamesh X1,180
"Who sheds the blood of man, by man his blood be shed" Genesis 9:6

"he touched our foreheads to bless us" Gilgamesh XI,192
"And God blessed Noah" Genesis 9:1
"elevated him to eternal life, like a god" Ziusudra vi,257
"they shall be like gods to us" Gilgamesh XI,194

"I lived in the temple of Ea, my lord" Atrahasis RS 22.421,7
"go down to dwell with my lord Ea" Gilgamesh XI,42
"he had gone to dwell with the gods" Berossus
"Noah walked with God." Genesis 6:9

On the basis of substantial historical evidence, what reasonable conclusions could we make?

1. There is a likelihood that a flood event actually happened.  Why would the ancients invent such a story unless there was some historical element?

2. Considering it as a historical event, it was local to southern Mesopotamia about 2900 BC, rather than having been a global deluge.

3.  Judgment would have been specific to the sinful, local Adamite population rather than a universal pronouncement upon all mankind.