Changing the way we look at the world
How long did the flood last? 

Genesis chapter 7 verse 17 states that: 
The flood continued for forty days on the earth; the water increased and lifted up the ark so that it rose above the earth.  

 That seems to be quite straight forward! So why do most people talk about the “flood year”? 

The reason is because Noah spent about one year in the Ark.
 
We get this by comparing Genesis 7 verses 11 and 12 with Genesis 8 verse 13 to 16: 
In the six hundredth year of Noah’s life, in the second month, on the seventeenth day of the month, on that day all the fountains of the great deep burst open, and the floodgates of the sky were opened. The rain fell upon the earth for forty days and forty nights.
[Genesis 7 verses 11 and 12] 
Now it came about in the six hundred and first year, in the first month, on the first of the month, that the water was dried up from the earth. Then Noah removed the covering of the ark, and looked, and behold, the surface of the ground had dried up. And in the second month, on the twenty-seventh day of the month, the earth was dry. Then God spoke to Noah, saying, “Go out of the ark, you and your wife and your sons and your sons’ wives with you.
 [Genesis 8 verses 13 to 16] 

 To correctly understand the duration of the flood we have to look at the Hebrew words for flood, water and waters of the flood. 

Genesis 7 verse 17 states that: 
The mabbul continued for forty days on the earth; the mayim increased and lifted up the ark so that it rose above the earth

Genesis 8 verse 13: 
Now it came about in the six hundred and first year, in the first month, on the first of the month, that the mayim was dried up from the earth.  

It would seem that the water (mayim) was used to cause the flood (mabbul) which happened because of the breaking up of the fountains of the deep and forty days and nights of rain. After the rain stopped and the fountains of the deep were closed, Noah had to stay in the ark for a whole year, until enough land had dried out. 

 The purpose of the mabbul was clear and is stated in Genesis 6 verse 17: 
Now behold, I Myself am bringing the flood of water upon the earth, to destroy all flesh in which there is the breath of life, from under heaven; everything that is on the earth shall perish. Now behold, I Myself am bringing the mabbul of mayim upon the earth, to destroy all flesh in which there is the breath of life, from under heaven; everything that is on the earth shall perish

Genesis 7 verses 21 to 23 tells us that this literally happened: 
So all creatures that moved on the earth perished: birds, livestock, animals, and every swarming thing that swarms upon the earth, and all mankind; of all that was on the dry land, all in whose nostrils was the breath of the spirit of life, died. So He wiped out every living thing that was upon the face of the land, from mankind to animals, to crawling things, and the birds of the sky, and they were wiped out from the earth; and only Noah was left, together with those that were with him in the ark.  

The most straight forward way to understand this is that by the end of the 40 days of the mabbul all air breathers were dead – wiped out is very strong language – and gone. 
Reference to Genesis 7 verse 17 indicates that the mabbul / cataclysm (aka flood) is the period of judgement on the earth. After no more than 40 days of judgement all air breathing land animals (including flying creatures) were wiped out. This cataclysm was caused by waters from the fountains of the deep and the rain from the windows of heaven. There was 40 days of unimaginabe destruction caused by water which wiped out all land-dwelling air breathers. The water remained for up to 150 days (which may include the 40 days) and then took another 150 days to drain away into newly formed deep oceans. 

 Genesis 6 verse 17, Genesis 7 verse 6, 7, 10 mention the water(s) of the flood or flood of water. 
Now behold, I Myself am bringing the flood of water upon the earth, to destroy all flesh in which there is the breath of life, from under heaven; everything that is on the earth shall perish. 
• Now Noah was six hundred years old when the flood of water came upon the earth. 
• Then Noah and his sons, his wife, and his sons’ wives with him entered the ark because of the waters of the flood. 
• Now it came about after the seven days, that the waters of the flood came upon the earth.
 
Literally the ‘flood waters’, ‘the flood happened water’, ‘water of the flood’, ‘waters of the flood’, respectively. 
The cataclysm of the flood was caused by water. Water which covered the whole earth within 40 days. Then another 11 months (approximately) passed before the water drained off the earth and the land was dry enough for Noah to leave the ark. 

 The age of Noah at his death confirms that the flood (mabbul) lasted much less than one year. 
Genesis 9 verse 28 states that Noah lived 350 years after the flood (mabbul). 
Genesis 9 verse 29 states that Noah lived a total of 950 years. 
We know that Noah was 600 years old when the mabbul started and if the ‘flood’ lasted for one year then Noah would have lived for 951 years (600 years before the flood plus one year during the flood and 300 years after the flood). The recorded age of Noah is consistent with the mabbul of only 40 days.