Significance of the year of the flood?
We have to admit the possibility that there is no actual significance to the fact that the flood happened 1556 years after creation!
Why was that year chosen? It took that long before men were evil enough in the eyes of God to merit such a punishment. It is that simple!
Why the hang up on the idea that a generation equals 40 years? Biblically speaking, a generation could be 40 years. Consider the following verses:
13 And the Lord'S anger was kindled against Israel, and he made them wander in the wilderness forty years, until all the generation, that had done evil in the sight of the Lord, was consumed. - Numbers 32:13
But then we know Moses was 70 years old when his son son was born. Issac was born when Abraham was 100 years old. Thus a biblical generation could be anywhere from 35 years to 70 years or even more.
The word generation in itself has different meaning within the Scriptures.
The original languages of the Bible used at least three different words that are translated “generation” in English. The Hebrew dor can refer to a normal, physical generation, as in Exodus 1:6. But it can also be used metaphorically to identify people of a distinguishable type. For example, Psalm 78:8 says, “They should not be like their fathers, a stubborn and rebellious generation, a generation whose heart was not steadfast, whose spirit was not faithful to God” (ESV). Here the word dor is used twice to refer to a group of people through a long period of time who were characterized by rebellion and sin. The “generation” in Psalm 78:8 is not limited to a normal thirty-year period but stretches back through the history of Israel to include all who were stubborn against God.
The other Hebrew word we translate as “generation” is toledot. This doesn’t refer to the character of a group or an age but to how that age was generated. So the “generations of the heavens and the earth” in Genesis 2:4 refers to the time periods that started with creation and continued organically from that point. The “generations of Adam” in Genesis 5:1 means the civilization of people that began with him. The next “generation” is that of Noah, to include the flood and the civilizations that came after. Shem’s influence is marked as a “generation” as he was the father of the Semites (Genesis 11:10). And Terah’s, because he left Ur with his son Abram (Genesis 11:27). Later, Ishmael (Genesis 25:12) and Isaac (Genesis 25:19) were the source of new generations. In each case, the men either experienced or caused a significant event that changed the course of their family line. They generated a culture-altering event. - How long is a generation in the Bible?
Let us recall that after the flood the life span of men was greatly shortened in comparison to the first Few generation prior to Noah. Why Did People Start to Have Shorter Lives After the Flood?
In the of all this, I can not see any real significance as to why the year 1556 was chosen as the year of the flood other that the crimes of men were great enough to stir God’s anger at that particular moment in human history.