The Talmud records a view (Gittin 60a) that Moses received the Torah m’gillah m’gillah, “section by section”.
If this assertion is true it helps to explain the series of announcements that punctuate this week’s reading, “These are the generations of…” – i.e. regarding each one as the opening of a new section.
M’gillah m’gillah does not in itself define the length of each section, so some sections could be quite short and others rather longer.
One of the longer sections would clearly be the “These are the generations of No’ach” which introduces the Flood story.
This approach also explains why sometimes we find certain information repeated. For example, we already know the names of No’ach’s children, but the information is given again because it is germane to the whole new episode that begins with “These are the generations of No’ach”.
We see the same thing at the end of the Book of B‘reshit, where we find Jacob’s family in Egypt; the information is repeated at the beginning of Sh’mot where the details of the family are needed as the introduction to the story of Moses and the Egyptian enslavement.