Where Parshas Bereishis begins with the creation of the world, Parshas Noach begins with the destruction and thus the re-creation of the world. The question is often asked why Noach, unlike Avraham who prayed for Sodom, did not pray for those who would be flooded. Indeed when the waters are called 'Mei Noach', the waters of Noach; it is said that he was partially responsible for them by, unlike Avraham, failing to influence his generation.
Various answers are given to this question but it's worth considering the difference betweeen Avraham and Noach. Noach is described as a Tzaddik and as Tamim, perfect. Noach was the son of generations of righteous men. He was righteous but he did not understand that it was possible to be away from G-d and then to come close to him. He did not pray or try to influence his generation because he had no reason to believe it was even possible. The world until now had been one where sin was followed by punishment. Adam and Chava sinned and were expelled from Gan Eden for it. Kain sinned and was forever marked by it. This was as far as Noach, righteous from birth and by descent knew, the nature of the world. In modern Orthodox parlance, Noach was an FFB.
By contrast Avraham was a Baal Tesuvah, son of an idol worshipper and idol maker who came through his own efforts to know and serve G-d. He understood that repentance was possible and that those far from G-d could be brought to G-d, because he had once been far from G-d himself. As such Avraham could pray even for Sdom because he knew that man's nature allowed him to change and that G-d's nature encompassed love for man despite his flaws and forgiveness and mercy.
Noach's very perfection made him flawed and Avraham's seeming flaw made him perfect. This is why Noach is described as walking with G-d, while Avraham is described as walking before G-d. The man or woman who never deviates is also never tested while the man or woman who is tested can truly serve G-d by walking before him.
One further point is a distinction between Sdom and the world at the time of Noach. Sdom's main crime was to deny others hospitality, namely their own property. Since all property ultimately comes from G-d, this was a crime against G-d. While the generation of Noach's crime was to take the property of others, a crime both against man and G-d. As with the tower, a crime against G-d, we see that G-d can be tolerant of crimes against him than against others as he forgives those sins we commit against him. Therefore Avraham thought it reasonable to pray for Sdom while Noach despaired of praying for the world.