The Portion, Noach, starts with the Flood; but before we can understand the Flood we need to understand the reason for it: the Nefilim, from the very end of last week’s Portion. The hybrids called the Nefilim were Satan’s attempt to destroy mankind’s gene pool – and he almost succeeded! By the time of Noah, it is estimated there were as many people on earth as there are today – but only eight of them were found to have a pure bloodline. So God sent the Flood to rid the Earth of the Nefilim and to preserve Noah and his family along with the unaffected animals. He used the waters from above and from below to flood the Earth.
When the water had receded God promised Noah He wouldn’t destroy the Earth again with water (Peter tells us it will be with fire next time). So Noah and his family started over again. They were to go out and replenish the Earth.
While they may have repopulated the Earth, they did not spread over the Earth. We find them gathered in one place – Babylon – and ruled by one man – Nimrod. And he was drawing all of mankind away from God. So God destroys all that Nimrod had built, He gave the people different languages and dispersed them so they couldn’t be controlled by a dictator again.