20 marks!!!

Actually, Nile river's annual flooding usually in July in the ancient. The egyption will put some sandbags near the Nile river to prevent the flooding. If the flooding is critical, they will also build up a sandbags wall (collected a lot of sand bags to build-up a small wall) to avoid the flooding. But when the flooding became more and more serious, the egyption started to keep their homes away from the Nile river, after they didn't live beside the Nile river, they became safety and won't lose their life.

I'm sorry that I don't know the answer, but I recommend you to visit the wikipedia.org, although it has no exact answer for this question, but I think if you watch more information at wiki, you will soon get your answer.

Sorry~~