Floods, when will people learn not to build on flood plains ?

Never.

When will they learn to build above the normal water line? Well... New Orleans is being rebuilt, with lots of residential areas below the water level of the canal that runs through town. In China, there are towns along the Yellow River that are the same way.

Humans have always settled on floodplains, at least ever since agriculture was invented. Floodplains are very fertile due since there is an abundance of water and lots of sediment deposition. Look at the Nile Valley: before irrigation, the annual floods were the only reason the area was habitable at all.

Still, you're right in that overbuilding with no flood mitigation is a huge mistake.

You are right.

When buying a house, an environmental survey is now recommended, which will show the chances of you being flooded.

Anyone who buys a house next to a river or on a flood plain is soft in the head. The information is out there.

Flood plains are locations for very rich soils for farming. When we lived in an mostly agrarian society it wasn't so bad. Flooding was just the trade-off for having good farmland. But over development has caused bigger flooding problems than we had in the past.

You are correct. However, it is common for people to just take the word of a developer that the land is "too high" for flooding problems when buying a house. We probably need to include more topographic map-reading in high school classes.