How were floods explained historically? There are plenty of myths about the reasons for floods in ancient cultures.

For example in Australian aboriginal culture was the Dreamtime story of Tiddalik which describes a frog that consumes all the water in the land but is then tricked into releasing it all at once.

See http://en.wikipedia.org/wiki/Tiddalik

In Greek mythology it was said that Jupiter sent down a giant flood. Here is an extract from the story. Which bears a similarity with the flood of Noah's Ark fame.

" The tops of the trees were hidden by the flood, and then the hills and then the mountains; and Deucalion and Pyrrha could see nothing anywhere but water, water, water–and they knew that all the people in the land had been drowned."

see http://www.authorama.com/old-greek-stories-6.html

Early 'scientific explanations'

Early 'scientific explanations' of the great flood were based on Christian interpretations of the classic Bible story.

Overall these ideas are based on a general field of study called Flood myths. According to the wikipedia entry about this it notes that "Adrienne Mayor's The First Fossil Hunters and Fossil Legends of the First Americans promoted the hypothesis that flood stories were inspired by ancient observations of seashells and fish fossils inland and on mountains."

So there was some truth to the idea of people actually observing evidence of sealife like shells and trying to come up with an explanation as to how they got there.

see also http://en.wikipedia.org/wiki/Flood_myth

More scientific approaches

Some ancient flood stories may also have been inspired from the result of Tsunamis that may have actually occurred during ancient times.

The first scientific ideas about the actual weather that relates to floods for example date back as far as 3000BC in India where they described the formation of clouds and formation of rain.

Later in 1027AD - Avicenna (A Persian scientist)publishes The Book of Healing, in which Part 2, Section 5, contains his essay on mineralogy and meteorology in six chapters: formation of mountains; the advantages of mountains in the formation of clouds amongst others things.

See http://en.wikipedia.org/wiki/Timeline_of_meteorology

In 1860 - Robert FitzRoy, with the help of the new telegraph system gathered daily observations from across England and produces the first synoptic charts.

The first international Meteorology Organisation was started in 1873 Vienna, Austria

see http://www.austehc.unimelb.edu.au/fam/0945.html

In 1905 the Australian Bureau of Meteorology was established by a law to unify the state based observatories.