Having been flooded (and we do not live on a flood plain) I can assure you that water (in our case sewerage) rushing into your home can happen VERY quickly. We were busy trying to stop water coming in the front door, turned round to find it coming up through our floor boards.
Now, maybe we should've spent the whole day moving all of our possessions upstairs, on the off chance, but having never flooded, and never expected to flood we didn't quite know the protocol. Same goes for all of our flooded neighbours.
0-12 inches of water was VERY fast. We moved some stuff (and of course we didn't plonk stuff into water, for the insurance) but actually, by the time water's coming in you are way too late to start moving things about.
One day later, when we turned up for the 'clean up' at the same time as the people appointed by the insurance company, we were shocked that essentially, if it was downstairs, and the water touched it, it was ruined - they asked if we were 'attached' to any of our stuff before ordering a skip to put it all in.
It is really, really shocking to lose half your stuff in one go. And then not be able to live in your house for months whilst it dries, the floorboards get replaced, everything has to be dried out, replastered, replaced.
We were really lucky to have insurance (unlike our next door neighbours), and we are even more lucky that we are not deemed, currently, to live in an area at risk from flooding, and can still get insurance.