Are backwater valves always necessary in flood-prone areas?

What does it look like if you build without a basement? Do you even need such a backflow valve then?

So as far as I understand it now, no. You don’t have any openings below the backwater levels. It would of course look different if you were to build your floor slab below the backwater level, which would probably be idiotic and I don’t think it is allowed. At least not here.

In the case of a foundation slab, normally not