The flood plain of a river is the zone in which flood water overflows.
It is therefore a zone that can be flooded from time to time and where any human activity and infrastructure is exposed to a natural risk.
Beyond the flood plain there is no river flooding risk although other types of flooding can occur.
The flood plain of the river Aa is flooded here. The course of its stream bed can still be seen on the left in the extension of of the concrete bridge. © Lamiot, Wikimedia CC by 3.0