Flood plain – The relatively flat area forming the valley floor on either side of a river channel, which is sometimes flooded
Floods occur here, plain (flood plain) and simple. No, you can't build a house here.
Flood plains have their advantages – chiefly that they are rich in fertile sediment that is deposited on the land by the floods, which can be to the benefit of both wildlife and agriculture.
Over recent years, however, they have become the focus of concern as climate change has brought more severe floods. This has resulted in misery for many people whose homes and businesses have been flooded. It has also brought into question the wisdom of building homes in certain areas.
For government, it has meant a rethinking of some policies and spending on often expensive systems to give relief to those with homes in flood-risk areas.