I'm not particularly clued up on all this type of thing so hopefully someone can explain it to me:
I was under the impression that a lot of the floods we had in this area (Berkshire) last July were caused by the sheer amount of rain that fell in a short period of time so the drains couldn't cope.
Certainly areas here which got flooded were well above sea level and away from the usual flood plains along the Thames valley.
So this type of flooding is different from the type when rivers overflow due to persistent rain and, therefore, it makes no difference whether the land was on a flood plain or not.
Hope this makes sense, and I hope someone can clarify because I'm getting myself confused just thinking about it!
