The huge amount that fell in certain areas over a shortish time meant there was too much water to cope with , coupled with the dreadful high winds damage was bound to ensue.
water soaks the ground, the ground becomes super-wet and then the rain passes from the hill straight into the river. And when you get hillside sheet runoff, it can be spectacular, a whole hillside awash with water flow......saw it in Fife for the first time.
Now if you are highly mathematical, I can lead you through the formula for super-saturation.
Most people associate flooding with the situation where there's already so MUCH water in the ground that no more can be absorbed. (i.e. flooding only occurs after LOTS AND LOTS of rain).
However flooding will also occur when there's so LITTLE water in the ground (making the surface hard, dry and nearly impermeable) that even a FAIRLY SMALL amount of rain can't be absorbed into the ground and thus remains on the surface.
It's because we've had largely dry conditions for the past few months that the latter situation now exists, meaning that a fairly small amount of rain can lead to flooding.
I watched an interview with a lady down Somerset way who said that they used to get these floods 40 years ago but this is the first time that it has rained this hard for decades to cause the floods.