Drains can't cope because the rain was unprecedented, but on top of that people throw rubbish in the street. And that washes away with the flow and ends up in drains. The "ME" generation. Yea yea, litter campaigns in the 50s, but the problem is worse, and the street cleaners are machines, it doesn't get into the corners that water will!
Basically civic pride is not high on the agenda today.
So what? They cry. Weeeeeeeell have a look at the way rubbish bags got thrown around by the deluge on this Lundun under water video @ 8:05. You won't see the Macdonalds trash - it is blocking the drains.
FFS even the approach to Tower Bridge is awash!

And Utah

Make no mistake, Global Warming = Global Floods. It ain't rocket science, it is not even chemistry.