On the evening of the flood I drove over the section of the A61 near High Green, where the road slipped. Water was running across the road. As this section of the road is in effect a bridge across a stream (Hallwood brook, or something like that), I presume that the valley on the west side of the road (ie upstream) must have been flooded up to road level, judging by the amount of water that was flowing across.

As I pass along that section now, it is clear that the valley upstream of the road is quite deep. So, if the valley was full up to road level, the pressure on the side of the road would have been enormous. I wonder if we were lucky that just part of the road shifted by a few inches, and that the whole section of road wasn't washed away.

I hope that they are increasing the size of the culvert under the road, to make sure it can carry all the flow and doesn't back up on the upstream side in future. This then makes we wonder -

Was it already big enough but blocked due to debris being carried down by the water?

or, more worryingly,

Was it already big enough, but not maintained, and so blocked?