Flash floods--how do the hills and valleys turn rain into a wall of water?

[TnDem]

26. Flash flooding most of the time...

Is just that...Flash flooding from a lot of rain in a short time in an area, (like the Texas hill country), that doesn't drain efficiently, nor fast enough.

Like I said, many years ago, i witnessed something like this near the Smoky Mountains....A TON of rain had fallen near the top of the mountain, and none at the bottom where we were. We were in a cabin way above the stream bed, but we heard a distant roar. In less than one minute, the entire tiny stream that had just had 10 inches of water in it, now had 15 FEET of raging water, boulders, tree trunks and everything else it could carry.

It was simply that, a FLASH flood that dumped three-four inches of rain on a near vertical surface several miles away from us.