Growing up in Austin, TX, I had experience with Hill Country flash floods [View all]
Geologically known as the Edwards Plateau, it was once covered by seawater. There is subsurface limestone as well as outcroppings, caliche soil littered with marine fossils. Rain water doesnt soak in, running down hill, thru old creek beds that only have water in them after rains.
In addition to hiking and camping, my Boy Scout Troop got into canoeing. We took summer canoe trips down the Colorado and Guadalupe Rivers.
The Guadalupe area is popular for tubing, camping, and venues for retreats. On one canoe trip down the Guadalupe in the early 60s, we were awakened in the middle of the night by rising water, caused by a thunderstorm near Comfort, TX.
We gathered up our gear and drug our canoes uphill to higher ground. My father, our Scoutmaster, hiked to a farmhouse, and used their phone to call parents for help.
When we returned the next day to retrieve my fathers station wagon parked upstream, we found it had been inundated by the high water. Luckily, it was the only casualty.