Fulltext is a long read, but you can browse through the article at Texas Monthly to see what it's like to live (or vacation) in the path of flash flooding.
The river registered around five feet at nine p.m. But as the measurements arrived by satellite every 15 minutes, Lenz watched in shock as the river crested to heights he had never seen. In just one hour—from 10:45 p.m. to 11:45 p.m.—it rose nearly 20 feet. Forecasters had predicted 1 to 4 inches of rain, up to 6 inches in isolated spots. Instead up to thirteen inches fell. They had predicted the river would rise to a moderate flood stage—roughly seventeen feet. But by midnight, the river registered at 32 feet—and was still rising.This is slightly different from the situation of people who foolishly decide to live in conventional "flood plains" of major rivers that exceed their banks after every snowmelt. What's happening here is a hundred-year-event. Note the speed of the water rise. The power of the water is described in the story at the link.