Rapid City Flash Flood

All Things Considered:

Survivors say the wall of water was like a tsunami that destroyed nearly everything in its path as it roared through a Black Hills canyon and into town. The flash flood that hit Rapid City, S.D., on June 9, 1972, was one of the worst floods in U.S. history. It killed 238 people and damaged or washed away more than 1,300 homes.
On Saturday, the city will read the names of those who died and reflect on how the flood changed the way the city and others towns across the country built themselves.On that night 40 years ago, a huge thunderstorm parked over the Black Hills west of Rapid City and dropped as much as 15 inches of rain in some places. The surge of water that barreled down Rapid Creek took with it almost everything it encountered, including houses, cars and people.
Rita Rosales, who was 20 at the time, says it was a terrifying scene.
"There was so many [people] in trees and screaming and crying and the sparks were flying from electric wires, houses were on fire, it was just — it was hell," she says.

That is just stupefying.  15 inches?  That is a fairly large disaster I'd never heard of.