For those not around in the early evening of Saturday, July 31, 1976 :
a large stationary thunderstorm released as much as 7.5 inches of rainfall in about an hour (about 12 inches in a few hours) in the middle reaches of the Big Thompson River Basin and to a lesser extent in parts of the Cache la Poudre River Basin.
http://pubs.usgs.gov/fs/2006/3095/
Wow. What a disaster.
Here is a good photo of the Big Thompson canyon, and you can see why there is so much damage. Upstream from this, there is a large area which will drain into this river. So when it has extremely heaving rains across the area, it will not have anywhere to go except to collect into this narrow canyon. On a good day it is barley wide enough to have the road and the river, but in a flood of this size, there can be a wall of water 30-40 feet going down the bottom of this canyon. There are campgrounds and fishing along this river. In this sort of situation, there is no place to escape to. Climbing the walls of the canyon is the only escape, but in many locations, not realistic for a lot of people.
i read a book on that and on the gelwood springs south mountain fire. to be in anything like that in the daytime has to be scary. i can’t imagine at nigh, hearing flash flood sirens at 0200.