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Disasters: Big Thompson Canyon, Colorado

Posted by feww on April 29, 2010

Each and every disaster can be interpreted as a warning sign, but to read the signs you have to understand the language—reader KMH

The Big Thompson Canyon flood killed 145 people (6 were never found), destroyed 418 houses and damaged another 138, destroyed 152 businesses, causing at least $40 million in damages, in 120 long minutes.

It was as if nature had laid a trap

Up to 3,500 people had escaped the summer heat,  traveling to the cooler mountain air in one of Colorado’s most popular holiday destinations, some celebrating the state’s  100-year statehood anniversary. It was July 31, 1976.

“At the height of the Colorado tourist season, several thousand people escaped city heat by traveling to a popular camping area an hour northwest of Denver for hiking, fishing, camping and relaxing in the cooler mountain air. By late afternoon, an estimated 2,500-3,500 people were enjoying themselves in one of Colorado’s most scenic river valleys. They had no way of knowing that unusual atmospheric conditions and the physical make up of the Big Thompson River valley were setting the stage for disaster.”

Northern Colorado’s Big Thompson River flows from the Rocky Mountains (west) to the Great Plains (east) through a steep-walled, boulder-strewn canyon. This image was captured by the Advanced Land Imager on NASA’s Earth Observing-1 (EO-1) satellite on March 22, 2010.  Image and caption: NASA
Download large image (4 MB, JPEG). Click image to enlarge.

By early evening “as campers frolicked, a witch’s brew began to develop in the atmosphere”

A combination of moist air rising up the mountain slopes and the summer heat formed thunderstorms which “lifted along the Front Range and began to dump heavy rain on the region about 6 p.m. Winds found at mountain crests of 10,000 feet are usually strong enough to push thunderstorms to the east and out of the area. On July 31, 1976, however, the upper winds were extremely weak and weren’t strong enough to push the storm away from the Big Thompson Valley.”

The Big Thompson River basin is similar geologically to many river basins along the eastern side of the Continental Divide. Sheer rock forms the canyon walls, with little soil and vegetation to absorb runoff from storms. The river starts high in the Rocky Mountains near Estes Park in north-central Colorado and flows eastward through the rugged, steep-walled canyon. In some places, the canyon walls jut almost straight up. From top to bottom, the river drops vertically more than half a mile and exits the canyon into the rolling, forested plains west of Loveland. Dotted with homes, restaurants and other businesses, U.S. Highway 34 stretched the length of the canyon. Image: USGS; Caption: NOAA. Click image to enlarge.

The worst natural disaster in Colorado’s history was about to occur

The quasi stationary storm lingered on above the canyon  for more than three hours,  dumping about 30cm (1 foot) of rain into the basin. “Eight inches of rain fell in one hour-long stretch, and turned the normally placid two-foot-deep trickle into a raging torrent of water 19 feet high. Sweeping 10-foot boulders in front of it, the wall of water sped down the canyon slope. Cars, campers, and buildings in its path had no chance of survival.”

The canyon received the average year’s worth of precipitation within the first 4 hours after the rainstorm began. “The gauging station at the mouth of the canyon recorded peak flow of 883 cubic meters per second, four times higher than the previous record flood.”

The Big Thompson Canyon flood killed 145 people (6 were never found), destroyed 418 houses and damaged another 138, destroyed 152 businesses, causing at least $40 million in damages, in 120 long minutes.

Image Source: Water Resources Archive, Colorado State University. Image may be subject to copyright. Click image to enlarge.

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