Utah towns get a mud-flow warning
By Joe Bauman
Deseret News science writer
The Utah Geological Survey has a warning for Santaquin and other towns where homes were built on geological debris flows: It can happen again.
The Sept. 12, 2002, debris flow in Santaquin damaged many homes. The Utah Geological Survey says it can happen again.
Utah Geological Survey
The stage was set for a crisis thousands of years before Santaquin was settled. Mud, rocks and other debris slid down nearby Dry Mountain, in mass earth movements dating to before Lake Bonneville and during modern times.
Homes were built on top of "active" debris flow fans that date to within the past 10,000 years, UGS senior geologist Barry J. Solomon wrote in 2001.
On Aug. 8, 2001, a human-caused wildfire, dubbed the Mollie fire, started on Dry Mountain two miles southeast of Santaquin. By the time it was extinguished late in the month, it had burned to within a few feet of homes on the city's east bench, forcing evacuations, and destroyed about 8,000 acres of vegetation.
It also set the stage for new debris flows.
Richard E. Giraud of the Utah Geological Survey told the Deseret News the fire not only took out vegetation that had held the ground in place, it also baked organic material in the soil, turning it into a waxy substance that repels water. Rain is more likely to run off and less likely to soak in.
Aug. 31, 2001, right after the fire was out, Solomon toured the burned area with Santaquin city manager Roger R. Carter.
"The purpose of this letter is to describe debris-flow and flood hazards and provide you with an assessment of areas at Santaquin at risk from these hazards," Solomon wrote Carter on Sept. 17, 2001. "In summary, heightened debris-flow and flood hazards exist at subdivisions in Santaquin east of Interstate 15 as a result of the fire. . . . "
In October 2001, Santaquin Mayor A. LaDue Scovill warned that as little as a quarter-inch of rain falling in an hour could send tons of mud and debris flowing down the mountain and into homes.
Residents and public agencies began working to fill sandbags, instal fences to catch debris flows and establish an early warning system.
It wasn't enough to prevent massive damage. "On the evening of Sept. 12, 2002, intense rainfall triggered fire-related debris flows in multiple drainages of Dry Mountain east of Santaquin and Spring Lake at the south end of Utah Valley," says the new UGS report.
The rainfall was only a bit more than half an inch, but the denuded, baked soil could not absorb it.
Gigantic slides ó a devil's porridge of mud, rocks, trees and floodwater ó poured into two subdivisions, causing severe property damage. The report by Greg N. McDonald and Giraud, dated Nov. 20, 2002, says five homes and two businesses had major damage while 27 homes had minor damage.
The report says the mud moved vehicles, pushing some into houses. Basements flooded and filled with debris as ground-level windows broke; doors buckled; a home's back wall was broken by the impact of the flow.
"For the most part, it (debris) flowed around the buildings and into basement windows, and into the buildings," said McDonald, who hiked through drainages soon afterward.
Official damage estimate is $500,000, but Giraud said the real cost would be much higher if the use of the trucks, equipment and labor provided by cleanup volunteers could be factored in.
Nobody was killed in the slide, perhaps because the residents were so alert to the possibility. But in other incidents, other localities, residents might not be so lucky. A flow can suddenly fill a basement bedroom, killing occupants. It could take out a power line or break a natural gas pipe.
A debris flow has the consistency of wet concrete. "Once it dries up, it's got an incredible dry strength. You could drive on top of it," said McDonald.
Most of the material from flows comes out of the channels of tributaries. Sometimes the channels are scoured down to bedrock and may pose little future danger. In Santaquin's case, Giraud said, "There is ample material remaining in all of the drainages of Dry Mountain from what we've seen."
"Santaquin's looking at . . . a bunch of options," McDonald said. "I think they're considering putting debris basins at the heads of any of these fans where there's developments planned." Diversion dikes are another possibility.
In some places debris flows are so infrequent that people don't recognize the danger. But they could be deadly.
Giraud delivered a warning that city officials throughout Utah may take to heart: "When you permit development on alluvial fans, where debris flows can run out and deposit . . . houses (are) at risk."
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Thread: Utah Towns get Mud flow warnings
02-03-2003, 05:13 PM #1
Utah Towns get Mud flow warnings
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