News of the Arroyo
Title: | Study: Mountains charred by Station Fire could yield killer landslides |
Subtitle: | |
Date: | 2009-10-07 |
Summary: | October 7, 2009 - Look for more fire-caused devastation this winter when rainstorms hit the Arroyo Seco and San Gabriel Mountains. |
Author: | Rebecca Kimitch, Staff Writer |
Publication: | Pasadena Star-News |
Content: | A rainstorm could cause massive debris flows on hillsides denuded by the Station Fire, sending mud and boulders pouring into foothill communities, geologists warned Tuesday. A U.S. Geological Survey study found that the fire- charred San Gabriel Mountains could face the worst landslides the Southland has seen since Christmas Day 2003, when a deluge of flowing debris killed 16 people in the San Bernardino Mountains. How bad the debris flows get depends on the winter\'s rain, but it doesn\'t look good, according to USGS research geologist Susan Cannon. \"We have calculated really high probabilities of really big flows,\" Cannon said. \"Some of the areas burned by the Station Fire show the highest likelihood for big debris The El Prieto area of the San Gabriel Mountains. (Courtesy of Paul Ayers) flows I\'ve ever seen.\" The 160,000-acre blaze cleared vegetation that normally catches rainwater and helps direct it into the soil. It also altered the chemistry of the soil, creating just below ground a waxy, impermeable surface, the study found. The first rains that hit the burned areas will flush soil and debris down the forest\'s steep slopes into canyons, experts said. The powerful flow could destroy bridges, roads, structures, and lives. \"The impact forces can just be huge. If you have a deck sitting over a stream, no matter how strong that deck is, the debris flow will carry it away,\" Cannon said. The study looked at the potential impacts of a rainstorm that lasts three hours, which has a near 100 percent Advertisement chance of happening this winter, and a longer 12-hour rainfall, of which there is a 50 percent chance. Under both circumstances, researchers found a more than 80 percent probability of large debris flows for \"all but the smallest basins along the San Gabriel mountain front between Big Tujunga Canyon and Arroyo Seco.\" The study points particularly to tributary basins that drain into Pacoima Canyon, Big Tujunga Canyon, Arroyo Seco, the West Fork of the San Gabriel River, and Devils Canyon. One of these basins alone could produce 100,000 cubic yards of mud and rocks - enough to cover a football field with 60 feet of debris. Even a tiny drainage area can produce \"pretty big\" debris flows, Cannon said. The Los Angeles County Department of Public Works has been contacting residents who live along charred slopes to provide engineering advice on how to best protect their homes in the event of heavy rain. Anybody who has not yet been contacted should call the department, according to deputy director Mark Pestrella. While no rains are on the horizon for at least the next week, the National Weather Service is preparing for El Nino conditions, which typically bring heavy rains to the region, according to spokeswoman Jamie Stern. The Weather Service will issue televised warnings in the event of an emergency. The public should heed such warnings, Cannon said. Once it starts raining, debris flows can move \"incredibly fast,\" faster than a grown person can run, Cannon said. One recorded flow started 8 minutes after rainfall started. \"If it starts raining hard, you should be on your way out,\" Cannon said. Debris basins intended to capture debris before it reaches neighborhoods and city streets need to be regularly cleared by county workers, she said. Under a worst-case scenario - if catch basins are full before the start of a 12-hour storm - debris could reach up to about a mile from the San Gabriel mountain front. Debris flows from Cooks, Blanchard, Pickens, Mullally, and Hall Beckley canyons could be deposited close to Foothill Boulevard; and debris flows from Blanchard, Dunsmore, Shields, Goss, Hay and Gould canyons could reach neighborhoods. County Supervisor Michael Antonovich has been tracking the department\'s daily progress on preparing catch basins for impending flows, according to his spokesman Tony Bell. \"We are very confident in their ability,\" Bell said. Cannon hopes advances in USGS flow research have agencies better prepared to predict potential debris flows this winter. The level of awareness also has changed in recent years. \"People seem to understand now that what comes after a fire is debris flow, after it rains, the debris comes,\" Cannon said. Antonovich is encouraging residents to seek preparation advice through the Coordinated Agency Recovery Effort Web site and hotline -- dpw.lacounty.gov/CARE/ and 800-214-4020 -- which he helped establish to handle the fire aftermath. While agencies attempt to prepare residents for the aftereffects of the Station Fire, Assemblyman Anthony Portantino, D-Pasadena is questioning whether the fire needed to be so bad in the first place. Following up on reports that emergency officials were misguided in their response the first day of the fire, Portantino has called for the California Emergency Management Agency to respond to public concerns in a town hall meeting. Portantino sent a letter to CalEMA Acting Secretary Matthew Bettenhousen requesting his participation in a public forum. \"I don\'t want to take anything away from the work done by firefighters ... but what concerns me is how the fire response was coordinated between various agencies. I think there are legitimate questions,\" Portantino said. He is particularly concerned about the use of air resources. Portantino said Bettenhousen \"seemed interested\" but wanted to talk to staff internally before moving forward with a public forum. rebecca.kimitch@sgvn.com (626) 962-8811, Ext. 2105 |
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