Thursday, January 21, 2010
Arroyo Seco from Devil's Gate Dam this morning
By Guy McCarthy
With more rain expected today on saturated, fire-denuded mountainsides, the U.S. Geological Survey and other officials warned evacuated residents to stay out of foothill homes below the Station Fire footprint.
"Based on all the rain we've had this week, if we get showers of any intensity there's still a chance of debris flows from the Station burn areas," Oxnard-based National Weather Service meteorologist Curt Kaplan said this morning.
"We do have potential for 2 to 4 more inches of rain in the mountains today," Kaplan said. "With a fast-moving thunderstorm, it may not take much to get things mobilized."
Evacuation orders issued to residents of more than 750 homes in the foothills remained in effect today, County Fire Inspector Matt Levesque and LAPD Officer Bruce Butterfield said in telephone interviews.
The evacuated areas of Little Tujunga, La Crescenta, La Canada and Glendale are in county and city jurisdictions.
Earlier this week, debris flows 8 feet to 12 feet high destroyed USGS monitoring equipment in Dunsmore Canyon and an unnamed tributary of Big Tujunga Canyon, USGS scientists in Pasadena said in a warning statement.
In the statement, titled "Southern California residents urged to heed evacuation orders as rain continues," USGS debris flow specialist Susan Cannon evoked two of the deadliest storms in recent Southern California history.
"The forecast rainfall for the next 48 hours is comparable to that which occurred during a 1969 storm that triggered landslides, debris flows and floods throughout Southern California, resulting in the deaths of 34 people," Cannon said.
"Because the hills above Glendora had been burned the previous fall, that area was particularly hard hit during the 1969 storm," Cannon said.
The storm forecast through today is also similar to the Christmas Day storm of 2003, which triggered debris flows from nearly every watershed burned by the Old and Grand Prix fires in the San Bernardino mountains, resulting in widespread destruction and the deaths of 16 people, the USGS stated.
The warnings might seem like overkill to evacuation-weary foothill residents.
But an aging flood-control system of debris basins and channels offers only partial protection below the burned areas, according to the county Department of Public Works.
Slope failures and debris flows are possible in some cases up to 72 hours after rains on burned areas, according to the Los Angeles County Fire Department.
"In Southern California, debris flows and floods have over history killed a comparable number of people as earthquakes," said USGS seismologist Lucy Jones. "These past deadly debris flows highlight that residents should not be complacent, and those with evacuation orders need to leave."
When Cannon evoked the post-fire storms' impact on Glendora 40 years ago, she was referring in part to two separate fires that denuded slopes above the community in July and August 1968, according to U.S. Forest Service records.
"The rainy season of 1968-69 provided a severe test to the disaster prevention facilities protecting Glendora," USFS researcher J.M. Rice wrote. The 1968 fires "denuded the slopes along 5 kilometers of the northern boundary of the city."
Subsequent debris flows during heavy rains in January 1969 destroyed six houses and damaged an additional 200 homes, according to Rice.
Glendora's vulnerability to post-fire and normal erosion stemmed from the fact that "immigrants to the San Gabriel Valley, where Glendora is located, failed to recognize the potential debris flow hazard to settlements on the debris cone," Rice said.
"They had little experience with mountains as precipitous as the San Gabriels north of Glendora. And they were probably unaware of the effects of intermittent brushfires that denuded the mountains of vegetation. Damages that resulted from the settlers' lack of foresight were modest until 1969."