Before the spring wildflowers, our foothill communities may be suffering from mudflows this winter. The USGS just released its assessment based on two scenarios, a gentle, sustained 12-hour rain and a hard 3-hr rain storm. In both of the scenarios, the USGS predicts that Pacoima Canyon, Big Tujunga Canyon, the Arroyo Seco, the West Fork of the San Gabriel River and Devils Canyon would have an 80% risk of mudflows. This risk includes the basins that are tributaries to each of these canyons, too. Our geomorphology works against us here. To quote the USGS report: “The high probabilities from basins burned by the Station fire reflects the combined effects of the steep slopes throughout the area and extensive areas burned at high and moderate severities.” To my surprise, a gentle soaking rain could actually generate more of a hazard than the short duration hard rain, and it looks like the communities of La Crescenta and La Canada Flintridge will suffer the most, potentially. The USGS report suggests that Altadena will be spared mudflow hazards because the major drainages that burned, El Prieto and Millard, drain into the Devil’s Gate Reservoir. Remember, these are the results of modeling and real life experiences will be different. The 28-page report is available on the USGS website. You can also listen to an interview by KPCC’s Larry Mantle with Lucy Jones.
This is the bad news. The good news is that we have this information in advance of the winter rains so people can plan. For an unfortunate few, their homes are reported to be at so much risk they’ve been advised to install plywood on the windows and get out when it rains, according to the article in the Los Angeles Times. The County Flood Control District and Department of Public Works staff are working overtime to reduce the hazards by cleaning out all of the 28 debris basins, including expanding some of them. They have been holding frequent meetings for emergency response personnel in the foothills to plan and coordinate. And they have been meeting with communities and individuals to assess risks and draw up plans for protection. You can learn more at the County’s website.