Southern California mudflows triggered by fire and rain, with more possible

By David Jacobson, Temblor

southern-california-mudflow
This photo shows one of many homes swept away by a massive mudflow in Montecito, CA, in Santa Barbara County. At this stage, 15 people are confirmed to have been killed and at least 100 homes were destroyed. (Photo by: Daniel Dreifuss via Quartz)

 

As rescue crews continue to search for those missing in mudflows that swept across areas scorched by the Thomas Fire just last month, another eye must be kept on parts of Southern California that could be hit by more mudflows. As of Wednesday afternoon, 15 people are confirmed to have been killed by mudflows in Santa Barbara and Ventura counties. Additionally, approximately 100 homes have been destroyed and hundreds of people are waiting to be rescued.

montecito-mudflow
This photo from the Ventura County Sheriff’s Office shows a massive mudflow that destroyed homes and killed at least 15 people in Montecito, CA.

 

In the map below, produced by ESRI, the likelihood of debris flows in Santa Barbara and Ventura counties is shown. What should be highlighted from this map is that over 10,000 people live in the colored areas, and 60% of those people are in moderate danger of debris flows.

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This map, produced by ESRI, shows the likelihood of debris flows in both Santa Barbara and Ventura counties.

 

Under normal circumstances, the likelihood of debris flows would not be so high. However, just last month the majority of this area burned in the Thomas Fire, one of the largest ever in the state’s history. This means that vegetation that would have normally soaked up water is gone. Therefore, it does not take much rain to produce mudflows. Combine this with the intense rain seen earlier this week (up to 5.5 inches in parts of Ventura County), and the steep slopes of the area, and you have a disaster waiting to happen.

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Mudflows Tuesday morning completely blocked roads in Southern California. Areas in Santa Barbara and Ventura counties were those hardest hit and hundreds of people are awaiting evacuation. (Photo from: Mike Eliason/Santa Barbara County Fire Department/Handout/Reuters)

 

The timing of these mudflows comes almost 13 years to the day of a deadly landslide that struck Ventura County in 2005 that killed 10 people and damaged or destroyed about three dozen homes. That slide, in La Conchita, occurred after several weeks of on-and-off heavy rain. The ability to forecast where damaging debris flows may occur highlights the need to inform those who may be at risk. Unfortunately, even though Santa Barbara County issued mandatory evacuations for 7,000 people, those killed in the mudflow in Montecito were not in a mandatory evacuation zone. If you live in Santa Barbara or Ventura counties and are interested in seeing if you are at risk of a debris flow, ESRI’s interactive map can be used below.

 

References
New York Times
CNN
ESRI

  • Eric Fielding

    The ESRI debris flow map looks like it only includes the possible source areas for debris flows, not the possible downstream runout areas that were heavily affected in Montecito. I read somewhere that the affected area on Olive Mill Road was included in a FEMA flood map.

  • Conrad

    I believe you’re right. I have seen how the runoff from a major mud/land slide can cause major damage quite a distance from the occurrence. I worry that these maps may give some people a false sense of security. That said, I am not a geological, flood plane expert, and do not know the areas well enough to give much opinion on the risk to that area.

  • Ross Stein

    Good point, Eric. The ESRI maps therefore do not forecast damage, but rather the source areas for the mud. It’s ironic to think that the ultimate cause of the mudflows are the active Mission Ridge Fault System that jack up and steepen the Santa Barbara coastal mountains.

  • Eric Fielding

    Yes, the active faults uplifting the coastal mountains of Santa Barbara are ultimately responsible for both the steep slopes that were the source of the debris flow and the topographic elevation of the ridge that caused the intense orographic precipitation that provided the water.