Yet another winter storm is slated to send more rain to Southern California this week and we at RedZone think it warrants a little inside info on the risks of post-fire debris flows. Winter rains mean a higher potential for land-moving rainfall rates in areas where fires have scorched the landscape. After 2018’s tragic events that occurred in the Thomas fire burn scar, officials have not taken the potential of these continued storms lightly. Last winter, residents were evacuated near the Holy, Thomas, Cranston, Napa/Sonoma, and Woolsey fires (all burn scars from the last couple years).
Yet another winter storm is slated to send more rain to Southern California this week and we at RedZone think it warrants a little inside info on the risks of post-fire debris flows. The nation is experiencing its second El Nino effect in three years with it forecast to last in to the summer. What that means, is a higher potential for land-moving rainfall rates in areas where fires have scorched the landscape. After last years tragic events that occurred in the Thomas fire burn scar, officials have not taken the potential of these continued storms lightly. So far this winter, residents have been evacuated near the Holy, Thomas, Cranston, Napa/Sonoma, and Woolsey fires (all recent burn scars from the last couple years).
While the threat of the Thomas fire just recently diminished in Ventura and Santa Barbra Counties, residents were weary to hear that they are now being threatened by mudslides originating from within the burn area. Rain started Monday afternoon in the areas of Santa Barbara and Ventura Counties, with the peak rainfall being around 2:30 AM Tuesday morning. The rain came down in amounts up to an inch per hour over the burned area, which the incident commander reported as being a critical factor in the amount of sediment and debris being carried by this amount of water. The devastating debris flow ranged from Cold Springs Canyon to Toro Canyon, and wreaked havoc all the way down to Highway 101. The debris flow was so strong in some locations that it pushed homes off of their foundations and carried them several hundred feet.
First responders have been preparing for this incident since Monday morning by preemptively staging resources in the areas that were forecasted to be impacted the most severely. This strategic placement of resources was followed by officials releasing evacuation zones. The warning stated that all residents within mandatory evacuation zones should leave by 12 noon on Monday in preparation for the heavy rains that were forecasted for the area. Since the early morning hours of Tuesday, first responders have been in a search and rescue mode: still actively engaged in performing helicopter and contact rescues. The threat from this debris flow still remains and first responders are warning residents to stay away from the area if at all possible.
Over the last couple weeks of the Thomas Fire the Federal Burned Area Emergency Response (BAER) Team has been evaluating the blaze’s impact to the fire area and watersheds by predicting debris flow hazards should rains like this impact the area. The orange, red, and dark red areas were determined to have the highest probability of debris flow during a heavy rain event. The BAER team is currently embedded with the Santa Barbara and Ventura Office of Emergency Services to assist them in implementing response plans for communities downstream of the fire.
Santa Barbara/Ventura Flooding at a Glance
- 17 confirmed deaths related to the storms (These numbers are subject to change as the incident continues)
- 13 missing people
- At least 25 injured
- 50 rescues via helicopter hoists have been performed during today’s search and rescue operations.
- 1-6.5 inches of rainfall over the Thomas Fire area.
- Search and rescue efforts still remain priority with approximately 75 percent of the primary search completed in the debris flow area.
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While the community of Gatlinburg, Tennessee grieves their losses and starts planning how to recover and rebuild, a new threat confronts the area. An inch and a half of winter rain helped put out the horrific fire over the weekend, but as we’ve covered in the past, wildfire-scorched areas often have an increased risk of landslides and mudslides. Local fire crews have thus far reported several small landslides that are slowing their ability to access damaged areas.
How a Fire Can Increase Landslide Risk?
Depending on soil type and topography, vegetation and land cover have a significant impact on the stability of the soil. Under normal conditions, leaf litter and other surface vegetation slow the rainfall water moving down a given slope. This allows much of that moisture to permeate through the soil and drain into the water table or aquifer below, leaving the surface soil relatively stable. Even during heavy rainfall when surface soil becomes saturated, root systems from brush and trees help to keep the soil from moving downhill.
However, when vegetation is lost due to wildfire (or other reasons such as construction), the factors that keep soil in place are minimized, and there is greater risk that the soil’s surface tension in a given area is overcome by gravity and washes down the slope.
Gatlinburg is in the Great Smoky Mountains, a very old mountain range within the Appalachian Mountain region. Due to the age of the mountains and the region’s climate, the mountains themselves are very weathered, and have much deeper soils than the mountains in the Western United States. The region is also heavily wooded, so the roots of the dense vegetation help to stabilize the nearby soil. When the wildfires recently moved through the area, the vegetation and leaf litter was burned out, and the stabilizing root systems were compromised. Storms then came and assisted in firefighting efforts, but the lack of vegetation due to the recent burns caused a few small slides. Before winter brings snow to the normally wet area, an increased likelihood for flooding, mudslides, and landslides remains a worrying possibility. As of December 7th, there is little rain forecasted in the region for several days.
Visit ready.gov to learn about how to better protect yourself, your family, and your property from landslides and other hazards.