SANTA CRUZ COUNTY (CBS SF) — The containment last week of the CZU Lightning Complex fires brings a sigh of relief, but residents shouldn’t relax too much as new natural disaster risks follow the fires in Santa Cruz County – debris flows.
Debris flows, essentially massive landslides, are triggered after high-intensity but short-duration bursts of rain. Short can mean as little as 15 minutes of heavy rain fall.
While landslides have always been a threat in steep, hilly areas like those of the Santa Cruz mountains; the threat has been exacerbated in and around burn areas following the CZU fire.
This is because the 37-day blaze eroded flora and their roots, leaving loose soil and rocks that, when saturated by rainfall, create a moving mass that can travel over 30 miles per hour, or at avalanche speeds.
“The debris flow threat affects not only homes on hillsides or within the burn area, but also homes on flat ground a mile or more from charred slopes,” county Senior Civil Engineer Carolyn Burke said during a presentation Tuesday to the Board of Supervisors.
That means that many residents who evacuated because of the recent fire are likely to evacuate again, and residents with houses burned in areas at risk of landslides could see rebuilding efforts delayed.
This reality poses a handful of challenges, Burke said.
The primary challenge is informing residents of the risks and helping them understand that early evacuation is essential and lifesaving.
“Unlike fires and floods, there is no way to fight or ride out a debris flow in place,” Burke said. “Early evacuation is the only sure way to survive a debris flow event. If you see or hear the debris flow, it is too late.”
Statistics from the 2018 landslide in Santa Barbara County following the Thomas Fire showed that 75 percent of residents evacuated for a fire but only 28 percent heeded evacuation orders for the landslide.
Statistics also showed that those who evacuated before for the fire were less likely to evacuate again, according to Burke.
“County staff recently met with Santa Barbara officials,” Assistant Director of Public Works Ken Edler said. “They said evacuation preparation must be done early. They stressed that we need to get the message out that debris flows need to be taken as seriously as a wildfire.”
The fire also significantly changed the county’s landscape, leaving those who have never been at risk of landslides unable to base predictions on the history of their property.
“Even if your home has been safe for the past 30 or 50 years, we are grappling with a post-fire burn scenario that we have not seen in decades longer,” Burke said.
In response to these looming challenges, geological experts and county leadership are acting fast before rain season starts in November, Burke said.
So far, the county has developed a Watershed Emergency Response Team (WERT) to create a map indicating high probability and locations of debris flows.
“The highest risk areas are in the western burn areas, which correlates with higher burn severities. This should not be confused with the areas of highest life safety risk,” Burke said. “One issue identified early on … was that WERT burn severity estimates were likely lower than field conditions. This means the probability shown on these maps should be assumed to be higher than listed.”
According to the preliminary WERT map, areas around Scott Creek, Big Basin State Park and Cascade State Ranch have a 60 to 100% risk of debris flow. The more east, the less risk.
There is already a preliminary map available to the public, but Burke said it is not accurate yet.
“The threat is real and worse now more than ever. The only sure way to keep yourself safe from debris flows is to evacuate and evacuate early,” Burke said.
The county will be releasing an updated map within the next two weeks and increased debris flow information to residents. It is also working on evacuation plans that will be shared later this month.
Residents are encouraged to create evacuation plans early, register with the county’s CodeRED alert system, and download the app for smart phones.
To register with CodeRED, visit http://www.scr911.org and add (866) 419-5000 to your contact list to know when CodeRED is calling.
To download the CodeRED app visit: https://www.onsolve.com/solutions/products/codered/mobile-alert-app/.
The preliminary WERT map is online at https://landslides.usgs.gov/hazards/postfire_debrisflow/detail.php?objectid=299.
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