After Wildfire, Geologists Prepare for the Next Hazard: Debris Flows

As firefighters bring some of this year’s most devastating wildfires to heel, California Geological Survey (CGS) geologists are already scouting the blackened ground for signs of a secondary hazard – potential debris flows.

Debris flows follow infernos and can sometimes be just as dangerous to life and property.

What Causes Debris Flows?

After wildfires eliminate the trees and undergrowth on sloped areas, only bare soil, rock, and fire debris is left on the surface. When it rains hard and long enough in a burned area, that material turns into a slurry several feet thick traveling downhill faster than a person can run, capable of damaging structures, burying roads, and clogging streams.

CGS staff Dave Longstreth (right)teaches CAL FIRE’s Jim Kral how to test soil for burn severity at the French Fire.

One of the most dramatic examples of the havoc post-wildfire debris flows can cause occurred in the Santa Barbara community of Montecito in 2018.

Watershed Emergency Response Teams

As major wildfires are extinguished, CAL FIRE assembles Watershed Emergency Response Teams (WERTs).

CGS staff assessing burn damage from the Caldor Fire.

Co-led by CGS, WERTs quickly assess and communicate debris flow, flood, and rock fall hazards to communities that may be at risk – a key part of DOC’s commitment to hazard management and watershed protection.

CGS began assessing post-wildfire debris-flow hazards in 1972, following the Marble Cone Fire which impacted Big Sur. Soon thereafter, interdisciplinary teams began to evaluate safety threats from debris flows, flooding, and rockfall on non-federal lands, leading to today’s WERT.

2021 WERT Efforts So Far

Currently, CGS is participating in three WERT efforts:

  • The nearly million-acre Dixie Fire: Spread over five counties in northeastern California and 94 percent contained as of September 28. CGS scientists John Oswald and Don Lindsay serve as co-leads with CAL FIRE; Chris Gryszan is also in the field.
  • The Caldor Fire: Nearly 222,000 acres in El Dorado, Alpine, and Amador counties and 76 percent contained as of September 28. Lindsay is the co-lead with CAL FIRE, joined in the field by CGS colleagues Kevin Doherty and Clay Allison.
  • The French Fire: 26,000 acres torched in Kern County and nearly completely extinguished as of mid-September. Dave Longstreth of CGS is co-lead with CAL FIRE.

Just as important as those recording data in the field are those processing incoming Geographic Information System (GIS) data in the office. Justin Epting, Deshawn Brown, Kate Thomas, Rachel Beard, Pete Roffers, and Michael Falsetto have provided GIS support for all three fires.

CGS staff at the Caldor Fire

CAL FIRE doesn’t convene WERT after every wildfire, only the ones posing a debris flow or flooding hazard. In fact, between 2003 and 2021, teams assessed post-wildfire hazards for 81 fires.

“We participated in more than a dozen WERT teams last year,” said Bill Short, Supervising Geologist in charge of CGS’ Forest and Watershed Geology program.

“Fire season used to be May through October or November. Now, it seems like it’s almost year-round. While these are the first three fires we’ve responded to this year, we expect to have more requests for WERT responses based on recent history.”

CGS staff studying burned landscape from the Caldor Fire.

Review CGS’ on-the-ground assessments of 2020 California wildfires at 2020 Recent Landslide Hazard Assessments page.

CGS has a new Twitter account! Follow to stay current with CGS’ hazard and watershed management work. Follow Department of Conservation’s Facebook, Twitter, and Instagram for more on DOC’s role in managing California’s hazards, watersheds, and carbon while supporting the state’s economic development.

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