Californians are familiar with the struggle to control large fires and the damage they cause. But the state’s record-breaking Thomas fire in late 2017 and early 2018 wasn’t over when the flames went out. On a late January night, more than an inch of rain in five minutes drenched the scorched landscape of Santa Barbara County, triggering landslides that killed 23 people and destroyed 130 structures. .
Among other cleanup efforts, local officials decided to scoop up mud, silt and wood debris and place them on nearby Goleta Beach.
Heili Lowman, a coastal biogeochemist who has studied the aftermath of disasters at the University of California, Santa Barbara, said: The debris overwhelmed the capacity of local landfills and was potentially unsuitable for construction projects, so the decision to move to the coast was the best and quickest, given the timeframe required for the agency’s “rapid recovery.” It was a great solution,” says Rowman. But she wondered what impact her swift action would have.
By analyzing sediment samples collected near deposition sites, both on land and in up to 20 meters (66 feet) of water, Rowman and her team investigated the effects of transport rates of mud and debris into the ocean. I checked. This research Comprehensive Environmental Science.
Accelerate the journey of debris to the beach
After a landslide, debris typically moves from the coastal environment to the waterway by natural decay and migration. Rapid transfer of burned material to the coastal environment meant fewer breakdowns along the way than usual. Lowman followed the decomposition of pyrogenic carbon, or charcoal, and lignin by-products. Because lignin is a structural component that is unique to land plants, measuring its byproducts over time allows the researchers to measure how quickly the displaced organic matter was decomposed in marine sediments from the original sediment deposited on the beach. I was able to compare and judge.
“The county’s approach [deposited material] They were taken away from the beaches and coasts and moved far out to sea,” Rowman said. “Instead, we found it stuck to the shore and buried, at least for the months we were sampling, while it was removed from the beach sediment.”
Roman and her team found that pyrogenic carbon, which makes up various types of partially burned vegetation, persists in coastal seafloor sediments for months. Debris of all kinds was degraded less than expected, probably because microbes didn’t have a chance to degrade it on its way to the coastal environment.
Subsequent measurements by other research groups revealed that the waters off Goleta were no longer safe for swimming due to fecal bacterial overgrowth, a potential consequence of nutrient-rich material from the litter. rice field.
Debris overwhelms existing infrastructure
The area has a debris basin built many years ago that caught part of the landslide. But Jonathan Warwick, a research geologist with the United States Geological Survey (USGS), said that although woody debris covered dozens of kilometers of coastline, the debris naturally found its way to Goleta Beach. would not have been.
“What’s unusual is that this time there was a lot of burned material in the sediment,” Warrick said, pointing to the conundrum faced by local authorities. If you try to add more sediment, it may not be complete,” he said. “It’s complicated, it’s not easy. ‘Oh no, don’t do this.'”
Climate change has added another layer of complexity, according to Amy East, USGS research geologist and editor-in-chief of the magazine. Journal of Geophysical Research: Earth’s Surface, AGU publications. “We know that the fire regime in California and the western United States are intensifying because of the warm, dry climate. “And there are also the types of intense fires that go from catastrophic fires to catastrophic post-fire debris flow conditions,” she said, citing the Thomas Fire as just one example. We expect to see an increase in rain events.” The warmer atmosphere also holds more water vapor, which could intensify future storms.
At the time of the tragedy, the Thomas Fire was the largest in California history. There have been several fairly large fires since then, including one that followed a mudslide. As populations and fire intensity increase, the balance between protecting structures and protecting ecosystems after a disaster must be carefully considered.
“We hope this study will give communities a reason to be more cautious about how much material is put into the ocean at any given time,” said Lowman. Just because you can’t see it, doesn’t mean it’s not having further impacts in nearby waters.”
—Robin Donovan (@Robin KD), science writer