Californians are no strangers to the struggle of managing big blazes and the damage they leave behind. But the state’s record-setting Thomas Fire of late 2017 and early 2018 didn’t end when the flames went out. Late one January night, a little more than a centimeter of rain fell in 5 minutes, drenching a charred landscape in Santa Barbara County and sparking a mudslide that killed 23 people and destroyed 130 structures.
Among other cleanup efforts, local officials decided to scoop up mud, silt, and woody debris and deposit them at nearby Goleta Beach.
“The county was sort of between a rock and a hard place,” said Heili Lowman, a coastal biogeochemist who studied the disaster’s aftermath while at the University of California, Santa Barbara. The debris could have overwhelmed capacity at local landfills and wasn’t suitable for construction projects, so the decision to move it to the beach was officials’ “best, fast solution given the quick time frame they had to recover,” Lowman said. But she wondered what the impact of that quick action would be.
Lowman and her team examined the impact of speeding the transport of mud and debris to the ocean by analyzing sediment samples collected near the deposition site, both on land and up to 20 meters (66 feet) underwater. The study was published in Science of the Total Environment.
Speeding Debris’ Journey to the Beach
Typically, after a mudslide, debris would make its way from inshore environments to waterways through natural breakdown and movement. Quickly relocating burned materials to nearshore environments meant less breakdown than usual occurred along the way. Lowman tracked the breakdown of pyrogenic carbon, or charcoal, and by-products of lignin. Lignin is a structural component specific to land-based plants, so measuring its by-products over time helped researchers determine how quickly relocated organic matter was degrading in marine sediment, compared to original sediment deposited on the beach.
“The county’s approach assumed that [deposited material] was being removed from the beach and from the nearshore and traveling far out to sea,” Lowman said. “Instead, we found that while it was removed from the beach sediments, it stuck around and was buried in the nearshore, at least for the couple of months that we were sampling.”
Lowman and her team discovered that pyrogenic carbon, which comprises various types of partially burnt vegetation, remained in nearshore marine sediment for months. Debris of all kinds was less degraded than expected, likely because microbes did not have a chance to break it down en route to the nearshore environment.
Subsequent measurements from other research groups revealed that the waters off Goleta were no longer safe to swim in because of an overgrowth of fecal bacteria, a potential outcome of nutrient-rich material from the debris.
Debris Overwhelms Existing Infrastructure
The region has debris basins, built years ago, that captured some of the mudslide material. Yet although woody debris covered tens of kilometers of coastline, none of that debris would have naturally ended up at Goleta Beach, according to U.S. Geological Survey (USGS) research geologist Jonathan Warrick.
“The unusual thing was that the sediment included a lot of burned material this time around,” Warrick said, pointing to the conundrum local officials faced. “You do nothing, and this park and beach erode away. You try to add sediment, and sometimes things are not perfect,” he said. “It’s complicated, and it’s not an easy ‘Yeah, duh, don’t do this.’”
Climate change has added another layer of complication, according to Amy East, a USGS research geologist who also serves as the editor in chief of the Journal of Geophysical Research: Earth Surface, an AGU publication. “We know that California’s fire regime, and the fire regime of the western U.S., is intensifying because of a warmer, drier climate. That leads to expanded fire seasons, the fires happening at times of year when they were formerly uncommon,” she said, pointing to the Thomas Fire as an example of just that. “And then also we expect to see an increase in the type of intense rain events that turned it from a devastating fire to a devastating postfire debris flow situation.” Warmer atmospheres also hold more water vapor, which could intensify future rainstorms.
At the time of the tragedy, the Thomas Fire was the biggest in California’s history. There have since been a handful of substantially larger fires, including some followed by mudslides. As populations and fire intensities grow, the balance between defending structures and protecting ecosystems after disasters must be considered carefully.
“I hope that this study can serve as a reason for communities to be more cautious with just how much material is placed in the ocean at a given time,” Lowman said. “Just because it’s off the beach and out of sight doesn’t mean that it isn’t having additional impacts on the nearshore ocean.”
—Robin Donovan (@RobinKD), Science Writer