How To Draw Burn Scars

CZU burn scar is geologically different — and that might be reducing debris-flow risk

Jeff Nolan, the geologist for Santa Cruz County, watched Tuesday night's storm pass over the CZU burn scar from the county's command incident center in Felton. Flanked by officials from the sheriff's office, several fire agencies and meteorologists, he monitored the pace of the rain passing over a landscape damaged months earlier by a fire unprecedented in size and intensity.

No one knew exactly how the burnt terrain would respond, but everyone knew there was a chance the rain could set off a deadly torrent of mud and rubble known as a debris flow.

The highest intensity rain observed overnight was a little more than half an inch — 0.56 inches to be exact — in a 15-minute span. That's well above the threshold of 0.3 inches in 15 minutes that was the county's threshold to trigger evacuations.

But no significant flows of earth were observed. And that has officials re-thinking whether the Santa Cruz Mountains might be less vulnerable to debris flows than previously thought.

"We've kind of tested some of the thresholds," Nolan said. "Going forward, there may be a chance for us to adjust the threshold upwards for when we start worrying."

Nolan stressed that the rain isn't over, "so we're still concerned," he said. But based on what he has observed so far, he's beginning to think that "the risk of fire-related type debris flows [is] probably not as high as the models predicted" for the CZU scar.

Predicting debris flows is incredibly complex, and geologists trying to forecast this risk in central and Northern California are hamstrung by the fact that they have rarely occurred in this area, so there is very little data on how this terrain reacts to intense fire followed by heavy rain.

The modeling done to understand debris flow risks after a fire is conducted mostly by the U.S. Geological Survey. The models they use are empirical, meaning they are based on observational data.

Noah Finnegan, a geologist at UC Santa Cruz who studies river erosion and landslides, says most of that data comes from southern California, particularly the San Bernardino and San Gabriel mountains, because that's where debris flows have occurred more frequently in the past.

"Folks in the California Geological Survey, or Cal Fire or in the USGS, they are all acutely aware of the fact that the models that they're using are not necessarily developed for the landscape of the Santa Cruz Mountains," he said. "It's different in pretty fundamental ways from the places where the data are drawn from that the model is actually built on."

The CZU burn scar (top) is among several scars at risk of debris flows in the Monterey Bay area.

Those southern mountains are covered in chaparral, while ours are covered in redwood trees. The rocks are different, even the storms are different, Finnegan explained. There are many reasons why the current models don't perfectly capture the dynamics of how debris flows will get started in the Santa Cruz Mountains.

"The fundamental physics are super complicated, and so you can't really start from f=ma and come up with a prediction, like you can with a weather model," Finnegan said. "Instead, what we're really doing is just relying on observations. And for that to happen, we have to have a few events to get a sense for how it works."

After the Lockheed fire burned a wide swath of land in the north Swanton/Davenport area in 2009, Finnegan and his team gathered data during the storm season that followed.

"There was an incredible storm that happened in October after the Lockheed fire, it rained like 12 inches in October in that year, and so we were able to make some observations there," he said. "Nothing too bad happened, but the landscapes got pretty close to producing debris flows up in the Swanton area. It's those kinds of data points where we can get fires and storms, and we can observe what happens — that's what enables people to get that local knowledge."

Finnegan wasn't surprised that the Tuesday night storm didn't end up triggering any flows. Like Nolan, he emphasized that this storm isn't over yet, as rain continues to fall Wednesday and is expected to intensify in the night.

"[It will be] interesting to see what happens tonight," Nolan said Wednesday morning. He was in the midst of surveying post-storm damage after just a few hours of sleep.

He'd been at the command center until 2:30 a.m. "It's too early to really draw any firm conclusions," he said.

But, "I'm feeling more confident" things will be OK, he added.