BOULDER CREEK — Past dozens of burned playsets, cars and properties where all that remains is a single chimney, lies a geologic relic. Where most would see a mishmash of jumbled boulders, California Geological Survey scientists spot a debris flow.
“Geology tends to repeat itself,” David Longstreth said, an engineering geologist with the California Geological Survey.
This Boulder Creek debris flow occurred around 40 years ago, according to Longstreth. Measuring by geologic time, which spans more than 4.9 billion years, the historic flow is mere seconds in time.
“Debris flows tend to occur in the same location over and over, so we’re thinking another one could come here,” Longstreth said.
Investigating historic debris flows is paramount to get insight on which areas of Santa Cruz County could be at risk this coming winter and beyond. California Geological Survey scientists working with Santa Cruz County, and the U.S. Geological Survey, are tracking what triggers debris flows using drones, rain gauges and cameras.
Past a “DEBRIS FLOW HAZARD” sign on Highway 9 where work crews are clearing culverts in preparation for the rainy season, Longstreth spots another.
“Once your eyes get tuned in and you’re driving around here, you start seeing debris flows everywhere,” Longstreth said.
Historic debris flows underlie Boulder Creek, Brookdale and Ben Lomond roads, homes and businesses. It takes a trained eye to “see” these hidden deposits. To find them, Longstreth and other scientists scan satellite maps.
Longstreth estimates that the most ancient debris flows along the Highway 9 corridor could be more than 2 million years old.
That flow history, combined with relatively high rainfall rates, as well as steep slopes that drain into creeks and rivers near densely populated areas, are some of the factors that make the Santa Cruz Mountains vulnerable, Longstreth said.
“There is a huge risk for debris flows here,” Longstreth said.
The CZU August Lighting Complex fire also stripped many of those slopes of trees, shrubs and root systems that armor landscapes from debris flows and floods. During a storm, that means more water will runoff and quickly accumulate in streams.
California Geological Survey scientists David Longstreth and Alex Morelan walk up Big Creek, searching for ancient debris flows.
The CZU Complex fire, ignited by lightning on Aug. 16, burned more than 86,500 acres in Santa Cruz and San Mateo counties. In Santa Cruz County alone, 911 homes were destroyed in the blaze.
For the last month, the California Geological Survey has been on the ground examining ancestral debris flows and setting up monitoring gear. It’s been a seven-day-a-week job, Longstreth said.
“We’re exhausted, everyone in our department, has been working at breakneck speed to try to get everything implemented,” Longstreth said.
Desperate for data
Debris flows are fast-moving slurries of rock, soil and sediment carried in water, according to Amy East, a U.S. Geological Survey researcher collaborating with Longstreth.
“They’re so powerful, large debris flows pick up boulders, cars and can absolutely destroy a house very quickly,” East said.
While some debris flow monitoring sites are near residential areas, in Swanton, or off of Highway 9, others are in more remote places. Debris flows that are smaller in size, or happen in rugged areas, often don’t cause such destruction and aren’t recorded. Those flows can also be more challenging to track, but are equally important to investigate, East said.
Near the merging of Berry Creek and Big Creek, lie historic debris flows. This area was badly burned in the CZU August Lighting Complex fire.
Monitoring and studying debris flows at large in Northern California is a relatively new science. Most post-fire debris flow studies have been done in the southern part of the state.
East and other geologists want to understand what level of rainfall triggers debris flows in different regions of Santa Cruz County. Tracking the level of rainfall that triggers a flow is crucial for emergency planning efforts.
“Having the timing, and knowing what rain conditions were going on when the debris flow happened, will eventually guide evacuation protocols for this area,” East said.
East and Longstreth with the California Geological Survey have set up rain gauges and cameras at more than a dozen sites around the CZU Complex burn scar.
“We’re desperate to get data from Northern California,” Longstreth said.
Drones take flight
On the western side of the CZU Complex burn scar, California Geological Survey scientists are investigating a severely burned area near Swanton. Longstreth and colleague Alex Morelan walk along Big Creek, past charred redwoods and downed pines. Longstreth points out loose sandy soil and broken up rock that litters the hillslopes.
“That mud, sand and silt is what’s providing the mass in a debris flow,” Longstreth said. “It’s like a locomotive coming down and pushing the boulders in front of it.”
Those boulders are what pummel houses, destroy infrastructure and kill people. They can range from a couple of feet in diameter, to the size of a car, weighing up to a couple of tons. In the 1982 Love Creek landslide and mudflow, 22 residents were killed and nine homes were destroyed. The natural disaster remains the deadliest in modern Santa Cruz County history.
The geologists are once again searching for a symptomatic jumble of boulders, sand and soil. At the confluence of Berry Creek and Big Creek, they find multiple ancestral debris flows.
Part of the team’s monitoring work includes taking drone surveys.
“These surveys will help us make a more quantitative check on whether a debris flow did or didn’t happen,” Morelan, a geologist with the California Geological Survey, explained. Morelan also has a Federal Aviation Administration commercial drone pilot’s license.
It’s crucial to finish these surveys prior to the rainy season. Historically that wet season runs from November to May. During that time, the Santa Cruz County area might see anywhere from 23 inches to more than 30 inches of rain.
Alex Morelan, a geologist with the California Geological Survey, flies a drone on top of an ancient debris flow.
Between thin trunked patches of doug firs, and tan oaks Morelan has a narrow path — it’s tricky flying here, he said.
The flight lasts about 30 minutes, and Morelan takes hundreds of photos that will be transformed into a 3D model of the debris flow.
The team is training Colleen Murphy, a UC Santa Cruz geology graduate student researcher, who’s also a drone pilot. After storm events, Murphy will fly at these same monitoring sites and take images.
That post-storm flight will be compared to the baseline drone surveys Morelan’s completed – using that data the scientists will be able to quantify the volume of boulders, sediment and woody debris that moved in a flow.
Murphy says she and the other scientists will track boulder and water level movement at these observation sites, as well as watch if a stream becomes cloudy, or muddy looking. They’ll also be monitoring nearby culverts and bridges.
“A lot of what we know about debris flows has come from Southern California,” Murphy said. “Because of different topography, vegetation, climate, the threshold that triggers a catastrophic debris flow down there, might be different than thresholds in Santa Cruz.”
Tracking any changes at these historic debris-flow sites during the winter will help Murphy and other geologists understand what conditions create flows across Santa Cruz County.
In the long term, this data could also aid officials in making more precise evacuation decisions.
“You can’t cry wolf,” said the U.S. Geological Survey’s East.
“Evacuations are costly, they’re risky for people that have a hard time moving, like in a nursing home,” East said. The COVID-19 pandemic further complicates evacuations.
Emergency planners must tow a challenging line, East said. If orders are issued often, residents could tire of heeding notices and get evacuation fatigue.
“You also don’t want to go the other way and tell people you probably don’t need to evacuate, and then there’s a January 2018 Montecito situation,” East said, referencing the Santa Barbara County debris flows that killed 20 people.
If the USGS or California Geological Survey scientists confirm a debris flow happens, first they’ll immediately alert Santa Cruz County officials. While this information won’t be used in the short-term for evacuation orders, Longstreth said he hopes that by notifying the county if a culvert or bridge is clogged, emergency responders could remove that chokepoint before the next storm.
“Then the second priority is gathering the data over the long term, so that we can better predict when debris flows happen,” Longstreth said.
Longstreth said he and his colleagues hope to launch additional debris flow monitoring sites to get more data.
But for the USGS and California Geological Survey scientists to expand their research, the monitoring would have to sustain into the future. Both government agencies are paying for this research using their own limited budgets, in an as-needed manner.
That means it’s not clear how long the science can continue, Longstreth said. For the work to be consistent, the California Geological survey would need a funded program.
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“I’ve worked on a lot of these debris flow projects and I’ve seen a lot of people killed in similar circumstances,” Longstreth said.
“It’s a really bad feeling when that happens, and so it’s as much as it’s in my power to use my skills to help more people, I want to do that.”
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