A Post Rainstorm Analysis: Debris Flows, A Highway Collapse And A Near Miss

Feb 3, 2021

 


With last week’s winter storm behind us, damage assessments are underway. KAZU News spoke with some of the people who predicted what could happen with a rain storm of this kind to see if their fears came to fruition. 

After the Dolan Fire scorched almost 125,000 acres in the Big Sur area, a team was called in to see how the landscape would react under heavy rainfall. The team studied hydrology, geology, soils and much more and came up with predictions. 

“So indeed, what we thought could happen, did happen,” said Kevin Cooper, a retired U.S. Forest Service Biologist. He led the team that looked into these post-fire hazards. 

Cooper hasn’t been able to inspect the damage himself, because of access restrictions. But from the information he’s gathered so far, some of his predictions came to life, including the possibility of debris flows, which he’s seen photos of, on an access road in the eastern section of the Dolan Fire burn scar. 

“So that had five, six foot diameter boulders come quite a bit down out of the mountains. We expect that if that happened there, that would probably happen in quite a few places,” Cooper said. 

Roads were a big concern for Cooper and his team when they looked at what a significant rain storm could do to this area. And those concerns played out with the collapse of Highway 1 near Rat Creek. 

Last week's rain storm caused the collapse of Highway 1 at Rat Creek and damaged other sections of the coastal roadway in approximately 60 locations. 
Credit Caltrans

He says this particular spot has a steep drainage system that’s about a mile and half long. When water comes off the mountain towards the ocean it's supposed to go through a culvert or drainage tube under Highway 1. That culvert is not that big, Cooper says. 

“The size of that culvert, it looked to be pretty minimum to handle this kind of event,” he said.  

And so with a huge amount of water, debris flowing in it, that culvert got plugged pretty quickly. Cooper explained how the collapse continued to play out. 

“Now you've got a full on just a dam face across this creek and it backs up and forms a small reservoir, a small lake if you will behind the highway. And then once that gets filled with water, it starts to flow, has to go somewhere, it goes over the top of the road,” Cooper said.  

At that stage, because water is rushing with such force and off a steep slope, the highway would... 

“It'll give way and just collapse to its original, if you will, topography,” said Cooper. 

More than 20 miles of Highway 1 has been closed because of impacts of the storm. Caltrans says they’re working to clear debris, remove water accumulation, and stabilize damaged sections of roadway at approximately 60 locations. 

Cooper says repairs at Rat Creek specifically may need to include some changes. 

“If they don't take into account this possibility, by making a bridge or something else, it's always going to be a risk,” said Cooper.   

The Dolan Fire area wasn’t the only burn scar of concern in Monterey County during last week’s storm. The River Fire burn scar, east of Salinas, produced mudflows that closed roadways and damaged homes. 

Monterey County Regional Fire Protection District worked with property owners around River Road in Salinas after they experienced mudflows when heavy rain fell over the River Fire burn scar.
Credit @mcrfd on Twitter.

Further north in Santa Cruz County, the fears of storm hazards around the CZU Lightning Complex fire burn scar were significant. That blaze burned more than 86,000 acres. 

But for those involved in predictions of what a major rain storm like last week’s could do, the county got lucky, this time.  

“We did get spared to a great extent,” said Jeffrey Nolan, the Santa Cruz County Geologist. “We didn't get the kind of rainfall totals that you got in Monterey County.” 

For Nolan, it was almost like a near miss. 

“You know if that storm track had been five or 10 miles further north than it was, we might have had a pretty different result,” said Nolan. 

But it's important to note that certain parts of this burn scar did get a significant amount of rainfall that exceeded thresholds that can cause debris flows. 

“We exceeded those intensities in a few areas in the burn scar for a relatively brief amount of time,” said Nolan. 

The geologist says after thorough analysis, those thresholds, that are used to order evacuations, could be updated for future storms. 

As we peer into the future, these natural disaster predictors are keeping one thing in mind -- winter isn’t over. And that means neither is the rainy season and the risk of life-threatening post-fire hazards.