MUDSLIDES IN MONTEREY COUNTY, leaking levees in San Joaquin County, crumbling corners on San Francisco’s Great Highway — the wear and tear of atmospheric rivers hits cities and farmlands at the ground level.
Mud in motion is dangerous, and the constant battle to tame the threat is waged by public works crews. In good weather, they cap potholes and knock down loose rocks. In torrents, they brave rains to clear the roads. As the Bay Area prepares for more atmospheric rivers this winter, and for increasingly intense storms with climate change, civil engineers are studying ways to build stronger landscapes. But the price tag is high.
A Feb. 4 rainstorm caused a mudslide across Arroyo Seco Road in Monterey County for the second time in two years. In March 2023, a river of rain flowed across the road like a waterfall, eventually breaking a chunk off. Arroyo Seco is a cut slope road — like most mountainside roads, a flat terrace was cut out of the hillside to build it. Then suddenly the curve is steepened dramatically.
“All slopes are getting less stable with time,” said John Erich Rauber, senior civil engineer for Monterey County. “Like a doctor, you first stop the bleeding. You have to quickly clean up the road and deal with the symptoms and then you can start to figure out why things are happening.”
Rauber’s team always watches fragile areas. Ideally, he said, it’s best to try to build where things are most stable, but people like living on hillsides, along coasts and near rivers.
“You build the berms higher, but the flood waters get higher,” Rauber said. “And so, it’s almost more of a defensive posture. You’re kind of fighting this losing battle in some ways. The slopes that were stable yesterday are not stable today,” he said. What’s happening is a combination of natural forces and human impacts.
“The rivers are going deeper. Slopes are getting steeper. Rock is getting weaker. … You know at the end of the day you’re going to fail.” John Erich Rauber, Monterey County senior civil engineer
“The rivers are going deeper. Slopes are getting steeper. Rock is getting weaker. So, you’re inevitably trying to keep the patient alive,” Rauber said. “You know at the end of the day you’re going to fail.”
He estimated the cost of cleanup in Monterey County last year, including levee repairs after the Pajaro River flooded, was about $20 million to $30 million. The Federal Emergency Management Agency usually pays for 50 to 100 percent of the costs, he said, but their budget must cover damage in surrounding counties as well, so it’s always a competition for public funds.
The ways of water
“Water is very opportunistic, in a whole lot of ways we wish it wasn’t,” said Laura Sullivan-Green, professor and chair of the civil and environmental engineering department at San Jose State University. “It’s going to look for the weakest link and go for it.”
Sullivan-Green is a geotechnical engineer, which means she studies the behavior of earth materials. She tries to apply the natural principles of soil and rock to solve engineering problems. Clay soils hold water and grow heavy with rain. Sand repels water and rides in the surf of a downpour. It cuts lines through the clay that binds boulders to mountainsides. Anything that stands in the path of resettling debris will eventually become part of it.
They tend to use mesh with rocks on the surface to help prevent erosion because it’s harder to wash away than sand or clay, she said.
“The common term is riprap. If you look on the coast, those big boulders at the base of cliffs are there to try to reduce erosion. We use gravel when we engineer levees and place those larger rocks on the surface to try to prevent and reduce erosion.”
In the winter of 2023, high water pressured a levee in the south end of the San Joaquin River. Water started boiling up through the base, threatening farmland and causing the evacuation of some areas.
“If we were using a soil to build a levee or an earthen dam now, we would control the exact composition of that soil,” Sullivan-Green said.
The peat soils that made the Delta so fertile are not very durable, she said.
“That was happening 150 years ago, they took what they knew to be a clay material and they compacted it with equipment that they had at the time. In some cases, they had horses walk back and forth across it.”
Delta construction has changed from trial and error to a materials science. According to Austin Elliott with the U.S. Geological Survey, it’s possible today to build a levee that can withstand a major earthquake, but that would be a very timely and expensive endeavor.
“We have a lot of levees, particularly in the Sacramento area,” said Sullivan-Green. “They are being engineered now because they recognize the significant risk that the dated levees pose.”
Every time you add a foot of height to a levee, you’re adding several feet of width but that’s not always feasible in developed areas, she said. They can use steel or concrete to build a wall that sits on top or add steel sheeting below to cut off the amount of water that goes through the base of the levee.
“We can use synthetics to improve the strength of a levee, but they tend to be expensive,” she said. “So, we have to use that sparingly.”
From potholes to sinkholes
On city streets, cracks need to be constantly sealed to keep water from getting under the pavement and softening the lower soil layers. Heavy trucks hit the top and a piece of the pavement pops off. A pothole is born, and it can grow into a sinkhole.
“Most of the time when there’s a sinkhole in San Francisco it has to do with a sewer or a water main break or leak,” said Rachel Gordon of San Francisco’s Department of Public Works. In addition to sealing roads, there is a list of areas she watches vigilantly. For example, a part of the Great Highway along Ocean Beach broke off in the last big round of storms.
“Hill slides can happen on Telegraph Hill, Twin Peaks or O’Shaughnessy near Glen Park,” said Gordon. “Last year, there was one on Russian Hill. We haven’t had any huge boulders come down, rocks just kind of slide down the hill.”
Sometimes they bring in a contractor that will manually, with hand tools, knock loose rock off the hillside.
“It’s like a planned little rockslide,” she said.
They capture rock and fix the surface with physical barriers like metal netting.
With climate change bringing more frequent, and more severe rainstorms, the system of bandaging the damage and moving on will have to pivot to a more offensive approach, said Rauber.
“From an engineering standpoint, the water levels are going up. The wave action on the slopes along the coast would get more significant. We need to prevent more bluff erosion and add stability along the coast. Same things could happen in rivers,” he said.
“That’s not the worst thing that global warming would cause,” he said, “but it certainly would exacerbate a lot of situations.”