A Potential Flood Threat Is Hidden in the East Bay Hills — Chabot Dam

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Chabot Dam, 1919.  (East Bay Municipal Utility District)


f the Lake Chabot Dam cracked open in a big earthquake, what kind of flooding should the communities below expect? A tsunami that knocks down houses? Sidewalk streams?”

Hollyann Vickers Keng posed that question to Bay Curious as she and her husband were checking out the possibility of buying a home in San Leandro. The house hunt in the East Bay town, just south of Oakland and stretching from the East Bay Hills to the shore of San Francisco Bay, included something unexpected.

“You know, we were expecting to see on the disclosure things about earthquakes,” Vickers Keng says. “But I was not expecting to see anything about a flood zone. It was totally surprising to me.”

Doing her own research, it didn’t take long to find out why there is a flood threat for many homes in San Leandro: Chabot Dam and the 3 billion gallons or so of water stored in Lake Chabot, the reservoir behind the dam.

But it wasn’t clear to Vickers Keng just what the threat might look like in the streets of San Leandro if the worst happened and the dam gave way.


“What kind of flood are we talking here?” she says. “Do I need to buy a canoe? Should we have life jackets in our emergency backpacks?”

If people know about Chabot Dam at all, it’s likely because Lake Chabot is the centerpiece of a popular regional park. But for the tens of thousands of people who travel through San Leandro every day on freeways, and even for many residents, the dam doesn’t register as a threat. The structure is virtually invisible even as you drive the winding road through the heavily wooded hills toward the lake.

When it was built, over an 18-year period from 1874 through 1892, the dam was one of the largest in the world. It’s a massive earthen embankment about 135 feet high, positioned strategically at the mouth of the rugged canyon where San Leandro Creek once cascaded from the hills toward San Francisco Bay.

The potential hazard the dam poses goes beyond the fact it’s a century and a half old and that it’s perched above a densely populated plain. There’s this, too: The structure was built within a few hundred yards of the Hayward Fault.

Why is that significant?

Scientists say that over the last 900 years, the fault has produced a powerful earthquake — magnitude 6.3 or higher — every century and a half on average. The last one? Just over a century and a half ago. That 1868 quake devastated the then sparsely populated East Bay, caused widespread damage in San Francisco and killed 30 people.


o go back to our original question, what would happen if Chabot Dam “cracked open” and emptied the lake behind it onto the neighborhoods below?

The dam’s owner, the East Bay Municipal Utility District, has a very straightforward response.

“The simple answer is that the dam would never crack open,” says Jimi Yoloye, EBMUD’s director of engineering and construction and the agency’s chief dam safety official.

“The reality is that the dams are designed such that they don’t just crack open,” he says. “You will see signs of a failure if one is to occur. And the dams are designed with monitoring equipment if that were to occur.”

The state Division of Safety of Dams lists the downstream hazard for Chabot Dam as “extremely high.”

It’s one of dozens of Bay Area dams and about 250 across the state with the highest hazard rating, which is based on the potential loss of life and property destruction that might occur in a catastrophic dam failure.

The state requires dam owners to create and file inundation maps depicting what would happen in that kind of disaster. And the inundation map for Chabot Dam shows an area stretching from the base of the hills in San Leandro all the way to Oakland International Airport that could suffer some degree of flooding if the dam broke.

Yoloye says the maps “are intended to show what is the worst possible scenario if the dam were just to suddenly disappear.”

He adds that decades of improvements, ongoing inspections and regular reevaluations of the dam’s condition and seismic vulnerability would prevent that “worst-possible-case” scenario from occurring.

But still: What if? What will happen if — or when, actually — a big quake shakes the dam?

Fifteen years ago, at the direction of the Division of Safety of Dams, EBMUD commissioned an independent study of that very question. The analysis estimated the effects of a 7.25 magnitude earthquake on the Hayward Fault, an event that would release nearly five times the energy of the 6.8 temblor that struck in 1868.

The study found the dam would remain standing, but not without damage.

The 7.25 magnitude quake “could cause the dam — the top of the dam — to slump by as much as 3 feet,” Yoloye says, summarizing the study’s findings.

Other damage could be expected, too, including some cracking of the main dam embankment. But crucially, the analysis found that because the top of the dam is 23 feet above Lake Chabot’s maximum surface level, there would be no uncontrolled release of water.

That analysis led to a 2017 project to reinforce the dam embankment and retrofit a seismically vulnerable outlet that dam operators can use to release water from the lake.

The work on the embankment focused on strengthening the soils in the structure’s downstream base, or toe, using a method called concrete deep soil mixing. The technique involved boring deep into the soil and injecting concrete to form soil-cement columns designed to maintain the embankment’s stability in an earthquake.

Crews working on the 2017 upgrade of Chabot Dam. The project included work to strengthen soils at the dam’s downstream base, or toe, using a technique called cement deep soil mixing. (Courtesy of A3GEO, Berkeley)


ollyann Vickers Keng’s question is a really good one because it reminds us that all dams pose risks of some kind.

Given the nature of the job they perform — holding back huge volumes of water that can turn from placid lake into deadly torrent if unleashed all at once — they deserve very close attention. When that attention lapses, catastrophes can and do happen.

In fact, history is full of examples of deadly, destructive dam failures.

One legendary collapse occurred in western Pennsylvania in May 1889. Floodwater overflowed a badly maintained private dam in western Pennsylvania, triggering its collapse. A wall of water raced down the valley of the South Fork of the Little Conemaugh River, sweeping everything before it as it descended on the community of Johnstown.

“Most of the people in Johnstown never saw the water coming; they only heard it; and those who lived to tell about it would for years after try to describe the sound of the thing as it rushed on them,” historian David McCullough wrote in “The Johnstown Flood. “… Everyone heard thudding and screaming, the earsplitting crash of buildings going down, glass shattering, and the sides of houses ripping apart.”

The disaster wiped entire towns off the map and killed more than 2,200 people.

A view of the Sisters of Charity Building in Johnstown, Pennsylvania, after the dam collapse and flood of May 31, 1889. (Library of Congress)

California owns its own special chapter in the history of dam disasters, with a tragedy more than 90 years ago in a remote canyon 50 miles north of downtown Los Angeles.

So much water leaked through and around sections of St. Francis Dam — a facility on a stream called San Francisquito Creek built to hold water shipped south from the Owens Valley via the Los Angeles Aqueduct — that locals had taken to joking about the structure’s imminent collapse.

Then, at 11:57 p.m. on March 12, 1928, the dam disintegrated — just hours after the 2-year-old concrete structure was pronounced sound by Los Angeles water chief William Mulholland.

When the dam broke apart, a wall of water raged more than 50 miles to the Pacific Ocean, killing about 450 people along the way.

It wasn’t clear at the time why the dam failed. Mulholland, who had designed and overseen construction of the dam, suspected Owens Valley saboteurs. Later investigations have concluded the dam collapsed because it was built on a massive but undetected ancient landslide that moved when the dam’s reservoir filled.

Remnants of the St. Francis Dam, in San Francisquito Canyon, northern Los Angeles County. The dam collapsed on March 12, 1928, killing about 450 people. (M.M. O'Shaughnessy Photograph Collection/The Bancroft Library, UC Berkeley)

Those events — 130 years ago, 90-some years ago — sound like ancient history. But there have been much more recent disasters and near-disasters, too. A string of dam failures across the United States in the 1970s killed at least 250 people.

Much more recently, California got a lesson in how dangerous and costly a failure of even part of a major dam can be.

The 2017 collapse of the spillway at Oroville Dam, in the northern Sierra foothills 130 miles northeast of San Francisco, touched off a series of events that led local officials to order 188,000 people to flee their homes. In the aftermath, an independent forensic investigation found the spillway was badly designed and poorly built. Inspections over the year had been cursory and maintenance ineffective.

Ruins of the main spillway at Oroville Dam reveal a blend of "fresh" (blue-gray) rock and "weahered" (reddish-brown) rock underneath.
Ruins of the main spillway at Oroville Dam reveal badly eroded areas of rock beneath the concrete structure. (California Department of Water Resources)


o how is Chabot Dam different from all those bad dams?

To understand EBMUD’s confidence that a dam built within a quarter-mile of a dangerous fault will stand up to a violent shaking, let’s take a look at how the dam was first built nearly 150 years ago and how it’s been maintained since.

Who built Chabot Dam? There’s more than one answer to that question.

Anthony Chabot, who had launched a series of profitable ventures supplying water to San Francisco, Oakland and other Bay Area cities, conceived the idea for the dam, scouted the site on San Leandro Creek, helped design the massive structure and supervised its construction.

Work began in early 1874. The San Leandro Record newspaper described the busy site and said “a grand thing is being achieved.”

The Record also noted a small army of workers, including 500 immigrant laborers from China. They did the backbreaking work of digging a series of foundation trenches and outlet tunnels, then building up the dam’s huge embankment.

Jacqueline Beggs, a retired East Bay Regional Parks ranger who led history tours for more than 15 years at Lake Chabot, says Chinese laborers were the unsung heroes in building what at the time was the world’s largest embankment dam.

She says few who look at the dam today can appreciate the enormous labor involved in building such a structure essentially by hand.

The process of building the dam’s dense clay core involved removing soil from the surrounding hills and carting it to the site. Wagonload after wagonload would be dumped, soaked with water, then packed down by teams of wild horses that had been captured in eastern Oregon.

“It was such horrific work, and they had to hand dig so much, and that’s why we call them the ‘shovel-men,’ ” Beggs says.

By 1876, the dam rose 100 feet above the original creek bed, and stretched 500 feet across the top. Chabot’s privately held company delivered water to homes throughout Oakland from the new reservoir.

It was about this time that the Oakland Tribune raised an alarm. What would happen, the paper asked, if the mighty new structure failed?

“The terrible consequences that would result from the bursting of this dam and the sudden rush, through the gorge below, of this vast body of water unloosed to instantaneously deluge the valley, can scarcely be conceived,” the Tribune wrote. “… In the twinkling of an eye it would annihilate the fruits of years of industry, destroying millions of dollars’ worth of property, and send more than a thousand human souls unheralded into oblivion.”

But the dam stood.

A 1920s view of Lake Chabot, in the East Bay Hills above San Leandro. The main embankment of Chabot Dam is at the lower right. (East Bay Municipal Utility District)

Work continued through the early 1890s raising the dam another 10 feet or so and adding huge amounts of fill sluiced onto the dam’s embankment, which is about 1,000 feet thick at its base.

Not long afterward, the dam got a real-world test. The Great 1906 San Francisco Earthquake devastated the city, likely killed thousands and ruptured the San Andreas Fault for 300 miles.

Although the quake was powerful enough to generate a 3-foot-high wave on Lake Chabot, the dam emerged with no apparent damage.

In more recent times, heightened awareness of the dangers posed by the Hayward Fault led to several studies in the 1960s and ’70s. Those analyses found the dam would remain stable in a major earthquake but prompted EBMUD to raise the embankment, add a modern spillway to supplement its original outlet tunnels and make other improvements.

Chabot Dam weathered the 1989 Loma Prieta earthquake unscathed. That was the last big event in the dam’s history before the new studies of the dam conducted in the early 2000s and EBMUD’s subsequent work to further strengthen the structure.

Jimi Yoloye, EBMUD’s engineering and construction chief, says the key to understanding the dam’s condition as it ages is continual attention. That includes a program of state-required annual inspections and the use of remote electronic monitoring — instruments that can detect changes in moisture levels or movement — to keep an eye on conditions in the dam embankment. Longer-term studies are needed, too.

“We routinely, probably every five to 10 years, we do an evaluation of the seismic stability of the dam, with the latest seismic engineering and information, to make sure nothing has changed and that the dam is still in safe condition,” Yoloye says.

The ongoing attention aside, there is something of a wild card in this discussion of what might happen to Chabot Dam and the waters of Lake Chabot in the event of some sort of cataclysm on the Hayward Fault.

And that is the presence of another, larger dam and reservoir just upstream from Chabot — Upper San Leandro Dam.

The state-mandated inundation map for the upstream dam and reservoir shows the catastrophic failure of Upper San Leandro Dam would breach Chabot Dam and submerge much of San Leandro and parts of East Oakland under 10 feet of water.

But EBMUD says Upper San Leandro Dam — a dam built in the 1970s after studies found a 1920s-era dam might suffer serious damage in a major earthquake — is extremely unlikely to suffer that kind failure.

The utility says the most recent analysis of the new dam, in 2011, found it would perform “satisfactorily” in a magnitude 7.25 quake on the Hayward Fault.

And that brings us back to Hollyann Vickers Keng, our Bay Curious “question asker.” How does she feel about the dam and the risks it poses now?

She and her husband wound up buying a house in San Leandro, downstream from the dam. She says hearing that the seismic risk to the dam is taken seriously — and that it’s been upgraded recently — does give her some peace of mind.

“In a way, I feel extremely reassured to know there are some kind of dam safety people and, like, engineers who are thinking about this,” she says.

And we’ll end with this observation:

Dams are a little deceiving. When we look at them, we see them as fixed, immovable, unchanging objects. But they’re really living, breathing structures that must withstand awesome physical forces. They are constantly changing in barely detectable ways.

It’s good to keep an eye on them, and it’s essential to make sure the agencies that own and operate them are doing that, too.