What Would Happen if the Lafayette Reservoir Dam Failed? A Detailed Look at the Nightmare Impact

Lafayette, CA — A breach of the Lafayette Reservoir Dam could unleash a catastrophic flood wave, impacting communities across Lafayette, Walnut Creek, and beyond, the EBMUD said in a statement.

According to a detailed inundation timeline released alongside the statement, floodwaters would spread rapidly, threatening critical infrastructure, neighborhoods, and public facilities.

The Bay Area Telegraph’s analysis estimates that the death toll could exceed 1,000 in the event of a sudden, catastrophic failure without warning, such as that caused by 7+ magnitude earthquake.

Here’s a breakdown of what would happen in the minutes and hours following a dam failure at the Lafayette Reservoir, based on inundation maps released by EBMUD.

15 Minutes: Floodwaters Inundate Western Lafayette

The initial flood surge would strike areas immediately downstream of the dam, and parts of Happy Valley Road. These locations would experience water depths up to 30 feet, making evacuation nearly impossible without prior warning.

30 Minutes: Floodwaters Submerge Lafayette Elementary School and Nearby Preschools

Floodwaters would rapidly advance, reaching Joyful Beginnings Preschool, Lafayette Elementary School, and Lafayette Nursery School. Buildings in central Lafayette would be inundated with water depths of up to 30 feet in low-lying areas.

45 Minutes: Floodwaters Cover Downtown Lafayette and Stanley Middle School

The flooding extends further into downtown Lafayette, impacting Stanley Middle School, Lafayette Police Department, and adjacent neighborhoods. Local roads, including sections of Mount Diablo Boulevard, would likely become impassable, hampering rescue and evacuation efforts.

1 Hour: Floodwaters Spread to Saranap and Western Walnut Creek

One hour after the breach, floodwaters would reach Saranap and begin affecting western Walnut Creek. Residential neighborhoods and key roadways, including stretches of Boulevard Way, would face significant flooding, with water depths ranging from 20 to 30 feet.

1 Hour 15 Minutes: Floodwaters Reach Walnut Creek Intermediate School and Broadway Plaza

Floodwaters would push further into Walnut Creek, inundating Walnut Creek Intermediate School and the busy retail and commercial area around Broadway Plaza. Flood depths of 15 to 20 feet would disrupt businesses and major traffic corridors in the area.

1 Hour 30 Minutes: Floodwaters Overwhelm Downtown Walnut Creek and Kaiser Permanente Medical Center

Downtown Walnut Creek would face severe flooding, with depths of 15 to 20 feet threatening Kaiser Permanente Medical Center and nearby residential areas. The flood would overwhelm much of the city’s infrastructure, putting residents, patients, and emergency services at risk.

2–4 Hours: Floodwaters Spread to Northern Walnut Creek and Pleasant Hill

Over the next several hours, floodwaters would continue spreading north, reaching neighborhoods near Pleasant Hill Middle School. Water depths would gradually decrease to between 7 and 15 feet, but the damage would remain extensive across homes, schools, and businesses. By the four-hour mark, much of northern Walnut Creek and Pleasant Hill would be affected.

Points of Interest Affected

• Hospitals: Kaiser Permanente Medical Center-Walnut Creek could experience water depths of up to 15 feet, potentially disrupting essential healthcare services.
• Schools: Multiple schools, including Lafayette Elementary, Stanley Middle School, and Walnut Creek Intermediate School, lie in the floodplain and could be submerged in floodwaters ranging from 10 to 30 feet.
• Transportation: Key transit routes, such as Highway 24 and parts of Interstate 680, would likely be rendered impassable during the flooding. The BART station in Walnut Creek, a critical transportation hub, could also face flooding.
• Emergency Services: Local fire stations and police departments would be affected, potentially delaying response efforts.

The Potential Toll

Based on the inundation data, the population density of the Lafayette and Walnut Creek communities, and related dam failures like the 1975 failure of the Banqiao dam in China and the 1928 failure of California’s St Francis dam, our analysis suggests that 500-2000 people could perish in the event of a total failure of the Lafayette Reservoir Dam.

It’s important to suggest that this is a rough estimate based on the data available to us. We’ll work to refine it further.

It’s also important to note that emergency preparedness can go a long way. A major incident at the Oroville dam in 2017 resulted in damage, but no deaths, partly because the community was prepared and evacuations were successful.

Why Aren’t We Talking About This?

The City of Lafayette and its citizens have been locked in a multi-year, contentious discussion with EBMUD about plans to seismically retrofit the Lafayette Reservoir’s tower.

EBMUD appears to have released the data about this nightmare breach scenario in response to that controversy.

But that begs the question: given the potential catastrophic impact of a dam failure–which the California Division of Safety of Dams’ own analysis indicates could happen to the Lafayette Reservoir Dam in the event of a major earthquake–why is the only response a retrofit of the existing tower?

The DSOD said the Lafayette Reservoir dam is in “fair” condition and that the impact of a breach is “extremely high.”

Is retrofitting the tower enough? Or are other upgrades to this nearly 100 year old structure being considered?

Could the tower be preserved aesthetically, and new, modern flow control equipment be installed elsewhere in the reservoir, rather than betting on the structural integrity of a 90+ year old structure–even one that has been upgraded?

Beyond the integrity of the tower, is the dam itself strong enough to withstand a major quake? It’s not very comforting to live in the shadow of a structure with a “fair” rating whose failure would result in a 30 foot wave sweeping through your neighborhood, and not even knowing what the “fair” rating actually means.

In short, there’s an open questions about what to do with the tower. But given the nightmare data which has come out as a result of those questions, many citizens are likely left wanting to know more.

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