Earthquake upgrades nearly complete at Los Angeles County dams
By Peter Fullam, Whittier Daily News
Posted: 02/03/14, 7:07 PM PST | Updated: 45 secs ago
LOS ANGELES >> Deep inside the Big Tujunga Dam in northern Los Angeles County, in a gallery just above the riverbed, engineer Michele Chimienti stands between the yellowish concrete face of the old dam and new light-gray concrete of the dam’s reinforcement wall.
“This dam can handle an 7.5-magnitude earthquake,” she says.
Four decades after the 1971 Sylmar earthquake prompted the state to invoke stringent standards for dams to withstand earthquakes, the Los Angeles County Department of Public Works is nearing completion of seismic rehab work on the county’s 14 major flood control dams.
The $94.8 million Big Tujunga seismic rehabilitation and spillway modification project was completed in July 2012. It was followed by a $2 million project to construct a riser — a reinforcement — on the Santa Anita Dam, completed in 2013.
There’s still valve and spillway work to finish on the Santa Anita Dam, work to seismically reinforce the Santa Anita Debris Basin Dam, seismic modification on the Eaton Wash Dam set for completion this month, and a study to determine whether work should be done on the Puddingstone Dam in Bonelli Regional Park.
The work should be wrapping up in 2016, according to Keith Lilley, principal engineer with the Department of Public Works, Water Resources Division.
Another operator of dams in Los Angeles County, the Army Corps of Engineers, is planning structural repairs for one of the four dams it owns and operates in the county, the Whittier Narrows Dam, which is at risk for overtopping or springing leaks in the event of a major storm – a once every 200 years event.
“Right now, we’re looking at completing the dam safety modification study in early 2016,” said Kathy Anderson, Whittier Narrows Dam Safety Modification Study Project Manager. “Shovels will be hitting the ground around late 2018 to early 2019.”
The flood control dams in the county serve a secondary role in water conservation. However, dams under seismic restrictions imposed by the state can’t fulfill the water conservation levels for which they were designed.
As Lilley explained in a series of interviews by telephone and email, a dam operating under a restriction may be required by the state to keep the reservoir at a level no greater than half full, or less in some cases.
During a large storm, the dam still will be filled.
“We can’t stop storm runoff from the mountains,” Lilley said.
The dam will be operated for flood control, with flood gates and valves opening to make controlled releases.
However, at the end of the storm, with the dam full or partially full, the department must quickly empty the water to the 50 percent level to meet the restriction requirement. Releasing the water into the ocean compromises the dam’s role in water conservation.
“If we are able to hold that water at the dam until there is capacity in the downstream spreading grounds, the water can percolate into the groundwater table,” said Lilley.
It can be pumped up later by water companies for local water consumption.
But the seismic restrictions have been necessary for safety, the engineer explained.
The higher the water level behind the dam, the more pressure on the dam. The force of the water combined with the force of an earthquake could exceed the strength of the dam.
The restrictions prevent the sudden release of water if the upper portions of a dam are not strong enough for a major earthquake.
“Upgrading the dams to the current very high standards prevents such an unlikely, but possible, dangerous event from occurring,” said Lilley.
The beginning of seismic restrictions can be traced to the 6.6 magnitude ‘71 Sylmar earthquake, which caused about $505 million damage in the San Fernando Valley area.
The quake almost felled the Van Norman Dam in Granada Hills. Officials ordered an evacuation downstream, affecting 80,000 people. Huge chunks of the dam fell off, leaving a gap only a few feet above the water level. Fortunately, the dam was only half full. A dam failure could have claimed more than 80,000 lives, a UCLA study later found.
The Pacoima Dam also sustained major damage, and three other dams had minor damage.
“After the ‘71 Sylmar earthquake, there was a whole different understanding of the seismic impacts on these large dam structures,” said Lilley. “The state, which oversees them, basically said you need to prove they’re safe under current standards or we’re going to put restrictions on them.”
The dams must to be able to withstand a “maximum credible earthquake,” the biggest earthquake that geologists think a particular earthquake fault can produce.
Initially, the department of public works conducted an analysis of all its dams and made modifications to the dams that required minimal upgrades.
But it wasn’t until after the 1994 Northridge Earthquake that the county seismic rehab project took off.
“After the 1994 earthquake, we began designing and implementing the more substantial rehabilitations to the remaining dams” that didn’t meet seismic standards, said Lilley.
Since 1994, the department has spent more than $165 million on dam rehabilitations for its Devil’s Gate, Big Dalton, Pacoima, San Gabriel, Sawpit, Thompson Creek, Eaton and Big Tujunga dams.
The department also plans to strengthen the Santa Anita Debris Dam, a 56-foot-high earth embankment dam downstream from the Santa Anita Dam, to meet the seismic requirements. That will allow the dam to hold more water for conservation.
In it’s final seismic strengthening project, the department is currently conducting geophysical tests at the Puddingstone Dam, a 147-foot-high earth and rockfill dam with a 250-acre reservoir that is a popular recreational attraction in Bonelli Regional Park. The analysis will determine whether any upgrades are needed, said Lilley.
“Our Reanalysis and Rehabilitation Program of our major dams should be wrapping up in 2016 with the completion of work at Santa Anita Dam and Santa Anita Debris Dam,” said Lilley.
“However, we are also looking at all of the auxiliary components on the dams, such as the inlet and outlet works, valves and gates,” he said.
That program is ongoing, he said.
The 56-year-old Whittier Narrows Dam, with about 1 million people downstream, is at risk of being undermined or overtopped during a major storm, Army Corps of Engineers studies have determined.
“After almost 60 years of settlement and damage from seismic events, the corps has determined that the Whittier Narrows Dam has a high risk of failure due to significant seismic, seepage and hydrologic issues,” said a corps statement.
Of the more than 600 dams the corps owns and operates, the Whittier Narrows Dam was determined to in the top 20 in need of repairs, said Douglas E. Chitwood, a senior engineer with the corps.
“It made the top 20 because there are very few dams with a million people downstream,” Chitwood added. “The concern is not that the dam fails, they’re concerned about the consequences should something happen.”
Chitwood said that after Hurricane Katrina devastated New Orleans, the corps began evaluating the structural risk of its dams.
A 2005 risk assessment found the 3-mile-long earth and rock Whittier Narrows Dam was at risk for overtopping, or springing leaks under the dam or through its wall.
There’s no immediate danger of the dam breaching, the corps said. (There’s no water behind the dam, currently.)
However, in the extremely unlikely event the dam failed while it was full, and the highest it’s ever been is just over half full, the consequences would extend all the way to Long Beach, said Greg Peacock, hydraulic engineer. The depth of the water would vary greatly depending on location, but in some areas it could be 10 feet deep.
A 2008 study confirmed that a heavy storm could wash away part of the dam.
However, no storm has come close to filling the 54-foot-high dam since it was completed in 1957. The highest level was recorded in a January 1969 storm when the water level reached 29.5 feet, The spillway gates don’t open until the pool level reaches 44.5 feet.
“Even in 1969, the pool was well short of causing a significant spillway flow event,” said a corps statement.
The corps is currently preparing to take the first step in the repair of the dam, preparing a Draft Environmental Impact Statement for the project.
The statement will identify and evaluate ways to correct the dams deficiencies.
Possible remedies include:
• Raising the dam by 5 feet;
• Building a weir or a barrier in the embankment to allow water to flow over the weir in the event of small floods;
• Placing a fuseplug, or a spillway, by taking out a section of the of the embankment and filling it with a concrete wall that would break at a lower elevation of water;
• Removing sediment and deepening the Santa Fe Dam Basin, which is upstream of the Whittier Narrows Dam, to increase upstream holding capacity.
“We are studying very carefully atmospheric conditions and updating them to state of the art,” said Brian Dela-Barre, dam safety program manager. “Originally, the dam design considered what they thought would be the highest possible maximum flood.
“That information has been updated,” he said. “If the stars aligned, theoretically, we could get an awful lot of water down there. So now the corps is developing a response to that improved information.”
