Assurances Fail to Halt Fears Over Dam : Water: Experts say chances are infinitesimal that three structures being built near Hemet will ever burst. But neighbors are leery. ‘It preys on your mind,’ one says.

They call themselves a support group, they meet monthly at the local, turn-of-the-century Methodist church, and if they have a single prayer, it’s for faith in the giant dams being built down the road.

The focus of their anxiety is the largest earth-and-rock dam project in the history of the United States, now under construction to create a huge reservoir.

These neighbors take little comfort in being told by engineers that the dams will be 99.999999% safe, or that the chance of a dam bursting during any given year is 1 in 100 million.

“It’s a potential for disaster, and it preys on your mind,” said Dave Ledbetter, a retired phone cable splicer who lives next to the reservoir site. “You hope and pray a disaster doesn’t happen. If it doesn’t, then this will have been an example of great engineering. But if it does, that means we’ve all been conned.”

The Metropolitan Water District of Southern California, which is building the $1.9-billion water storage project near Hemet, says Ledbetter and his neighbors can relax.

“We consider these to be the safest of all man-made structures,” said Dennis Majors, manager of the Domenigoni project. The site is free of earthquake faults and the dams are of proven design, the MWD and its consultants insist.

When completed, the three dams will close the open ends of the rural Domenigoni Valley in Riverside County and create the largest reservoir in Southern California. It will contain more water than Lake Havasu on the Colorado River, and will be available for public boating and fishing, with land around it set aside for camping, golfing and other outdoor recreational activities.

Most importantly, the reservoir, 4 1/2 miles long and 2 miles wide, will hold enough water to serve 15 million Southern California residents for about six months.

Its prime function will be to provide water in times of drought and peak summer use.

But if an earthquake along the San Andreas fault cuts off the region’s life-sustaining supply of imported water from Northern California or the Colorado River, the water of Domenigoni Reservoir will flow by gravity feed through pipes and canals--assuming they survive a quake--to water agencies serving parts of San Diego, Orange, Los Angeles, Ventura, San Bernardino and Riverside counties.

One dam with a base 800 feet thick will stretch two miles along the gaping, eastern opening of the valley and crest at 185 feet. A second dam, with a base 1,200 feet thick, will run 1.7 miles across the valley’s lower west floor and tower 285 feet above it. And along a saddle-shaped gap in one of the two ridgelines that parallel the valley, a third, half-mile-long dam will rise 130 feet.

The dams won’t plug a river and be at risk of overflowing because of floods--the most common cause of dam failures. Rather, the dams will rise above the agricultural valley where alfalfa and onions once were harvested. The site will then be filled with water delivered by aqueducts and canals--a process that may take as long as four years once the dams are completed in 1999.

Ledbetter and more than a dozen of his neighbors meet monthly to commiserate about the project.

Among other concerns, they worry that if a cataclysmic earthquake doesn’t fracture the dams, the mere weight and depth of the 260 billion gallons of water may trigger earthquakes by lubricating heretofore unknown tectonic faults nearby or adding quake-inducing stress to the local bedrock.

In addition to MWD’s own consultants, seismologists at the U.S. Geological Survey and Caltech have concluded that no active faults run beneath the site. An inactive fault approaches the reservoir from the south but stops short of it by a mile or so.

Daniel Pradel, a private geotechnical engineer, remains skeptical about the project. He was hired to challenge the safety of the dams by the Domenigoni family, which gave the valley its name but which has sued the MWD for its takeover of some of the family farm and the alleged depreciation of the family’s remaining property.

Pradel said there may be undiscovered faults beneath the reservoir site as in the previously undetected thrust fault that caused the 1994 Northridge earthquake.

But Clarence Allen, a renowned Caltech geologist and one of six outside consultants hired by the MWD to provide independent design oversight, said the region sits atop the Perris Block, whose geological characteristics would make a blind thrust fault “exceedingly unlikely.”

“The Perris Block is an amazingly coherent piece of real estate as compared to most of the rest of California, and much more coherent than anything around Los Angeles,” he said.

And USGS research geologist Doug Morton, who is not connected with the MWD project but who has studied the region’s geology as a UC Riverside professor, said the Domenigoni Valley is “as stable a site as you’ll get in Southern California.”

Still, so-called reservoir-induced seismicity is a slim possibility, the experts concede. At deep-water reservoirs around the world--including Lake Oroville in Northern California--earthquakes have been attributed to the stress the water loads have put on the local geology.

In 1975, a magnitude 5.9 earthquake struck about seven miles south of Lake Oroville, along a fault that runs beneath the lake. The temblor did no damage to the 770-foot-high earth-and-rock structure, which is the tallest dam in the country and holds back more than four times as much water as the capacity of Domenigoni.

But neither the depth of the Domenigoni Reservoir--260 feet at its deepest point--nor the bedrock geology beneath the site lend themselves to reservoir-induced seismicity, Allen and others say.

There is less than a 1% chance that the reservoir might trigger “small” tremors in the vicinity--quakes that would have occurred over time anyway, but triggered prematurely and unleashing less energy, said Joseph Ehasz, designer of the Domenigoni dams.

Like others working on this project, Ehasz--who has designed dams around the world--said that despite the project’s size, the construction of the Domenigoni dams does not pose unique challenges.

“This isn’t rocket science,” he said. “These dams are long, but otherwise they’re not real exciting. And it’s comforting to know that we’ve done this before.” There are about 74,000 dams in the United States--most of them earth-fill.

As the Domenigoni construction progresses, the work is being inspected by the state’s Division of Safety of Dams, which must approve the finished project before the reservoir can be filled.

Domenigoni project manager Majors said the slow filling of the reservoir will give safety inspectors even more time to check the dams’ integrity.

Still, few public works projects stir such visceral reaction and anxiety about public safety as the construction of large dams, because of deadly dam breaks over the decades. The MWD knows this.

More than five years ago--even before the Domenigoni site was selected--an MWD public relations consultant prepared a “strategy paper” on how to address the dam-safety issue with the public.

And even before the MWD completed its design of the Domenigoni dams, the agency went on record stating that the chances of a catastrophic dam failure were 1 in 100 million.

That level of probability was determined by multiplying the various probabilities of circumstances that could lead to a dam burst, based on worst-case scenarios.

It assumed, for instance, that a 7.5 magnitude earthquake would erupt on a segment of the San Jacinto fault where it most closely approaches the valley, about eight miles away near Hemet. Scientists then calculated how much the ground would shake at Domenigoni. The largest earthquake ever recorded on that fault was 6.8, near Hemet in 1918. A 7.5 magnitude quake would be about seven times more powerful.

According to a separate panel of scientists, this San Jacinto Valley segment of the fault is in a location with the highest probability in Southern California for the next major quake. The experts, working under the auspices of the Southern California Earthquake Center at USC, in January assessed the probability of a large quake along the San Jacinto Valley segment at 43% in the next 30 years.

They said the maximum magnitude of such a quake would be 6.96, more than twice as powerful as Northridge but several times less powerful than a 7.5.

Because the law requires it, the MWD is preparing--for the state Office of Emergency Services--a flood inundation study based on a worst-case scenario--the one that engineers say is all but impossible: the total collapse of the dams at Domenigoni with the reservoir at full capacity.

The worst-case scenario was developed by Tom McDonald, a hydrologist with Woodward-Clyde Consultants on behalf of the MWD for local officials. The scenario--which is not required to be filed with the state until the dams are completed--is horrifying:

* The total collapse of the eastern dam would flood the western fringe of Hemet with up to 10 feet of water within six hours. Portions of Sun City, near Interstate 215, would be under as much as 20 feet of water eight hours after the breach. As the water coursed along the San Jacinto River and dumped into Lake Elsinore, it would raise the lake level by as much as 35 feet and flood adjoining areas.

* Some of the water would then flow north, through Corona, before dumping into the U.S. Army Corps of Engineers’ Prado Flood Control Basin, where its release into the Santa Ana River would be governed. Some of the Lake Elsinore overflow would travel south into Murrieta, flooding parts of that city by as much as 10 feet of water, and then pass through Temecula.

* With the unimaginable total collapse of the taller western dam, the waters would rampage down Warm Springs Creek and into the Santa Margarita River where it narrows in a gorge at Temecula. The lowest parts of Temecula would be under 60 feet of water within 10 hours of the breach, and Interstate 15 would be impassable. The water would then flow in a more governed fashion along the Santa Margarita River through Camp Pendleton and out to sea.

Such a worst-case scenario is simply the stuff of late-night horror flicks bearing no semblance to reality, according to the MWD’s consultants and independent experts.

They say that earthen dams are the best for earthquake country because of their ability to “self-heal.” That is, unlike a brittle concrete dam, an earthen dam is designed to plug its own leaks as its soils resettle under weight and hydraulic pressure.

When the 6.7 magnitude earthquake struck Northridge last year, the earthen, 122-foot-high Los Angeles Dam seven miles away in Granada Hills settled about three inches and began to leak water, but the leaking later stopped as predicted, said Mitchell Sakado, a geotechnical engineer with the Los Angeles Department of Water and Power.

Ehasz, the designer of the Domenigoni dams, said the structures might settle about a foot if a 7.5 magnitude earthquake strikes eight miles away.

While failures of modern dams are exceedingly rare, they leave legacies of finger-pointing and questions of why they occurred.

In 1963, the asphalt-encased, earth-and-rock Baldwin Hills Dam burst with just a few hours’ warning, unleashing 300 million gallons of water that killed five people and destroyed or damaged 300 structures.

Scientists later surmised that the dam break was triggered either by the extraction of oil in the area or the high-pressure injection of water into the ground to improve oil production, activating one of three faults that ran beneath the reservoir.

After the disaster, a feud embroiled Los Angeles city and county officials over how the tragedy could have been anticipated and avoided, based on pre-existing information about ground movement beneath the dam before it was even built.

Similarly, the federal Bureau of Reclamation was skewered by congressional and independent investigations after its just-completed, 305-foot-tall, earth-fill Teton Dam in Idaho was breached in 1976, killing 11 people downstream and causing more than $500 million in property damage.

The dam failure was blamed on inadequate exploration of the fissure-riddled abutments alongside the dam, which allowed water that was swiftly filling the reservoir for the first time to erode the dam. The Bureau of Reclamation also was criticized for not employing outside experts for independent oversight of the dam’s construction, and for not heeding a warning about geological hazards at the site by the USGS.

Pradel, the private engineer critical of the Domenigoni project, said he will always remain wary of dam safety. “Even with the best engineering and the best consultants and the best knowledge, we can only reduce the risk, but we cannot avoid failures,” he said.

Dave Achterberg, chief of dam safety for the Bureau of Reclamation, says such cynicism is healthy. “There’s always some potential risk,” he said. “If we go a long time without a dam failure, we as an industry can become complacent. And we can’t, when there’s so much at stake.”

Cindy Domenigoni said concern about the dams’ safety “is a fear we carry with us all the time.”

But she said she is not sure whether her family, which lives in a farmhouse about half a mile below the western dam site, will move.

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Building the Dam

The Metropolitan Water District is constructing Southern California’s largest reservoir, in a valley near Hemet, to provide emergency storage of water. The lake will be contained within the 4 1/2-mile-long, 2-mile-wide Domenigoni Valley by three earth dams encased in rock, which engineers say are more resilient in earthquakes than concrete dams, which are more brittle.

Dam Material

Material is being excavated from the valley site.

1) Each dam’s pyramid-shaped core will consist of a nearly impermeable mixture of fine sand and clay.

2) A three-foot-wide concrete wall will extend below the base of the dam to bedrock.

3) The core material will be protected on the reservoir side by a shell of quartzite, silt and sand.

4) The core will be protected on the downstream side by quartzite.

5) About five gallons of water per minute is expected to seep through the core and will be funneled into crushed quartzite and drained into the downstream valley.

6) The dam will be supported by a compacted shell of quarried rock up to three feet in diameter, interlocked by smaller rock.

Source: Metropolitan Water District of Southern California