That is the biggest news in this story about the unreliability f the levees, particularly in the case of an earthquake, that the minimum level of protection the area should settle for is 500 years and the only option providing that is the Auburn Dam.
An excerpt.
Expert sees Katrina-like risk in state
Earthquakes, not hurricanes, could destroy levees built on unstable land, professor says.
By Matt Weiser - Bee Staff WriterPublished 12:00 am PDT Tuesday, October 24, 2006
Raymond Seed, a professor of civil engineering at the University of California, Berkeley, calls it the most expensive inch of soil in the history of the world.
Seed and a team of 37 engineers and hydrologists found that layer of soil beneath the 17th Street Canal levee in New Orleans when they investigated why the city flooded so badly during Hurricane Katrina last year.
It was a layer of organic material laid down across much of the city by another hurricane 250 years earlier. It had a consistency similar to "peanut butter and jelly," Seed said, and it was not detected by soil tests prior to construction of the modern 17th Street levee.
When Hurricane Katrina hit, a long stretch of the levee was pushed inland 52 feet, sliding on that slippery 1 inch of soil "like a cake sliding across a pan," he said.
That failure and others in New Orleans killed 1,482 people, caused at least $150 billion in damages, and displaced 400,000 people. A year later, 280,000 are still displaced.
Seed called these levee failures "the most costly peacetime engineering failure in history," and he said it could happen again in California.
"We can build levees better, but we didn't in this case. We either need to do this properly or do nothing at all. There's no middle ground. We can't have levees that are 'pretty good.' "
He spoke Monday in Sacramento at a science conference presented by the CalFed Bay-Delta Program, a joint state and federal agency charged with improving water supply and environmental quality in the Sacramento-San Joaquin Delta.
His drew parallels between the New Orleans disaster and similar risks in California.
The California Delta, for instance, is laced by 1,100 miles of levees, many built atop similarly unstable layers of organic matter.
Those levees keep salty ocean water out of the Delta's interior, helping to convey 60 percent of the state's freshwater to more than 22 million people statewide.
We don't have hurricanes, Seed noted. But we do have earthquakes, which could dissolve Delta levees by liquefaction. Multiple Delta levee failures would cause seawater to be drawn into the estuary, contaminating the freshwater supply.
"What keeps me awake at night is the seismic Armageddon," he said.
Another example is underseepage. In New Orleans, some levees were built atop porous layers of eroded and crushed seashells. Water pressure from Katrina's storm surge got into this material and "vaporized" levees.
In urban Sacramento, layers of porous sand exist beneath levees protecting thousands of people. Many have been fixed by building deep, impermeable barriers, called slurry walls, inside levees.
But Sacramento's Natomas area is one place where seepage remains a persistent problem. Just this spring, soil tests showed the problem is much worse than previously thought. In some areas of Natomas, seepage potential still exists below 30-foot-deep slurry walls that were completed in 1993.
Fixing the problem and attaining 200-year flood protection for Natomas could cost $400 million.
Addressing several hundred scientists from around the country at Monday's conference, Seed said it is vital to spend whatever it takes to understand the geology beneath levees. Then, he said, don't cut corners with the repairs, especially on levees protecting people.
"Why is 200-year protection enough? Five hundred would seem to be the minimum responsible level," he said.
