EARTHQUAKE RISK IN DELTA WORSE THAN EXPECTED, USGS FINDS……BUT, WATER SUPPLY SURVIVES

By Patricia McBroom

When California water officials predicted four years ago that many delta levees would breach in a bad earthquake, flooding the region and threatening the State’s main water supply, they didn’t have any specific site information to prove their point (see Delta Seismic Risk Report, 2005).  Now they do.  And the situation looks worse than ever.

            Soft delta soils amplify seismic waves by up to a factor of 15, compared to recordings in rock at Black Diamond Mines in Pittsburgh, the United States Geological Survey (USGS) in Menlo Park has found.  They monitored the action of ten small earthquakes (up to 4.2 magnitude) that have occurred in the East Bay since 2007 and were picked up by monitors at eight places in the delta, including four sites on levees.

Levees on Sherman Island, across the bridge from
Antioch, are among the most vulnerable.

            There had been some hope that the peat soils characteristic of the  Sacramento-San Joaquin delta would dampen seismic waves.  But the USGS recordings found no evidence of such an effect. On the contrary, the wave configuration had an unusual shape with a strong, consistent peak at a single frequency, as well as a strong amplitude.  As a result, vibrations were stronger at the top of the levee than at the bottom.

            “This means these levees are going to shake a lot harder than we thought and will probably lead to multiple failures, said Jon Fletcher, chief of the Earthquake Effects Project at USGS Menlo Park. Prior reports from the Department of Water Resources had projected that as many as 50 levee breaks could occur at once from Bay Area earthquakes, such as one on the Hayward Fault that runs from Oakland to Berkeley.

            Fletcher said soil conditions in the delta resemble those in the San Francisco Marina District where buildings collapsed in the 1989 Loma Prieta earthquake; in both cases, the soils are very soft and seismic waves move through with large amplitudes.

            The USGS findings, due for release later this year, were not included in California’s controversial “Delta Risk Management Study” (DRMS) which predicted that as many as 20 islands in the delta could flood simultaneously in a major earthquake, potentially causing salt water from the bay to contaminate the source of California’s largest water supply for urban and agricultural users.

            “I think the risk is worse than what DRMS has reported,” said Fletcher.

            But, fortunately for California water users, the consequences of such a catastrophe might not be as bad as the scenario offered up to the public for the past three years, according to new studies of an earthquake aftermath prepared for the Bay Delta Conservation Plan.

            “It’s not true that a major earthquake would mean the end of the delta and we’d never be able to use it again,” said Greg Gartrell, a hydrologist with the Contra Costa County Water District, who is familiar with the new modeling.

            “Yes, you get a lot of salt water coming in, but as soon as it rains, that water can get washed out.”  Pumps that supply California’s urban and agricultural water would have to stop for about three to four months, under the conditions studied, said Gartrell, and then could become operational again.  Most urban water districts have local water supplies to cover such a period.

            Agricultural communities would suffer the most (depending on the season) because they don’t all have backup, but for urban users, the challenge would be “difficult but not severe.” The Contra Costa district, which is normally supplied by delta pumps, has up to a year’s supply in accessible reservoirs.

            Loss of fresh water to the delta’s pumps in an earthquake has been a major justification water contractors have used for planning to build an “isolated conveyance” (aka, peripheral canal/tunnel) from further north on the Sacramento River. If urban water users can withstand such a catastrophe, there is less reason to build a huge tunnel that delta residents strongly oppose.

            The new modeling shows that after the salt water is pushed out by winter rains, the delta actually freshens, rather than staying salty – contrary to many scary scenarios. Flooded islands then act as a buffer to reduce salt water flowing in, said Gartrell, adding that there is time then to go in and fix some of the levees. 

            The new study is based on a major 7.0 earthquake on the Hayward Fault, using severe conditions for salt water intrusion, including a multi-year drought and simultaneous levee failures.  It found that salts can flush out fairly quickly, depending on how soon rains or releases from the reservoirs can get water flows going again – a period of 3 to 4 months, in this case.

            “A lot of hysteria is leading people to believe it (massive levee failure due to earthquake) will shut down the delta for a long period of time and maybe forever.  That’s not correct, so long as we plan properly,” said Gartrell.

“The world doesn’t come to an end,” even with massive levee failures in the delta, but the State “needs to do emergency planning now. We need a sense of urgency about this.”

State agencies have lagged behind on creating an emergency response plan and are now stockpiling rock on some delta islands – a bare beginning.  Local districts, however, have been working on emergency plans for years, as many engineers and local officials told the Delta Stewardship Council this summer.

But they have no money to implement their plans or bring them together into a regional emergency response.

“The agencies that have the money (Department of Water Resources, for one) don’t respond.  And the locals, who want to respond, don’t have the money,” said Ronald E. Baldwin, director of emergency operations for San Joaquin County.

“Empower people at the local level,” Baldwin urged council staff at the meeting last July. “If we can do that, and let the lowest (governance) level deal with the problem, we can take the burden off the State and Federal agencies,” he said.

This problem of depriving local authorities of power and money is not new for California or the delta.  It has consistently tied the hands of local engineers and experts who know what to do from close observation and are ready to move ahead with levee repair and flood control plans.  Two weeks ago, the State finally released the money for $120 million in levee repairs we wrote about in the August 10 post.

MBK engineer Gilbert Cosio welcomed the move but was cautious about the money flow.  “We’ll see how it goes,” he said.