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Quake-Proofing Delta Levees

Yet another use has been found for unwanted sand dredged up from the Bay bottom - earthquake safety. State agencies plan to undertake studies this summer to determine if dredge spoils can be used to help shore up Delta levees against earthquakes, as well as floods.

The stability of 1,100 miles of loosely constructed levees on the 60 islands of the Bay-Delta system could be threatened in an earthquake, according to Greg Zlotnick of the Bay Delta Oversight Council. "If a major failure or breach should occur, it is generally conceded that a water quality crisis would ensue, likely leading to a complete and extended halt to the export and use of Delta waters," he says. Thousands of acres of wildlife and waterfowl habitat would also be permanently inundated.

The state Department of Water Resources plans to install half a million dollars worth of seismic monitoring equipment this summer to study the problem, officials say. Getting a handle on the problem is a department "priority," according to director David Kennedy.

Delta levees, which protect uninhabited farmland, differ from levees that protect towns and cities in that they are constructed simply by piling up a mixture of peat and other soils. Subsidence in the Delta's peat soils has left some islands up to 15 feet below sea level. There's a shortage of earth to repair the levees surrounding the islands. The Department's Curt Schmutte says it will take 50 million cubic yards of dirt to shore up the levees, mostly to keep fresh water from spilling on the islands.

The idea of tapping dredged material to supply that dirt got its first test in 1990 when 1,500 cubic yards were piled on the Sherman Island levee without adverse effect, says Schmutte. Last year tests adding 50,000 cubic yards to Twitchell Island levees produced no problems. This summer, the Department will conduct the biggest test yet when over 100,000 cubic yards are added to Jersey Island's banks.

"We've found no indication that dredged sediment imported from Simmons Island for levee rehabilitation has had an impact on water quality," Schmutte told the Central Valley Regional Board. But officials at the Board and U.S. Fish & Wildlife say no deter-mination has yet been made that would allow large-scale use of salt and contaminant-laden sands to repair levees.

Dredged materials could provide more earthquake-resistant repair materials than peat. "The problem is nobody knows what peat does in an earthquake," says the Department's Les Harder. "There's one school of thought that says peat could amplify the motion, and other scientists who say the soft soil could absorb it. These tests could tell us."

But the Department's $500,000 investment in the tests, including new instruments and seismographs installed on levees, won't be well spent until a quake actually hits. "In the 1980s we had three or four earthquakes that could have served as a test," says Harder, "and I think it's reasonable to assume there will be a similar number in the 1990s." Despite their investment in the tests, officials are quick to point out that Delta levees have not failed in a hundred years and that the water quality risk from a major earthquake remains remote. A comprehensive report on the levees will be released this February (see Now in Print).

Contact: Curt Schmutte (916)653-5422

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