This article appears in the Sept. 3, 2012 issue of Engineering News-Record’s Global Digital Edition
Warnings of dire consequences from earthquake-induced levee failures in the Sacramento-San Joaquin Delta of Northern California have frequently been raised over the years, but now a team of engineers from Southern California is testing the unusual soils from which the levees are built and upon which they stand to see if the oft-predicted failures during a quake are likely to occur.
Researchers from the University of California, Los Angeles’ George E. Brown, Jr. Network for Earthquake Engineering (NEES) have been conducting shake tests of model levees on Sherman Island in the Delta to “better understand how the unique peat soil of the Delta may respond to a seismic event,” according to a NEES statement.
The Delta is a region where two of the state’s largest rivers – the Sacramento and San Joaquin – join, and it is where freshwater from the rivers pushes against saltwater from the Pacific Ocean in San Francisco Bay, creating the West Coast’s largest estuary. It is composed of 700 mi of sloughs, winding channels, and 1,100 mi of levees. It is scattered with islands laced together with narrow roads and bridges, and with small communities, busy ship ports, farmlands, ranches, industrial sites, highways, historical sites and marinas.
The Delta also is a vital reservoir from which fresh water is withdrawn and distributed via aqueducts for hundreds of miles to supply 30% of Southern California’s water needs, including drinking water for 23 million people and irrigation water for most of the Central Valley’s farms.
Experts and state officials, including the California Department of Water Resources (DWR), a partner in the test, fear that a major levee breach in the Delta and the rapid flooding of the dry land it protects would draw a rush of saltwater into the freshwater of the Delta at the aqueduct’s pumping station, and the whole freshwater collection and transport system would have to be shut down.
Levee failures have caused flooding at least 140 times in the past 160 years, says the DWR, though none of the failures have been linked to an earthquake. But with major faults just west of the Delta, such as the very dangerously active San Andreas Fault, as well as the Midland faults and the West Tracy fault that are in the Delta and are capable of triggering earthquakes in the magnitude-6 range, researchers say it’s scientifically “worth knowing” what can happen to those levees in the event of a quake.
The team conducting the tests ran them on dry peat soil a year ago and recently returned to run a similar test on saturated peat soil, which is more representative of the actual conditions. Sherman Island, one of 57 levee-ringed islands in the Delta, is predominately a cow pasture owned by the state of California.
According to DWR, the underlying peat soil of the Delta came from decomposed tules — a large bullrush plant still common in the area. The peat extends as deep as 80 ft in some places.
Gold Rush miners started settling the Delta in the 1850s and they reclaimed the land from the swamplike conditions by building the levees, which were raised and strengthened over time. The peat was mixed with mounds of packed-down silt, clay and sand to build the levees, which can now be 20 ft or more above the islands they protect. Settlement of the drying peat on island interiors and wind erosion have also contributed to a lowering of the elevation of the protected land. Some Delta islands are deeply bowl-shaped, dipping as far as 25 ft below sea level.