Tyler Island levee was breached during a 1986 flood

Subsidence and Levee Stability in the Bay Delta

Building up land surface through sequestering carbon has the added benefit of reducing the hydraulic force on Delta levees, the only barriers that protect below-sea-level islands from the surrounding waterways. In the long term, this could significantly reduce the risk of levee failure and the cost of maintenance while providing greater security to California's water supply.

LIVING WITH POSSIBLE LEVEE FAILURE

Approximately 1,100 miles of levees need to be maintained. Levee failure has been common in the Sacramento-San Joaquin Delta since reclamation began in the 1850s. About 100 levee failures have occurred since the early 1890s.

EARTHQUAKES

The Delta sits atop a blind fault system on the western edge of the Central Valley. Moderate earthquakes in 1892 near Vacaville and in 1983 near Coalinga demonstrate the seismic potential of this structural belt.

The increasing height of the levee system has prompted growing concern about the seismic stability of the levees. The concern is based on the proximity of faulting, the nature of the levee foundations, and the materials used to build the levees.

Many levees consist of uncompacted weak local soils that may be unstable under seismic loading. The presence of sand and silt in the levees and their foundations indicates that liquefaction is also a possibility. Although no historic examples of seismically induced levee failure are known in the Delta, the modern levee network has not been subjected to strong shaking. Levees were either smaller or nonexistent in 1906 when the region was strongly shaken by the great San Francisco earthquake.

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U.S. Department of the Interior | U.S. Geological Survey
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Page Last Modified: Wednesday, 19-Oct-2011 13:22:05 EDT