UCLA

The approximately 1,100 miles of levees in the Sacramento-San Joaquin Delta is critical to aquatic and terrestrial habitat, agriculture, California’s water supply and distribution system, and other infrastructure investments, and the levee system protects them from flooding and salt water intrusion. However, the levee system is threatened by a variety of hazards. Land due to oxidation of the rich Delta peat soils, and due to sea level risk act together to effectively increase the levee hydraulic loading. Consolidation of peat soils beneath levees can lead to their continued settlement over time. Delta levees are also threatened by potential sudden shocks from floods events and earthquakes. Numerous advances with greater proliferation and more sophisticated methods of risk assessments have been made since the most recent risk study of the Delta was completed. Therefore, assessing multi-hazard risks of the Delta levee system by leveraging newly available data and knowledge is of great importance for decision makers to implement improvements in response to those long-term and short-term stressors.This study primarily focuses on seismic risk assessment of Bacon Island in the central Delta. The seismic capacity, demand, spatial correlations of levee systems, and system reliability analysis are four essential components throughout the seismic risk assessment. Newly available LiDAR, bathymetry data, geotechnical site investigation results, and measurements from advanced geophysical tests significantly facilitate determining geometry, soil stratigraphy/layering, and soil property of levees. Consequently, the levee fragility functions which reflect the system seismic capacity are developed from a large number of time-series nonlinear finite element simulations using OpenSees. An overview of updated probabilistic seismic hazard analysis results for the Delta region is discussed. Moreover, an algorithm for selecting a subset of events for hazard-consistent analysis of spatially distributed infrastructures is introduced, and performed to analyze the regional probabilistic seismic hazard analysis of the Bacon Island levee system, which quantifies seismic demand of the levees. The correlation functions of capacity are derived based on field geophysical measurements and geo-statistics analysis. Furthermore, the system reliability analysis using level crossing statistics method is implemented to assess seismic risk for Bacon Island levees based on the developed levee fragility, correlation lengths, and selected event subset.