Composite Risk Assessment for the Sacramento–San Joaquin Delta Levee System

Abstract

The objectives of this project were to analyze available Sacramento–San Joaquin Delta, California, levee risk information in a Composite Risk Management matrix and examine the results for management decision support. U.S. Army Corps of Engineers (USACE) guidance documents define risk as the “probability and severity of loss linked to hazards” and prescribe a composite risk assessment method. The Delta Risk Management Strategy performed for a group of state and federal partners provided analyses of the relative probability of hazards and severity of risks in the Delta and provide the information needed for a risk analysis compliant with USACE requirements. Composite Risk assessment provides rank-ordered lists of the highest risk zones–those with the greatest probability of failure combined with the most severe consequences–for several hundred protected areas in the Delta. Although uncertainties in the absolute magnitude of the results make them most useful for comparisons, the actual values of the probabilities and consequences are alarming. For example, Sargent Barnhart Tract, northwest of Stockton, has a mean annual failure rate of 0.07, or an expected levee failure every 14 years, with a probable 96 fatalities for a nighttime seismic-induced failure. Adjacent tracts with only slightly lower failure probabilities put another 500 lives at risk. An area of the Suisun Marsh has a projected failure rate of 0.5, or once every two years, with maximum possible damages exceeding $250 million. The Sacramento Pocket Area, with a mean annual failure rate of 0.006, has over $9 billion at risk. Although refinements to these risk estimates are possible, this paper and the Delta Risk Management Strategy analyses provide more than sufficient evidence that flooding in the Sacramento–San Joaquin Delta presents significant risks to California and the nation. Hundreds of lives and billions of dollar damages are at risk. Urgent action is necessary to manage those risks.