Where's the Risk? An Analysis of the Sacramento-San Joaquin Delta Levee Risks

摘要

This paper presents the methods and results of a two-level risk analysis of the Sacramento-San Joaquin Delta and Suisun Marsh levees. The first-level objective of the analysis is to assist the Delta Stewardship Council in prioritizing and managing competing demands for investments in the 1,100 miles of levees protecting 140 Delta islands and tracts. The analysis included computation of Expected Annual Damages (EAD) and Expected Annual Fatalities (EAF) for each island and tract and development of an updateable application for estimating the relative risk among all islands and tracts. The risk analysis methodology also incorporated options for including the effects of climate change and sea level rise to determine hazards faced by Delta levees. After the levee investment priorities have been determined, the second-level objective is to reduce risk: should a levee be raised or strengthened? This paper is focused on the second-level analysis and is based on an evaluation of the current EAD and EAF risk curves for the Delta islands to determine which hazard- (river stage or return period) and consequence-levels contribute the most to total risk. The results of the analysis imply that, in many cases, the greatest risk reduction for a given Delta island or tract can be accomplished by improving the levees to be more resilient at the lower river or channel stages that occur during the more frequent, lower-hazard return periods. In only certain cases, does it appear that substantially raising the levee crest elevation would be needed to provide a significant reduction in EAD and EAF risk.